DEPARTMENT OF COMMERCE

BUREAU OF STANDARDS

George K. Burgess, Director

REPORT OF THE

NATIONAL SCREW THREAD

COMMISSION

(REVISED, 1924)

MISCELLANEOUS PUBLICATIONS, BUREAU OF STANDARDS, No. 61

DEPARTMENT OF COMMERCE

BUREAU OF STANDARDS

GEORGE K. BURGESS, Director

REPORT

OF THE

NATIONAL SCREW THREAD COMMISSION

(REVISED, 1924)

(AUTHORIZED BY CONGRESS, JULY 18, 1918, H. R. 10852)

AS APPROVED AUGUST 19, 1924

FEBRUARY 11, 1925

MISCELLANEOUS PUBLICATIONS

OF THE

BUREAU OF STANDARDS

No. 61

PRICE 35 CENTS

Sold only by the Superintendent of Documents, Government Printing Office Washington, D. C.

WASHINGTON

GOVERNMENT PRINTING OFFICE

1925
PREFACE

This report is the first revision of the progress report of the National Screw Thread Commission published in 1921. The material

has been generally rearranged and subdivided into sections, all specifications relating to a given class of product being included in a

single section. Among the more important changes made in the

specifications as previously published are the following: The classes

of fit have been renamed and renumbered; the tolerance on major

diameter of screws of classes 3 and 4 has been made the same as for

class 2; specifications for gages have been extensively revised, and

the allowances and tolerances on fire-hose coupling threads have been

revised to decrease the maximum permissible looseness. In general,

any screw thread product which met the previous specifications will

meet the revised specifications. New material added includes specifications for threading tools, recommended tool shapes, tap dimensions, and tap drill sizes; specifications for screw threads of special

diameters, pitches and lengths of engagement; specifications for

gages for fire-hose coupling threads; specifications for wood screws;

and the appendixes, which embody information supplementing the

specifications.

Acknowledgment is made to the many individuals, firms, and. other

organizations, whose cooperation and assistance have made possible

the completion of this report. The thanks of the commission are

especially due the manufacturers and users of screw-thread products,

tools, and gages; the American Society of Mechanical Engineers;

the Society of Automotive Engineers; the American Engineering

Standards Committee and its sectional committees on screw threads

and plain limit gages; The Federal Specifications Board; the National

Board of Fire Underwriters; the United States Army; the United

States Navy; and the Bureau of Standards.

Attention is directed to the fact that in so far as the same ground

is covered by this revised report of the commission and by report

(B la—1924) of the American Engineering Standards Committee

(Sec. II, Sec. Ill— 1, 2, 3, and 4, and Appendix 1, herein), the two

reports are in substantial agreement.

Criticisms and suggestions for the improvement of the report are

invited and should be addressed to the National Screw Thread Commission, Bureau of Standards, Washington, D. C.

ii
APPROVAL BY THE COMMISSION AND TRANSMITTAL TO

THE SECRETARIES OF WAR, NAVY, AND COMMERCE

Hon. John W. Weeks,

Secretary of War.

Hon. Curtis D. Wilbur,

Secretary of the Navy.

Hon. Herbert Hoover,

Secretary of Commerce.

August 19, 1924.

To the honorables the Secretary of War, the Secretary of the

Navy, the Secretary of Commerce.

The National Screw Thread Commission, having revised its

Progress Report, dated January 4, 1921, herewith submits its report

revised 1924, for your acceptance and approval, in accordance with

Public Act No. 201 (H. R. 10852, 65th Cong.), approved July 18,

1918; as amended by Public Act No. 324 (H. R. 15495, 65th Cong.),

approved March 3, 1919; Public Resolution No. 34 (H. J. 299, 66th

Cong.), approved March 23, 1920; and Public Resolution No. 43

(H. J. 227, 67th Cong.), approved March 21, 1922.

George K. Burgess,

Chairman.

E. C. Peck, Lieut. Colonel, U. S. A.,

J. 0. Johnson, Major, U. S. A.,

Appointed by the Secretary of War.

M. A. Libbey, Commander, U. S. N.,

John B. Rhodes, Commander, U. S. N.,

Appointed by the Secretary of the Navy.

F. O. Wells,

'

Ralph E. Flanders,

Appointed by the Secretary of Commerce from nominations

by the American Society of Mechanical Engineers.

Earle Buckingham,

George S. Case,

Appointed by the Secretary of Commerce from nominations

by the Society of Automotive Engineers.

in
IV NATIONAL SCREW THREAD COMMISSION

APPROVAL BY THE SECRETARIES OF WAR, NAVY, AND COMMERCE

October 2, 1924.

The attached report prepared by the National Screw Thread

Commission, in accordance with the law establishing the commission, Public Act No. 201 (H. R. 10852, 65th Cong.), amended by Public Act No. 324 (H. R. 15495, 65th Cong.), is hereby accepted and

approved.

John W. Weeks,

Secretary of War.

Curtis D. Wilbur,

Secretary of the Navy.

Herbert Hoover,

Secretary of Commerce.
1924 REPORT OF THE NATIONAL SCREW THREAD

COMMISSION

(Authorized by Congress, July 18, 1918, H. R. 10852)

AS APPROVED AUGUST 19, 1924

CONTENTS

Page

Preface n

Section I. Introduction 4

1. Historical 4

2. Authorization 5

(a) Commission authorized by Congress 5

(b) Life of commission extended by Congress 6

3. Organization of the commission 6

(a) Preliminary meeting 6

(6) Members 7

(c) Officers 8

(d) Committees 8

(e) Later committees 1 8

(/) Personnel on European trip 1 9

(g) Present organization 1 9

(/i) General procedure 10

4. Arrangement of report 1 11

5. General 12

(a) Strict interchangeability 12

(b) Need of definite specifications 12

Section II. Terminology 13

1. Definitions 13

(a) Terms relating to screw threads 13

(b) Terms relating to classification and toler- ances 14

2. Symbols 16

3. Illustrations showing terminology 18

Section III. Screw threads for bolts, nuts, commercial tapped holes, etc.. 18

1. National form of thread 18

(a) Specifications 18

(b) Illustration 18

2. Thread series 19

(a) National coarse-thread series 19

(b) National fine-thread series 20

3. Classification and tolerances 21

(a) General specifications 22

(b) Classification of fits 23

4. Tables of dimensions 37

1 New material not included in the progress report of Jan. 4, 1921. 1
2 NATIONAL SCREW THREAD COMMISSION

Section III. Screw threads for bolts, etc.—Continued. Page

5. Specifications for threading tools 1 53

(a) Form of tools for producing screws 53

(b) Taps 58

(c) Tap-drill sizes 63

6. Gages 1 67

(a) Fundamentals 67

(b) Gaging practices and types of gages 69

(c) Specifications for gages 73

Section IV. Screw threads of special diameters, pitches, and lengths of

engagement 1 88

1. Form of thread 88

2. Thread series 88

3. Classification and tolerances 88

(a) General specifications 88

(6) Classification of fits 90

4. Specifications for threading tools 101

(a) Form of tools for producing screws 101

(b) Taps 101

(c) TapdriUs 102

5. Gages 103

(a) Specifications for gages 103

Section V. National hose-coupling and fire-hose coupling threads 106

1. Form of thread 108

2. Thread series 108

(a) National hose-coupling threads 108

(b) National fire-hose coupling threads 109

3. Allowances and tolerances 109

4. Tables of dimensions 112

5. Gages 1 113

(a) Gages for national fire-hose coupling threads. 1 13

Section VI. National pipe threads 114

1. Form of thread 115

(a) Specifications 115

(6) Illustration 115

2. Symbols ... 116

3. Thread series 116

(a) National (American Briggs') taper pipe

threads 116

(b) National straight pipe threads 120

(c) National locknut threads 122

4. Tables of pipe dimensions 124

5. Threading tools 1 126

(a) Taps 126

lb) Tap driUs 126

6. Gages 128

(a) Classification of gages 128

(b) Gaging practices 130

(c) Specifications for gages 130

Section VII. Wood screws 1 139

1. General specifications 139

2. Thread series 141

1 New material not included in the progress report of Jan. 4, 1921.
1924 REPORT 3

Section VII. Wood screws—Continued. Page

3. Tolerances on length 141

(a) Flat and oval-head screws 141

(&) Round-head screws 141

4. Standard sizes of wood screws 142

(a) Steel screws 142

(6) Brass screws 142

Appendix 1. Derivation of tolerances 1 145

1. Pitch diameter tolerances 145

(a) Tolerances for fastening screws 145

(6) Tolerances for screw threads of special diameters, pitches, and lengths of engagement 145

2. Relation of lead and angle errors to pitch diameter

tolerances 146

(a) Diameter equivalent of lead error 146

(6) Diameter equivalent of angle error 146

Appendix 2. Wire methods of measurement of pitch diameter 1 147

1. Size of wires 147

2. Specification for wires 149

3. Methods of measuring and using wires 149

4. Measurement of pitch diameter of national straight

threads 150

5. Measurement of pitch diameter of national taper

threads 151

6. Measurement of pitch diameter of thread ring gages_ 153

Appendix 3. Control of accuracy of thread elements in the production of

threaded product 1 156

1. Fundamental factors 156

(a) Tool controlled by lead screw 156

(b) Self-leading threading tool 157

2. Cutting of screw threads 157

(a) Single-point tool 157

(b) Thread chaser 158

(c) Tap or die 158

(d) MiUing cutter 159

(e) Threading hob 159

3. Rolling of screw threads 160

(a) Threading roll 160

(b) Thread rolling dies 160

4. Finishing of screw threads 161

(a) Grinding 162

(6) Lapping 162

Appendix 4. Design and construction of gages 163

1. Material 168

2. Plug thread gages 163

(a) Type 163

(6) Design of "go" thread plug gages 163

(c) Design of " not go " thread plug gages 164

3. Ring thread gages 164

(a) Type 164

(&) Design of "go" thread ring gages 164

(c) Design of "not go" thread ring gages 164

1 New material not included in the progress report of Jan. 4, 1921.
4 NATIONAL SCREW THREAD COMMISSION

Appendix 4. Design and construction of gages—Continued. p3ge 4. Plain plug gages 164

5. Plain ring gages 164

6. Plain snap gages 164

Appendix 5. Future work of the commission 1 165

1. Screw threads used in electrical industry 165

2. Bolt and nut proportions and wrench openings 165

3. Machine-screw and stove-bolt proportions 166

4. Oil-well casing threads 166

5. Wrench fit of threaded studs 167

6. Wire gages, and stock sizes of wire, metal sheet, and

plate 167

7. Acme threads 168

8. Other standardization projects 169

(a) Tolerance specifications for pipe threads

subject to high pressures 169

(6) Instrument-tubing threads 169

(c) Threads on instrument screws 169

(d) Form of thread for valve stems 169

(e) Threads on condenser-tube ferrules 169

(J) Plumbers' fine threads 169

(g) Special threads 169

Index 171

SECTION I. INTRODUCTION

1. HISTORICAL

The initial accomplishment in the standardization of screw threads

in the United States was the report under date of December 15, 1864,

of the special committee appointed by the Franklin Institute on April

21, 1861, for the investigation of a proper system of screw threads,

bolt heads, and nuts to be recommended by the institute for adoption and general use by American engineers.

In its report this committee recommended a thread system designed

by William Sellers. This thread system specified a single series of

pitches for certain diameters from one-fourth inch to 6 inches, inclusive. The threads had an included angle of 60° and a flat at the

crest and root equal to one-eighth of the pitch. This system came

into general use and was known as the Franklin Institute thread,

the Sellers thread, and commonly as the United States thread.

The accomplishments realized in the adoption of the Franklin Institute, or United States standard thread, in 1864 were brought

about largely by the great need of standard threads by American

railroads for the development of their lines and equipment. In May,

1868, this thread was adopted by the United States Navy.

In recent years numerous organizations have carried forward the

standardization of screw threads. The American Society of Mechani1 New material not included in the progress report of Jan. 4, 1921.
1924 REPORT 5

cal Engineers, the Society of Automotive Engineers, the Bureau of

Standards, and prominent manufacturers of specialized threaded

products have been the chief influences in standardization of screw

threads in this country. In England the standardization of screw

threads began with the efforts of Sir Joseph Whitworth in ascertaining shop practice in the manufacture of screw threads resulting in

the standardization and adoption of the Whitworth thread system,

which found extensive use in England. This work has been carried

forward by the British Engineering Standards Association, an organization formed in 1901.

While the United States standard thread system fulfilled a great

need in the period of the development of our great railway systems,

it did not fully meet the requirements of modern manufacture be- cause of the need for additional standard sizes and pitches developed

in other industries, and especially because of the need for definitely

specified limiting sizes of threaded parts. To fulfill the first of these

needs, a thread system having finer pitches than the United States

standard system was recommended by the Society of Automotive

Engineers, and a machine-screw thread series which provided smaller

sizes of screws than the United States standard threads was recommended by the American Society of Mechanical Engineers. The

progress of machine design and manufacture has established an extensive use of these fine thread series.

With the great extension of quantity production in this and other

countries, particularly during the World War, the need for national

standard limiting dimensions was emphasized, as one of the prerequisites of quantity production is standardization of form and dimensions of parts, in order that interchangeability may be established.

This is especially important in the matter of screw-thread parts, since

there are two mating parts that must fit and these parts in many

cases are made in different places. Standardization of screw threads

is important to both the manufacturer and the user of a machine, as the user should be able to buy locally a screw or nut for replacement

in case of breakage or wear.

2. AUTHORIZATION

Through the efforts of several of the engineering societies, the

Bureau of Standards, and prominent manufacturers of screw-thread

products, a petition was presented to Congress requesting the appointment of a commission to investigate and promulgate standards of screw threads to be adopted by manufacturing plants under the

control of the Army and Navy and for adoption and use by the public.

(a) Commission Authorized by Congress.—As a result of this

action the National Screw Thread Commission was authorized by
6 NATIONAL SCREW THREAD COMMISSION

the following act of Congress, approved July 18, 1918 (Public Act

No. 201, H. R. 10852, 65th Cong.):

AN ACT To provide for the appointment of a commission to standardize screw threads. Be it enacted by the Senate and House of Representatives of the United States of America in Congress assembled, That a commission is hereby created, to be

known as the Commission for the Standardization of Screw Threads, hereinafterreferred to as the commission, which shall be composed of nine commissioners, one of whom shall be the Director of the Bureau of Standards, who shall be chairman of the commission; two commissioned officers of the Army, to be appointed

by the Secretary of War; two commissioned officers of the Navy, to be appointed

by the Secretary of the Navy; and four to be appointed by the Secretary of Commerce, two of whom shall be chosen from nominations made by the American

Society of Mechanical Engineers and two from nominations made by the Society

of Automotive Engineers.

Sec. 2. That it shall be the duty of said commission to ascertain and establish

standards for screw threads, which shall be' submitted to the Secretary of War,

the Secretary of the Navy, and the Secretary of Commerce for their acceptance

and approval. Such standards, when thus accepted and approved, shall be

adopted and used in the several manufacturing plants under the control of the

War and Navy Departments, and, so far as practicable, in all specifications for screw threads in proposals for manufactured articles, parts, or materials to be

used under the direction of these departments.

Sec. 3. That the Secretary of Commerce shall promulgate such standards for use by the public and cause the same to be published as a public document.

Sec. 4. That the commission shall serve without compensation, but nothing

herein shall be held to affect the pay of the commissioners appointed from the

Army and Navy or of the Director of the Bureau of Standards.

Sec. 5. That the commission may adopt rules and regulations in regard to

its procedure and the conduct of its business.

Sec. 6. That the commission shall cease and terminate at the end of six months from date of its appointment.

Approved, July 18, 1918.

(b) Life of Commission Extended by Congress.—Prior to the

expiration of the original term of six months for which the commission was appointed, it became apparent that it would be impossible

to complete in a satisfactory manner the work outlined by the commission. Extensions of time were, therefore, asked- by the commission and granted by Congress in accordance with the following

acts: Public Act No. 324 (H. R. 15495), 65th Cong.; Public Resolution No. 34 (H. J. Res. 299), 66th Cong. ; and Joint Public Resolution

No. 43 (H. J. Res. 227), 67th Cong. The term of the commission,

as last extended, expires March 21, 1927.

3. ORGANIZATION OF THE COMMISSION

(a) Preliminary Meeting.—As soon as nominees were selected

by the various organizations to be represented in the commission

a preliminary meeting was called at Washington, D. C, on September 12, 1918, by Dr. S. W. Stratton, Director of the Bureau of

Standards and chairman of the commission. At this meeting the
1924 REPORT 7

organization of the commission was planned in order that work

could be started as soon as formal appointments of the various

members of the commission were made. The various commissioners

were formally appointed under date of September 21, 191S.

(&) Members.—In accordance with the act, the following members

were appointed:

Appointed by the Secretary of Commerce:

Chairman

Dr. S. W. Stratton, Director of Bureau of Stand- Date appointed

ards, Washington, D. C September 21, 1918.

Dr. G. K. Burgess, Director of Bureau of Standards, Washington, D. C, succeeding Dr. S. W.

Stratton April 23, 1923.

On nomination by the American Society of Mechanical

Engineers:

James Hartness September 21, 1918.

F. O. Wells September 21, 1918.

Ralph E. Flanders, succeeding James Hartness December 15, 1920.

On nomination by the Society of Automotive Engineers

H. T. Herr 1 September 21, 1918. E. H. Ehrman September 21, 1918.

Earle Buckingham, succeeding H. T. Herr April 8, 1921.

George S. Case, succeeding E. H. Ehrman October 3, 1922.

Appointed by the Secretary of War:

E. C. Peck, lieutenant colonel, Ordnance, U. S. Army-. September 21, 1918.

O. B. Zimmerman, major of Engineers, U. S. Army September 21, 1918.

John O. Johnson, major of Ordnance, succeeding Maj.

0. B. Zimmerman May 23, 1919.

Appointed by the Secretary of the Navy:

E. J. Marquart, commander, U. S. Navy, Bureau of Ordnance September 21, 1918.

S. M. Robinson, commander, U. S. Navy, Bureau of Steam Engineering September 21, 1918.

N. H. Wright, commander, U. S. Navy, Bureau of

Steam Engineering, succeeding Commander S. M.

Robinson July 14, 1919.

L. N. McNair, commander, U. S. Navy, Bureau of

Ordnance, succeeding Commander E. J. Marquart, . October 7, 1919.

Joseph S. Evans, commander, U. S. Navy, Bureau of

Steam Engineering, succeeding Commander N. H.

Wright r May 10, 1920.

S. M. Robinson, commander, U. S. Navy, Bureau of

Steam Engineering, succeeding Commander J. S. Evans November 8, 1921.

J. N. Ferguson, commander, U. S. Navy, Bureau of

Ordnance, succeeding Commander L. N. McNair January 9, 1922.

C. A. Jones, lieutenant commander, U. S. Navy, Bureau

of Engineering, succeeding Commander S. M. Robinson March 21, 1922.

M. A. Libbey, commander, U. S. Navy, Bureau of

Engineering, succeeding Commander C. A. Jones July 19, 1922.

John B. Rhodes, commander, U. S. Navy, Bureau of

Ordnance, succeeding Commander J. N. Ferguson — February 20, 1924.
8 NATIONAll SCREW THREAD COMMISSION

(c) Officers.—The following officers were elected by the commission at the first meeting.

Lieut. Col. E. C. Peck, vice chairman for meetings held in Washington.

James Hartness, vice chairman for meetings held outside of Washington.

H. L. Van Keuren, executive secretary.

H. W. Bearce, general secretary.

Robt. Lacy, first lieutenant of Engineers, U. S. Army, assistant secretary. A. W. Coombs, stenographic reporter.

(<Z) Committees.—The commission resolved itself into the following subcommittees, with authority to call to their aid one or more

experts for counsel. These subcommittees were responsible for

compiling and auditing data pertaining to the subject of each committee and for compiling reports for presentation to the commission

as a whole, for the action of the commission.

Pitches, systems, and form of thread

Classification and tolerances

Terminology

Gages and methods of test

Order of business

Research

F. O. Wells, chairman.

Commander S. M. Robinson.

E. H. Ehrman.

H. W. Bearce, secretary.

Lieut. Col. E. C. Peck, chairman.

James Hartness.

E. H. Ehrman.

H. L. Van Keuren, secretary.

F. O. Wells, chairman.

Commander E. J. Marquart.

Maj. O. B. Zimmerman.

Lieut. Robert Lacy, secretary.

James Hartness, chairman.

Lieut. Col. E. C. Peck.

Commander E. J. Marquart.

H. L. Van Keuren, secretary.

James Hartness, chairman.

Lieut. Col. E. C. Peck.

F. O. Wells. E. H. Ehrman, chairman.

Maj. O. B. Zimmerman.

Commander S. M. Robinson.

Taps, dies, tap drills, and wire

gages

(e) Later Committees.—Since the publication of the progress

report of 1921 the following additional subcommittees have been

appointed

Lieut. Col. E. C. Peck, chairman.

Ralph E. Flanders.

Earle Buckingham.

F. O. Wells, chairman.

Commander L. N. McNair (replaced by

Commander J. N. Ferguson).

E. H. Ehrman (replaced by George S. Case). E. H. Ehrman, chairman (replaced by

George S. Case).

Maj. J. O. Johnson.

Commander Joseph S. Evans (replaced by

Commander M. A. Libbey).

Boltheads, nuts, and wrenches.

Bar sizes.
1924 REPORT 9

Instrument threads and brass

tubing

Acme and special threads.

Rearrangement of progress report.

Commander L. N. McNair, chairman (replaced by Commander J. N. Ferguson).

F. O. Wells.

Dr. S. W. Stratton (replaced by Dr. George

K. Burgess).

Maj. J. O. Johnson, chairman.

Ralph E. Flanders.

Commander Joseph S. Evans (replaced by

Commander M. A. Libbey).

Earle Buckingham.

Lieut. Col. E. C. Peck, chairman.

Ralph E. Flanders.

George S. Case.

Ralph E. Flanders, chairman.

. , Mai. J. O. Johnson.

Revision of progress report „ , Commander J. N. Ferguson.

[Earle Buckingham.

(/) Personnel on European Trip.—In July, 1919, the commission conferred with British and French engineers and manufacturers of screw-thread products, for the purpose of discussing

the tentative report prepared by the commission with reference to

its suitability to serve as a basis for international standardization

of screw threads.

The commission was represented by the following persons

E. C. Peck (chairman), representative U. S. Army, lieutenant colonel, Ordnance, U. S. Army.

F. O. Wells (vice chairman), representative A. S. M. E.

L. D. Burlingame, representative A. S. M. E., alternate for James Hartness.

E. Buckingham, representative S. A. E., alternate for H. T. Herr. H. L. Horning, representative S. A. E., alternate for E. H. Ehrman.

J. 0. Johnson, representative U. S. Army, major, Ordnance, U. S. Army.

L. B. McBride, representative U. S. Navy, commander, U. S. Navy.

H. C. Dickinson, Representative Department of Commerce, U. S. Government, advisory member.

H. W. Bearce, representative Bureau of Standards, U. S. A. (general secretary).

Robert Lacy, representative U. S. Army, first lieutenant Engineers, U. S. Army (technical secretary)

(g) Present Organization.—At the time of publication of this

1924 revised report the commission comprises the following members

Dr. George K. Burgess, chairman.

Lieut. Col. E. C. Peck, vice chairman.

F. O. Wells.

Maj. John O. Johnson.

Earle Buckingham.

Ralph E. Flanders.

George S. Case.

Commander John B. Rhodes.

Commander M. A. Libbey.

H. W. Bearce, general secretary. E. G. Hubbell, stenographic reporter.
10 RATIONAL, SCREW THREAD COMMISSION

(70 General Procedure.—In its work of establishing standards

for screw threads, the commission has made particular efforts to secure actual facts concerning the need of standardization and the

economic conditions to be provided for in the production and use

of screw threads.

The work of the commission has proceeded rapidly, inasmuch as, in recent years, the accomplishments of the American Society of

Mechanical Engineers, the Society of Automotive Engineers, and

the British Engineering Standards Association have paved the way

toward the adoption of necessary screw-thread standards. Furthermore, the commission has availed itself of the opportunity to se- cure from the Tap Makers Association and representatives of prominent manufacturing concerns valuable information and data regarding the production of tools and appliances for making threaded products, as well as information and data regarding the application and

use of screw-thread products.

Steps were taken to secure from various screw-thread authorities,

and representative manufacturers and users, testimony as to the

nature of the standards to be adopted for the use of the Government

and for American manufacturers. To secure this information public

hearings were conducted in various industrial centers throughout the

country; and Government officials, authorities on screw threads,

manufacturers, and users of screw-thread products, as well as manufacturers of taps, dies, gages, and other tools required for producing

screw-thread products, were invited to attend these hearings and

present their views on various phases of the subject. In addition,

announcements of the meetings, extending invitations to all interested

to be present, were published in the technical magazines. Topic

sheets were distributed in advance of the hearings in order that wit- nesses could prepare their views on the subjects of the meeting in a definite, concise, and authentic form.

A large amount of evidence was collected in this way and the opportunity was available for the various members of the commission

to bring out by cross-examination information which could have been

secured in no other way. This evidence was tabulated for the consideration of the commission in formulating its report.

A large number of experiments and tests were made by the Bureau

of Standards to verify the results obtained at the various hearings

and also in connection with the development of tolerances and of

other technical subjects considered by the commission. In addition

to the experiments conducted by the Bureau of Standards, the members of the commission individually conducted experiments and re- search work at their ovm expense.

With regard to the possibilities of international standardization

of screw threads, it is the opinion of the commission that not only
1924 REPORT 11

is such international standardization very desirable, but that the

present time is most opportune for accomplishments in this direction.

Furthermore, international standardization is of great importance in

connection with the development of foreign trade.

In July, 1919, the commission visited Europe to confer with

British and French engineering standards organizations, and while

no definite agreements were reached in regard to international

standardization of screw threads, it was apparent in both France

and England that the engineers and manufacturers in these countries were anxious to cooperate with the United States in this work.

The time is very opportune for accomplishments along this line,

and it is the opinion of the commission that, as a result of the war,

it should be possible to reach an agreement on an international

standard thread. Such an international standard should be established by giving consideration to the predominating sizes and

standards used in manufactured products, as well as to the possibilities of providing a means for producing this international screw

thread by the use of either the English or the metric system of

measurement.

The advances made by the commission up to date will facilitate

manufacture in case of war, make the best use of labor in our industries in time of peace, increase the safety of travel by rail, steamship, and airplane, and, in general, will increase the dependability

of all mechanisms. Every step toward standardization will result

in increased production with a minimum expenditure of materials,

energies, and other resources. The commission, in formulating this report, has acted largely

in a judicial capacity, basing its decisions upon evidence received

from authorities on screw-thread subjects and upon the conclusions

drawn by other organizations having to do with standardization of screw threads. In addition, the various subjects dealt with have

been considered with a knowledge of present manufacturing conditions and with anticipation of further development in the production of screw-thread products. Above all, it is the intention of

the commission to facilitate and promote progress in manufacture.

4. ARRANGEMENT OF REPORT

There are included in the body of the report matters of particular

importance and of general interest, while in the appendixes there is arranged supplementary information of both a general and a technical nature. There is included in the body of the report sufficient

information to permit the writing of definite and complete specifications for the purchase of screw-thread products.

The specifications in the report have been arranged, as far as

possible, by products. For example, one section deals with threads
12 NATIONAL. SCREW THREAD COMMISSION

for bolts and nuts, etc., another with hose-coupling threads, another

with pipe threads, etc. As far as practicable, each section is arranged

in the following order:

1. Form of thread.

2. Thread series.

3. Classification and tolerances.

4. Tables of dimensions.

5. Specifications of threading tools.

6. Gages.

5. GENERAL

One of the most important phases of standardization of screw- thread products is that of interchangeability. The direct result of

establishing a national thread system will be the elimination of

many unnecessary sizes. Of even more importance are the

advantages to be gained in the manufacture of interchangeable

screw-thread parts, especially when made in different manufacturing

plants located at a distance from each other, which will assemble

without difficulty and in a dependable manner.

(a) Strict Interchangeability.—Many manufacturers, previ- ous to the war, were making interchangeable machine parts in

their own shops where there was but one master gage or reference

standard, but one individual who had authority to pass on parts in

dispute, and where it was possible to secure assembly and satisfactory operation by fitting the parts.

The experience gained by manufacturers producing war material

has demonstrated the economic advantage of producing interchangeable parts, especially where large quantities of parts are manufactured. In addition to the direct saving in the cost of

manufacture, the numerous advantages to be gained make it mandatory that a definite procedure for producing interchangeable work

to specified tolerances be explicitly followed, if we, as manufacturers,

are to keep pace with or lead in the world's progress.

A screw-thread fit can not be accurately made with the same

facility as the fit of a plain hole and shaft. In the fit of a plain hole

and shaft only three elements are taken into account in securing a given class of fit, namely, rotundity, diameter, and length; whereas

in a screw-thread fit it is necessary to consider rotundity, length,

major diameter, pitch diameter, minor diameter, angle of thread,

and pitch or lead. A variation in any one of these elements of a screw thread will prevent a perfect fit, so that it is much more diffi- cult to make a perfect screw-thread fit than it is to make a plain

bearing fit.

(&) Need of Definite Specifications.—The difficulties en- countered in obtaining enormous quantities of war material needed
1924 REPORT 13

by the United States Government during the recent World War

pointed out to Government authorities, as well as manufacturers,

the need of writing definite and complete specifications for material

required. All specifications should be so written that the qualities

in the product desired are stated in definite terms of known measurable standards and correctly defined by the largest tolerance limits

compatible with the satisfactory operation or performance of the

articles or material for the purpose intended. To this end every

factor involved in the acceptability of the manufactured product

required should be comparable within specified limits with a known

measurable standard. Every specification should be so definite

that no dispute regarding the limiting lines of acceptance can arise.

SECTION II. TERMINOLOGY

In this report there are utilized, as far as possible, nontechnical

words and terms which best convey alike to the producer and user of screw threads the information presented.

1. DEFINITIONS

The following definitions are given of the more important terms

used in the report. Definitions of terms which are obviously elementary in character are intentionally omitted.

(a) Terms Relating to Screw Threads.—1. Screw thread.—

A ridge of uniform section in the form of a helix on the surface of a

cylinder or cone.

2. External and internal threads.—An external thread is a thread

on the outside of a member. Example: A threaded plug.

An internal thread is a thread on the inside of a member. Example: A threaded hole.

(These terms are here defined because of possible confusion arising

from the fact that an "internal member" has an "external thread,"

and vice versa.)

3. Major diameter {formerly Tcnown as "outside diameter").—-The

largest diameter of the thread of the screw or nut. The term "major

diameter" replaces the term "outside diameter" as applied to the

thread of a screw and also the term "full diameter" as applied to

the thread of a nut.

4. Minor diameter (formerly Tcnown as "core diameter").—The

smallest diameter of the thread of the screw or nut. The term

"minor diameter" replaces the term "core diameter" as applied to

the thread of a screw and also the term "inside diameter" as applied

to the thread of a nut.

5. Pitch diameter.—On a straight screw thread, the diameter of

an imaginary cylinder, the surface of which would pass through the

16802°—25t 2
14 NATIONAL SCREW THREAD COMMISSION

threads at such points as to make equal the width of the threads

and the width of the spaces cut by the surface of the cylinder. On a taper screw thread, the diameter, at a given distance from a refer- ence plane, of an imaginary cone, the surface of which would pass

through the threads at such points as to make equal the width of

the threads and the width of the spaces cut by the surface of the cone.

6. Pitch.—The distance from a point on a screw thread to a cor- responding point on the next thread measured parallel to the axis.

The pitch in inches=c^ , t-t, ; :

r

JN umber of threads per men

7. Lead.—-The distance a screw thread advances axially in one

turn. On a single-thread screw, the lead and pitch are identical;

on a double-thread screw the lead is twice the pitch; on a triplethread screw, the lead is three times the pitch, etc.

8. Angle of thread. —-The angle included between the sides of the

thread measured in an axial plane.

9. Helix angle.—The angle made by the helix of the thread at

the pitch diameter with a plane perpendicular to the axis.

10. Crest.—The top surface joining the two sides of a thread.

11. Root.—The bottom surface joining the sides of two adjacent

threads.

12. Side.—The surface of the thread which connects the crest

with the root.

13. Axis of a screw.—The longitudinal central line through the

screw.

14. Base of thread. —The bottom section of the thread; the greatest

section between the two adjacent roots.

15. Depth of thread.—The distance between the crest and the base

of thread measured normal to the axis.

16. Number of threads.—Number of threads in 1 inch of length.

17. Length of engagement.—The length of contact between two

mating parts, measured axially.

18. Depth of engagement.—-The depth of thread contact of two

mating parts, measured radially.

(&) Terms Relating to Classification and Tolerances.—

1. Allowance.—An intentional difference in the dimensions of mating

parts. It is the minimum clearance or the maximum interference

which is intended between mating parts. It represents the condition

of the tightest permissible fit, or the largest internal member mated

with the smallest external member. It is to provide for different

classes of fit. Examples

One-half inch, class 1, loose fit, national coarse thread:

Minimum pitch diameter of nut 0. 4500

Maximum pitch diameter of screw . 4478

Allowance (positive) . 0022
1924 REPORT 15

One-half inch, class 4, close fit, national coarse thread

Minimum pitch diameter of nut 0. 4500

Maximum pitch diameter of screw . 4504

Allowance (negative) . 0004

2. Tolerance.—The amount of variation permitted in the size of a

part. Example

One-half inch screw, class 1, loose fit, national coarse thread:

Maximum pitch diameter 0. 4478

Minimum pitch diameter . 4404

Tolerance . 0074

3. Basic size.—The theoretical or nominal standard size from

which all variations are made.

4. Crest clearance.—Defined on a screw form as the space between

the crest of a thread and the root of its mating thread.

5. Finish.—The character of the surface on a screw thread.

Fig. 1.Screw thread notation

6. Fit.—The relation between two mating parts with reference to

the conditions of assembly; for example: Wrench fit; close fit; medium fit; free fit; loose fit. The quality of fit is dependent upon

both the relative size and finish of the mating parts.

7. Neutral zone.—A positive allowance. (See " Allowance. ")
16 NATIONAL SCREW THREAD COMMISSION

8. Limits.—The extreme permissible dimensions of a part. Example :

One-half inch screw, class 1, loose fit, national coarse thread:

Maximum pitch diameter 0. 44781 These are Minimum pitch diameter . 4404 J the limits.

2. SYMBOLS

For use in formulas for expressing relations of screw threads, and

for use on drawings and for similar purposes, the following symbols

should be used

Major diameter D

Corresponding radius d

Pitch diameter E

Corresponding radius e Minor diameter K

Corresponding radius A; Angle of thread A

One-half angle of thread a Number of turns per inch N

Number of threads per inch n

Lead

Pitch or thread interval V=~

n

Helix angle s

Tangent of helix angle S==zHIE9XE

Width of basic flat at top, crest, or root F

Depth of basic truncation /

Depth of sharp V-thread H

Depth of national form of thread h

Length of engagement Q

Included angle of taper Y

One-half included angle of taper y Additional symbols for national pipe threads are given in Section VI.

Symbols are for use on correspondence, drawings, shop and storeroom cards, specifications for parts, taps, dies, gages, etc., and on

tools and gages.

The method of designating a screw thread by means of symbols

is by the use of the initial letters of the thread series, preceded by

the diameter in inches (or the screw number) and number of threads

per inch, all in Arabic characters, followed by the classification of

fit in Arabic numerals. If the thread is left hand, the symbol

"L. H. " sball follow the number of threads. No symbol is used to

distinguish right-hand threads. Examples:
1924 KEPOET 17

Fig. 2.

Screw thread notation

National Coarse Thread Series:

To specify a threaded part 1 in. -diameter, 8 threads per Mark

inch, class 1 fit , l"—8—NC—l

National Fine Thread Series:

Threaded part 1 in. -diameter, 14 threads per inch,

class 4 fit 1"—1J+—NF—4

National Form, Special Pitch:

Threaded part 1 in. -diameter, 12 threads per inch,

class 5 fit l"—12—N—5

National Form, Left-Hand Thread:

Threaded part 1 in.-diameter, 12 threads per inch,

class 5 fit 1"—12L. H.—N—5

National Pipe Threads:

National taper pipe thread. Threaded part 1 in.- diameter, 113^ threads per inch 1"—lty2—NPT

National straight pipe thread 1"—

NPS

National Fire-Hose Coupling Threads and National HoseCoupling Threads:

Threaded part 3 in.-diameter, 6 threads per inch 8"—6—NH

Threaded part 1 in.-diameter, llj^ threads per inch.. 1"—11}/^—NH

The number of threads per inch must be indicated in all cases,

irrespective of whether it is the standard number of threads for that

particular size of threaded part or special.
18 NATIONAL SCREW THREAD COMMISSION

Symbols for Wire Measurements

Measurement over wires M

Diameter of wire G

Corresponding radius g

3. ILLUSTRATIONS SHOWING TERMINOLOGY

Figures 1 and 2 illustrate the use of the terms used in the report as previously denned.

SECTION III. SCREW THREADS FOR BOLTS, MACHINE

SCREWS, NUTS, COMMERCIAL TAPPED HOLES, ETC.

1. NATIONAL FORM OF THREAD

The form of thread profile specified herein, known previously as the "United States standard or Sellers' profile," is adopted by the

commission and shall hereafter be known as the "national form of

thread."

The national form of thread shall be used for all screw-thread

work except when otherwise specified for special purposes.

(a) SPECIFICATIONS

1. Angle op Thread.—The basic angle of thread (A) between the

sides of the thread measured in an axial plane is 60°. The line

bisecting this 60° angle is perpendicular to the axis of the screw thread.

2. Flat at Crest and Root.—The flat at the root and crest of

the basic thread form is X p, or 0.125 X p.

3. Depth of Thread.—The depth of the basic thread form is

X X 0.649519

A = 0.649519 Xp, or h =

where

p = pitch in inches,

?i = number of threads per inch,

h = basic depth of thread.

4. Clearance at Minor Diameter.—A clearance shall be provided at the minor diameter of the nut by removing from the crest of

the basic thread form an amount equal to one-sixth to one-fourth of

the basic thread depth.

5. Clearance at Major Diameter. —A clearance at the root of

the thread at the major diameter of the nut shall be provided by

decreasing the depth of the truncation triangle any desired amount not

exceeding two-thirds of its theoretical value.

(b) ILLUSTRATION

There are indicated in Figure 3 the relations as specified herein for

the national form of thread for the minimum nut and maximum

screw, free or medium fits. These relations are further shown in

Figures 7 and 9.
4

1924 REPORT 19

2. THREAD SERIES

It is the aim of the commission, in establishing thread systems for

general use, to eliminate all unnecessary sizes and, in addition, to utilize as far as possible present predominating sizes. While from cer- tain standpoints it would have been desirable to make simplifications

in the thread systems and to establish more thoroughly consistent

standards, it is believed that any radical change at the present time

would be out of place and interfere with manufacturing conditions,

and would involve great economic loss.

The testimony given at the various hearings held by the commission

is very consistent in favoring the maintenance of the present coarse- thread and fine-thread series, the coarse-thread series being the

" United States standard" threads, supplemented in the sizes below

one-fourth inch by sizes taken from the standard established by the

American Society of Mechanical Engineers (A. S. M. E.). The fineP H- p

Fig. 3.

National form of thread

Note: No allowance is shown. This condition exists in class 2, free fit, and class 3, medium fit, where

both the minimum nut and the maximum screw are basic NOTATION n=number of threads per inch H=0.866025 p depth of 60° sharp V thread

ft =0.649519 p depth of national form thread

•/«ft =0.541266 p maximum depth of engagement

F=0.125000 p width of flat at crest and root of national form

/=0.108253 pi =Y% H Idepth of truncation =KA J thread series is composed of standards that have been found necessary

and consists of sizes taken from from the standards of the Society of

Automotive Engineers (S. A. E.), and the fine-thread series of the

American Society of Mechanical Engineers (A. S. M. E.).

(a) RATIONAL COARSE-THREAD SERIES

In Table 1 are specified the nominal sizes and basic dimensions of

the "national coarse-thread series."

The national coarse threads are recommended for general use

in engineering work, in machine construction where conditions are favorable to the use of bolts, screws, and other threaded components

where quick and easy assembly of the parts is desired, and for all work where conditions do not require the use of of fine-pitch threads.
20 NATIONAL, SCREW THREAD COMMISSION

Table 1.National coarse-thread series

Identification Basic diameters Thread data

„. . JVLini*

IVJ. CL11L mum Helix Ma] or Pitch onn i i7o „ '

1 i ii J v a T>pr»th J-'opcu Basic width angle per diame- Minor lent of Pitch Of width of flat at basic Sizes inch ter diameter diameter major V thread of flat at major pitch n j) E (\v_A ilame amp- ft PI a diameter

ter s VI "

1 2 3 4 5 6 7 8 9 10 11

Inches Inches Inches 771771 Inch Inch Inch Inch Deo. Min.

\ 64 0. 073 0. 0629 0. 0527 1. 854 0. 01562 0. 01015 V. UwUJ 4 31 2 56 !o86 !0744 !0628 2. 184 '. 01786 ! 01160 .00223 .00074 4 22

3 48 .099 .0855 .0719 2. 515 .02083 . 01353 .00260 .00087 4 26 4 40 . 112 .0958 .0795 2. 845 . 02500 . 01624 .00312 .00104 4 45

5 40 . 125 .1088 .0925 3. 175 .02500 . 01624 .00312 . 00104 4 11

g 32 . 138 . 1177 . 0974 3. 505 . 03125 . 02030 . 00391 . 00130 4 .50 8 32 ! 164 !l437 '. 1234 i. 166 '. 03125 '. 02030 ! 00391 ! 00130 3 58

10_„__ 24 . 190 . 1629 .1359 4.826 .04167 .02706 .00521 .00174 4 39 12 24 .216 .1889 .1619 5.486 . 04167 .02706 . 00521 .00174 4 1 M— 20 .2500 .2175 .1850 6.350 .05000 . 03248 .00625 .00208 4 11

-5, lS %—

18 . 3125 . 2764 . 2403 7. 938 . 00694 . 00231 3 40

16 !3750 !3344 !2938 9] 525 .06250 .04059 ! 00781 ! 00260 3 24

7 TS 14 .4375 .3911 .3447 11. 113 . 07143 .04639 .00893 .00298 3 20 Vi—

13 .5000 .4500 .4001 12.700 . 07692 .04996 .00962 .00321 3 7 A—

12 . 5625 .5084 .4542 14.288 .08333 . 05413 . 01042 .00347 2 59 %— 11 . 6250 . 5660 . 5069 15. 875 . 09091 . 05905 . 01136 . 00379 2 56 24—- 10 .7500 .6850 .6201 19. 050 . 10000 .06495 . 01250 .00417 2 40 Vs.— 9 .8750 .8028 .7307 22. 225 . 11111 . 07217 . 01389 .00463 2 31

l 8 1.0000 .9188 .8376 25. 400 . 12500 . 08119 . 01562 .00521 2 29 m— 7 1. 1250 1. 0322 .9394 28. 575 . 14286 .09279 .01786 . 00595 2 31 M— 7 1.2500 1. 1572 1. 0644 31. 750 . 14286 . 09279 . 01786 . 00595 2 15 M— 6 1. 5000 1. 3917 1.2835 38. 100 . 16667 . 10825 .02083 .00694 2 11 m~. 5 1. 7500 1. 6201 1. 4902 44. 450 .20000 . 12990 .02500 .00833 2 15 2 2.0000 1. 8557 1. 7113 50.800 . 22222 . 14434 . 02778 .00926 2 11 2X— 2. 2500 2. 1057 1. 9613 57. 150 . 22222 . 14434 . 02778 .00926 1 55 2V2-- 4 2.5000 2. 3376 2. 1752 63. 500 .25000 . 16238 .03125 . 01042 1 57 2M-- 4 2. 7500 2. 5876 2. 4252 69. 850 .25000 . 16238 .03125 . 01042 1 46

3 4 3. 0000 2. 8376 2. 6752 76. 200 . 25000 . 16238 . 03125 .01042 1 36

(b) NATIONAL FINE-THREAD SERIES

In Table 2 are specified the nominal sizes and basic dimensions of

the "national fine-thread series."

The national fine threads are recommended for general use in

automotive and aircraft work, for Use where the design requires both

strength and reduction in weight, and where special conditions re- quire a fine thread, such as, for instance, on large sizes where sufficient

force can not be secured by hand to set properly a screw or bolt of coarse pitch.
1924 REPORT 21

Table 2.

National fine-thread series

Identification Basic diameters Thread data

Mini- Metric mum Helix Threads Major equiva- Depth Basic width angle

Sizes per diame- Pitch Minor lent of Pitch of width of flat at basic inch ter diameter diameter major V thread of flat at major pitch n D E K diame- ft p/8 diameter diameter

ter of nut s p/24

1 2 $ 4 g g 1 g 9 10 11

Inches Inches Inches mm Inch Inch Inch Inch Deg. Min.

80 0.060 0. 0519 0.0438 1. 524 0. 01250 0.00812 0. 00156 0. 00052 4 23

1 72 .073 .0640 .0550 1. 854 . 01389 . 00902 . 00174 .00058 3 57

2 64 . 086 . 0759 . 0657 2. 184 . 01562 . 01015 . 00195 . 00065 3 45

3 56 . 099 . 0874 . 0758 2. 515 . 01786 . 01160 . 00223 . 00074 3 43

4 48 . 112 .0985 . 0849 2. 845 . 02083 . 01353 . 00260 . 00087 3 51

5 44 .125 .1102 .0955 3. 175 .02273 . 01476 .00284 .00095 3 45

6 40 .138 . 1218 . 1055 3. 505 .02500 .01624 . 00312 .00104 3 44

8 36 . 164 . 1460 . 1279 4. 166 . 02778 . 01804 . 00347 . 00116 3 28

10 32 . 190 . 1697 . 1494 4. 826 . 03125 . 02030 . 00391 . 00130 3 21

12.... 28 .216 .1928 .1696 5. 486 . 03571 . 02320 .00446 .00149 3 22 H—- 28 .2500 .2268 .2036 6. 350 . 03571 . 02320 .00446 .00149 2 52 ft—

24 .3125 .2854 .2584 7. 938 .04167 .02706 .00521 .00174 2 40 k~~ 24 .3750 .3479 .3209 9. 525 .04167 .02706 .00521 .00174 2 11 ft—

20 .4375 .4050 .3725 11. 113 .05000 .03248 .00625 . 00208 2 15 Vi — 20 .5000 .4075 .4350 12.700 .05000 .03248 .00625 .00208 1 57

ft

18 . 5625 .5264 .4903 14.288 . 05556 .03608 .00694 .00231 55

Vs 18 .6250 .5889 .5528 15. 875 .05556 . 03608 .00694 .00231 43 K—

Vs.—

16 .7500 .7094 .6688 19.050 .06250 .04059 .00781 .00260 36

14 .8750 .8286 .7822 22. 225 . 07143 .04639 .00893 .00298 34

1 14 1.0000 .9536 .9072 25.400 . 07143 .04639 .00893 .00298 22 1H— 12 1. 1250 1.0709 1. 0167 28. 575 .08333 .05413 .01042 .00347 25

12 1.2500 1. 1959 1. 1417 31. 750 .08333 .05413 . 01042 .00347 16 1H— 12 1.5000 1. 4459 1. 3917 38. 100 .08333 .05413 .01042 . 00347 3

3. CLASSIFICATION AND TOLERANCES

There are established herein for general use five distinct classes

of screw-thread fits as specified in the following brief outline. These

five classes of fits, together with the accompanying specifications, are

for the purpose of insuring the interchangeable manufacture of screw- thread parts throughout the country.

It is not the intention of the commission arbitrarily to place a general class or grade of work in a specific class of fit. Each manufacturer and user of screw threads is free to select the class of fit best adapted to his particular needs. The tolerances and dimensions

for four classes of fit are given in Tables 3 to 14, inclusive, and summarized in Tables 15 and 16. Tolerances and allowances for class 5,

wrench fit, are under consideration by the commission and will be

published as soon as determined.

Class 1, loose fit.

Includes screw-thread work of rough commercial quality, where the threads must

assemble readily, and a certain amount

of shake or play is not objectionable.
22 NATIONAL SCREW THREAD COMMISSION

Class 2, free fit

Class 3, medium fit

Includes the great bulk of screw-thread

work of ordinary quality, of finished

and semifinished bolts and nuts, machine screws, etc. Includes the better grade of interchangeable screw-thread work.

C Includes screw-thread work requiring a

Class 4 close fit J ^ne snuS n% somewhat closer than the

| medium fit. In this class of fit selective

assembly of parts may be necessary.

{Subdivision A _ f Includes screw threads used in light

1 sections with moderate stresses. Subdivision B [ Includes screw threads used in heavy

1 sections with heavy stresses.

(a) GENERAL SPECIFICATIONS

The following general specifications apply to all classes of fit specified for the "national coarse-thread series" and the "national

fine-thread series."

1. Uniform Minimum Nut.—The pitch diameter of the minimum

threaded hole or nut corresponds to the basic size, the tolerances

being applied above the basic size.

2. Uniform Minor Diameter of Nut.—The minor diameter of

the threaded hole or nut, of any given size and pitch, is uniform for

fits of classes 1 to 4, inclusive.

3. Length of Engagement.—A length of engagement equal to

the basic major diameter is the basis of the tolerances specified

herein for screw-thread products.

4. Tolerances.2—(a) The tolerances specified represent the

extreme variations permitted on the product.

(b) The tolerance on the nut is plus, and is applied from the basic

size to above basic size.

(c) The tolerance on the screw is minus, and is applied from the

maximum screw size to below the maximum screw size.

(d) The pitch diameter tolerances for a screw and nut of a given

class of fit are the same.

(e) Pitch diameter tolerances include lead and angle variations.

(See footnote 1, Tables 3, 4, 5, and 6.)

(/) The tolerances on the major diameter of class 1, loose fit, or

class 2, free fit screws are twice the tolerance values allowed on the

pitch diameters of the same respective classes, with the exception

that on class 2, free fit, "national coarse-thread series," externally

threaded parts of unfinished, hot-rolled material, the same tolerances

on major diameter are applied as on class 1, loose fit screws. The

tolerances on the major diameters of class 3, medium fit, and class 4,

close fit screws are the same as those on class 2, free fit finished screws.

2 Recommendations and explanations regarding the application of tolerances are given in Appendix 1.
1924 REPORT 23

(g) The minimum minor diameter of a screw of a given pitch is such as to result in a basic flat Q/% Xp) at the root when the pitch

diameter of the screw is at its minimum value. When the maximum

screw is basic, the minimum minor diameter of the screw will be below

the basic minor diameter by the amount of the specified pitch diameter

tolerance.

(h) The maximum minor diameter of a screw of a given pitch may

be such as results from the use of a worn or rounded threading tool,

when the pitch diameter is at its maximum value. In no case, however, should the form of a thread, as results from tool wear, be such

as to cause the screw to be rejected on the maximum minor diameter

by a "go" ring gage, the minor diameter of which is equal to the

minimum minor diameter of the nut.

(i) The maximum major diameter of the nut of a given pitch is such as to result in a flat equal to one-third of the basic flat X p)

when the pitch diameter of the nut is at its maximum value. When

the minimum nut is basic, its maximum major diameter will be above

the basic major diameter by the amount of the specified pitch diameter tolerance plus two-ninths of the basic thread depth.

(j) The nominal minimum major diameter of a nut is the basic

major diameter. In no case, however, should the minimum major

diameter of the nut, as results from a worn tap or cutting tool, be

such as to cause the nut to be rejected on the minimum major diameter by a "go" plug gage made to the standard form at the crest.

(k) The tolerance on the minoi diameter of a nut of a given pitch

is one-sixth of the basic thread depth regardless of the class of fit being produced.

(b) CLASSIFICATION OF FITS

1. Class 1, Loose Fit.—(a) Definition.—The loose-fit class is intended to cover the manufacture of threaded parts where quick

and easy assembly is necessary and a considerable amount of shake

or play is not objectionable.

This class has an allowance on the screw to permit ready assembly . even when the threads are slightly bruised or dirty.

(b) Minimum nut basic.—The pitch diameter of the minimum nut

of a given diameter and pitch corresponds to the basic pitch diameter,

as specified in the tables of thread series given herein, which is computed from the basic major diameter of the thread. The pitch

diameter of the minimum nut is the theoretical pitch diameter for

that size.

(c) Maximum screw below basic. 3—The dimensions of the maximum

screw of a given pitch and diameter are below the basic dimensions

as specified in the tables of thread series given herein, which are

3 The maximum minor diameter of the screw is above the basic minor diameter, as shown in Figure 4.
24 NATIONAL SCREW THREAD COMMISSION

computed from the basic major diameter of the threads, by the

amount of the allowance given in Table 3.

(d) Allowance and tolerance values.—Allowances and tolerances are

specified in Table 3.

Table 3.

Class 1, loose fit, allowances and tolerances for screws and nuts

Threads per inch Allowances

Pitch diameter

tolerances i

Lead

errors consuming

one-half

of pitch- diameter

tolerances 1

SO. 72- 64.

56.

48..

44.,

40.. 36.

32. 28.

24.

20.

18.

16. 14.

13.

12.

11.

10. 9-

8__

7__ 6..

5-. 4H

4_.

Inch

0. 0007 .0007

.0007

.0008

.0009

.0009

.0010

.0011 .0011 .0012

.0013

.0015

.0016 .0018

.0021

.0022

.0024 .0026

.0028

.0031

.0034

.0039

.0044

.0052 .0057

.0064

Inch

0.0024

.0025

.0026 .0028

.0031

.0032

.0034

.0036

.0038 .0043

.0046

.0051 .0057

.0063 .0070

.0074

.0079

.0085 .0092 .0100

.0111

.0124

.0145 .0169

.0184

.0204

Inch

0.0007 .0007

.0008 .0008

.0009

.0009

.0010 .0010

.0011 .0012

.0013

.0015

.0016

.0018 .0020

.0021 .0023

.0025 .0027

.0029

.0032 .0036

.0042 .0049

.0053

.0059

1 The tolerances specified for pitch diameter include all errors of pitch diameter, lead, and angle. The

full tolerance can not, therefore, be used on pitch diameter unless the lead and angle of the thread are

perfect. Columns 4 and 5 give, for information, the errors in lead (per length of thread engaged) and in

angle, each of which can be compensated for by half the tolerance on the pitch diameter given in column 3. If lead and angle errors both exist to the amount tabulated, the pitch diameter of a bolt, for example, must be reduced by the full tolerance or it will not enter a basic nut or gage.

2 Between any two threads not farther apart than the length of engagement.
1924 EEPOKT 25

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H
26 NATIONAL SCREW THREAD COMMISSION

NOTATION

Z)=major diameter

iS=pitch diameter

j5T= minor diameter

ft =0.0325= basic thread depth

Fig. 5.

Illustration of loosest condition for class 1, loose fit, one-fourth inch, 20

threads

NOTATION D=major diameter

2?= pitch diameter K=minor diameter

ft =0.032S= basic thread depth

Fig. 6.Illustration of tightest condition for class 1, loose fit, one-fourth inch, SO

threads
1924 REPORT 27

2. Class 2, Free Fit.—(a) Definition.—The free-fit class is intended to apply to interchangeable manufacture where the threaded

members are to assemble nearly, or entirely, with the fingers, where

fl - B.AS/C THKEAO

Hsavy Ltne. Shows'

a moderate amount of shake or play between the assembled threaded

members is not objectionable, and where no allowance is required.

This class includes the great bulk of fastening screws.
28 NATIONAL SCREW THREAD COMMISSION

(6) Minimum nut basic.—The pitch diameter of the minimum nut

of a given diameter and pitch corresponds to the basic pitch diameter, as specified in tables of thread series given herein, which is computed from the basic major diameter of the thread.

(c) Maximum screw basic.4—The major diameter and pitch diameter of the maximum screw of a given pitch and diameter corre- spond to the basic dimensions, as specified in tables of thread series

given herein, which are computed from the basic major diameter

of the thread.

(d) Allowance and tolerance values.—Allowances and tolerances are

specified in Table 4.

Table 4.Class 2, free fit, allowances and tolerances for screws and nuts

Threads per inch

80.

72. 64.

56. 48.

44. 40.

36.

32.

28.

24.

20.

18.

16. 14.

13- 12.

11.

10. 9.. 8-

7„

6-

a_.

4._

Allowances

Inch

0.0000 .0000

.0000

.0000

.0000

.0000

. 0000 .0000

.0000 .0000

.0000

.0000

.0000

.0000

.0000

.0000

.0000 .0000

.0000

.0000

.0000

.0000

.0000

.0000 .0000

.0000

Pitch diameter

tolerances

Inch

0. 0017 .0018

.0019 .0020

.0022

.0023

.0024

.0025

.0027

.0031

.0033

.0036

.0041 .0045

.0049

.0052 .0056

.0059

.0064 .0070

.0076 .0085

.0101 .0116

.0127

.0140

Lead

errors consuming

one-half

of pitch- diameter

tolerances b

Inch

0. 0005 .0005

.0005

.0006

.0006

.0007 .0007

.0007 .0008

.0009

.0010

.0010

.0012

.0013 .0014

.0015

.0016

.0017

.0018 .0020

.0022

.0025 .0029

.0033

.0037

.0040

Errors in

half-angle consuming

one-half

of pitch- diameter

tolerances

Deg. Min.

36

28

19

565043

39

39

3122

25

22

19

1717

11

1312

108

9

65

4

» The tolerances specified for pitch diameter include all errors of pitch diameter, lead, and angle. The

full tolerance can not, therefore, be used on pitch diameter unless the lead and angle of the thread are perfect. Columns 4 and 5 give, for information, the errors in lead (per length of thread engaged) and in angle, each of which can be compensated for by half the tolerance on the pitch diameter given in column 3. If lead and angle errors both exist to the amount tabulated, the pitch diamerer of a bolt, for example, must

be reduced by the full tolerance or it will not enter a basic nut or gage.

b Between any two threads not farther apart than the length of engagement.

1 The maximum minor diameter of the screw is above the basic minor diameter, ns shown in Figure 7.
1924 REPORT 29

NOTATION

2>= major diameter E=pitch diameter K=minor diameter

A =0.0325= basic thread depth

Fig. 8.Illustration of loosest condition for class 2, free fit, one-fourth inch, 20

threads
30 NATIONAL SCREW THREAD COMMISSION

3. Class 3, Medium Fit.—(a) Definition.—The medium-fit class

is intended to apply to the manufacture of the higher grade of

threaded parts which are to assemble nearly or entirely with the

fingers and must have the minimum amount of shake or play between

0£FTH=O. £495 /£>h

MeAY/ Z//v£" Shows

3as/c S/z. £.

the threaded members. It is the same in every particular as class

2, free fit, except that the tolerances are smaller.

(b) Minimum nut basic.—The pitch diameter of the minimum nut

of a given diameter and pitch corresponds to the basic pitch diameter, as specified in tables of thread series given herein, which is computed from the basic major diameter of the thread.
1924 REPORT 31

NOTATION

Z> = major diameter E= pitch diameter K=minor diameter

A =0.0325 = basic thread depth

Fig. 11.

Illustration of loosest condition for class 3, medium fit, one-fourth inch,

20 threads

NOTATION D=major diameter

-E=piteh diameter K=minor diameter

/i=0.0325=basic thread

depth

Fig. 12.Illustration of tightest condition for class 3, medium fit, one-fourth inch,

20 threads
32 NATIONAL, SCREW THREAD COMMISSION

(c) Maximum screv: basic.5 —The major diameter and pitch diameter

of the maximum screw of a given pitch and diameter correspond to

the basic dimensions, as specified in tables of thread series given

herein, which are computed from the basic major diameter of the

thread.

(d) Allowance and tolerance values.—Allowances and tolerances are

specified in Table 5.

Table 5.

Class S, medium fit, allowances and tolerances for screws and nuts

Threads Der inch Allowances

Pitch- diameter

tolerances

Lead

errors consuming

one-half

of pitch- diameter

tolerances b

Errors in

half-angle consuming

one-half

of pitch- diameter

tolerances

80.

72. 64.

56. 48.

44.

40.

36.

32.

28.

24.

20.

18.

16.

14.

13.

12. 11.

10. 9..

8.. 7..

6..

5..

4..

Inch

0.0000 .0000

.0000

.0000

.0000

.0000

.0000

.0000

.0000

.0000

.0000

.0000

.0000

.0000 .0000

.0000

.0000

.0000

.0000

.0000

.0000

.0000 .0000

.0000

.0000

.0000

Inch

0.0013

.0013

.0014 .0015

.0016

.0016

.0017

.0018

.0019

.0022

.0024 .0026

.0030

.0032

.0036

.0037

.0040

.0042

.0045

.0049

.0054

.0059

.0071 .0082

.0089

.0097

Inch

0.0004

.0004

.0004

.0004

.0005

.0005

.0005

.0005 .0005 .0006

.0007

.0008

.0009

.0009 .0010

.0011 .0012

.0012

.0013 .0014

.0016

.0017

.0020

.0024

.0026

.0028

Deg. Min.

59

47

43

36

28

21

1814

10

11

6

2

59

58

55

55

5352

51

60

4749

47

46

44

° The tolerances specified for pitch diameter include all errors of pitch diameter, lead, and angle. The

full tolerance can not, therefore, be used on pitch diameter unless the lead and angle of the thread are perfect. Columns 4 and 5 give, for information, the errors in lead (per length of thread engaged) and in angle, each of which can be compensated for by half the tolerance on the pitch diameter given in column 3. If lead and

angle errors both exist to the amount tabulated, the pitch diameter of a bolt, for example, must be reduced by the full tolerance or it will not enter a basic nut or gage.

* Between any two threads not farther apart than the length of engagement.

4. Class 4, Close Fit. —(a) Definition. —The close-fit class is intended for threaded work of the finest commercial quality where very

little shake or play is desirable, and where a screw driver or wrench

may be necessary for assembly. In the manufacture of screw-thread

products belonging in this class it will be necessary to use precision

tools, selected gages, and other refinements. This quality of work

should, therefore, be used only in cases where requirements of the

1 The maximum minor diameter of the screw is above the basic minor diameter, as shown in Figure 10.
1924 EEPOET 33

mechanism being produced are exacting, or where special conditions

require screws having a precision fit. In order to secure the fit desired

it may be necessary in some cases to select the parts when the product

is being assembled.

(b) Minimum nut basic. —The pitch diameter of the minimum nut

of a given diameter and pitch corresponds to the basic pitch diameter,

as specified in tables of thread series given herein, which is computed

from the basic major diameter of the thread.

(c) Maximum screw above basic. —The pitch diameter of the maximum screw of a given diameter and pitch is above the basic dimensions,

as specified in tables of thread series given herein, which are computed from the basic major diameter of the thread, by the amount of

the allowance (interference) specified in Table 6.

(d) Allowance and tolerance values. —Allowances and tolerances are

specified in Table 6.

Table 6.Class 4, close fit, allowances and tolerances for screws and nuts

Threads per inch

80..

72..

64.. 56..

48..

44..

40..

36.. 32..

28..

24..

20..

18..

16..

14..

13..

12..

11..

10.. 9..

8...

7...

6...

5... iy2

4..

Interfer- ences or negative

allowances

Inch

0.0001

.0001

.0001 .0002 .0002

.0002

.0002

.0002

.0002

.0002

.0003

.0003

.0003 .0004

.0004

.0004

.0005

.0005 .0006

.0006

.0007

.0008

.0009 .0010

.0011

.0013

Pitch- diameter

tolerances 1

Inch

0. 0006

.0007

.0007

.0007

.0008

.0008

.0009

.0009

.0010

.0011

.0012

.0013

.0015 .0016

.0018

.0019 .0020

.0021 .0023

.0024

.0027 .0030

.0036

.0041 .0044

.0048

Lead

errors consuming

one-half of pitch- diameter

tolerances 2

Inch

0.0002

.0002

.0002

.0002 .0002

.0002

.0003

.0003

.0003 .0003

.0003

.0004

.0004

.0005

.0005

.0005

.0006

.0006

.0007

.0007

.0008

.0009 .0010

.0012

.0013

.0014

Errors in

half-angle consuming

one-half of pitch- diameter

tolerances

Beg. Min.

55

585145

44

40

•11 37

37

35

3330

31

2929

28

28

2626

25

2524

25

23

23

22

1 The tolerances specified for pitch diameter include all errorsof pitch diameter, lead, and angle. The full tolerance can not, therefore, be used on pitch diameter unless the lead and angle of the thread are perfect. Columns 4 and 5 give, for information, the errors in lead (per length of thread engaged) and in angle, each

of which can be compensated for by half the tolerance on the pitch diameter given in column 3. If lead and

angle errors both exist to the amount tabulated, the pitch diameter of a bolt, for example, must be reduced by the full tolerance or it will not enter a basic nut or gage.

2 Between any two threads not farther apart than the length of engagement.
34 NATIONAL SCREW THREAD COMMISSION

Fig. 13.

Illustration of tolerances, allowance (interference) , and crest clearances

for class 4, close fit
1924 EEPOET 35

—Illustration of loosest condition for class 4, close fit, one-fourth inch,

20 threads

Fig. 15.—Illustration of lightest condition for class 4, close fit, one-fourth inch,

20 threads
36 NATIONAL SCREW THREAD COMMISSION

Pitch-Inch

JQ . J,2 , J4 . 16

iUk&Zt « 2016 fG 14 13 12 II IO 3 & 7 G 3

&XUX Number or Threads per /nch.

Fig. 16.Relation of maximum and minimum pitch diameters of classes 1, 2, 3,

and 4 fits to basic pilch diameters
1924 REPORT 37

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38 NATIONAL SCREW THREAD COMMISSION

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1924 REPORT 39

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1924 REPORT 53

5.- SPECIFICATIONS FOR THREADING TOOLS

The limiting sizes of the threaded product are always the essential

requirements of the manufacturer and user of threaded parts. To

maintain these sizes, not only the tools but also the processes employed

must be suitable. So many varying factors are involved that it is not always possible to predetermine the exact sizes of the tools

necessary to accomplish the required results. Variations in the

nature of the material, the lubrication, the method of holding the

product and tool, the sharpness of the tool, etc., all have an effect

on the final results.

The effort has been made, in the following specifications, to establish sizes for various commonly used tools such that, when the tools are used with reasonable care under normal conditions, a threaded

product will be produced in conformity with the product specifications. Occasions will undoubtedly arise when, as a result of peculiar

combinations of circumstances, special tools will be required.6

(a) FORM OF TOOLS FOR PRODUCING SCREWS

Screws, or external threads are commonly produced by lathe

tools, solid or adjustable dies, adjustable or opening die heads with

removable chasers, thread milling cutters, threading hobs, and

roller dies.

Of these, the dies, die-head chasers, and hobs are all multiple

toothed, cutting in several thread spaces simultaneously, and finishing the operation at one pass. Lathe tools are ordinarily singlepointed and operate in a single thread, which is finished by repeated

passes; but multiple pointed chasers for use as lathe tools are sometimes made.

All rolled threads and many cut threads are produced with dies,

chasers, or hobs made with master tools, such as hobs, taps, or milling

cutters. These master tools are frequently made with forming

cutters or other tools, but the primary tool is always made with a

single-point tool. Angle and pitch errors tend to accumulate in a

series of master tools and must be carefully considered in the design

and use of this single-point tool.

The tables and charts given in this section apply only to the tool

used to produce the finished standard thread, and such a tool must

be used on an axial section of the work.

1. Inspection of Tooth Outlines.—All threading tools, whether

for use in a lathe, die head, thread miller, or roller, and whether

single or multiple pointed, must produce the proper tooth shape on

an axial section of the work. In Figure 17, for instance, the lathe

threading tool is shown tipped up to the helix angle of the thread

to be cut. This is sometimes done for the sake of a better cutting

6 An analysis of the various factors which determine the accuracy of a screw thread in production is given in Appendix 3.
54 NATIONAL SCREW THREAD COMMISSION

action, but necessitates a change in the tool shape, which must show

an included angle of less than 60°, when projected on line y—y of the

illustration, if the resulting thread as measured on the axial section

x—x, is to be exactly 60°. The ordinary thread milling cutter, set

at the helix angle, and roller dies must have the same correction

if accurate results are desired. In brief, the final test of accuracy in

any threading tool is its ability to produce a thread of the proper

axial section as defined in Section III, division 1 (a), "Specifications,"

of this report.

Most cutting tools for standard threads have their cutting edges

in the axial plane of the work, so that the shape of those edges tends

to reproduce itself on the screw thread. In forming and inspecting

the cutting edges of these tools, their forms may be directly compared

with standard outlines. This can be done by means of accurately

formed templets, carefully applied under the microscope. A more

satisfactory and practical way is to draw the desired outline on a

chart to a magnification of 100 or 200 times, and then project on this

chart the image of the cutting tool under inspection magnified to the

corresponding degree. By this means the tool shape may be quickly

compared with the standard shape to a degree of accuracy much greater

than that required for commercial work. Care must be taken to use

a lens system free from distortion. Optical projection machines and

comparators are available for this work in commercial designs.

2. Outline for Single-Point Tool or Cutter.—In Figure 18

is shown the form of outline, measured on an axial section of the

work, required for a single-point lathe tool or a milling cutter to

produce directly a standard thread. Such a diagram, if drawn to a magnification of 100 or 200 times, will serve as a chart for projection

testing. Note the shaded area at the point of the tool, within which

a tool point of any shape is permissible. While sharp-cornered tools are easier to make, one with a permissible round might be more

durable.

3. Outline for Multiple-Tooth Chasers and Hobs.—In

Figure 19 are given similar instructions for determining the outline

of multiple-toothed chasers and hobs. The tolerance at the point or

crest of the tool is the same as in Figure 18. At the roots of the

teeth, however, the depth of the thread space is carried far enough to

clear the crests of the screw thread being cut. This is in accordance

with modern practice in threading alloy steels, which tend to tear at

the crests. Relieving the chasers as shown minimizes this trouble.

It should be noted that this clearance at the crest of the screw is certain only on classes 2, 3, and 4 fits. If a class 1 screw is being cut

to a minimum pitch diameter from stock of maximum major diameter

size, the crests will be trimmed by the chaser. It is not possible to

avoid this without carrying the grooves in the chasers nearer to a sharp bottom than is desirable.
1&24 REPORT 55

Fig. 18.

Shape for cutter or single point lathe tool not intended to trim crest oj

thread
56 NATIONAL, SCREW THREAD COMMISSION

Side offool must le

straight a6e¥f

line

rxnec/or sTiafae ofpoint

Fig. 19.

Shape for teeth of chaser or hob not intended to trim crest of thread

Shaded area, shouis tol- erance (or t&gf>» of root\

of fjfreaa' cAascrorJto&

SiA' o{tootmast I* SfraitAt • tcrf tnisl

trance for sink' efhainf

Fig. 20.

Shape for teeth of chaser or hob intended to trim crest of thread
1924 REPORT 57

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NOOOM CN CO © -H C O C O OCOOQH hOiOCNm iOCOOOh ^lOiOtOh- NXOO-h 0©000 © © © —

I

© so o cn X O O CN S C) CO C) IN C C M r- - - N

J3

Eh ^0X0* CO CN ^ © CN CN --. ^ ^ — ^ -< --i 00OV COCN^O© cot*o»o^ ^«
58 NATIONAL SCREW THREAD COMMISSION

When it is desired to follow the older practice which permits

trimming of the crests, the outline shown in Figure 20 may be used.

It is somewhat easier to make than the one shown in Figure 19.

Note that the tolerance allowed on the shape of the points or

crests is the same as in Figures 18 and 19. In both Figures 19 and

20 the shape of the bottom of the thread spaces may be given any

curved form which can be contained within the shaded tolerance

area, and still produce screw threads which will agree with the

standards set up in this report.

In Table 17 are given the required dimensions for drawing the

charts of Figures 18, 19, and 20 correctly for any standard pitch.

(b) TAPS

1. Tap Dimensions.—The tap dimensions given in Tables 18 to

24, inclusive, were prepared with the cooperation of a committee of

tap and die manufacturers, and represent present practice for the

production of tapped holes of the classes indicated. Whether or not

the following specifications for taps will prove to be those most

acceptable can only be determined by their use and trial. These

specifications, therefore, are tentative, and are subject to additions

and revisions as experience proves necessary.

Dimensions are given for three classes of taps : First, commercial

taps; second, class X ground taps; and third, class Y ground taps.

(a) Commercial taps. —The commercial taps should be found

suitable, in most cases, for tapped holes made to class 1, loose-fit

specifications, and for the fractional sizes of class 2, free fit specifications. With proper selection by trial, under the actual manufacturing conditions involved, it should be found possible to use some

of the commercial taps for tapped holes made to the class 2, free-fit

specifications in the numbered sizes, and for the fractional sizes of

class 3, medium-fit specifications.

(&) Class X ground taps.—The class X ground taps should be

found suitable when used with reasonable care for tapped holes made

to class 3, medium-fit specifications and class 4, close-fit specifications.

(c) Class Y ground taps.—The class Y ground taps should be found

suitable when used with reasonable care for tapped holes made to

class 3, medium-fit specifications. The minimum size of the class Y

tap is above the basic size to give a longer wearing life. Where

accuracy requirements permit, class Y ground taps should be used

in preference to class X taps because of their longer useful life.

2. Marking of Taps.—Taps shall be marked indicating the diameter, pitch, and thread series as follows (see Section II, division 2,

''Symbols")- Ground taps shall be marked "X" or "Y" according

to their classification.
1924 REPORT 59

A 1-inch, 8-pitch tap shall be marked 1"—8—NC.

A 1-inch, 14-pitch, class Y ground tap shall be marked 1"—14—NF—Y.

3. Shape of Cutting Edge for Taps (and other Internal

Threading Tools).—In Figure 21 is shown the form of outline,

measured on an axial section of the "work, required for a tap or other internal threading tool intended for cutting threads of national

form. These outlines are the same for all pitches and classes of fit. Such a diagram, if drawn to a magnification of 100 or 200 times, will serve as a chart for projection testing. Note the shaded area at the

Skacted area shouts toferaxee

for s/itz/ie of /point

Fig. 21.

Shape for teeth of tap or internal thread-chasing tool

point of the tool, within which a tool point of any shape is permissible. Also note the shaded area at the root of the thread which

shows permissible variations in tooth outline at this point.

While sharp-cornered tools are easier to make, one with a permissible round at the point might be more durable. In any event, the

maker of the tap or other tool should keep the outline well away from

the lower part of the permissible, area, represented by the basic flat

of }i X p. The danger here is that after a few holes have been

tapped the corners will be rounded over so that the tapped hole

rejects the basic gage.

Table 17 gives the necessary dimensions for drawing the diagram

shown in Figure 21 for any standard pitch.
60 NATIONAL SCREW THREAD COMMISSION

Table 18.

Dimensions of hand taps

Length over all Length of thread Diameter of shank Size of square

XT * Nominal size 01 tap in inches Nominal Toler- Nominal Toler- Nominal Toler- Nominal Toler- ance ance ance ance

1 2 3 4 6

<*l 8 9

Inches Inch Inches Inch Inches Inch Inches Inch M VA ±ft 1 ±ft 0. 2530 -0.005 0. 1897 -0.004 A - . 3155 — . 005 . 2366 _ 004 % —- ±ft IX ±ft .3785 - [ 005 .2839 - !o04

7I* 3ft ±ft 1ft ±ft .3232 - .005 .2424 - .004 y2 —

SVs ±ft ift ±ft .3667 — .005 .2750 - .004

IT, 3H ±ft IB ±ft .4292 - .005 .3219 - .006 H CO lit i 3 ±ri — . UUO . 3597 — . 006

4)4 ±ft 2 ±ft .5900 - .005 .4425 - .006

Vt 4H ±ft 2ft ±ft .6973 - .005 .5230 - .006

l 5Vs ±ft 2H ±A .8000 - .005 .6000 - .006 m— 5ft ±rV 2ft ±ft .8965 - .007 .6724 - .008

lJi m ±ft 2ft ±ft 1. 0215 - .007 .7661 - .008 m ±ft 3 ±ft 1.2333 - .007 .9250 - .008 m — 7 ±ft 3ft ±ft 1.4300 - .007 1. 0725 - .008

2 m ±ft 3ft ±ft 1. 6445 - .007 1.2334 - .008

±ft 3rV ±ft 1.8944 - .009 1.420S - .010

- ±ft 4 ±A 2. 1000 - .009 1. 5750 - .010

2'A- — 9M ±ft 4 ±ft 2.3500 - .009 1. 7625 - .010

3 - 9% ±rV 4ft ±ft 2.5429 - .009 1. 9072 - .O'O

Table 19.

Dimensions and tolerances for commercial taps, national coarse-thread

series

Sizes Threads

per inch

Major diameter Pitch diameter Basic diameter

Mini- mum

Maximum—

measured

pitch diameter

plus

Mini- mumToler- ance Maximum Major Pitch

1 •X 3 i 5 6 7 8 9

Inches Inch Inches Inch Inch s Inches Inches

1 64 0. 0740 0. 0124 0. 0634 0.0010 0.0644 0.073 0.0629

2 56 .0870 .0142 .0749 .0010 .0759 .086 .0744

3 - — 48 . 1000 .0165 .0860 .0010 .0870 .099 . 0855

4. 40 . 1130 .0198 .0963 .0015 .0978 .112 .0958

5 40 .1260 .0198 . 1093 .0015 .1108 .125 .1088

6... 32 . 1390 .0248 . 1182 .0015 .1197 .138 .1177

8... 32 .1650 .0248 . 1442 .0015 . 1457 .164 .1437

10 24 . 1910 .0331 . 1634 .0015 . 1649 .190 . 1629

12 24 .2170 .0331 . 1894 .0015 . 1909 .216 .1889 U- 20 .2510 .0397 .2180 .0020 . 2200 .2500 . 2175

ft 18 .3135 .0441 . 2769 .0020 .2789 .3125 .2764 % - — 16 .3760 .0496 .3349 .0020 3369 .3750 . 3344

ft 14 .4385 .0567 .3916 . 0025 .3941 .4375 . .3911

Yi 13 .5010 .0611 .4505 .0025 .4530 .5000 .4500

ft- 12 .5635 .0662 .5089 .0025 .5114 .5625 .5084 X 11 .6260 .0722 .5665 .0025 .5690 . 6250 .5660 H- —

%

10 .7510 .0794 .6855 .0030 .6885 .7500 .6850

9 .8760 .0882 .8033 . 0030 .8063 .8750 .8028

1 8 1. 0010 .0992 .9193 .0030 .9223 1.0000 .9188

lYs 7 1. 1265 .1134 1. 0327 .0035 1. 0362 1. 1250 1. 0322 V4 7 1.2515 . 1134 1. 1577 .0035 1. 1612 1.2500 1. 1572 W2 — 6 1. 5015 .1323 1. 3922 .0035 1. 3957 1.5000 1.3917 M~ 5 1.7515 . 1588 1. 6211 .0040 1. 6251 1.7500 1. 6201 2 4H 2. 0015 .1764 1. 8567 .0045 1. 8612 2.0000 1. 8557

2Ji 4M 2. 2520 . 1764 2. 1067 .0045 2.1112 2. 2500 2. 1057 m 4 2. 5020 . 1985 2. 3386 .0050 2. 3436 2.5000 2. 3376 2M 4 2. 7520 . 1985 2. 5886 .0050 2. 5936 2.7500 2. 5876

3 4 3. 0020 . 1985 2. 8386 .0050 2. 8436 3.0000 2. 8376

Note.—Maximum lead error =±0.003 inch in 1 inch of thread.

1 If a tap should be of maximum diameter and contain the maximum lead error, it may not produce

tapped holes within the specified tolerances for the product.
1924 REPORT 61

Table 20.

Dimensions and tolerances for commercial taps, national fine-thread

series

Sizes

l nreacls per inch

Major diameter Pitch diameter Basic diameter

Mini- mum

Maximum 3

measured

pitch diameter

plus

Mini- mumToler- ance Maxi- mum i Major Pitch

ii iM 3 4 5 6 j 8 9

Inches Inch Inches Inch Inches Inches Inches

n 80 0. 0610 0. 0099 0. 0524 0. 0010 0. 0534 0. 060 0. 0519

i .0740 .0110 . 0645 . 0010 .0655 .073 .0640

2 64 . 0870 .0124 .0764 . 0010 .0774 .086 .0759

3 __ 56 . 1000 . 0142 .0879 . 0010 .0889 . 099 .0874

4 _ 48 . 1130 .0165 .0990 . 0010 . 1000 .112 .0985

K 44 , . 1260 .0180 .1107 .0015 . 1122 .125 .1102

fl 40 . 1390 .0198 . 1223 . 0015 . 1238 . 138 . 1218

8 36 . 1650 . 0221 . 1465 . 0015 . 1480 . 164 . 1460

10 _ 32 .1910 .0248 .1702 .0015 . 1717 .190 . 1697

12 28 . iyoo 1QAQ Ol ft 1 GOO X - 28 .2510 .0284 .2273 .0015 .2288 .2500 .2268 A - 24 .3135 .0331 .2859 .0015 .2874 .3125 .2854 % - 24 .3760 .0331 .3484 .0015 .3499 . 3750 .3479 A - - 20 .4385 .0397 .4055 .0020 .4075 .4375 .4050

Vt 20 .5010 .0397 .4680 .0020 .4700 .5000 .4675 A - - 18 .5635 .0441 .5269 .0020 .5289 .5625 .5264 %—- - 18 .6260 .0441 .5894 .0020 .5914 .6250 .5889

*A 16 .7510 .0496 . 7099 .0020 .7119 .7500 .7094 % — —

14 .8760 .0567 .8291 .0025 .8316 .8750 .8286

1 14 1. 0010 .0567 .9541 .0025 .9566 1.0000 .9536 m — 12 1. 1265 .0662 1. 0714 .0025 1. 0739 1. 1250 1. 0709

12 1. 2515 .0662 1. 1964 .0025 1. 1989 1.2500 1. 1959 M~~ 12 1. 5015 .0662 1.4464 .0025 1. 4489 1. 5000 1. 4459

Note.—Maximum lead error =±0.003 inch in 1 inch of thread.

1 If a tap should be of maximum diameter and contain the maximum lead error, it may not produce

tapped holes within the specified tolerances for the product.

Table 21.Dimensions and tolerances for class X ground taps, national coarse- thread series

Sizes Threads

per inch

Major diameter Pitch diameter Basic diameter

Toler- ance on

half angle

of thread Mini- mum

Maximum=

measured

pitch diameter

plus

Mini- mum

Toler- ance Maximum Major Pitch

1 3 3 4 5 6 7 8 9 10

Inches Inch Inches Inch Inches Inches Inches Dea. Min.

M. 20 0. 2520 0. 0397 0. 2175 0. 0008 0.2183 0. 2500 0. 2175 15 A 18 .3145 .0441 .2764 .0008 .2772 .3125 .2764 15

16 .3770 .0496 .3344 .0008 .3352 .3750 .3344 15 1::::::::: 14 .4400 .0567 .3911 .0009 .3920 .4375 .3911 15 H 13 .5025 .0611 .4500 .0009 .4509 .5000 .4500 14 A- 12 .5650 .0662 .5084 .0010 .5094 .5625 .5084 14

Vs 11 .6275 .0722 .5660 .0010 .5670 .6250 .5660 13 %- 10 .7530 .0794 .6850 .0011 .6861 .7500 .6850 13 %- 9 .8780 .0882 .8028 0012 .8040 .8750 .8028 13 I.. ' 8 1. 0030 .0992 .9188 .0013 .9201 1.0000 .9188 12 1H- 7 1. 1290 .1134 1. 0322 .0014 1. 0336 1. 1250 1. 0322 12 IH- 7 1. 2540 .1134 1. 1572 .0014 1. 1586 1.2500 1. 1572 12 m-- 6 1. 5040 . 1323 1.3917 .0015 1. 3932 1.5000 1.3917 12

5 1. 7550 .1588 1. 6201 .0015 1.6216 1. 7500 1. 6201 12 2 V/2 2.0050 .1764 1. 8557 .0015 1. 8572 2.0000 1. 8557 • 12 M— 4H 2. 2560 . 1764 2. 1057 .0015 2. 1072 2. 2500 2. 1057 12 254- - 4 2. 5060 . 1985 2. 3376 .9015 2. 3391 2. 5000 2. 3376 11 2H- 4 2. 7570 . 1985 2. 5876 .0015 2. 5891 2.7500 2. 5876 11 3 4 3. 0070 . 1985 2.8376 .0015 2.8391 3.0000 2. 8376 11

Note.—Maximum lead error= ±0.0005 inch in 1 inch of thread. 16802°—251 5
62 NATIONAL SCKEW THREAD COMMISSION

Table 22.Dimensions and tolerances for class X ground taps, national fine-thread

series

Sizes

TVi 1 111roiric t i

1 1 > nor l n/">Vi

I '1 1 il II 1 1

Major diameter Pitch diameter Basic diameter

i oier- 8.DC6 on

half angle

of thread Mini- mum

Maximum=

measured

pitch diameter

plus

Mini- mumToler- anceM!axi- mum Major Pitch

1 2

A

*± o

aDQO oy 10

Inches Inch Inches Inch Inches Inches Inches Uty . JxL I It. M- 28 0. 2520 0. 0284 0. 2268 0.0008 0. 2276 0. 2500 0. 2268 18 A—%-

24 .3145 .0331 .2854 .0008 .2862 .3125 .2854 16 24 .3770 .0331 .3479 .0008 .3487 .3750 .3479 16 ft-- 20 .4400 .0397 .4050 .0008 .4058 .4375 .4050 15 V2- 20 .5025 .0397 .4675 .0008 .4683 .5000 .4675 15

ft- 18 .5650 .0441 .5264 .0008 .5272 .5625 .5264 15 18 .6275 .0441 .5889 .0008 .5897 .6250 .5889 15 %-

%-

16 . 7530 .0496 .7094 .0008 .7102 .7500 .7094 15

14 .8780 .0567 .8286 .0009 .8295 .8750 .8286 15 1- 14 1.0030 .0567 .9536 .0009 .9545 1.0000 .9536 15 m 12 1. 1290 .0662 1. 0709 .0010 1. 0719 1. 1250 1.0709 14 m~ 12 1. 2540 .0662 1. 1959 .0010 1. 1969 1.2500 1. 1959 14 m 12 1. 5040 .0662 1. 4459 .0010 1. 4469 1. 5000 1. 4459 14

Note.—Maximum lead error= ±0.0005 inch in 1 inch of thread.

Table 23.Dimensions and tolerances for class Y ground taps, national coarse- thread series

Sizes Threads

per inch

Major diameter Pitch diameter Basic diameter

Toler- ance on

half angle

of thread Mini- mum

Maximum=

measured

pitch diameter

plus

Mini- mumToler- ance Maximum Major Pitch

1 2 3 4 5 6 7 8 9 10

Inches Inch Inches Inch Inches Inches Inches Deg. Min.

y* 20 0. 2525 0. 0397 0. 2180 0. 0010 0. 2190 2500 0. 2175 20

ft 18 .3150 .0441 .2769 .0010 .2779 3125 .2764 20

SA- 16 .3776 .0496 .3350 .0011 .3361 3750 .3344 19

7 14 .4407 .0567 .3918 .0012 .3930 4375 .3911 19 K 13 .5032 .0611 .4507 .0013 .4520 5000 .4500 18

ft—- - 12 . 5658 .0662 .5092 .0014 .5106 5625 .5084 18

hA 11 .6283 .0722 . 5668 .0014 .5682 6250 .5660 18

SA 10 .7539 .0794 .6859 .0014 .6873 7500 .6850 17 Vs 9 .8790 .0882 .8038 .0015 .8053 8750 .8028 17

l 8 1.0041 .0992 .9199 .0016 .9215 1 0000 .9188 17 1H 7 1. 1302 .1134 1. 0334 .0018 1. 0352 1 1250 1. 0322 16 m 7 1. 2552 . 1134 1. 1584 .0018 1. 1602 1 2500 i. 1572 16 m 6 1. 5054 . 1323 1. 3931 .0022 1. 3953 1 5000 1. 3917 16 m 5 1. 7566 .1588 1. 6217 .0025 1.6242 1 7500 1. 6201 16 2 2. 0068 .1764 1. 8575 .0027 1. 8602 2 0000 1. 8557 15 2^ iyt

2. 2578 .1764 2. 1075 .0027 2. 1102 2 2500 2. 1057 15 2H 4 2. 5060 . 1985 2. 3396 .0030 2. 3426 2 5000 2. 3376 15 m 4 2. 7590 . 1985 2. 5896 .0030 2. 5926 2 7500 2. 5876 15 3- 4 3. 0090 .1985 2. 8396 .0030 2. 8426 3 0000 2. 8376 15

Note.—Maximum lead error= ±0.0005 inch in 1 inch of thread.
1924 REPORT 63

Table 24.

Dimensions and tolerances for class Y ground taps, national fine- thread series

CI' bizes Threads

per inch

Major diameter Pitch diameter Basic diameter

Toler- ance on

half angle

of thread Mini- mum

Maximum=

measured

pitched diameter

plus

Mini- mum

Toler- ance Maximum ir iLLil

1 2 3 4 5 6 7 8 10

Inches Inch Inches Inch Inches Inches Inches Deg. Min.

a —

A

28 0. 2525 0.0284 0. 2273 0. 0010 0. 2283 0.2500 0. 2268 24

24 .3150 .0331 .2859 .0010 .2869 .3125 .2854 22

Vi —

24 .3775 .0331 .348* .0010 .3494 .3750 .3479 22

_2_ 20 .4405 .0397 .4055 .0010 .4065 .4375 .4050 20 y2 20 .5030 .0397 .4680 .0010 .4690 .5000 .4675 20 A 18 .5655 .0441 .5269 .0010 .5279 .5625 .5264 20 % 18 .6280 .0441 .5894 .0010 .5904 .6250 .5889 20 % 16 .7536 .0496 .7100 .0011 .7111 .7500 .7094 19 Vs— 14 .8787 .0567 .8293 .0012 .8305 .8750 .8286 19

1 14 1. 0037 .0567 .9543 .0012 .9555 1.0000 .9536 19 iys 12 1. 1298 .0662 1. 0717 .0014 1. 0731 1. 1250 1.0709 18 1H 12 1. 2548 .0662 1. 1967 .0014 1. 1981 1. 2500 1. 1959 18 iy2 12 1. 5048 .0662 1. 4467 .0014 1.4481 1. 5000 1. 4459 18

Note.—Maximum lead error= ±0.0005 inch in 1 inch of thread.

(c) TAP-DRILL SIZES

Classes 1 to 4, inclusive The essential requirement of a tap drill is that the hole produced

by it shall be such that, when tapped with a screw thread, the minor

diameter of the tapped hole shall be within the specified limits

shown in Tables 7 to 14, inclusive, herein.

If the drill is too large, the minor diameter of the tapped hole will

also be too large and the thread in the nut will be too shallow; that is,

too small a percentage of a full thread. As an extreme case the threads

in the nut will engage only the tops of the threads on a bolt of cor- rect size, and under stress the nut member will not sustain the

proper load.

If, on the other hand, the tap drill is too small, the tap will be

forced to cut a thread of full depth, and in the extreme case to act as a reamer also. This will result in excessive power consumption and

tap breakage, and will also make the minor diameter of the tapped

hole dependent upon the minor diameter of the tap. This is undesirable, since the minor diameter of the tap is not, in general, held to the

same close limits as the other tap elements, and as a result the

minor diameter of a hole tapped under these conditions may be in error even though the tap is otherwise correct.

It is a well-known fact that the size of the hole produced by a tap

drill depends to some extent upon the method of grinding the drill,

the material drilled, the lubricant used, and the speed and feed of
64 NATIONAL SCREW THREAD COMMISSION

operation. This being true, it is apparent that fixing the diameter of

the tap drill does not completely fix the diameter of the drilled hole.

The most that can be accomplished is to fix the drill diameters

between certain limits and to depend upon correct grinding, lubrication, and operation to keep the diameter of the holes within prescribed limits.

In establishing tap-drill sizes, the starting point is the mean between

the maximum and minimum minor diameter of the tapped hole.

This permits a variation of the minor diameter, both above and below

the ideal tap-drill size, equal to half the tolerance on minor diameter.

Thus the tap drill may cut oversize by this amount by reason of

incorrect grinding, lubrication, or operation, or on account of differ- ences in the materials tapped; and it may be allowed to wear undersize until the tapped hole reaches the lower limit on minor diameter.

It should be noted that the minor diameters of the tapped holes are the same for classes 1 to 4, inclusive. This condition results from the

requirement on page 18 with reference to clearance on minor diameter:

4. Clearance on Minor Diameter.—A clearance shall be provided at the

minor diameter of the nut by removing from the crest of the basic thread form

an amount equal to one-sixth to one-fourth of the basic thread depth. , The minor diameter of the tapped hole is therefore greater than

the basic minor diameter by one-third to one-half the basic thread

depth, and the tolerance on the minor diameter is one-sixth the

basic depth.

Minimum minor diameter of tapped hole = basic minor diameter +

^ Maximum minor diameter of tapped hole =basic minor diameter +^

in which

A = basic thread depth = 0.649519 p

p = pitch =i

n = number of threads per inch.

There are given in Tables 25 and 26 the limiting dimensions

of the minor diameter of the threaded hole, as established by the

commission, together with the tap-drill sizes recommended. There

are also given the diameters of all the drills regularly carried in

stock that fall between the limiting dimensions of the minor diameter

of the threaded hole. Accordingly, drills of these diameters should

produce holes which result in a thread depth of 75 to 83M per cent

of the basic depth of thread.

The commission recommends to manufacturers of tap drills the

making up of complete sets of drills of the sizes required for both

the "national coarse" and "national fine thread series." The user
1924 REPORT 65

will then be able to select at once the correct drill for any national

standard thread without hunting through the several sets of numbered, lettered, fractional, and metric sizes ordinarily carried in

stock. The drills should be marked in accordance with the instructions on page 17. For example, 1"—8—NO would be used to designate the drill to be used for a 1-inch, eight threads per inch, national coarse thread.

Table 25.Sizes of tap drills, national coarse-thread series

Size of thread Threads

per inch

m

1 Not a stock size.

64

56

48

40

40

32

32

24

24

20

18

16

14

13

12

11

10

Minor diameter of nut

Basic

Inches

0. 0527

.0628

.0719

.0795

.0925

.0974

.1234

. 1359

. 1619

. 1850

.2403

.2938

.3447

.4001

.4542

.5069

.6201

.7307

.8376

.9394

1. 0644

1. 2835

1. 4902

Maximum

Inches

0. 0578

.0686

.0787

.0876

. 1006

. 1076 .1336

. 1494

. 1754

.2013

.2584

.3141

.3679

.4251

.4813

.5364

.6526

.7667

.8782

.9858

1. 1108

1. 3376

1. 5551

Mini- mum

Inches

0. 0561

.0667

.0764

.0849

.0979

.1042

. 1302

. 1449

.1709

.1959

.2524

.3073

.3602

.4167

.4723

.5266

.6417

.7547

.8647

.9704

1. 0954

1.3196

1. 5335

Recom- mended

tap drill diameter

Inches

i 0. 0575

i

. 0682

.0781

.0866

.0995

. 1063

1. 1324

. 1476

71732

.1990

.2559

"."3110

.3642

."4219

1. 4776

.5315

1. 6480

{---

7615

1. 8723

"i.~9789

1. 1024

"i."3281

n."5453

Present stock tap drills leaving between 83^

and 75 per cent of basic thread depth

Nominal size

1.7 mm.

No. 51.. A in No. 47..

2 mm. _

No. 44..

2.2 mm.

No. 40..

2.5 mm.

No. 39 _. 2.7 mm.

No. 36..

3.7 mm..

No. 26...

3.75 mm.

in. No. 17...

4.4 mm..

No. 9....

5 mm

No. 8_...

5.1 mm..

No. 7...,

6.5 mm.. F

7.9 mm.. A in

9.2 mm..

9.25 mm.

9.3 *i in

12 mm..

i* in---.

13.5 mm.

16.5 mm.

Hin...

22 mm.

: m.

25 mm.

Hin---

28 mm.

1& in..

IJi in..

39 mm.

l*f in—

39.5 mm.

Diameter

Inches

0. 0669 .0670

.0781 .0785 .0787

.0860

.0866 .0980

.0984

.0995

. 1063

. 1065

.1457

. 1470

. 1476

. 1719 .1730

. 1732 .1960

.1968

.1990

.2008

.2010

.2559

.2570

.3110

.3125 .3622

.3642

.3661

.4219 .4724

.5312

.5315

.6496

.7656

.8661

.8750

.9842 .9844

1. 1024

1. 1094

1. 3281

1. 5354

1. 5469

1. 5551

Per cent

of depth

of basic thread

8282

7777

76

7580

78

8382

78

82

82

7979

8382

78

76

75

78

77

7977

81

79

77

78

83

7880

797977

79

7682

7977

79

80

76

79

8379

7875
66 NATIONAL SCREW THREAD COMMISSION

Table 25.Sizes of tap drills, national coarse-thread series—Continued

Size of thread

1

Threads

per inch

Minor diameter of nut

Basic Maximum

Mini- mum

Recom- mended

tap drill diameter

Present stock tap drills leaving between 8314 and 75 per cent of basic thread depth

Nominal size Diameter

Per cent

of depth

of basic thread

2__

3__

m.

4

Inches

1. 7113

1. 9613

2. 1752

2. 4252

2. 6752

Inches

1. 7835

2. 0335

2. 2564

2.5064

2. 7564

Inches

1. 7594

2. 0094

2. 2294

2. 4794

2. 7294

Inches

1. 7716

1 2. 0225

Hi in.. 45 mm.

Hi in.. 2A in..

2. 2441

51.5 mm.

2^ in 2M in.... 57 mm...

214 in-..

63 mm...

I 2. 4939

in.

63. 5 mm.

2V2 in 2*i in

69.5 mm..

i 2. 7439

2

34in_.

70 mm.

Inches

1. 7656

1. 7716 1.7812

2. 0156

0276

0312

2344

2441 2500

4803

4844

2. 5000

2. 5000

2. 7344

2. 7362

2. 7500

2. 7559

8179

76

8179

77

76

82

79

77

83

82

79

77

7782

81

7977

75

1 Not a stock size.

Table 26.

Sizes of tap drills, national fine-thread series

Size of thread Threads

per inch

Minor diameter of nut

Basic Maximum

Mini- mum

Recom- mended

tap drill diameter

Present stock tap drills leaving between 83^

and 75 per cent of basic thread depth

Nominal size Diameter

Per cent

of depth

of basic thread

0...

1

2

34

5 - -

6...

8

10

12M

A —

%

1 Not a stock size.

80

72

64

56

48

44

40

36

32

28

28

24

24

Inch

0. 0438

.0550

.0657

.0758 .0849

.0955

.1055

. 1279

. 1494

.1696

.2036

.2584

.3209

Inch

0. 0478

.0595 .0708

.0816

.0917

.1029

. 1136

.1369

.1596

.1812

.2152

.2719

.3344

Inch

0. 0465

.0580

.0691 .0797

.0894

.1004

. 1109

.1339

.1562

.1773

.2113

.2674

.3299

Inch

0. 0472

.0591

.0700

.0810

i

. 0911

'""."1624'

. 1130

\~136o'

.1590

.1800

.2130

"l72703"

No. 56.. &

1.2 mm.

1.5 mm.

No. 53.. No. 50.. No. 46..

2.3 mm.

Inch

0.0465

.0469 .0472

. 0591 .0595 .0700

.0810

.0906

No. 38..

2.6 mm.

No. 34.. No. 33..

3.4 mm.

No. 29.. A in.... No. 22..

4 mm...

No. 21.. No. 15..

4.6 mm.

5.4 mm.

No. 3...

6.8 mm..

.1015 .1024

. 1110

. 1130

. 1339 .1360

. 1562

. 1570

. 1575

. 1590

. 1800

. 1811

. 2126 .2130

.2577

.3320

6.9 mm.

8.4 mm.

Q

.2717

.3307 .3320

83

81

7977

7579

7879

7780

77

8377

8378

83

8180

76

78

75

8180

83

787582

79
1924 REPORT 67

Table 26.Sizes of tap drills, national fine-thread series—Continued

Size of thread

&-

HrV- HK-

%-

i._

lVs- MmThreads

per inch

Minor diameter of nut

Basic

20

20

18

18

16

1414

12

12

12

Inches

0. 3725

.4350

.4903

.5528

.6688

.7822

.9072

1. 0167

1. 1417

1.3917

Maximum

Inches

0. 388S

.4513 .5084

.5709-

.6891 .8054 .9304

1. 0438

1. 1688

1. 4188

Mini- mum

Inches

0. 3834

.4459

.5024

.5649

.6823

.7977 .9227

1. 0348

1. 1598

1. 4098

Kecom- mended

tap drill diameter

Inches

0. 3860

i

. 4492

i

. 5062

1 . 5687

'""."6875

i

. 8024

1 . 9274

1 1. 0401

i 1. 1651 ' 1 1. 4153

Present stock tap drills leaving between 83J-3 and 75 per cent of basic thread depth

Nominal size

9.75 mm.

9.8 mm.. W

14.5 mm.

«in..._.

17.5 mm.

23.5 mm.

26.5 mm.

29.5 mm.

36 mm_

Diameter

Inches

0. 3839

.3858 .3860

.5709

.6875

.9252

1. 0433

1. 1614

1. 4173

Per cent

of depth

of basic thread

8280

79

7878

78

75

77

75

7881

78

78

75

82

78

78

76

1 Not a stock size.

6. GAGES

The art of measuring screw threads has developed very rapidly

during the past few years. This development still continues, so that

it would be inadvisable to attempt to specify any definite method as standard for this purpose. The object is to establish the fundamentals of this subject, and to point out practices now successfully

used.

(a) FUNDAMENTALS '

1. Object of Gaging.—The final results sought by gaging are to secure interchangeability ; that is, the assembly of mating parts without selection or fitting of one part to another, and to insure that the

product conforms to the specified dimensions within the limits of

variation establishing the closest and loosest conditions of fit permissible in any given case, as provided for in the foregoing specifications.

This requires the use of gages representing the limit of maximum

metal which must assemble with the product, known as "go'-' gages,

and gages representing the limit of minimum metal which must not

assemble with the product, known as "not go" gages.

Gaging should be as much employed to prevent unsatisfactory

parts from being produced as to sort out the correct from the incorrect parts.

2. Purpose of "Go" and "Not go" Gages.—The "go" gage,

which is the gage representing the maximum limit of the internal

member or the minimum limit of the external member, controls the

allowance between mating surfaces, and also controls interchange-
68 NATIONAL. SCREW THREAD COMMISSION

ability. "Go" gages control the maximum tightness in the fit of

mating parts. Parts which are acceptable to proper "go" gages will

always interchange. Successful interchangeable manufacturing has

been carried on for many years with the use of "go" gages only.

The "not go" gage, which is the gage representing the minimum

limit of the internal member or the maximum limit of the external

member, controls the extent of the tolerance in one direction. " Not

go" gages limit the amount of looseness between mating ' parts,

and thus control, in large measure, the proper functioning of mating

parts.

3. Gage Classification.—The limiting dimensions of the

threaded parts to be produced must be represented in: (a) Gages

used in checking the product as it is machined, known as "working

gages"; (b) gages for use in the acceptance of the product, known

as "inspection gages"; and (c) gages used to determine the accuracy

of the two preceding classes of gages, known as "master gages."

4. Gages Used to Measure the Product.—The gages used to

check the product may be divided into two general types: "Mechanical" and "optical." Both types, however, are controlled by the master gages. Most of the product accepted by one type of gaging with

a correct gage will be accepted by the other. It should be pointed

out, however, that those parts which are near either rejection point

may be accepted by one system and rejected by the other.

(a) Mechanical gages.

1—Mechanical gages ordinarily comprise the

inspection and working gages as above defined, and these two classes are generally of the same design. The dimensions of inspection gages

are such that they represent very nearly the extreme limits of the

part. It is recommended that, when successive inspections are re- quired, the working gages, either by design or selection, be of such

dimensions that they are inside the limits of the gages used in suc- ceeding inspections.

(&) Optical gages.—When gages of the optical type are employed,

the elements of wear and "feel" are not involved, hence no differ- ence in size between inspection and working gages is necessary, but

is desirable.

5. Gages for Reference.—(a) Master gage.—The "master gage"

is a plug-thread gage which represents the physical dimensions of

the nominal or basic size of the part. A master gage shall be

accompanied by a record of its measurement. In case of question,

the deviations of this gage from the basic size shall be ascertained

by the Bureau of Standards at Washington, D. C.

(6) Setting gage {check gage).—A setting gage is a plug-thread

gage to which adjustable ring-thread gages and other thread comparators are adjusted for size.

Recommendations as to the design and construction of mechanical gages are given in Appendix 4.
1924 REPORT 69

6. Direction of Tolerances on Gages.—The extreme sizes for

limit gages shall never exceed either extreme limit of the part being

produced. AU variations in the gages, whatever their cause or purpose,

shall bring these gages within these extreme limits. Thus, a gage

which represents a minimum limit may be larger, but never smaller,

than the minimum size specified for the part, while the gage which

represents a maximum limit may be smaller, but never larger, than

the maximum size specified for the part.

7. Temperature at Which Gages Shall be Standard.—Gages

and product shall have their correct nominal dimensions at a temperature of 68° F. (20° C). . As gages and products are ordinarily checked at room temperature,

whatever it may happen to be, it is desirable that the thermal coefficient of expansion of gages be the same as that of the product on which

they are used. Inasmuch as the majority of threaded products consist of iron and steel, and as screw-thread gages are ordinarily made

of hardened steel, because- of its high wear-resisting qualities, this

condition is ordinarily fulfilled without giving it special attention.

8. Measuring Pressure for Three-Wire Measurements.—

contact pressure of 2 to 3 pounds shall be used in measuring the pitch

diameter of screw-thread gages by means of wires.

(b) GAGING PRACTICES AND TYPES OF GAGES

The production of accurate parts is primarily a matter of constant

vigilance and of training of workmen. The smaller the tolerances

which are to be maintained, the more complete the inspection or gaging system must be. In order to secure satisfactory results, the

manufacturing tools provided must be sufficiently accurate, and the

manufacturing methods must be sufficiently reliable, to produce the

required results. After tools and methods of proved reliability are provided, it is necessary to watch the wear on the tools or changes

in their set-up to insure that the required conditions are maintained.

This is accomplished by periodical tests of the tools, by periodical

gaging of the product, and sometimes by both.

There is a broad, general principle in regard to limit gages which

should be kept in mind; a "go " gage should check simultaneously as many elements as possible, a "not go" gage, to be effective, can check

but one element. By effective inspection is meant assurance that

specified requirements in regard to size are not exceeded. A "not

go" thread gage made to check the pitch diameter only is usually

sufficient for practical purposes.

A screw thread comprises several elements: First, the major diameter; second, the pitch diameter; third, the minor diameter; fourth,

the angle of the thread; and fifth, the lead. The most difficult element of a screw thread to gage is the lead. Lead-testing devices for

checking tools and gages are available, but, in general, their opera-
70 NATIONAL SCREW THREAD COMMISSION

tion is too slow for use as production inspection equipment. In addition, the lead is the most important element of a screw thread as regards the nature of the contact between the surfaces of the mating

parts. Furthermore, the result of an error in lead is almost double

that of an equal error in diameter as regards interchangeability. For

exacting threaded work, if the method of inspection of the product

does not effectively detect lead errors, the tools used must be carefully

inspected for lead. In order to reduce the possibilities of disagreement to a minimum, the manufacturer should strive to produce parts

well within the specified limits rather than close to the limiting sizes.

1. Thread Micrometers.—Thread micrometers are extensively

used to measure the pitch diameter of taps and screws. Thread

micrometers should be calibrated periodically against a master gage,

to avoid errors due to fyear on the anvils of the instrument. As

thread micrometers give no indication of lead and angle errors, the

results of tests made with thread micrometers alone can not be taken

as conclusive.

2. Thread Snap Gages.—Thread snap gages are generally adjustable and have contact points consisting of cone-pointed anvils, wedgeshaped prisms with rounded edges, serrated or grooved plates, or grooved or threaded cylinders adjustably mounted and suitably

spaced in a U-shaped frame. These gages are used to some extent

in gaging external threads, and have the advantages that work may

be inspected with great rapidity by the single motion of passing it between the anvils of the gage, and given a visual examination for

clearance as well as a tactile inspection. The positions of the anvils are set to a setting gage, and the anvils are then clamped in position

and sealed.

The usual form of cone-pointed snap gage has a single point on

each side of the frame, and is an effective "not go" gage. It does

not, however, fully meet the requirements for a "go" gage, as it does not check the lead, and, therefore, must be supplemented with

some type of indicating gage to check the lead when used for checking

pitch diameter, angle, and thread form. Also, as it checks only a

single diameter at a time, the "go " snap gage must be tried at a series

of points to determine whether the maximum diameter of an external

thread is within the tolerance. When provided with three contact

points, two on one side spaced an integral number of threads apart

and one on the other, such a gage checks the lead for progressive,

but not always for local or periodic lead errors, and, thus, it more

nearly fulfills the requirements for a "go" thread gage.

Thread snap gages having multiple toothed contact points; that is,

toothed blades, serrated or grooved plates, or grooved or threaded

cylinders are made in a variety of forms, either as separate or combined "go" and "not go" gages. The fit of a screw in such a gage

is affected by variations in pitch diameter, lead, and angle of the screw,
1&24 REPORT 71

and the gage accordingly may be used as a "go" gage. Such gages

have been found suitable only for the less accurate classes of work,

such as the loose and free fit classes.

Thread micrometers and thread snap gages are extensively used

for testing the product as it is produced. As these instruments do

not test all elements of the screw thread, a "go" gage should always

be used as a supplementary test. Thread micrometers are very

effective means of checking against the change in set-up due to wear

on tools, etc.

3. Ring Thread Gages.—Ring thread gages are extensively used

to measure the threads on screws. These are usually adjustable

to suitable setting gages. When the product is to be within specified

limits, "go" and "not go" gages are required. The use of such

gages gives some information as to lead and angle errors as well as pitch diameter errors.

4. Thread Comparators.—A recent development in the art

of measuring threaded parts is the optical thread comparator, which

embodies the principle of gaging in an optical projection system. In

addition to giving a rapid indication of whether the elements of the screw thread lie within the limiting dimensions specified, such instruments furnish more detailed information as to the errors in screw

threads than is usually obtained- by means of mechanical gages,

particularly as to irregularities in thread form, lead and diameter.

These instruments can be adapted to measure taps and other threading tools.

The available forms of projection comparators differ somewhat in

design and principle, but each consists primarily of a source of

parallel light, such as an electric arc or concentrated filament lamp

with condensing lens system, a projection lens system, a screen upon

which the magnified shadow image of the work is projected, and a

device for holding the work in position in front of the projection

lenses. The screen consists of a tolerance chart on which two outlines of the correct thread form at the magnification used are spaced

one above the other a distance equal to the tolerance multiplied by

the magnification. The chart and gage holder are adjusted to position by projecting the shadow image of a setting gage and adjusting

to bring the outline of the shadow image and certain lines of the chart

into coincidence, after which the system may be used as a gaging

device.

5. Indicating Gages.—An indicating thread gage has movable

contact points, which are set to a setting gage, and is intended to

give an exact indication of the variations of the dimensions of a screw

thread within the specified limits, rather than to show merely that the

thread is within, or outside of, the specified limits, as is the case with

limit gages. In such gages, the movable contact points actuate a

multiplying lever system, or other means for magnifying their motion,
72 NATIONAL SCREW THREAD COMMISSION

and the amount of the motion is registered on a graduated dial or

scale. Indicating gages are made according to a variety of designs,

some to indicate progressive lead error only, some to indicate pitch

diameter only, some to indicate both separately but on the same gage,

others to indicate the major and minor diameters as well, and still others to indicate the apparent size. They have been applied almost

exclusively to external threads. Those which indicate the apparent

size may be considered as most nearly fulfilling the requirements of a gaging system. However, those indicating lead errors are very

useful in controlling lead errors in threading tools and screw-thread

products. Also certain types can be used to indicate the variation

in roundness on pitch or major diameters.

6. Plug Thread Gages.—At the present time the most practical

means of gaging threaded holes or nuts is by the use of plug thread

gages. When the product is to be within specified limits, "go" and

"not go" gages are required. The use of such gages gives some

information as to lead and angle errors as well as pitch diameter

errors. A correct "go" plug gage will reject any parts which exceed

the minimum dimensions specified.

One practice of inspecting tapped holes is to first inspect the tap,

and then to test the tapped holes periodically with "go" and "not

go" gages. The tap can be watched for wear by testing the tapped

holes with a "go" thread gage. One widely used practice consists

of using a "go" plug thread gage, and a "not go" plain plug gage

for the minor diameter.

One practice of inspecting taps is to measure the several elements,

such as pitch diameter, angle, and lead. Another practice consists

of tapping a hole with each tap before it is issued from the tool

crib and testing these tapped holes with "go" and "not go" plug

thread gages.

Sometimes the tap is tested after it is returned to the tool crib.

If it is correct, it is replaced in its proper compartment. If it has

worn below the limits, it is discarded and work produced by it is checked and corrected when necessary.

7. Plain Gages.—"Go" and "not go" plain cylindrical plug

gages are used for inspecting the minor diameter of the tapped hole.

Plain ring or snap gages are used for inspecting the major diameter

of the screw. When used, it is recommended that the "go" inspection gage be a ring gage and the "not go" inspection gage be a snap

gage. The working gages may be a combined "go" and "not go"

snap gage.

8. Testing or Gages.—Gages should be tested periodically for

wear and to insure that the gages are properly distributed. When

successive inspections in the same plant are involved, it is good

practice to inspect all gages of the same nominal size against each
1924 REPORT 73

other periodically, and to distribute these gages so that the earlier

inspections are made with those which are the greatest amount

inside of the component limits, while the later inspections are made

with those gages closest in size to the component limits.

The original testing of a thread gage should include measurements

for diameters, angle, and lead. If these elements test satisfactorily,

the later inspection need be measurements of pitch diameter only. 8

(c) SPECIFICATIONS FOR GAGES

The following specifications are for the purpose of establishing

definite limits for thread gages rather than for the purpose of specifying the gages required for the various inspection operations.

1. Classification of Gages, and Gage Tolerances.—Screw

thread gages are classified according to accuracy into classes X, Y,

and Z, the class X being the most accurate. The tolerance limits

on classes Y and Z "go" gages are placed inside of the extreme

product limits to provide allowance for wear of the gages. The tol- erances on all "not go" gages, however, are applied from the extreme

product limit as the starting point, as no allowance for wear is necessary. The selection of gages from among these classes for use in the

inspection of threaded product depends entirely upon the specifications for the product. For example, in the production of parts to

class 4, close fit specifications, class X gages may be required for all purposes. On the other hand, for parts made to class 1, loose fit specifications, class Z gages may be sufficiently accurate for all purposes.

(a) Master gages.—No fixed tolerances are specified for master

gages. These should be made to the basic size as accurately as possible and be within the tolerances specified for class X gages. The

variations from basic size shall be plus. Each master gage shall be

marked with an identification number or symbol, and be accompanied by a record of its measurement on major diameter, pitch

diameter, lead, and angle. In case of question, the deviations of

such gages from the exact standard shall be ascertained by the

Bureau of Standards at Washington, D. C.

(6) Class X gages.—Class X gages should be suitable for inspection and setting gages for all classes of fits. They may also be neces- sary for working gages for class 4 fits. The tolerances on these gages

shall be as given in Table 28. In all cases the tolerances shall be such

that the gage does not fall outside of the component tolerances. For

example, if a plug thread gage is used as the "go" gage for checking a tapped hole, it can be larger, but not smaller than the minimum size specified. On the other hand, if a plug thread gage is used

as the "go" setting plug for ring thread gages or for optical or other

comparators, it can be smaller, but never larger than the maximum

size of the screw.

'Methods of measuring pitch diameter of screw thread gages are described in Appendix 2.
74 NATIONAL SCREW THREAD COMMISSION

Class X tolerances, as given in Table 28, are specified for all "not

go" gages.

(c) Class Y gages.—Class Y gages should be suitable for inspection gages for classes 1, 2, and 3 fits. They may also be desired as working gages for classes 2 and 3 fits. The tolerances on these

gages shall be as given in Table 29.

(d) Class Z gages.—Class Z gages should be suitable for working

gages for class 1, loose fit. The tolerances on these gages shall be as given in Table 30.

(e) Wear on gages.—-"Go" gages may be permitted to wear to the

extreme product limits. It is desirable, however, that working and

inspection gages be so selected that the dimensions of the working

gages are inside of the limiting dimensions represented by the inspection gages, in order that all parts passed by the working gage

will be accepted by the inspection gage.

As to wear on "not go" gages, it is purely a question of economy

as to when the "not go" gage should be discarded. Continued use reduces the available working tolerance on the product, and the re- sulting loss must be balanced against the cost of a new gage.

(f) Tolerance on lead.—The tolerances on lead given in Tables 28

to 30, inclusive, are specified as an allowable variation between any

two threads not farther apart than the length of engagement of the

assembled threaded product. When this length of engagement is

equal to the diameter, the permissible progressive lead errors per

inch may be determined by dividing these lead tolerances by the

corresponding diameters.

(g) Tolerances on angle of thread.—The tolerances on angle of

thread, as specified in Tables 28 to 30, inclusive, for the various

pitches, are tolerances on one-half of the included angle. This

insures that the bisector of the included angle will be perpendicular

to the axis of the thread within proper limits. The equivalent

deviation from the true thread form caused by such irregularities as convex or concave sides of thread,* rounded crests, or slight projections on the thread form, should not exceed the tolerances permitted on angle of thread.

2. Tolerances for Plain Gages.—For plain plug gages, plain

ring gages, and plain snap gages required for measuring diameters of

screw-thread work, the gage tolerances specified in Table 31 should

be used. Attention is directed to the fact that the tolerances on

thread diameters vary in accordance with the number of threads per

inch. In manufacturing a plain plug, ring, or snap gage, in the

absence of information as to the number of threads per inch of the screw to be made, or for gage dimensions other than thread diameters, the tolerances for plain gages given in Table 31 may be used.

This table contains recommended tolerances for classes X, Y, and Z
19*24 REPORT 75

Table 27.Recommended uses for classes X, Y, and Z gages

Class of fit Setting gage Inspection gage Working gage

1 3 3 4

Class 1, loose fit Class X Class Y Class Z. Class Y.

Do.

Class X.

Class 2, free fit do do.--

Class 3, medium fit do... _ Class X

Class 4, close fit do do--

Table 28.

Tolerances for class X "go" thread gages and all "not go" thread gages

Threads per inch

Tolerance on pitch diameter 1 Tol6ra.nc6

in li !< I

Tolerance on half angle of thread

Tolerance on major or minor diameters 1

From

To— FromTo—

1 2 3 4 5 6 7

Inch Inch Inch Beg. Min. Inch Inch ± ±

80 0.0000 0.0002 0.0002 30 0.0000 0. 0003

72 .0000 .0002 .0002 30 .0000 .0003

64 .0000 .0002 .0002 30 .0000 .0004

56 .0000 .0002 .0002 30 .0000 .0004 48 .0000 .0002 .0002 30 .0000 .0004

44 .0000 .0002 .0002 20 .0000 .0004 40 .0000 .0002 .0002 20 .0000 .0004

36 .0000 .0002 .0002 20 .0000 .0004 32 .0000 .0003 .0003 15 .0000 .0004

28 .0000 .0003 .0003 15 .0000 .0005

24 .0000 .0003 .0003 15 .0000 .0005

20 .0000 .0003 .0003 15 .0000 .0005

18 .0000 .0003 .0003 10 .0000 .0005

16 .0000 .0003 .0003 10 .0000 .0006

14 .0000 .0003 .0003 o 10 .0000 .0006

13 .0000 .0003 .0003 10 .0000 .0006

12 .0000 .0003 .0003 10 .0000 .0006

11 .0000 .0003 .0003 10 .0000 .0006

10 .0000 .0003 .0003 10 .0000 .0006

9 .0000 .0003 .0003 10 .0000 .0007

8 .0000 .0004 .0004 5 .0000 .0007

7 .0000 .0004 .0004 5 .0000 .0007

6 .0000 .0004 .0004 5 .0000 .0008

5 .0000 .0004 .0004 5 .0000 .0008 4H- .0000 .0004 .0004 5 .0000 .0008

4 .0000 .0004 .0004 5 .0000 .0009

1 On "go" plugs the tolerance is plus, and on "go" rings the tolerance is minus. On "not go" plugs

the tolerance is minus, and on "not go" rings the tolerance is plus.

3 Allowable variation in lead between any two threads not farther apart than the standard length of engagement, which is equal to the basic major diameter.

gages, which have been tentatively adopted by the A. E. S. C. Sectional Committee on the Standardization of Plain Limit Gages for

General Engineering Work.

3. Recommended Gage Practice.—There are given in Table 27 the

recommended uses for the foregoing classes of gages. Tables 32, 33,

34, and 35 give limiting dimensions of gages of the several classifications for the "national coarse" and "national fine thread series."
76 NATIONAL SCREW THREAD COMMISSION

It is suggested that, in case of question between the manufacturer

and purchaser of threaded products in regard to their size, if the

manufacturer produces limit gages which do not measure outside of the

specified limits for the threaded components and which pass the parts

in question, they be accepted as meeting the specifications for size.

In case the dimensions of the gages are questioned, their sizes shall

be determined by a disinterested third party, preferably the Bureau

of Standards at Washington, D. C, which maintains a department

for this service.

4. Marking of Gages.—Gages should be plainly marked, for

identification, with the diameter, pitch, and thread series. See

Section II, division 2, "Symbols."

Table 29.Tolerances for class Y "go" thread gages

Threads per inch

Tolerance on pitch diameter 1

1 will (llJvC in lead 1

Tolerance on half angle of thread

Tolerance on major or minor diameters 1

From— To— From— To—

1 2 3 i 5 6 7

Inch Inch Inch Deg. Min. Inch Inch ± ±

80 0. 0001 0.0003 0. 0002 45 0. 0000 0.0003 72 .0001 .0003 .0002 45 .0000 .0003 64 .0001 .0004 .0002 45 .0000 .0004

56 .0001 .0004 .0002 45 .0000 .0004 48 .0001 .0004 .0002 45 .0000 .0004

44 .0001 .0004 .0002 30 .0000 .0004

40 .0001 .0004 .0002 30 .0000 .0004

36 .0001 .0004 .0002 30 .0000 .0004 32 .0001 .0004 .0003 20 .0000 .0004 28 .0002 .0005 .0003 20 .0000 .0005

24 .0002 .0005 .0003 20 .0000 .0005 20 .0002 .0005 . 0003 20 .0000 .0005

18 .0002 .0005 .0003 15 .0000 .0005

16 .0002 .0006 .0003 15 .0000 .0006

14 .0002 .0006 .0003 15 .0000 .0006

13 .0002 .0006 .0003 15 .0000 .0006

12 .0002 .0006 .0003 10 .0000 .0006

11 .0002 .0006 .0003 10 .0000- .0000

10 .0002 .0006 .0003 10 .0000 .0006

9 .0002 .0007 . . 0003 10 .0000 .0007

8 .0002 .0007 .0004 5 .0000 .0007

7 .0002 .0007 .0004 5 .0000 .0007

6 .0003 .0008 .0004 5 .0000 .0008

5 .0003 .0008 .0004 5 .0000 .0008

i'A- .0003 .0008 .0004 5 .0000 .0008

4 .0003 .0009 .0004 5 .0000 .0009

1 On "go" plugs the tolerance is plus and on "go" rings the tolerance is minus.

1 Allowable variation in lead between any two threads not farther apart than the standard length of engagement, which is equal to the basic major diameter.
1924 REPORT

Table 30.Tolerances for class Z "go" thread gages

77

Threads per inch

Tolerance on pitch diameter 1

From- To—

Tolerance

in lead 2

Tolerance on half angle of thread

Tolerance on major or minor diameters 1

From- To—

72...

64...

56... 48—

44... 40..

36..

32..

28...

24..

20..

18..

16.. 14-

13..

12.. 11-

10.. 9..

8_. 7-

6..

5_. iV2 4-

IncJi

0.0002

.0002

.0002

.0002 .0002

.0002

.0002

.0003

.0003 .0003

.0003 .0003

.0004

.0004

.0004

.0004

.0004

.0004 .0005

.0005

.0006

.0006

.0006

.0007 .0007

.0007

Inch

0. 0006 .0006

.0006

.0007

.0007

.0007

.0007 .0008

.0008

.0008

.0009

.0009 .0010

.0010 .0010

.0011

.0011

.0011 .0012

.0012

.0013

.0013 .0014

.0015

.0015

.0016

Inch ±

0.0002

.0002

.0002

.0002 .0002

.0002 .0002

.0002

.0003

.0003

.0003

.0003 .0004

.0004

.0004

.0004

.0004

.0004 .0004

.0004

.0005

.0005

.0005

.0005 .0005

.0005

Deg. Min.

±

4545

4545

45

30

30

30

20

20

20

20

1515

15

15

10

101010

5

55

5

5

Inch

0.0000

.0000

.0000

.0000 .0000

.0000

.0000

.0000 .0000

.0000

.0000

.0000

.0000 .0000

.0000

.0000 .0000

.0000 .0000

.0000

.0000

.0000

.0000

.0000

.0000

.0000

Inch

0. 0003 .0003

.0004

.0004 .0004

.0004 .0004

.0004

.0004

.0005

.0005

. 0005 .0005

.0006

.0006

.0006 .0006

.0006

.0006

.0007

.0007

.0007 .0008

.0008

.0008 .0009

1 On "go" plugs the tolerance is plus, and on "go" rings the tolerance is minus. . 2 Allowable variation in lead between any two threads not farther apart than the standard length of engagement, which is equal to the basic major diameter. Table 31.

Tolerances for plain gages 1

Size of gage in inches

Class X 2 Class Y Class Z

FromTo— From— To— From— To—

1 2 3 i 5 6 7

to 1, inclusive

1 to 3, inclusive

Inch

0.0000

.0000

Inch

0. 0001 .0002

Inch

0. 0001 .0001

Inch

0. 0002

.0003

Inch

0.0002

.0003

Inch

0. 0003 .0005

1 On "go" plugs the tolerance is plus, and on "go" rings the tolerance is minus. On "not go" plugs

the tolerance is minus, and on "not go" rings the tolerance is plus.

2 All "not go" gages are made to class X tolerances. 16802°—251 6
78 NATIONAL, SCREW THREAD COMMISSION

Minimum or Go Plug Gage Maximum ouNorGoPmGQflse

Maximum or "Qo Ring Gags Minimum oqNotGo"Ring Gage

% Cl£AIT£0 Abov£

6 S~ro. Flat to r/tcmr/m,

wm^/////a

'

Fig. 22.—Thread form of "go" and "not go" thread plug and ring gages

Note.—"Not go" gages check pitch diameter only.
1924 REPORT 79

2 3 liiii 11 1111 IHillll II

sfPi Illillllilli ii liiilllllllill

S s Inch 0.

3732

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3726 Illlllilllllll llllllllllll

2 s lllll llllllllllll 11 Illlllilllllll

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2485

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2495

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.

1959

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19

64

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2428

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2139

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2142

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2168

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1959

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2067

si lllll nisiiiiiiii ii iniiiiiiiiiii

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!1

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Inch 0.

1369

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1365

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1380

.1376 .1166

.

1163

.

1165

.

1162

.

1163

.1158

.

1177

.

1174

.

1176

.

1173

.

1179

.

1176

.1042 .1038

.

1293

.

1297

.

1326

.

1330

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1131

.1150

.

1153

.

1158

.

1161

.

1169

.

1172

.

1042

.

1109

Inch 0.

1240

.

1236

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1250

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1246

.1078

.

1076

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1077

.

1074

.

1076

.

1071

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1088

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1086

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1087

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1084

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1090

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1088

.0979 .0975 .1172

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1176

.

1202

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1206

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1044

.1046

.

1064

.1060

.

1071

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1073

.

1081

.

1083

.0979

.

1033

§ llllllllllll If Illlllilllllll

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1

1924 REPORT 81

2

1

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1924 REPORT 87

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88 NATIONAL SCREW THREAD COMMISSION

SECTION IV. SCREW THREADS OF SPECIAL DIAMETERS,

PITCHES, AND LENGTHS OF ENGAGEMENT

The tolerances specified in Section III of this report are based on

the pitch of the threads only, and apply, in general, to bolts, nuts,

etc., of standard pitches and diameters and for lengths of engagement

not greater than the diameter.

In addition to the foregoing threaded components, there are large

quantities of threaded parts produced, such as hub and radiator

caps in the automotive industry, threaded collars on machine tools,

etc., where the diameters are larger, the pitches finer, and the lengths

of engagements shorter than for bolt and nut practice. The following specifications have been adopted for such threaded parts, taking

into consideration, when determining the tolerances, the diameter,

pitch, and length of engagement of the particular components.

1. FORM OF THREAD

The national form of thread profile as specified in Section III,

and known previously as the United States form or Sellers' profile,

shall be used.

2. THREAD SERIES

It is recommended that one of the following pitches be selected

whenever it is necessary to use a screw thread of special diameter

and pitch: 4, 6, 8, 10, 12, 14, 16, 18, 20, 24, 28, 32, 36, 40, 48, 56, 64.

3. CLASSIFICATION AND TOLERANCES

There are established herein for general use five distinct classes of

screw-thread fits, which are lettered to distinguish them from the

regular classification of fits. These five classes, together with the

accompanying specifications, are for the purpose of insuring a uniform

practice on screw-thread production throughout the country.

It is not the intention of the commission to arbitrarily place a

general class or grade of work in a specific class of fit. Each manufacturer and user of screw threads is free to select the class of fit best

adapted to his particular needs.

(a) GENERAL SPECIFICATIONS

The following general specifications apply to all classes of fit specified for screw threads of special diameters, pitches, and lengths

of engagement.

1. Uniform Minimum Nut.—The pitch diameter of the minimum

threaded hole or nut corresponds to the basic size, the tolerances

being applied above the basic size.

2. Tolerances.9—(a) The tolerances specified represent the

extreme variations allowed on the product.

! Recommendations and explanations regarding the application of tolerances are given in Appendix 1.
1924 REPORT 89

(b) The tolerance on the nut is plus, and is applied from the basic

size to above basic size.

(c) The tolerance on the screw is minus, and is applied from the

maximum screw size to below the maximum screw size.

(d) The pitch diameter tolerances for a screw and nut of a given

class of fit are the same.

(e) Pitch diameter tolerances include lead and angle variations.

(/) The pitch diameter tolerances are obtained by adding three

values, or increments, one dependent upon the basic major diameter,

another upon the length of engagement, and the third upon the pitch

of the thread. These increments are based on formulas given under

" Classification of fits."

(g) The tolerances on the major diameters of the screws and

minor diameters of the nuts are based on the pitch of the thread, as these control the depth of engagement; they are, therefore, based

on the pitch alone.

Qi) The minimum minor diameter of a screw of a given pitch is such as to result in a basic flat {}/$ X p) at the root when the pitch

diameter of the screw is at its minimum value. When the maximum

screw is basic, the minimum minor diameter of the screw will be

below the basic minor diameter by the amount of the specified pitch

diameter tolerance.

(i) The maximum minor diameter of a screw of a given pitch

may be such as results from the use of a worn or rounded threading

tool, when the pitch diameter is at its maximum value. In no case,

however, should the form of the screw, as results from tool wear,

be such as to cause the screw to be rejected on the maximum minor

diameter by a "go" ring gage, the minor diameter of which is equal

to the minimum minor diameter of the nut.

(j) The maximum major diameter of the nut of a given pitch is such as to result in a flat one-third of the basic flat (-faXp) when

the pitch diameter of the nut is at its maximum value. When the

minimum nut is basic, its maximum major diameter will be above

the basic major diameter by the amount of the specified pitch diameter tolerance plus two-ninths of the basic thread depth.

Qc) The nominal minimum major diameter of a nut is the basic

major diameter. In no case, however, should the minimum major

diameter of the nut, as results from a worn tap or cutting tool, be

such as to cause the nut to be rejected on the minimum major diameter

by a "go" plug gage made to the standard form at the crest.

(Z) The tolerance on minor diameter of a nut of a given pitch is one-sixth of the basic thread depth regardless of the class of fit being

produced.
90 NATIONAL SCREW THREAD COMMISSION

(b) CLASSIFICATION OF FITS

1. Class A.—This class is intended to cover the manufacture of

threaded parts where quick and easy assembly is necessary and a

considerable amount of shake or play is not objectionable.

This class is made with an allowance on the screw, so as to permit

ready assembly, even when the threads are slightly bruised or dirty,

in conformity with the practice in Section III.

Special cases will arise, however, when a class A thread is required

on finished drawn tubing with thin walls, and in such cases the

allowance should be made on the nut.

The pitch diameter tolerances are obtained by adding three values, or increments, as follows

Diameter increment = 0. 002 -JD

Length of engagement increment = . 002 Q

Pitch increment = . 020 -sfp

where

D = basic major diameter

Q = length of engagement

p = pitch of thread in inches.

Tables 36, 37, and 38 give the clearances and tolerances for major

and minor diameters, and tolerance increments for pitch diameters

for a range of diameters, lengths of engagement, and pitches. The

total pitch diameter tolerance may be obtained by adding increments

obtained from the tables, or it may be obtained directly from the

chart, Figure 23.

Example: 3%-inch, 16-thread, class A, with allowance on screw,

one-half inch length of engagement.

From Table 36 we get for the screw: Maximum major diameter=3.250 —0.0018=3.2482

Minimum major diameter =3.2482— .0126 = 3.2356

Maximum minor diameter = 3.2482— .0767=3.1715

Maximum pitch diameter=3.2482— .0406=3.2076

And for the nut: Minimum major diameter =3.250

Minimum minor diameter =3.250 — .0677=3.1823

Maximum minor diameter =3. 1823+ .0068=3.1891

Minimum pitch diameter =3.250 - .0406=3.2094

Also for the pitch diameter tolerance:

Pitch increment = .00500

From Table 37, diameter increment (3J^ inches) = .00360 (by interpolation)

From Table 38, engagement increment (J^ inch) = .00100

Total .00960

Increment values are given to five decimal places for purposes of

calculation. The final result may be rounded off to the nearest third
1924 REPORT 91

or fourth decimal place. The pitch diameter tolerance in this example for both screw and nut can.be 0.0096, or 0.010 inch.

The pitch diameter tolerance should never be appreciably greater

than the tolerance on the major diameter of the screw. If the calIS

All dimensions in inches

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Fig. 23.Chart for determining tolerances for various classes of fit of special threads

culated pitch diameter tolerance exceeds the tolerance on the major

diameter of the screw, it is evident that too fine a pitch or too loose a fit has been selected for the particular service. The foregoing

example should be sufficient to illustrate the use of these tables.
92 NATIONAL SCREW THREAD COMMISSION

Table 36.Class A, allowances and pitch increments

Number of threads per inch

Allowance

64.

56. 48.

40. 36.

32.

28. 24.

20.

18.

16.

14.

12.

10. 8..

6..

4..

Inch

0. 0007

.0008

.0009

.0010

.0011

.0011 .0012

.0013

.0015

.0016 .0018

.0021

.0024 .0028

.0034

.0044

.0064

Pitch diameter

tolerance, pitch increment

Inch

0. 00250

. 00267

. 00289

.00316

. 00333

.00354 .00378

.00408 .00447

. 00471 .00500

.00535

.00577 .00632

.00707

.00816 .01000

Tolerance on

major

diameter,

minus

Screw

Maximum

minor

diameter =

maximum

major

diameter minus

Inch

0. 0052

.0056 .0062

.0068

.0072

.0076

.0086

.0092 .0102

.0114

.0126 .0140

.0158

.0184

.0222 .0290

.0408

Inch

0. 0192

.0219

. 0256

.0307

.0341

.0383 .0438

.0511

.0613 .0682

.0767 .0876

.1022

.1227

.1534

.2045

.3067

Maximum

pitch

diameter=

maximum

major

diameter minus

Inch

0. 0101 .0116

.0135 .0162

.0180

.0203 .0232

.0271 .0325

.0361

.0406 .0464

.0541 .0650 .0812

.1083 .1624

Tolerance on minor

diameter,

plus

Nut

Minimum

minor

diameter=

basic major

diameter minus

Inch

0.0017

.0019

.0023 .0027

.0030

.0034 .0039

.0045

.0054 .0060 .0068

.0077 .0090

.0108

.0135

.0180

.0271

Minimum

pitch

diameter=

minimum

major

diameter minus

Inch

0. 0169

.0193

.0226

.0271 .0301

.0338

.0387

.0451 .0541

.0601 .0677

.0773

.0902

.1083

.1353 .1804

.2706

Inch

0. 0101 .0116

.0135 .0162

.0180

.0203 .0232

.0271 .0325

.0361

.0406 .0464

.0541 .0650

.0812

.1083

.1624

Table 37.Class A, diameter increments of pitch diameter tolerances

Diameter Diameter

increment

2

Inches 0.0625

.125 .1875 .250

.375

Inch

0.00050

.00071 .00087

.00100

.00122

.500

.750

1.000

1.500 2.000

.00141

.00173 .00200

.00245

.00283

3.000 4.000

6.000 8.000

12. 000

. 00346 .00400

.00490

.00563

. 00693

Table 38.

Class A, length of engagement increments of pitch diameter tolerances

Lenth of engagement Increment

1 2

Inches

0.250 .500

1.000 2.000

3.000 4.000

Inch

0. 00050 .00100

.00200

.00400

.00600 .00800
1924 REPORT 93

2. Class B.—This class is intended to cover the manufacture of

threaded parts which are to assemble nearly, or entirely, with the

fingers, and a moderate amount of shake or play is not objectionable; and no allowance is required. It is the same in every particular

as class A, except that it has no allowance.

The pitch diameter tolerances are the same as for class A and are obtained by adding three values, or increments, as follows:

Diameter increment =0.002 -yjD

Length of engagement increment = .002 Q

Pitch increment = .020 V?

where

Z) = basic major diameter

Q = length of engagement

p = pitch of thread in inches.

Tables 39, 40, and 41 give the tolerances for major and minor

diameters, and tolerance increments for pitch diameters for a range

of diameters, lengths of engagement, and pitches. The total pitch

diameter tolerance may be obtained by adding increments obtained

from the tables, or it may be obtained directly from the chart,

Figure 23.

Example: 3^-inch, 16 thread, class B, one-half inch length of en- gagement.

From Table 39 we get for the screw: Maximum major diameter =3. 250

Minimum major diameter = 3. 250 -0.0126 = 3.2374

Maximum minor diameter = 3. 250 — . 0767 = 3. 1733

Maximum pitch diameter = 3. 250 - .0406 = 3.2094

And for the nut: Minimum major diameter =3. 250

Minimum minor diameter = 3. 250 — . 0677 = 3. 1823

Maximum minor diameter = 3. 1823+ . 0068 = 3. 1891

Minimum pitch diameter = 3. 250 - . 0406 = 3. 2094

Also for the pitch diameter tolerance:

Pitch increment = . 00500

From Table 40, diameter increment (3Minches) — . 00360 (by interpolation)

From Table 41, engagement increment i}/2 inch) = .00100

Total . 00960

Increment values are given to five decimal places for purposes of

calculation. The final result may be rounded off to the nearest

third or fourth decimal place. The pitch diameter tolerance in this

example for both screw and nut can be 0.0096, or 0.010 inch.

The pitch diameter tolerance should never be appreciably greater

than the tolerance on the major diameter of the screw. If the calculated pitch diameter tolerance exceeds the tolerance on the major

diameter of the screw, it is evident that too fine a pitch or too loose a fit has been selected for the particular service. The foregoing

example should be sufficient to illustrate the use of these tables.

16802°—25t 7
94 NATIONAL SCREW THREAD COMMISSION

Table 39.—Class B, pitch increments

Number of threads per inch

Pitch diameter

tolerance, pitch incrementTolerance on major

diameter,

minus

Screw

Maximum

minor

diameter=

maximum

major

diameter minus

Maximum

pitch diameter

maximum

major

diameter minus

Tolerance on minor

diameter,

plus

Nut

Minimum

minor

diameter

basic major

diameter minus

Inch

0. 00250

. 00267

.00289

. 00316

.00333

.00354

.00378

. 00408

.00447

.00471

.00500

.00535 .00577

.00632

.00707

.00816 .01000

Inch

0.0052

.0056

.0062 .0068

.0072

.0076

.0086 .0092

.0102

.0114

.0126

.0140

.0158

.0184 .0222

.0290 .0408

Inch

0. 0192

.0219

.0256

.0307

.0341

.0383

.0438

.0511 .0613 .0682

.0767

.0876 .1022

.1227 .1534

.2045

.3067

Inch

0. 0101 .0116

.0135 .0162

.0180

.0203

.0232 .0271

.0325

.0361

.0406

.0464

.0541 .0650 .0812

.1083 .1624

Inch

0.0017

.0019

.0023 .0027 .0030

.0034

.0039

.0045 .0054

.0060

.0068

.0077

.0090

.0108

.0135

.0180

.0271

Inch

0. 0169

.0193 .0226

.0271

.0301

.0338

.0387

.0451 .0541

.0601

.0677

.0773 .0902

.1083

.1353

.1804

.2706

Table 40.Class B, diameter increments of pitch diameter tolerances

Diameter Diameter

increment

1 2

Inches

0. 0625

.125

.1875

.250 .375

Inch

0. 00050

.00071 .00087

.00100

.00122

.500

.750

1.000

1.500 2.000

.00141 .00173

.00200

. 00245

.00283

3.000

4.000

6.000

8. 000

12.000

. Q0346

. 00400

.00490 .00566

.00C93

Table 41.

Class B, length of engagement increments of pitch diameter tolerances

Length of engagement Increment

1 2

Inches

0. 250

.500

1.000 2.000

3.000

4.000

Inch

0.00050

.00100 .00200

. 00400 .00600

.00800
1924 REPORT 95

3. Class C.—This class is intended to cover the manufacture of

threaded parts which are to assemble nearly, or entirely, with the

fingers, and where a shght amount of shake or play between the as- sembled threaded members is not objectionable. It is the same in

every particular as class B except that the tolerances are smaller.

The pitch diameter tolerances are obtained by adding three increments, as follows:

Diameter increment =0.002 VZ5

Length of engagement increment = 0.002 Q

Pitch increment =0.010 Vj?

where

Z> = basic major diameter

Q = length of engagement

p = pitch of thread in inches.

Tables 42, 43, and 44 give the tolerances for major and minor

diameters, and tolerance increments for pitch diameters for a range of

diameters, lengths of engagements, and pitches. The total pitch

diameter tolerance may be obtained by adding increments obtained

from the tables, or it may be obtained directly from the chart,

Figure 23.

Example: 4-inch 24-thread, class C, three-eighths inch length of

engagement.

From Table 42 we get for the screw: Maximum major diameter =4. 000

Minimum major diameter =4. 000 -0. 0066 = 3. 9934

Maximum minor diameter = 4. 000 — .0511 = 3.9489

Maximum pitch diameter =4. 000 - . 0271 = 3. 9729

And for the nut: Minimum major diameter = 4. 000

Minimum minor diameter =4. 000 — . 0451 = 3. 9549

Maximum minor diameter = 3. 9549+ . 0045 = 3. 9594

Minimum pitch diameter =4.000 — .0271 = 3.9729

Also for the pitch diameter tolerance:

Pitch increment = . 00204

From Table 43, diameter increment ( 4 inches ) _ _ = . 00400

From Table 44, engagement increment

inch) = . 00075 (by interpolation)

Total . 00679

Increment values are given to five decimal places for purposes

of calculation. The final result may be rounded off to the nearest

third or fourth decimal place. The pitch diameter tolerance in this

example for both screw and nut can be 0.0068, or 0.007 inch.

The pitch diameter tolerance should never be appreciably greater

than the tolerance on the major diameter of the screw. If the calculated pitch diameter tolerance exceeds the tolerance on the major
96 NATIONAX/ SCREW THREAD COMMISSION

diameter of the screw, it is evidence that too fine a pitch or too loose a

fit has been selected. In the foregoing example, the pitch diameter

tolerance is slightly greater than the tolerance on the major diameter of the screw. This example is a limiting case.

Table 42.

Class C, pitch increments

dumber of threads per inch

Pitch diameter

tolerance,

Screw Nut

Tolerance on major

diameter,

minus

Maximum

minor

diameter—

maximum

major

diameter minus

Maximum

pitch diameter

maximum

major

diameter minus

Tolerance on minor

diameter,

plus

Minimum

minor

diameter=

basic major

diameter minus

Minimum

pitch

diameter—

minimum

major

diameter minus

1 % 3 4 5 6 7 8

Inch Inch Inch Inch Inch Inch Inch 64-. 0. 00125 0. 0038 0. 0192 0. 0101 0. 0017 0. 0169 0. 0101 56-__ - .00134 .0040 .0219 . 0116 .0019 .0193 .0116 48 . 00144 .0044 .0256 .0135 .0023 .0226 .0135 40 .00158 .0048 .6307 .0162 .0027 .0271 .0162

36 . 00167 .0050 .0341 .0180 .0030 .0301 .0180

32 . 00177 .0054 .0383 .0203 .0034 .0338 .0203 28 .00189 .0062 .0438 .0232 .0039 .0387 .0232 24 .00204 .0066 .0511 .0271 .0045 .0451 .0271 20.- . 00224 .0072 .0613 . 0325 .0054 .0541 .0325 18.. .00236 .0082 .0682 .0361 .0060 .0601 .0361

16 .00250 .0090 .0767 .0406 .0068 .0677 * .0406 14-.. .00267 .0098 .0876 .0464 .0077 .0773 .0464 12._ .00289 .0112 . 1022 .0541 .0090 .0902 .0541 10__ _ .00316 .0128 . 1227 .0650 .0108 . 1083 .0650 8_- _ .00354 .0152 . 1534 .0812 .0135 . 1353 .0812

6.-- —

.00408 .0202 .2045 . 1083 .0180 .1804 .1083

4 .00500 .0280 .3067 . 1624 .0271 .2706 .1624

Table 43.

Class C, diameter increments of pitch diameter tolerances

Diameter Diameter

increment

1 2

Inches

0. 0625 .125

.1875 .250

.375

Inch

0.00050

. 00071 .00087

.00100

.00122

.500

.750

1.000

1. 500 2.000

. 00141

. 00173

. 00200

.00245

. 00283

3.000

4. 000 6.000

8. 000

12. 000

. 00346

.00400

.00490

.00566

. 00693
1924 REPORT 97

Table 44.

Class C, length of engagement increments of pitch diameter tolerances

Length of engagement Increment

1 2

Inches

0. 250 .500

1.000 2.000

3.000

4. 000

Inch

0. 00050 .00100

.00200

.00400

.00600

.00800

4. Class D.—This class is intended to cover the manufacture of

the higher grade of threaded parts which are to assemble nearly or

entirely with the fingers and must have the minimum amount of

shake or play between the threaded members. It is the same as

class C in every particular except that the tolerances are smaller.

The pitch diameter tolerances are . obtained by adding three

increments, as follows

Diameter increment = 0.002

Length of engagement increment = .002 Q

Pitch increment = .005

where

D = basic major diameter

Q = length of engagement

p = pitch of thread in inches.

Tables 45, 46, and 47 give the tolerances for major and minor

diameters, and tolerance increments for pitch diameters for a range

of diameters, lengths of engagements, and pitches. The total pitch

diameter tolerance may be obtained by adding increments obtained

from the tables, or it may be obtained directly from the chart,

Figure 23.

Example: 33^-inch, 20-thread, class D, five-eighths inch length of

engagement.

From Table 45 we get for the screw: Maximum major diameter =3.500

Minimum major diameter = 3. 500— 0.0072 =3.4928

Maximum minor diameter =3. 500— .0613 =3.4387

Maximum pitch diameter = 3.500- .0325 =3.4675

And for the nut:

Minimum major diameter =3.500

Minimum minor diameter =3.500 —.0541 =3.4459

Maximum minor diameter = 3.4459 + . 0054 =3.4513

Minimum pitch diameter =3.500 —.0325 =3.4675

Also for the pitch diameter tolerance:

Pitch increment = .00112

From Table 46, diameter increment (3^2 inches) = .003731

From Table 47, engagement increment {% inch)= .00125/ (by interpolation)

Total .00610
98 NATIONAL SCREW THREAD COMMISSION

Increment values are given to five decimal places for purposes of

calculation. The final result may be rounded off to the nearest

third or fourth decimal place. The pitch diameter tolerance in this

example for both screw and nut can be 0.0061, or 0.006 inch.

The pitch diameter tolerance should never be appreciably greater

than the tolerance on the major diameter of the screw. If the

calculated pitch diameter tolerance exceeds the tolerance on the

major diameter of the screw, it is evidence that too fine a pitch or too loose a fit has been selected.

Table 45.Class D, pitch increments

Number of

t~ Vl rr\Q c nor in f^Yi ILircaUb ptT lliCIl

Pitch diameter

tolerance, pitch increment

Screw Nut

Tolerance on major

diameter minus

Maximum

minor

diameter

maximum

major

diameter minus

Maximum

pitch

diameter=

maximum

major

diameter minus

Tolerance on minor

diameter,

plus

Minimum

minor

diameter

basic major

diameter minus

Minimum

pitch

diameter=

minimum

major

diameter minus

1 2 3 4 5 6 7 8

Inch Inch Inch Inch Inch Inch Inch

64 0. 00062 0. 0038 0. 0192 0. 0101 0. 0017 0. 0169 0. 0101 56.. .00067 .0040 .0219 .0116 .0019 .0193 .Olio 48.. .00072 .0044 .0256 .0135 .0023 .0226 .0135 40 .00079 .0048 .0307 .0162 .0027 .0271 .0162

36 .00083 .0050 .0341 .0180 0030 .0301 .0180

32__ .00088 .0054 .0383 .0203 .0034 . 0338 .0203

28 .00094 .0062 .0438 .0232 .0039 .0387 .0232

24 .00102 .0066 .0511 .0271 .0045 0451 .0271 20_. .00112 .0072 .0613 .0325 .0054 .0541 .0325

18 .00118 .0082 .0682 .0361 .0060 .0601 .0361

16 .00125 .0090 .0767 .0406 .0068 .0677 .0405 14___ .00134 .0098 .0876 .0464 .0077 .0773 .0464

12 .00144 .0112 .1022 .0541 .0090 .0902 .0541 10.. .00158 .0128 . 1227 .0650 .0108 .1083 .0650 8_. .00177 .0152 . 1534 .0812 .0135 .1353 .0812

6 .00204 .0202 .2045 .1083 .0180 .1804 .1083

4 .00250 .0280 .3067 .1624 .0271 .2706 .1624

Table 46.

Class D, diameter increments of pitch diameter tolerances

Diameter Diameter

increment

1 i

Inches

0. 0625

.125

.1875 .250

.375

Inch

0. 00050

.00071 .00087

. 00100

. 00122

.500

.750 1.000

1.500

2. 000

. 00141

. 00173 .00200

. 00245

.00283

3.000

4. 000 6.000

8. 000

12.000

. 00346

. 00400

.00490

.00566

.00693
1&34 REPORT 99

Table 47.Class D, length of engagement increments of pitch diameter tolerances

Length of engagement Increment

1 1

Inches 0.250

.500

1. 000 2.000 3.000

4. 000

Inch

0. 00050

. 00100

.00200

. 00400

. 00600

. 00800

5. Class E.—This class is intended to cover the manufacture of

threaded parts of the finest commercial quality, where very little

shake or play is desirable, and where a screwdriver or wrench may be

necessary for assembly.

In the manufacture of screw-thread products belonging to this

class it may be necessary to use precision tools, selected gages, and

other refinements. This quality of work should, therefore, be used

in cases only where requirements of the mechanism being produced

are exacting. In order to secure the fit desired, it may be necessary

in some cases to select the parts when the product is being assembled.

The maximum pitch diameters of the screws are slightly larger

than the minimum pitch diameters of the nuts as determined from

Table 48.

The pitch diameter tolerances are obtained by adding three increments, as follows:

Diameter increment =0.001 -yjD

Length of engagement increment = .001 Q

Pitch increment = . 0025 Vz>

where

D = basic major diameter

Q = length of engagement

p = pitch of thread in inches.

Tables 48, 49, and 50 give the tolerances for major diameter and

minor diameter, and tolerance increments for pitch diameter for a range of diameters, lengths of engagement, and pitches. The total

pitch diameter tolerance may be obtained by adding increments

obtained from the tables, or it may be obtained directly from the

chart, Figure 23.
100 NATIONAL SCREW THREAD COMMISSION

Example: 434-inch, 16 thread, class E, three-fourths inch length of

engagement.

From Table 48 we get for the screw: Maximum major diameter =4. 250

Minimum major diameter =4. 250 — 0. 0090 = 4. 2410

Maximum minor diameter = 4. 250— . 0767 = 4. 1733

Maximum pitch diameter = 4. 250— . 0402 = 4. 2098

And for the nut: Minimum major diameter =4. 250

Minimum minor diameter = 4. 250—0. 0677 = 4. 1823

Maximum minor diameter = 4. 1823 +. 0068 = 4. 1891

Minimum pitch diameter =4. 250— . 0406 = 4. 2094

Also for the pitch diameter tolerance:

Pitch increment = . 00062

From Table 49, diameter increment (4J4 inches) = . 00206

From Table 50, engagement increment (94 inch) . = . 00075J ^y interpolation.

Total . 00343

Increment values are given to five decimal places for purposes of

calculation. The final result may he rounded off to the nearest

fourth decimal place. The pitch diameter tolerance in this example

for both screw and nut would be 0.0034 inch.

The pitch diameter tolerance should never be appreciably greater

than the tolerance on the major diameter of the screw. If the calculated pitch diameter tolerance exceeds the tolerance on the major

diameter of the screw, it is evidence that too fine a pitch has been

selected.

Table 48.Class E, -pitch increments

Number of threads per inch

Pitch diameter

tolerance, pitch increment

Screw Nut

Tolerance on major

diameter,

minus

Maximum

minor

diameter=

maximum

major

diameter minus

Maximum

pitch diameter

maximum

major

diameter minus

Tolerance on minor

diameter,

plus

Minimum

minor

diameter

basic major

diameter minus

Minimum

pitch diameter

minimum

major

diameter minus

1 2 3 4 5 6 7 8

Inch Inch Inch Inch Inch Inch Inch

64 0. 00031 0.0038 0. 0192 0.0100 0. 0017 0. 0169 0. 0101

56 .00033 .0040 .0219 .0114 .0019 .0193 0. 0116

48 .00036 .0044 .0256 .0133 .0023 .0226 .0135 40... .00040 .0048 .0307 .0160 .0027 .0271 .0162

36 .00042 .0050 .0341 .0178 .0030 .0301 .0180

32 .00044 .0054 .0383 .0201 .0034 .0338 .0203

28 .00047 .0062 .0438 .0230 .0039 .0387 .0232 24.. . 00051 .0066 .0511 .0268 .0045 .0451 .0271 20 .00056 .0072 .0613 .0322 .0054 .0541 .0325

18 .00059 .0082 .0682 .0358 .0060 .0601 .0361

16 . 00062 .0090 .0767 .0402 .0068 .0677 .0406

14 .00067 .0098 .0876 .0460 .0077 .0773 .0464

12 .00072 .0112 . 1022 .0536 .0090 .0902 .0541

10 .00079 .0128 . 1227 .0644 .0108 . 1083 .0650

8 .00088 .0152 .1534 .0805 .0135 . 1353 .0812

6 .00102 .0202 .2045 .1074 .0180 .1804 .1083

4 .00125 .0280 .3067 . 1611 .0271 .2706 . 1624
1924 REPORT 101

Table 49.

Class E, diameter increments of pitch diameter tolerances

Diameter Diameter

increment

1 jg

Inches Inch

0. 0625 0. 00025

. 125 . 00035 .1875 .00043 .250 . 00050 .375 .00061

. 500 . 00071

. 750 . 00087

1.000 . 00100

1. 500 .00122

2.000 . 00141

3.000 .00173 4.000 . 00200

6. 000 .00245 8.000 .00283

12.000 .00346

Table 50.Class E, length of engagement increments of pitch diameter tolerances

Length of engagement Increment

1 2

Inches

0. 250 .500

1. 000 2.000

3. 000

4. 000

Inch

0. 00025 .00050

. 00100

. 00200

. 00300 .00400

4. SPECIFICATIONS FOR THREADING TOOLS

(a) FORM OF TOOLS FOR PRODUCING SCREWS

The specifications given in Section III, division 5(a), for tolerances

on chasers, thread milling cutters, roller dies, threading hobs, etc., are also applicable to screw threads of special diameter and pitches.

(b) TAPS

1. Tolerances for Taps.—Tables of tolerances for three classes

of taps are given: First, commercial taps; second, Class X ground

taps; and third, class Y ground taps. (See Tables 51, 52, and 53.)

The selection of the tap will depend upon the tolerances on the

tapped hole. A tap should be selected whose tolerance does not

exceed 75 per cent of the tolerance on the tapped hole.
102 NATIONAL SCREW THREAD COMMISSION

2. Marking of Taps.—Taps shall be marked indicating the

diameter, pitch, and thread form as follows (see Section II, division 2, " Symbols"):

A 1-inch tap, 16 threads per inch, national form of thread, will be marked

1"—16—N.

A 1-inch tap, 16 threads per inch, left hand, national form of thread, will be

marked: 1"—16 L. H.—N.

A multiple threaded tap will be marked as above, and also with the number of

starts. Where the size of the shank is large enough, the lead should also be given.

A double-threaded tap; 1 inch diameter, 16 threads per inch, national form

of thread, with a lead of 0.125 inch will be marked: 1"—16—N Double,

Lead—0.125.

3. Shape of Cutting Edge for Taps.—The specifications given

in Section III, division 5(b), for shape of cutting edge of taps and

other internal threading tools are also applicable to screw threads of

special diameters and pitches.

(c) TAP DRILLS

The essential requirement of a tap drill is that the hole produced by

it shall be such that, after tapping, the minor diameter of the tapped

hole shall be within the specified limits. For tapped holes of special

diameters and pitches, the standard drill that comes nearest in siz*

to the mean of the maximum and minimum minor diameters of the

tapped hole should be selected, if the diameter is within the specified

limits.

Table 51.Tolerances for commercial taps, special diameters and pitches

Sizes Majordiameter

Pitch diameter

Coarser pitches than "national

fine-thread series" Finer pitches than "nationalfine-thread series"

From— To—

Minimum

= basic major diameter plus

Minimum

= minimum

pitch diameter of hole plus

Tolerance,

plus

Maximum

=minimum

pitch diameter of hole plus

Minimum

= minimum

pitch diameter of hole plus

Tolerance,

plus

Maximum

=minimum

pitch diameter of hole plus

1 2 3 i 5 C 7 8

M

k

lVs

V/s 2Vs 2%

3H

%

%

1m22M

3

4

. Inch

0. 0010 .0010

.0010

.0015

.0015

.0015

.0015 .0015

Inch

0. 0005 .0005

.0005

.0005

.0010

.0010 .0010

.0010

Inch

0. 0020

.0025

.0030

.0035

.0040

.0045

.0050

.0055

Inch

0. 0025

.0030

.0035

.0040

.0050

.0055 .0060

.0065

Inch

0. 0005

. 0005

.0005

.0005

.0005

.0005

.0005 .0005

Inch

0. 0015

.0020

.0025 .0030

.0030

.0035 .0035

.0045

Inch

0. 0020

.0025 .0030

. 0035

.0035 .0040 .0040

.0050

Note.—Maximum lead error =± 0.003 inch in 1 inch of thread.
1924 REPOBT 103

Table 52.

Tolerances for class X ground taps, special diameters and pitches

Major

Pitch diameter

Sizes diameter

Coarser pitches than "national

fine-thread series" Finer pitches than "national

fine-thread series"

From

To—

Minimum

=basic major diameter plus

Minimum

= minimum

pitch diameter of hole plus

Tolerance, plus

Maximum

=minimum

pitch diameter of hole plus

Minimum

=minimum

pitch diameter of hole plus

Tolerance,

plus

Maximum

=minimum

pitch diameter of hole plus

1 2 3 i 5 6 1 8

H

i

u

2VS 2%

VA

Vt

Vi%

1m22V2 34

Inch

0. 0020

.0025

.0025 .0030

.0040

.0050 .0050

.0050 .0050

Inch

0. 0000

.0000

.0000

.0000

.0000 .0000

.0000

.0000 .0000

Inch

0. 0008

.0009

.0010

.0012

.0015

.0015 .0015

.0015

.0015

Inch

0. 0008

.0009

.0010

.0012

.0015

.0015

.0015

.0015

.0015

Inch

0. 0000

.0000

.0000

.0000

.0000

.0000

.0000 .0000

.0000

Inch

0. OOOS

.0008

.0008 .0009

.0010

.0015

.0015

.0015 .0015

Inch

0. 0008

.0008

.0008

.0009

.0010

.0015

.0015 .0015

.0015

Note.—Maximum lead error = ± 0.0005 inch in 1 inch of thread. Tolerance on half angle of thread = ± 15 minutes. Table 53.

Tolerances for class Y ground taps, special diameters and pitches

Major

Pitch diameter

Sizes diameter

Coarser pitches than "national

fine-thread series" Finer pitches than "nationalfine-thread series"

From— To—

Minimum

= basic major diameter plus

Minimum

=minimum

pitch diameter of hole plus

Tolerance,

plus

Maximum

=minimum

pitch diammeter of hole plus

Minimum

=minimum

pitch diameter of hole plus

Tolerance, plus

Maximum

= minimum

pitch diameter of hole plus

1 2 3 I 5 6 7 8

H

A

A

%

Wtm2Vs 2%

a

Vi

Y%

1m2

2Vi

3

4

Inch

0. 0025

.0030

.0035 .0040

.0050

.0065 .0080

.0080 .0080

Inch

0. 0006

.0007 .0008

.0010

.0012

.0015

.0020 .0020

.0020

Inch

0. 0010

.0011

.0013

.0015 .0020 .0025

.0030

.0030 .0030

Inch

0. 0016

.0018

.0021 .0025

.0032 .0040

.0050

.0050

.0050

Inch

0. 0005

.0005

.0006

.0007 .0008

.0010 .0010

.0015 .0015

Inch

0. 0010

.0010

.0010

.0013

.0015 .0018 .0020

.0025

.0025

Inch

0. 0015

.0015

.0016

.0020

.0023 .0028 .0030

.0040

.0040

Note.—Maximum lead error = ± 0.0005 inch in 1 inch of thread. Tolerance on half angle of thread = ± 20 minutes.

5. GAGES

(See also Section HI, division 5, "gages")

(a) SPECIFICATIONS FOR GAGES

1. Classification of Gages, and Gage Tolerances.—Screwthread gages are classified according to accuracy into classes X, Y,

and Z, the class X being the most accurate. The tolerance limits on

classes Y and Z "go" gages are placed inside of the extreme product
104 NATIONAL SCREW THREAD COMMISSION

limits to provide allowance for wear of the gages. The tolerances

on all "not go" gages, however, are applied from the extreme product

limit as the starting point, as no allowance for wear is necessary.

Also, only one set of tolerances, namely, class X, is specified for

"not go" gages. The selection of "go" gages from among these

classes for use in the inspection of threaded product depends entirely

upon the specifications for the product. For example, in the production of parts to class E specifications, class X gages may be re- quired for all purposes. On the other hand, for parts made to classes

A or B specifications, class Z gages may be sufficiently accurate for

all purposes. The recommended uses for the various classes of gages

are given in Table 54. In Tables 55, 56, and 57 are specified toler- ances on diameter, lead, and angle for each class of gage.

"Go" gages may be permitted to wear to the extreme product

limits. It is desirable, however, that working and inspection gages

be so selected that the dimensions of the working gages are inside of

the limiting dimensions represented by the inspection gages, in order

that all parts passed by the working gage will be accepted by the

inspection gage. As to wear on "not go" gages, it is purely a question of economy as to when the "not go" gage should be discarded.

Continued use reduces the available working tolerance on the product, and the resulting loss must be balanced against the cost of a new gage.

2. Major and Minor Diameters op Gages.—The maximum

minor diameter of the "go" ring thread gage should be the same as the minimum minor diameter of the tapped hole. The minimum

minor diameter of the "not go" ring thread gage should not be

smaller than the maximum minor diameter of the "go" gage. Also

the maximum major diameter of the "not go" plug thread gage

should not be larger than the minimum major diameter of the corre- sponding "go" gage; that is, not larger than the basic major diameter. This will insure contact of the "not go" thread gage on the

sides of the threads rather than at the corners of the crest and root.

Table 54.Recommended uses for classes X, Y, and Z gages

Class of fit Setting gage Inspection gage Working gage

1 3 i

Class A Class X Class Y Class Z. Do.

Class Y. Do.

Class X.

Class B do _ do

Class C do do.

Class D._ _ do Class X

Class E do do
Table 55.

1924 REPORT 105

Tolerances for class X "go" thread gages and all "not go" thread gages

Threads per inch

Tolerance on pitch diameter 1 Tolerance

in lead 3

Tolerance on half angle of thread

Tolerance on major or minor diameters 1

FromTo— From— To—

1

9A

o

o 4 S f

Inch Inch Inch Beg. Min. Inch Inch ±

64 0. 0000 0. 0002 f* Anno

0. UUU2 30 0. 0000 0.0004

56 .0000 .0002 . 0002 30 .0000 .0004

48 . 0000 . 0002 .0002 30 . 0000 . 0004

40 !oooo !0002 .0002 20 ioooo !o004 36 .0000 .0002 . 0002 20 .0000 .0004

32 .0000 .0003 .0003 15 .0000 .0004

28 .0000 .0003 .0003 15 .0000 .0005

24 .0000 .0003 .0003 15 .0000 .0005

20 .0000 .0003 .0003 15 .0000 .0005

18 .0000 .0003 .0003 10 .0000 .0005

16 .0000 .0003 .0003 10 .0000 .0006

14 .0000 .0003 .0003 10 .0000 .0006

12 .0000 .0003 .0003 10 .0000 .0006

10 .0000 .0003 .0003 10 .0000 .0006 8 .0000 .0004 .0004 5 .0000 .0007

6 .0000 .0004 .0004 5 .0000 .0008

4 .0000 .0004 .0004 5 .0000 .0009

1 On "go" plugs the tolerance is plus, and on "go" rings the tolerance is minus. On "not go" plugs the

tolerance is minus, and on "not go" rings the tolerance is plus.

1 Allowable variation in lead between any two threads not farther apart than the length of engagement.

Table 56.Tolerances for class Y "go" thread gages

Threads per inch

Tolerance on pitch diameter 1 Tolerance

in lead 2

Tolerance on half angle of thread

Tolerance on major or minor diameters 1

FromTo— From— To—

1 2 3 i 5 6 7

Inch Inch Inch Beg, Min. Inch Inch ± ±

64 0. 0001 0. 0004 0. 0002 45 0.0000 0. 0004

56 .0001 .0004 0002 45 .0000 .0004

48 .0001 .0004 .0002 45 .0000 .0004

40 .0001 .0004 .0002 30 .0000 .0004

36 .0001 .0004 .0002 30 .0000 .0004

32 .0001 .0004 .0003 20 .0000 .0004

28 .0002 .0005 .0003 20 .0000 .0005

24 .0002 .0005 .0003 20 .0000 .0005

20 .0002 .0005 .0003 20 .0000 .0005

18 .0002 .0005 .0003 15 .0000 .0005

16 .0002 .0006 .0003 15 .0000 .0006

14 .0002 .0006 .0003 15 .0000 .0006

12 .0002 .0006 .0003 10 .0000 .0006

10 .0002 .0006 .0003 10 .0000 .0006 8 .0002 .0007 .0004 5 .0000 .0007

6 .0003 .0008 .0004 5 .0000 .0008

4 .0003 .0009 .0004 5 .0000 .0009

1 On "go" plugs the tolerance is plus, and on "go" rings the tolerance is minus.

2 Allowable variation in lead between any two threads not farther apart than the length of engagement.
106 NATIONAL SCREW THREAD COMMISSION

Table 57.Tolerances for class Z "go" thread gages

Threads per inch

Tolerance on pitch diameter 1 Tolerance

in lead 3

Tolerance on half angle of thread

Tolerance on major or minor diameters 1

_

r rom— I o— I rom

To

1 2 3 S 6 7

Inch Inch Inch Beg. Min. Inch Inch ± ±

64 0. 0002 0. 0006 0. 0002 45 0. 0000 0.0004

56 .0002 .0007 . 0002 45 .0000 .0004

48 .0002 .0007 . 0002 45 .0000 .0004

40 .0002 .0007 .0002 30 .0000 .0004

36 .0003 .0008 .0002 30 .0000 .0004

32 .0003 .0008 .0003 20 .0000 .0004

28 .0003 .0008 . 0003 20 .0000 .0005

24 .0003 .0009 !0003 20 .0000 .0005

20 .0003 .0009 .0003 20 .0000 .0005

18 .0004 .0010 .0004 15 .0000 .0005

16 .0004 .0010 .0004 15 .0000 .0006

14 .0004 .0010 .0004 15 .0000 .0006

12 .0004 .0011 .0004 10 .0000 .0006

10 . 0005 .0012 .0004 10 .0000 .0006

8 .0006 .0013 .0005 5 .0000 .0007

6 .0006 .0014 .0005 5 .0000 .0008

4 .0007 .0016 .0005 5 .0000 .0009

1 On "go" plugs the tolerance is plus, and on "go" rings the tolerance is minus.

2 Allowable variation in lead between any two threads not farther apart than the length of engagement.

SECTION V. NATIONAL HOSE-COUPLING AND FIRE-HOSE

COUPLING THREADS

Several years ago specifications for "national standard fire-hose

coupling threads" were approved by the National Board of Fire

Underwriters, National Fire Protection Association, American Society

of Mechanical Engineers, American Society of Municipal Improvements, New England Water Works Association, American Water

Works Association, the Bureau of Standards, and other interested

organizations. These specifications were published in 1911 as the

Specifications of the National Board of Fire Underwriters, recommended by the National Fire Protection Association and approved by

the various other organizations. They were also published in 1914 as Circular No. 50 of the Bureau of Standards. This circular was

revised and republished in 1917.

When the National Screw Thread Commission took up its work on

the standardization of screw threads, the specifications for fire-hose

coupling threads above referred to were accepted as the basis of

its work on fire-hose coupling threads. It was found, however, that

the specifications as originally drawn were inadequate in that they

specified nominal dimensions only, with no maximum and minimum

limits.

From the best information available when fire-hose coupling threads

were under discussion, the commission concluded that they should be

placed in the loose-fit class and that the allowance provided in the

original specifications should be interpreted as the minimum difference
1924 REPORT 107

in pitch diameter of the couplings and nipples ; that is, that a looser fit could be tolerated than that represented by the nominal .dimensions,

but that a tighter fit would be objectionable. The nominal size of

the coupling was, therefore, taken as the minimum coupling, and the

nominal size of the nipple as the maximum nipple; the loose-fit

tolerances being applied in such a direction as to make the maximum

coupling larger and the minimum nipple smaller than the nominal

size by the amount of the tolerance. The dimensions for "national

fire-hose coupling threads" appearing in the 1921 Progress Keport of

the commission were calculated on that basis.

It later appeared from evidence presented by the National Board of

Fire Underwriters that the fits resulting from the limiting dimensions

specified in the 1921 Progress Report may in extreme cases be looser

than good practice warrants. The commission was, therefore, re- quested to revise its specifications for fire-hose coupling threads in

such a manner that the resulting fits would be somewhat closer than

those originally permitted. The revision requested was accomplished

by reducing the allowance between the minimum coupling and the

maximum nipple; the minimum coupling dimension being retained as published in the 1921 Progress Report. However, from evidence

presented by certain manufacturers, it appeared desirable to provide

a larger wear limit for gages and also permit the use of field inspection

gages which were outside of the limits set for the manufacturers'

gages, in order to insure the approval, by the purchaser, of threaded

couplings which had passed the manufacturers' gages. To permit

this, the tolerances on both the coupling and nipple were increased

slightly.

Furthermore, since the publication of the 1921 Progress Report, it has been found desirable, in order to facilitate production and simplify

the manufacture of threading tools and gages, to adopt the national

form of thread for "national fire-hose coupling threads." This,

however, is a very minor change and only slightly affects the thread

form at crest and root.

The specifications have been further supplemented by recommended

limiting dimensions of gages such that every coupling and nipple

accepted will be known to be within the limiting dimensions specified.

Only such a system can insure that the finished product will assemble

and function properly.

Comparison of the resulting dimensions with those contained in the

original specifications of the National Board of Fire Underwriters and

in Bureau of Standards Circular No. 50, shows that the two are not in

conflict. The nominal dimensions originally specified for couplings

and nipples are not inconsistent with the above limiting dimensions,

and the revised dimensions should not be considered as changing in

any way the original specifications for "national fire-hose coupling

threads," but as supplementing those specifications, and as expressing
108 NATIONAL SCREW THREAD COMMISSION

them in definite and measurable terms. It should be understood that

these dimensions for "national fire-hose coupling threads" are the

limiting dimensions for the finished product and not for the threading

tools.

With regard to the "national hose-coupling threads," which range

in size from three-fourths to 2 inches, the series of sizes and pitches

originally adopted by the commission are essentially the same as the

corresponding sizes of the "national straight pipe threads," with the

exception of the three-fourths-inch size. In this case 11 and 11%

threads, with a diameter of 1^ inches, was already in very extensive

use. The adoption of the so-called "straight iron pipe thread" was

indorsed by the Nation Association of Brass Manufacturers, and at

the time of its adoption was recommended by the committee on

Small Hose Couplings of the National Fire Protection Association

and tentatively adopted by that association.

In ordering threading tools for producing 'national hose-coupling

and fire-hose coupling threads," it should be pointed out that new

taps should be near the maximum permissible size of the coupling,

and new dies near the minimum permissible size of the nipple, in

order that reasonable wear may be provided. As the threading tools

wear by use, the couplings will become smaller and the nipples larger

until the limiting dimensions are reached. These must not be ex- ceeded. When the product reaches, or comes dangerously close to the

limiting size, the threading tools should be readjusted or replaced.

1. FORM OF THREAD

1. Angle of Thread.—The basic angle of thread (A) between the

sides of the thread measured in an axial plane is 60°. The line bisecting this 60° angle is perpendicular to the axis of the screw thread.

2. Flat at Crest and Root.—The flat at the root and crest of

the basic thread form is l/% X p, or 0.125 X p.

3. Depth of Thread.—The depth of the basic thread form is

T. r,n A ~rm 7 0.649519

7i = 0.649519Xp, or7i = —

where

p = pitch in inches

n = number of threads per inch

h = basic depth of thread.

2. THREAD SERIES

(a) National Hose-Coupling Threads.—There is specified in

Table 58 a thread series and basic dimensions for hose-coupling

threads from three-fourths to 2 inches in diameter, which will be

known as the "national hose-coupling threads."

The "national hose-coupling thread" shall be used on all couplings

and connections where sizes between three-fourths and 2 inches in

diameter are required.
1924 REPORT 109

Table 58.National hose-coupling threads

BASIC MINIMUM COUPLING DIMENSIONS

Nominal size Number of threads

per inch Pitch Depth of thread Major

diameter

Pitch diameter Minor

diameter Allowance

1 2 3 4 5 6 7 8

U

ny2 uy2 ny2

Inch

0. 08696

. 08696

. 08696

. 08696

. 08696

Inch

0. 05648

. 05648

. 05648

. 05648

. 05648

Inches

1. 0725

1. 3051

1. 6499

1. 8888

2. 3628

Inches

1. 0160

1.2486

1. 5934

1. 8323

2. 3063

Inches

0. 9595

1. 1921

1. 5369

1. 7758

2. 2498

Inch

1V4m2 _

BASIC MAXIMUM NIPPLE DIMENSIONS

u - 0. 08696 0. 05648 1.0625 1.0060 0.9495

1 . 08696 . 05648 1. 2951 1. 2386 1. 1821 m —- . 08696 . 05648 1. 6399 1. 5834 1. 5269 m — — ny2 . 08696 . 05648 1. 8788 1. 8223 1. 7658

2.... ny2 . 08696 . 05648 2. 3528 2. 2963 2. 2398

(&) National Fire-Hose Coupling Threads.—There is specified

in Table 59 a thread series and basic dimensions for fire-hose couplings

from 2y2 to 43^ inches in diameter which will be known as the

"national fire-hose threads." These basic sizes and dimensions cor- respond in all details to those recommended by the National Fire

Protection Association and by the Bureau of Standards.

The "national fire-hose thread" shall be used on all couplings and

hydrant connections for fire-protection systems and for all other purposes where hose couplings and connections are required in sizes

between 2% and 4}4 inches in diameter.

Table 59.National fire-hose coupling threads

BASIC MINIMUM COUPLING DIMENSIONS

Nominal size Number of threads per inch Pitch Depth of thread Major

diameter

Pitch diameter Minor

diameter Allowance

1 % 3 i 5 6 7 8

2.5000 7.5

6.0 6.0

4.0

Inch

0. 13333

. 16667

. 16667 .25000

Inch

0. 08660

. 10825

. 10825 .16238

Inches

3. 0836

3. 6389

4. 2639

5. 7859

Inches

2. 9970

3.5306

4. 1556

5. 6235

Inches

2. 9104

3. 4223

4. 0473

5. 4611

Inch

3.0000

3.5000 4.5000

BASIC MAXIMUM NIPPLE DIMENSIONS

2.5000. 7.5 0. 13333 0. 08660 3. 0686 2. 9820 2. 8954 0.015

3.0000 6.0 . 16667 . 10825 3. 6239 3. 5156 3. 4073 .015

3.5000.. 6.0 . 16667 . 10825 4. 2439 4. 1356 4. 0273 .020

4.5000 4.0 .25000 . 16238 5.7609 5. 5985 5. 4361 .025

3. ALLOWANCES AND TOLERANCES

(a) Specified allowances and tolerances, given to Table 60, apply to " national hose-coupling" and " national fire-hose coupling " threads.

16802°—25t 8
110 NATIONAL SCE.EW THREAD COMMISSION

The tolerances represent extreme variations permitted on the product.

There are shown, in Figure 25, the relations between nipple and

coupling dimensions and thread form as specified herein.

(b) The tolerance on the coupling is plus, and is applied from the

minimum coupling dimension to above the minimum coupling

dimension.

(c) The tolerance on the nipple is minus, and is applied from the

maximum nipple dimension to below the maximum nipple dimension.

(d) The pitch diameter tolerances provided for a mating nipple

and coupling are the same.

(e) Pitch diameter tolerances include lead and angle variations

(see footnote 1, Table 60).

Fig. 24.

National hose-coupling and national fire-hose coupling threads

See Tables 60, 61, and 62 for dimensions and tolerances

(f) The tolerance on the major diameter is twice the tolerance

on the pitch diameter.

(g) The tolerance on the minor diameter of the nipple is equal to

the tolerance on pitch diameter plus two-ninths of the basic thread

depth. The minimum minor diameter of a nipple is such as to result

in a flat equal to one-third of the basic flat 0^ X p) at the root when

the pitch diameter of the nipple is at its minimum value. The maximum minor diameter is basic, but may be such as results from the use of a worn or rounded threading tool.
1924 KEPOKT 111

(h) The tolerance on major diameter of the coupling is equal to

the tolerance on pitch diameter plus two-ninths of the basic thread

depth. The minimum major diameter of the coupling is such as to

result in a basic flat Xp) when the pitch diameter of the coupling

is at its minimum value. The maximum major diameter of the

COUPUNG FORM

/VO 7~A T/O/sl - = Pitch

r Basic T~h rea

: B a. s/c Depth of

J~ru.fi cat/ on

A/£/ivy c/a/e Shows £

Fig. 25.

National hose-coupling and national fire-hose coupling threads

coupling is that corresponding to a flat equal to one-third the basic

flat (^Xp).

(i) The tolerance on the minor diameter of the coupling is twice

the tolerance on pitch diameter of the coupling. The minimum
112 NATIONAL SCREW THREAD COMMISSION

minor diameter of a coupling is such as to result in a basic flat (3^ X p)

at the crest when the pitch diameter of the coupling is at its minimum

value.

Table 60.

Tolerances and allowances for national hose-coupling and national

fire-hose coupling threads

Lead TTrrnrQ 11/1 L KJ L O 1X1 in errors half angle Threads Tolerances consuming consuming

Nominal size Allowances on pitch one-half of one-half of per inch diameter 1 pitch- pitch- diameter diameter

tolerances 2 tolerances

1 2 3 i 5 6

Inch Inch Inch Deg. Min.

0. 010 0. 0085 0. 0025 1 52

1. .010 .0085 .0025 1 52 M im .010 .0085 .0025 1 52 m ny2 .010 .0085 .0025 1 52

2 ny2 .010 .0085 .0025 1 52 2^ m .015 .0160 .0046 2 17

3 6 .015 .0180 .0052 2 4

6 .020 .0180 .0052 2 4 4V2 4 .025 .0250 .0072 1 55

1 The tolerances specified for pitch diameter include all errors of pitch diameter, lead, and angle. The

full tolerance can not therefore be used on pitch diameter unless the lead and angle of the thread are perfect. Columns 5 and 6 give, for information, the errors in lead (per length of thread engaged) and in angle, each of which can be compensated for by half the tolerance on the pitch diameter given in column 4. If lead and

angle errors both exist to the amount tabulated, the pitch diameter of a nipple, for example, must be reduced by the full tolerance or it will not enter the "go" gage.

2 Between any two threads not farther apart than the length of engagement.

4. TABLES OF DIMENSIONS

Table 61.National hose couplings, thread dimensions

COUPLING THREAD

Nominal

size

Threads

per inch Pitch Depth

of thread

Major diameter Pitch diameter Minor diameter

MaximumToler- ance Mini- mumMaximumToler- ance Mini- mumMaximum

Toler- ance Mini- mum

1 2 3 i 5 6 , 7 8 9 10 11 12 13

u

im

iva

Inch

0. 08696

. 08696

. 08696

. 08696

. 08696

Inch

0. 05648

. 05648

. 05648 .05648

. 05648

Inches Inch Inches

1. 0725

1 1. 3051

1 1. 6499

1 1. 8888

i 2. 3628

Inches

1.0245

1. 2571

1. 6019

1. 8408

2. 3148

Inch

0.0085 .0085

.0085 .0085

.0085

Inches

1. 0160

1. 2486

1. 5934

1. 8323

2. 3063

Inches

0. 9765

1. 2091

1. 5539 1.7928

2. 2668

Inch

0. 0170

.0170

.0170

.0170

.0170

Inches

0. 9595

1. 1921

1. 5369

1. 7758 2.2498

1IXVA

2

NIPPLE THREAD

54 11%

nte imimn%

0. 08696

. 08696 .08696

. 08696

. 08696

0. 05648

. 05648

. 05648

. 05648

. 05648

1. 0625

1. 2951

1. 6399

1. 8788 2.3528

0. 0170 .0170

.0170

.0170 .0170

1. 0455

1. 2781

1. 6229

1. 8618

2. 3358

1.0060

1. 2386

1. 5834

1. 8223 2.2963

0.0085

.0085

.0085 .0085

.0085

0.9975

1.2301

1. 5749

1. 8138

2.2878

2 0.9495

2 1. 1821

2 1. 5269

2 1. 7658

2 2. 2398

1IX—- m—2

1 Dimensions for the minimum major diameter of the coupling correspond to the basic flat (J^Xp),

and the profile at the major diameter produced by a worn tool must not fall below the basic outline. The

maximum major diameter of the coupling shall be that corresponding to a flat at the major diameter of the maximum coupling equal to AXp.

2 Dimensions given for the maximum minor diameter of the nipple are figured to the intersection of the worn tool arc with a center line through crest and root. The minimum minor diameter of the nipple

shall be that corresponding to a flat at the minor diameter of the minimum nipple equal to -hXV-
1924 REPORT 113

Table 62.National fire-hose couplings, thread dimensions

COUPLING THREAD

Nominal

size

X IireaUo per inch

Pitch of thread

Major diameter Pitch diameter Minor diameter

MaximumToler- ance Mini- mum

MaximumToler- ance Mini- mum

MaximumToler- ance Mini- mum

1 2 3 4 5 G 7 8 9 10 11 12 13

— 7.5 6

64

Inch

0. 13333

. 16667

. 16667 .25000

Inch

0. 08660

. 10825+

. 10825+

. 16238

Inches Inch Inches

1 3. 0836

1 3. 6389

1 4. 2639

1 5. 7859

Inches

3. 0130

3. 5486

4. 1736

5. 6485

Inch

0. 0160

.0180

.0180 .0250

Inches

2. 9970

3. 5306

4. 1556

5. 6235

Inches

2. 9424

3. 4583

4. 0833

5. 5111

Inch

0. 0320 .0360

.0360 .0500

Inches

2. 9104

3. 4223

4. 0473

5. 4611

3

3V2 m

NIPPLE THREAD

2%

3

7.5

6

64

0. 13333

. 16667

. 16667 .25000

0. 08660

. 10825+

. 10825+

. 16238

3. 0686

3. 6239

4. 2439

5. 7609

0. 0320

.0360

.0360

.0500

3. 0366

3. 5879

4. 2079

5. 7109

2. 9820

3. 5156

4. 1356

5. 5985

0. 0160 .0180

.0180

.0250

2. 9660 2 2. 89.54

3. 4976 ' 3. 4073

4. 1176 2 4. 0273

5. 5735 2 5. 4361 3V2 4K2 - -

'Dimensions for the minimum major diameter of the coupling correspond to the basic flat (

l/iXp),

and the profile at the major diameter produced by a worn tool must not fall below the basic outline. The

maximum major diameter of the coupling shall be that corresponding to a flat at the major diameter of the maximum coupling equal to ?'4 Xp.

2 Dimensions given for the maximum minor diameter of the nipple are figured to the intersection of the worn tool arc with a center line through crest and root. The minimum minor diameter of the nipple

shall be that corresponding to a flat at the minor diameter of the minimum nipple equal to j'jXp.

5. GAGES

(a) Gages for National Fire-Hose Coupling Threads.—It is

recommended that "national fire-hose coupling threads" be inspected

in the field by means of gages made within the tolerances given in

Table 63. Limiting dimensions for these gages are given in Tables

64 and 65. These gages will be in the hands of the representative of

the purchaser, who will in most cases be the fire chief, or superintendent of water works, or both.

It is further recommended that "national fire-hose coupling

threads" be given final inspection by the manufacturer by means of

gages made within the limiting dimensions given in Tables 64 and 65,

by whatever amount may be desired, in order to avoid, as far as

possible, disagreements which might otherwise arise as the result of

slight differences in the sizes of gages.

Table 63.

Tolerances on gages for national fire-hose coupling threads

Allowable variation in lead between any two threads not

farther apart than length of engagement

Allowable

variation

in one-half angle thread

Tolerance on diameter of minimum

thread

gage

Tolerance on diameter of maximum

thread

gage

1 2 3 4

Inch ±0.0005 -

Beg. Min.

±0 10

Inch

1 +0.000

\ +.001

Inch —0.000 —.001
114 NATIONAL, SCREW THREAD COMMISSION

Table 64.Limiting dimensions of field inspection plug thread gages for couplings

(internal) 1

Nominal

size Threadsper inch

"Go" or minimum gage "Not go" or maximum gage

Major diameter Pitch diameter Major diameter Pitch diameter

MaximumMini- mumMaximumMini- mum

MaximumMini- mum

Maximum

Mini- mum

1 2 3 4 5 6 7 8 9 10

2. 500 3.000

3.500

4.500

7.5

6.0

6.0 4.0

Inches

3. 0846

3. 6399

4. 2649

5. 7869

Inches

3. 0836

3. 6389

4. 2639

5. 7859

Inches

2. 9980

3. 5316

4. 1566

5. 6245

Inches

2. 9970

3. 5306

4. 1556

5. 6235

Inches

3. 0836

3. 6389

4. 2639

5. 7859

Inches

3. 0826

3. 6379

4. 2629

5. 7849

Inches

3. 0130

3. 5486

4. 1736

5. 6485

Inches

3. 0120

3. 5476

4. 1726

5. 6475

i The minor diameters of plug gages and the major diameters of ring gages are undercut beyond the nominal diameters to give clearance for grinding or lapping. The allowable variation in lead between any two threads not farther apart than the length of engagement is ±0.0005 inch. The allowable variation in

one-half angle of thread is ±10 minutes. Table 65.

Limiting dimensions of field inspection ring thread gages for coupling

nipples (external) 1

Nominal

size Threads

per inch

"Go" or maximum gage "Not go" or minimum gage

Pitch diameter Minor diameter Pitch diameter Minor diameter

Maximum

Mini- mum

Maximum

Mini- mumMaximumMini- mumMaximumMini- mum

1 2 3 4 5 6 7 8 9 10

2.500

3.000

3.500 4.500

7.5

6.0

6.0

4.0

Inches

2. 9820

3. 5156

4. 1356

5. 5985

Inches

2. 9810

3. 5146

4. 1346

5. 5975

Inches

2.9104

3. 4223

4. 0473

5. 4611

Inches

2. 9094

3. 4213

4. 0463

5. 4601

Inches

2. 9670

3. 4986

4. 1186

5. 5745

Inches

2. 9660

3. 4976

4.1176

5. 5735

Inches 2.9114

3. 4233

4. 0483

5. 4621

Inches

2. 9104

3. 4223

4. 0473

5. 4611

1 The minor diameters of plug gages and the major diameters of ring gages are undercut beyond the nominal diameters to give clearance for grinding or lapping. The allowable variation in lead between any

two threads not farther apart than the length of engagement is ±0.0005 inch. The allowable variation in one-half angle of thread is ±10 minutes. SECTION VI. NATIONAL PIPE THREADS

The material on the subject of pipe threads presented herewith is essentially the same as that in the report prepared by a special committee of the Committee of Manufacturers on Standardization of

Fittings and Valves, acting in cooperation with pipe and gage manufacturers and the A. S. M. E. committee on International Standards

for Pipe Threads. It was published in October, 1919, under the title

"Manual on American Standard Pipe Threads." It has been indorsed by the American Society of Mechanical Engineers and the

American Gas Association, and is adopted by the commission with

only such changes as are necessary to bring it into conformity with

the remainder of the report.
1&24 REPORT 115

The " national (American Briggs') pipe-thread standard" for taper

threaded pipe joints was formulated prior to the year 1882 by Robert

Briggs, of Philadelphia, Pa. This standard, with certain modifications and additions, is now in general use throughout the United

States and Canada.

1. FORM OF THREAD

(a) Specifications.—1. Angle of thread.—The angle between the

sides of the thread is 60° when measured in an axial plane, and the

line bisecting this angle is perpendicular to the axis of the pipe for

either taper or straight threads.

2. Depth of thread.10—The crest and root of the thread form are truncated an amount equal to 0.033 p; the depth of thread is, therefore, equal to 0.8 p.

Fig. 26.

National taper pipe thread form and notation

NOTATION A =60° angle of thread

o =30° one-half angle of thread

y = 1° 47' approx. taper angle=one-sixteenth inch per inch on diameter H =0.866025 p depth of 60° sharp V thread

A{=a923761 ff}dePth of thread on work

|=0]033012 p

J{ =0.038120 Hf Depth of truncation

1=0.041266 ftj p =]/n pitch n =number of threads per inch

8. Taper of thread.—The taper of the thread is 1 in 16, or threefourths inch per foot, measured on the diameter.

(Jo) Illustration.—There are shown in Figure 26 the relations as specified herein for form of thread, and general notation. Special

notation is given in Figures 27, 28, and 30.

10 While Mr. Briggs originally advocated a slightly rounded crest and root, cutting tools are actually

slightly flattened at the crest and root.
116 NATIONAL SCREW THREAD COMMISSION

2. SYMBOLS

The list of symbols given in Section II—2, together with additional symbols given below, should be used in formulas for expressing

relations of pipe threads and for use on drawings, etc.

Pitch diameter of thread at end of pipe E

Pitch diameter of thread at gaging notch Ei

Pitch diameter of thread at L2 from end of pipe E2 Maximum pitch diameter, external locknut thread Ee Minimum pitch diameter, internal locknut thread E\

Distance from gaging notch to end of pipe =normal engagement by hand L\

Length of effective thread L2 Nominal outside diameter of pipe =major diameter of pipe thread at L2 from

end of pipe ^ D

Internal diameter of pipe d

3. THREAD SERIES

(a) National (American Briggs') Taper Pipe Threads.—Taper

external and internal pipe threads are recommended for threaded

pipe joints and pipe fittings for any service. The sizes and basic

dimensions of the "national (American Briggs') taper pipe threads"

are specified in Table 66.

1. Outside diameter of pipe.—The outside diameters of pipe are given in column 5 of Table 66.

2. Diameters of taper threads.—The pitch diameters of the taper

threads are determined by formulas based on the outside diameter

of pipe and the pitch of thread. These are as follows 11 (see Symbols above) : # = Z>-(0.05 D + l.l) p

Et = E + 0.0625 L1

3. Length of thread.—The length of the taper external thread is determined by a formula based on the outside diameter of pipe and

the pitch of the thread. This is as follows 11 (see Symbols above)

i3 =(0.8 D + 6.8) p

4- Length of engagement.—The normal length of engagement between taper external and internal threads, when screwed together by

hand, is shown in column 6 of Table 66. This length is controlled

by means of gages.

5. Tolerances.—The tolerance on diameter is the equivalent of the

variation in diameter due to taper over one and one-half turns

either way from the basic dimensions.12

11 These formulas are not expressed in the same terms as the formulas originally established by Mr.

Briggs, because they are used to determine directly the pitch diameter and the length of effective thread, which includes two threads slightly imperfect at the crest; whereas the Briggs formulas determined the major diameter and the length of perfect thread, the two threads imperfect on the crest not being included

in the formula. However, both forms give identical results.

13 On account of the gage tolerance of one-half turn on working gages, this is equivalent to one turn plus

or one turn minus from the gaging notch when using working gages (see figs. 34 and 35).
1924 REPORT 117

Fig. 27.

National taper pipe thread notation

NOTATION £o = D-(0.05Z)+l.l)p

Ei=Eo +0M25Li

U=P(0.8D+6.S)

A=0.8p
118 ITATIONAL SCREW THREAD COMMISSION

o

-a

os cO'-' co tCN COOO t— rH ? t-- OOCN CO X co tp co r- as

tp — r-H Tp

CN ic 00 CN 12

oo o i—t cn r--

t~- O tP CN n"J CD »0 tp t—

t— t— t— 1» co co oo co oo tp

>0 N rH »C N CN CO CO CN CNJ d 5 x co Q

i—I rH i—" CN CN CO CO TP ^p iO CD t- 00 OS ©

CO OS rH CO t— NCOO^OO «5 05 QO O0

r- oo cm r- co

CO tP CO I*" os

CO 00 Tp CO

CO CO CO CN i—

00 CO CN CO CN O —I CN tP OS

iO GO —i CD CN CO CO r— T> OS CN lO OO CN r~ COCOCOQOt

t- tp co t- tp

OS co © as os iCMONO SO Q OS CN

tO 05 Tp O

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MOCNCON iNHNrHlCJ

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r- >o tp co

CN CO Tf O H CN iO OO CO O 800DNH O OS OS CO

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CO O I- CO CO

t- tp i-< as cn

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CO i—i O O iO co r- co os os

rH lO OS CO r—i cn m t— cn i—

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co co cn r— ON^rnO Tp CO CO CO OS

CO CO CO 00 CO

CO CO T(i Tp >o

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CP T3 "3 ? CD V- CN g-pH fa ftuO

rHh-NCDCO —CO "rp TP

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CO CO CO CO rH TP TP TP TP 00 ;ss8ss sssss

CO CO CO CO 00

r- t- r-- r- O OO Q O O ©OO 38888

•Go -a ?c>2 Pa .,

iOCOCOhh

2* co rH r- co ic

CO 00 CO CN O t— i^. tp io to

cn co co co r— CO O CN iO CO CO N N N H SO O © CO

CN CN CO CO TP

© © © o ©

CN CN CN CN >C rHHHHtN

IO CO t- 00 OS

.a 2 . >> O O O O OS CO TP CN CO 2 rH CN CN CO CO

CO -H TP »C t- CD CN TP t- CO

t- co oo co as

00 O CO O O O CO CO rH OiOO <-<CN

21 "etc o

5 Q TP TP 25

J Tp iO CO O0 O

inoioioo

CN CN CN CN iO CD CO CD CD

rH HHHNN CO rP TP iC iO cot- CO OS o

CO tP tP tP CO Tp -^p rH rH rC as tp tp O CN tP Tp IO u0 £ OOO© Or- 1— r— t- © IO "C iO iCC

os os as as c_ SCO CD CO O OOO rH

tP CO CD CO CO

. O lO IO Tj* Tp

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cj co io i-o r— « O O © O ©

CO CO CO CO C as as as os c_ CO CO CO co «o O0 00 CO 00 CN O O O O rH

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r, -O O

B o g — S

3 fa. <-< r> CD

t- O0 OO TP TP 0O 00 00 0OO0 0000000000

rXr^eS\f-\e< rH rH rH CN CN COCOTPTplO CoV- 00 OS rH
1924 REPORT

OO OO OS CN

CO CO QC CO O 03 CO iO 00 00 00 OO 00

S?S2 TP 0> IO

g) co GO 00 00

CO 00 CO i-H I

s" o r— cm tt1 io CO CO 00 00 00

l-S CN CO Tt< IO

O Q CO CO OS

I

s- r- co co

00 CO CO 00 00

' CO 00

CN iO 00

CO >—1 CO 00 00 OO

as co ,-< ^- o o co os HOlTf ION

i-H cn co id

CM CN CO CO

r^- 1-— o co co

co co c5 r- cn co oo co r- r- 00 CO 00 00 00

t-- 00 OS

OS <N IO

!> CO 00

to co »o 00 00 00

iC t-' OS

CN CN CN

CDhOOO

CN >S 00 CM CO CO t— CO CO "O O N N 1^

rH CN CO t** tf-j

SSS88 SSSS8 SSS

OOOOO C5 io o c o OOOOO O O Q Q O

inOOOQO io ic io lq

iO CO 00 O CN

CN CN CN CN CN CN CN CN CO CO CO CO CO

tfJOMC-CN

CO CO CO 00 i—1 CN CO iO CO 00 S O »-t CN CO

i—i CN CN CN CN CN CN CN

»-i cn m co

5 O O O : oi g o

; o o o OOOOO OOOOO o o o o

- OO o s sOOO

O O OO

OOOOOO Q O O Q

CM CN CN CN IN O O Q O O OOOOO

CN CN CN CN CN

coooooooco oooooocooo ooooco

;ddd ddddd odd

i-H CN Tfl iO CO f- 00 Q CN COOOO rtHHHH i—I i—I CN CN CN CN CN CO
120 NATIONAL SCREW THREAD COMMISSION

(&) National Straight Pipe Threads.—The specified sizes and

basic dimensions on the "national straight pipe threads" are given

in Table 67.

1. Diameters of straight threads.—The basic pitch diameter of

the straight thread is equal to the diameter at the gaging notch of

"national (American Briggs') taper pipe threads," and is determined by the following formula based on the outside diameter of

pipe and the pitch of thread (see Symbols above)

Et = D- (0.05 # + 1.% + 0.0625 Lt 2. Tolerances.—The tolerance on pitch diameter of a straight

pipe thread is the equivalent of the variation in diameter over one and

one-half turns either way from the gaging notch of the "national

(American Briggs') taper pipe thread." 13 (See columns 4 and 6 of

Table 67.)

3. Application to internal threads.—Straight threaded internal

wrought iron or wrought steel couplings of the weight known as

Fig. 28.National straight pipe thread notation (internal)

Note.—This thread is gaged with the taper threaded plug gage and should gage flush at the face with

the gaging notch, allowing a maximum variation of one turn plus or minus from the notch

"standard" may be used with taper threaded pipe for ordinary

pressures, as they are sufficiently ductile to adjust themselves to the

taper external thread when properly screwed together.

For high pressures, only taper external and internal threads should

be used.

4- Application to external threads.—Straight external threads

are recognized only for special applications, such as long screws and

tank nipples.

5. Application to long screw joints.—Long screw joints are used

to a limited extent. This joint is not considered satisfactory when

13 The coupling thread may be gaged with a taper threaded plug gage. On account of the gage tolerance

of one-half turn on working taper pipe thread gages, the working tolerance is equivalent to one turn either way from the gaging notch. In gagiDg, care must be taken to gage at the first thread and not at the end

of the coupling when the thread is chamfered.
1924 REPORT 121

subjected to high temperature or pressure. In this application

the coupling has a straight thread and must make a joint with a

" national taper pipe thread." (See fig. 28.) It is necessary that the

coupling be screwed on the straight external thread for the full

length of the coupling and then back until it engages the taper

external thread. The straight thread on the pipe enters the coupling

freely by hand, the joint being made by a packing material between

the locknut and the coupling. (See fig. 29.)

Fig. 29.Illustration of "long screw" joint between straight threaded coupling

and taper threaded pipe

On account of the long engagement of thread, imperfections in

pitch affect the fit when the coupling is screwed on the pipe its full

length. Refinements of manufacture and gaging to insure a properly interchangeable product are more costly than the commercial

use warrants; therefore, the use of this type of joint is not recommended. For this reason, specifications for tolerances and gaging

are not included herein.

Table 67.Dimensions of national straight pipe threads (for couplings)

[For notation, see fig. 28]

Nominal sizes in inches Threads

per inch

Major

diameter, 1 basic

Pitch diameter Minor

diameter 1

Maximum Basic Minimum Basic

1 2 3 4 5 6 7

Inches Inches Inches Inches Inches H - 27 0. 40439 0. 37823 0. 37476 0.37129 1 0.34513 X - 18 . 53433 . 49510 . 48989 . 48468 .44544 % 18 . 67145 . 63222 . 62701 .62181 . 58257

Vi —- 14 . 83557 .78513 . 77843 .77173 .72129

SA - 14 1. 04600 . 99557 .98887 . 98217 . 93172

l mi 1. 30819 1.24678 1. 23863 1. 23048 1. 16907 VA im 1. 65294 1. 59153 1. 58338 1.57523 1.51382 m - ny2 1.89190 1. 83049 1.82234 1.81418 1. 75277

2 ny2 2. 36583 2. 30442 2. 29627 2. 28812 2. 22671 m 8 2. 86216 2. 77388 2. 76216 2. 75044 2. 66216

3 8 3. 48850 3. 40022 3. 38850 3. 37678 3. 28850 m.~ - 8 3. 98881 3. 90053 3. 88881 3.87709 3. 78881

4 8 4. 48713 4. 39884 4. 38712 4. 37541 4. 28713 VA 8 4. 98594 4. 89766 4. 88594 4. 87422 4. 78594

5 8 5. 54929 5. 46101 5. 44929 5. 43757 5. 34929

6 8 6. 60597 6. 51769 6. 50597 6. 49425 6. 40597

1 The national pipe thread form is maintained; therefore, the major and minor diameters vary with the

pitch diameter and are determined by the threading tools.
122 NATIONAL, SCREW THREAD COMMISSION

(c) National Locknut Threads.—Occasional requirements make

it advisable to have a straight thread of the largest diameter it is possible to cut on a pipe. This practice has been standardized and

is known as "maximum external and minimum internal locknut

threads." For dimensions, see Table 68. The "tank nipple" shown

in Figure 31 is an example of this thread. In this application a "national (American Briggs') standard taper pipe thread" is cut on

the end of the pipe after having first cut the "external locknut

thread."

Table 68.Dimensions of national locknut threads

[For notation, see fig. 30]

Nominal size in inches Threads

per inch &

(Maximum) 1 (Minimum) 1 Depth of thread

H-XHlAH1.. m2.. 2M

3.. 3K2 4... iH

5..

6..

7_. 8..

flit)-

11.

12.

27

18

18

14

14ny2 ny2 ny2 uy2 8

8

8

88

8

8

8

8

Inches

0. 38402 .50378

.64090 .79629

1.00672

1. 26037

1.60512

1. 84408

2. 31801

2. 79341

3. 41975

3. 92006

4, 41838

4. 91719

5. 48054

6. 53722

7. 53359

8. 53128

9. 52922

10. 65219

11. 65063

12. 64906

Inches

0. 38633

. 50725

.64437 .80075

1.01119

1. 26580

1. 61055

1. 84951

2. 32345

2. 80122

3. 42756

3. 92787

4. 42619

4. 92500

5. 48836

6. 54503

7. 54141

8. 53909

9. 53703

10. 66000

11. 65844

12. 65688

Inch

0. 02963 .04444 .04444

. 05714

. 05714

.06957

. 06957

.06957

. 06957

. 10000

.10000

.10000

.10000

.10000 .10000

.10000

.10000 .10000

.10000

.10000

.10000

.10000

1 A tolerance equivalent to one and one-half turns of the "national taper pipe thread' the tolerance being minus on E* and plus on Ei.

' is recommended,
1924 REPORT

Fig. 30.National lo-knut thread notation

NOTATION E\= pitch diameter at gaging notch of national taper plug gage £e =£i+(4pX0.0625)

£i = £i+(5pX0.0625) Note.—See Table 68 for relation to taper pipe thread

Fig. 31.

Illustration of "tank nipple" thread
124 NATIONAL SCREW THREAD COMMISSION

4. TABLES OF PIPE DIMENSIONS

Tables 69, 70, 71, and 72, which follow, are not a part of the

thread standard, but are reprinted as part of the "Manual on American Standard Pipe Threads."

Table 69.Dimensions of standard wrought pipe

Nominal sizes in inches

K-

%-

V*- %-

1__

V4, m2..

3..

4.. VA

5—

6_. 7.. 8..

8..

9..

10.

10. 10.

11.

12.

12.

Inside diameter

Inches

0. 269

.364

.493

.622

.824

1.049

1. 380

1.610

2. 067

2. 469

3. 068 3.548

4.026

4. 506 5.047

6. 065

7. 023

8.071

7.981

8. 941

10. 192

10. 136

10. 020

11.000

12. 090

12. 000

Outside diameter

Inches

0.405 .540 .675

.840

1.050

1.315

1. 660

1. 900

2. 375

2. 875

500

0UU 500

000

5. 563

6. 625 7.625

8. 625 8.625

9.625

10. 750

10. 750

10. 750

11.750

12. 750

12. 750

Nominal

thickness

Inch

0.068

.088

.091 .109

.113

. 133 .140

.145

. 154 .203

.216

.226

.237

.247 .258

.280

.301 .277 .322 .342

.279

.307

.365

.375

.330

.375

Transverse areas

Internal

Square

inches 0.057

. 104 .191 .304

.533

.864

1. 495

2.036

3. 355 4.788

7.393

9.886

12. 730

15. 947

20. 006

28.891

38. 738

51. 161

50. 027

62. 786

81. 585

80. 691

78. 855

95. 033

114. 800

113.097

Metal

Length

of pipe per square

foot of external

surface

Square

inches

0. 072

. 125 .167 .250

.333

.494

.669

.799

1. 075

1. 704

2.228

2.680

3. 174 3.688 4.300

5.581

6. 926

7.265 8.399

9. 974

9. 178 10.072

11. 908

13. 401

12. 876

14. 579

Feet

9. 431 7.073

5. 658

4. 547 3.637

2.904

2.301 2.010

1. 608

1. 328

1. 091 .954 .848

.763

.686

.576

.500

.442 .442

.396

.355

.355

.355

.325 .299

.299
1924 REPORT

Table 70.

Dimensions of extra strong wrought pipe

125

Nominal sizes in inches

Ys- XYs3A1-

VA

m2__

3„

VA

4.- 4H

5-

6.. 7„

8„

9..

10.

11-

12.

Inside diameter

Inches

0. 215 .302

.423

.546 .742

.957

1. 278 1.500

1. 939 2.323

2.900

3. 364

3. 826

4. 290

4. 813

5.761 6.625

7. 625

8. 625 9.750

10. 750

11. 750

Outside

diameter

Inches

0. 405 .540

.675 .840

1. 050

1.315

1. 660

1.900

2.375 2.875

3.500

4.000 4.500

5. 000

5.563

6. 625

7. 625

8. 625 9.625

10. 750

11. 750

12. 750

Nominal

thick- ness

Inch

0. 095 .119

.126

. 147

. 154

.179

.191 .200

.218 .276

.300

.318

.337

.355 .375

.432

.500

.500

.500

.500

.500

.500

Transverse areas

Internal

Square

inches

0. 036 .072

. 141 .234

.433

.719

1.283

1. 767 2.953 4.238

6.605

8.888

11. 497

14. 455

18. 194

26. 067

34. 472

45. 663

58. 426

74. 662

90. 763

108. 434

Metal

Square

inches 0.093

. 157 .217 .320

.433

.639

.881

1.068

1. 477

2.254

3. 016

3. 678

4. 407

5. 180

6. 112

8. 405

11. 192 12.763

14. 334

16. 101

17. 671

19. 242

Length of pipe per square

foot of

external

surface

Feet

9. 431

7. 073

5.658 4.547

3.637

2.904

2. 301 2.010

1.608

1. 328

1.091 .954

.848

.763 .686

.576 .500

.442

.396 .355

.325

.299

Tablti 71.

Dimensions of double extra strong wrought pipe

Nominal sizes in inches Inside diameter

Outside diameter

Nominal

thick- ness

Transverse areas

Internal Metal

Length of pipe per square

foot of external

surface

YiYi- 1—

m

2.. VA

3_-

4..

5-

6„

7_.

Inches

0. 252 .434

.599

.896

1. 100

1. 503

1. 771

2. 300 2.728

3. 152

3. 580

4. 063 4.897

5. 875 6.875

Inches 0.840

1. 050

1.315

1. 660

1. 900

2.375

2.875 3.500

4.000

4. 500

5. 000

5. 563

6. 625 7.625

8. 625

Inch

0.294

.308

.358

.382 .400

.436

.552

.600

.636

.674

.710 .750

.864

.875

.875

Square

inches

0. 050

.148

.282

.630

.950

1. 774

2. 464

4. 155

5. 845

7. 803

10. 066

12. 966

18. 835

27. 109

37. 122

Square

inches

0. 504 .718

1.076

1. 534 1.885

2.656 4.028

5. 466

6. 721

8. 101

9. 569

11. 340

15. 637

18. 555

21. 304

Feet 4.547

3. 637

2. 904

2.301

2. 010

1. 608

1. 328

1. 091 .954 .848

.763

.686

.57 .500 .442

16802°—25t 9
126 NATIONAL SCREW THREAD COMMISSION

Table 72.Diameters of large 0. D. pipe

Nominal sizes in inches

Outside diameter

Inside diameter

% inch

thick

fa inch

thick % inchthick

fa inch

thick % inch

thick

fa inch

thick % inch

thick % inch

thick

1 inch

thick

1 2 3 i 5 8 J 8 9 10 11

Inches

14 Inches izH

14%

15%

16%

17%

Inches 13%

14%

Inches 13%

14%

15%

16%

17%

19%

21%

23%

25%

Inches 13%

14%

15%

16%

17%

19%

21%

23%

25%

27%

29%

Inches

13 Inches 12%

13%

14%

15%

16%

18%

20%

22%

24%

26%

28%

Inches 12%

13%

14%

15%

16%

18%

2054 22%

24%

26%

28%

Inches 12%

13%

14%

15%

16%

18%

20%

22%

24%

26%

28%

Inches

12

13

14

15-.- 15 14 16 16 15% 15

17- 17 16% 16 15 18.... 18 17%

19%

21%

17 16

20. 20 19 18 22 22 21 24 24 23

26 26 25

28. 28 27

30 30 29

5. THREADING TOOLS

(a) Taps.—Recommended dimensions of standard taps for

"national taper pipe threads" are given in Table 73.

Recommended dimensions of standard taps for " national straight

pipe threads" are given in Table 74.

(&) Tap Drills.—Sizes of tap drills recommended for "national

taper pipe threads" are given in Table 75.

Sizes of tap drills recommended for "national straight pipe threads "

are given in Table 76.

Table 73.Dimensions of standard taps for national taper pipe threads

Projection of Length Length of Diameter of Size of small end

overall thread shank square through

taper gage Nominal sizes in inches Nom- NomNom- Toler- Nom- Toler- inal Toler- inal Toler- Nom- Tolerinal ance inal ance (maxi- ance (maxi- ance inal ance mum) mum)

1 2 3 i 5 6 7 8 9 10 11

Inches Inch Inches Inch Inches Inch Inches Inch Inch Inch % 2% ±ft % ±A 0. 4375 -0.007 0. 328 -0.006 0.312 ±0.0625 %- - 2rV ±A irV ±A . 5625 -.007 .421 -.006 .459 ±. 0625 %-

%

2rV ±& ±A .7000 -.007 .531 -.006 .454 ±. 0625 3% ±A 1% ±A .8650 -.007 .640 -.006 .579 ±. 0625 % 3% ±A 1% ±A 1. 0750 -.009 .812 -.010 .565 ±. 0625

1 3% ±A 1% ±A 1. 1250 -.009 .843 -.010 .678 ±. 0938 1% ----- 4 ±A 1% ±A 1.3125 -.009 .984 -.010 .686 ±.0938 1%-— 4% ±A 1% ±A 1.5000 -.009 1. 125 -.010 .699 ±.0938

2 4% ±A 1% ±A 1. 8750 -.009 1. 406 -.010 .667 ±.0938 2% 5% ±rV 2fa ±A 2.2500 -.009 1. 687 -.010 .925 ±.0938

3.. 6 ±rV 2% ±A 2. 6250 -.009 1.968 -.010 .925 ±.0938 3% 6% ±rV 2H ±A 2. 8125 ' -.009 2. 108 -.010 .938 ±. 1250

4 6% ±rV 2% ±A 3.0000 -.009 2. 250 -.010 .950 ±. 1250
1924 KEPOBT 127

Table 74.Dimensions of standard taps for national straight pipe threads

Nominal

sizes in inches

Length

overall

NominalToler- ance

Length of thread

Nominal Toler- ance

Diameter of shank

Nominal (maximum)

Toler- ance

Size of square

Nominal (maximum)

Toler- ance

Pitch diameter

Maximum Basic Mini- mum

Major

diameter

Nominal

10 11 12 13

Ys- kA-

*Al._ IK

VA

2__ m

3__ VA

4.-

Inches 2A

2rV zy» 3M

3M

4

iAl iA

6A

Inch ±A

±A

±A

±A

±A

±A

±A

±A

±A

±A

±ft ±tV ±A

Inches

SA

1A

If. 1%

1J*

154 1M

2A

2%

2tt 2%

Inch

=fcA ±A

±A

±A

±A

±A

±A

±A

±A

±A

±A

±A

±A

Inches

0. 4375

.5625

.7000

.8650

1. 0750

1. 1250

1. 3125

1. 5000

1. 8750

2. 2500

2. 6250

2. 8125

3. 0000

Inch

-0. 007 -.007

-.007

-.007

-.009

-. 009 -.009

-.009

-.009

-.009

-.009

-.009

-.009

Inches

0. 328

.421 .531

.640 .812

.843 .984

1. 125

1. 4C6 1.687

1. 968

2. 108

2. 250

Inch

-0. 006 -.006

-.006

-.006

-. 010

-.010 -.010

-.010

-.010

-.010

-.010

-.010

-.010

Inches

0. 3763 .4914

.6288

.7802 .9909

1. 2406

1. 5856

1. 8246

2. 2988

2. 7649

3. 3913

3. 8916

4. 3899

Inches

0. 3748 .4899

. 6270 .7784

1. 2386

1. 5834

1. 8223

2. 2963

2. 7622

3. 3885

3. 8888

4. 3871

Inches

0. 3733 .4884

.6253

.7767

1.2366

5811 8201

2938 7594

3. 3858

3. 8861

4. 3844

Inches

0. 4044

. 5343 .6715

.8356

1.0460

1. 3082

1. 6529 8919

3658 8622

3..4885 3.9888

4. 4871

Table 75.Sizes of tap drills, national taper pipe threads

Nominal sizes in inches Threads

per inch

Minor

diameter

at small end of pipe

Tap drill

Ys- AA1... VA. m2...

iYz.

3...

4... iA

5...

6...

27

1818

14

14UA

iVA

nA

ny2 8

8

8

Inches

0. 33388 .43294

. 56757

. 70129

. 91054

1. 14407 L 48757

1. 72652

2. 19946

2. 61953

3. 24063

3. 73750

4. 23438

4. 73125

5. 29073

6. 34609

Inches a

A

A

14 1A

m

2A

V4

iA

i'A 5A

6»
128 NATIONAL SCKEW THREAD COMMISSION

Table 76.

Sizes of tap drills, national straight pipe threads

Nominal sizes in inches Threads

per inch

Minor

diameter,

basic

Vi- y2 -

SA1... m2...

3__. 3Ji

4...

5... 6—

27

18

1814

14ny2 uy2

Inches

0. 34513 .44544

. 58257 .72129

. 93172

1. 16907

1. 51382

1. 75277

2. 22671

2. 66216

28850

78881

28713

78594 34929

6. 40597

6. GAGES

In order properly to maintain interchangeability of pipe threads,

gages should consist of "master," " check" or "setting," "inspection,"

and "working" gages. The same fundamentals apply as those

outlined in Section III covering gages for fastening screws, with

the single exception that, with taper threaded gages, separate "go"

and "not go" gages are not necessary.

(a) Classification of Gages.—1. Master gage.—The master

gage is a taper threaded plug gage. The roots of the threads are cut

to a sharp V or may be undercut below the sharp V to facilitate

the making of the thread. The crests are truncated an amount

equal to 0.1 p (see fig. 32) , 14 Basic dimensions of taper pipe

thread gages are given in Table 81. This gage is provided with the

gaging notch as illustrated in Figure 32. The master gage is the

gage to which all other gages are ultimately referred either by transfer of measurements or direct comparison by engagement. It is

intended primarily for the use of gage and thread tool manufacturers.

2. Check {or reference) gages.—The check gages consist of a plug

gage, similar in all respects to the master gage, and one ring gage.

The ring gage has a thickness equal to dimension Lv

is the same

diameter at the small end as the small end of the plug gage, and is

flush with the plug gage at the small end and at the gaging notch

when screwed on tight by hand (see fig. 32). The check plug gage

is used to inspect inspection and working taper threaded ring gages.

The check ring gage is used to compare the check plug with the

h The object of truncating the crests on gages (truncation 0.1 p) is to insure that, when gaging commercial threads cut with a slightly dull tool, the gage bears on the sides of the thread instead of on

the roots.
1924 REPORT 129

master plug, or the inspection and working plug gages with the check

plug gage.

3. Inspection gages.—Inspection gages consist of one taper threaded

plug gage and one taper threaded ring gage. The roots of the

threads are cut to a sharp V or may be undercut to facilitate making

the thread. The crests are truncated an amount equal to 0.1 p,

but otherwise the gages are made to the dimensions given in Table

66.14 The ring gage has a thickness equal to dimension Lv and the

same diameter at the small end as the small end of the plug gage

(see fig. 33).

Inspection gages are for the use of the purchaser of pipe thread

products. When used, the extreme tolerances on the work should

be applied. This tolerance is one and one-half turns either way

from the gaging notch in the case of internal threads inspected with

L PITCH £)/Afi7ET£i?-*m\ J

JR/A/G G/ICrE AO/? CHECf/KG SMALL.

£j*o o/r PLUG G*US&

Fig. 32.

Master gages or check gages for checking working gages

the inspection plug gage, and when inspecting external threads the

tolerance is one and one-half turns either way from the small end

of the inspection ring. Inspection gages should be checked frequently against the check gages, and in use their errors should be

taken into account.

4- Working gages.—The working gages consist of one taper threaded

plug and one taper threaded ring gage. These gages are similar in

all respects to the inspection plug and ring gages. The working gages

are used by the manufacturer to inspect his product. In using the

working gages, the tolerance to be applied is one turn either way from

the gaging notch in case of internal threads inspected with the plug
130 NATIONAL SCREW THREAD COMMISSION

gage, and in the case of external threads the tolerance is one tum

either way from the small end of the working ring gage.

(b) Gaging Practices.—1. Gaging internal threads.—The inspection and working plug gages, Figure 34, should screw tight by hand

into the fitting or coupling until the notch is flush with the face.

When the thread is chamfered, the notch should he flush with the

bottom of the chamfer. The fitting or coupling is within the working

or net tolerance if the working gage notch is within one turn of the

coupling or fitting face when screwed in tight by hand. In the same

way the coupling or fitting is within the inspection or extreme toler- ance if the inspection gage notch is within one and one-half turns of

the coupling or fitting when screwed on tight by hand.

This method of gaging is used either for taper internal threads or

for straight internally threaded couplings which screw together with

taper external threads.

cessrs

A=PITCH DIAMETER--?*

/IT S/*7/H-I- END

taps./? //a/16measured o/v Q//wtaref?

Fig. 33.

Inspection or working gages for checking product

2. Gaging taper external threads.—The ring gage, Figure 35, should screw tight by hand on the pipe or external thread until the small

end of the gage is flush with the end of the thread. The pipe or external thread is within the working or net tolerance if the working

ring gage screws on until the end of pipe or external thread is within

one turn of the small end of the gage. The pipe or external thread

is within the inspection or extreme tolerance if the inspection ring screws on until the end of pipe is within one and one-half turns of

the small end of the gage.

(c) Specifications for Gages.—1. Master gages.—Master gages

shall be made within the narrowest possible limits of error. In no
1924 REPORT 131

case should the accumulative error exceed the total accumulative

tolerance on diameter given in Table 77. Each master gage shall

be accompanied by a report showing the error on each of the elements of thread and a statement of the accumulative error derived

from the errors in the various elements. In case of question, the

deviations of this gage from the basic size shall be ascertained by

the Bureau of Standards at Washington, D. C.

PLUS, NATIONAL TAPER VYO&/0/VG

-PLf/G CAGE

*" \*-QN£ Tt/RN l-A&GG - \*^QN£TVRNS/>1/ILU

/NVERNAL THREAD

BASIC SfZE

INTERNAL. TH&EAB

MAXIMUM SIZE

//VTEfNAL. THREAD

MINIMUM SIZE

Fig. 34.Gaging of internal national taper pipe threads

2. Check {or reference) gages.—Column 2 of Table 77 gives the

maximum allowable accumulation of all errors in the thread surface

of a check gage, expressed in terms of diameter, as illustrated in

Figure 36. No point on the thread surface of the gage should be

outside of the zone of tolerance indicated by the shaded portion of

NATIONAL TAPER

WOP/T/NG P/a/G GAGE

ONE TI//?N Lfir?G£ ONE T(/RN S/VAAi.

EXTERNAL THREAD

BASIC SIZE

EXTERNAL THREAD

MAXIMUM S/-Z.E

EXTERNAL. THREAD

MINI1*1 ur*1 SIZE.

gages by

Fig. 35.Gaging of external national taper pipe threads

the illustration. This column is used when checking

measurement. If the errors in the gage are reported in terms of

pitch, angle of thread, and diameter, Tables 79 and 80 may be used

to determine the accumulation of these errors for comparison with

column 2. In Table 79 the results of errors in angle are expressed

in terms of diameter. In Table 80 the results of errors in pitch are expressed in terms of diameter.
132 NATIONAL SCREW THREAD COMMISSION

For example: A three-fourths inch, 14 thread, pipe thread plug

gage is reported as follows:

Pitch diameter, large end, 0.98881 inch.

Pitch diameter, small end, 0.96775 inch.

One-half included angle of thread, 29° 58'. Maximum error in lead, 0.00007 inch.

The correct pitch diameter at large end is 0.98886 inch. The error

is 0.00005 inch. The correct pitch diameter at small end is 0.96768

inch. (See Table 66.)

Error of 2' in angle is equivalent to 0.00005 inch. 0.00007 inch

error in lead is equivalent to 0.00012 inch. (See Table 66.) The

accumulative error at large end in terms of diameter = 0.00023 inch.

The accumulative error at small end equals 0.00025 inch.

The gage falls within the li mits of the check gage (0.00028 inch as given in Table 77).

Column 3 of Table 77 gives the equivalent of column 2, expressed

in terms of distance parallel to the axis, and represents the maximum distance which a check ring gage of perfect thickness, or a check plug gage of perfect length from small end to gaging notch,

may vary from being flush at the gaging notch, or at the small end,

when referred to basic dimensions. It is equal to 16 times column 2

because of the basic taper of 1 in 16, measured on the diameter.

This column is used when checking check gages by comparison with

a master gage. The necessary allowance must be made for the error in the master gage.

Column 4 of Table 77, gives the equivalent of column 3, expressed

in terms of the decimal part of a turn. This column is also used

when checking check gages by comparison with a master gage.

The necessary allowance must be made for the error in the master

gage.

A tolerance of plus or minus 0.0002 inch is allowed on the distance

between the gaging notch and the small end of the check plug gage, or on the thickness of the check ring gage.

It is possible for check plug and ring gages, which come within all of the above tolerances, to vary from being flush with each other at

the small end, or at the gaging notch, when screwed together tight

by hand. The maximum variation which might occur, expressed in

terms of distance, is given in column 5 of Table 77, and gages which

come within these limits should be checked by measurement before

being rejected.

3. Inspection gages.—The tolerances on new inspection gages are the same as on working gages. (See Table 78.)

4. New working gages.—Column 2 of Table 78 gives the maximum

allowable accumulation of all errors in the thread surface of new

working gages, expressed in terms of diameter, as illustrated in

Figure 36. No point in the thread surface of the gage should be
1924 REPORT 133

outside of the zone of tolerance indicated by the shaded portion of the

illustration. This column is used when checking gages by measurement.

Column 3 of Table 78 gives the equivalent of column 2, expressed

in terms of distance parallel to the axis, and represents the maximum

distance which a new working ring gage of perfect thickness, or a new working plug gage of perfect length from small end to gaging

notch, may vary from being flush at the gaging notch, or at the small

end, when referred to basic dimensions. It is equal to 16 times

column 2, because of the basic taper of 1 in 16, measured on the

diameter. This column is used when checking working gages by

comparison with a gage, the error of which is known. The necessary

allowance must be made for this error.

TAPER I irtl&MEASURED ON DtAftl.

AXIS

C £ B

Fig. 36.

Relation of tolerance to basic dimensions of a correct taper pipe thread gage

JE =basic pitch diameter at small end of gage £=minimum pitch diameter at small end of gage C— maximum pitch diameter at small end of gage B= E„—column 2 from Table 77 for check gages, or column 2 from Table 78 for new working gages C= £o+column 2 from Table 77 for check gages, or column 2 from Table 78 for new working gages Note.—No point of the thread surface of the gage should be outside of the zone of tolerance indicated by the shaded portion of the illustration. The dotted line indicates the outline of a perfect gage made

exactly to the basic dimensions.

Column 4 of Table 78 gives the equivalent of column 3, expressed

in terms of the decimal part of a turn. This column is also used

when checking working gages by comparison with a gage, the error

of which is known. The necessary allowance must be made for

this error. A tolerance of plus or minus 0.0005 inch is allowed on the distance

between the gaging notch and the small end of the working plug

gage, or on the thickness of the working ring gage.

It is possible for working plug and ring gages which come within

all of the above tolerances to vary from being flush with each other

at the small end or at the gaging notch, when screwed together tight

by hand. The maximum variation which might occur, expressed in

terms of distance, is given in column 5 of Table 78, and gages which

come within these limits should be checked by comparison with

reference gages before being rejected.
134 NATIONAL SCREW THREAD COMMISSION

It is also possible for working plug and ring gages which come

within all of the above tolerances to vary from being flush at the

small end or at the gaging notch, when screwed tight by hand on a

reference gage which comes within the tolerances specified for refer- ence gages. The maximum variation which might occur, expressed

in terms of distance, is given in column 6 of Table 78, and gages which

come within these limits should be checked by measurement before

being rejected.

5. Worn working gages.—The maximum wear on working gages

must not be more than the equivalent of one-half turn from the basic

dimensions.

In order that no work passed by the working gage shall be rejected

by the inspection gage, it will be necessary to discontinue the use of

the working gage when it has worn one-half turn; that is, the working

gage should always be kept within the tolerance equivalent of one-half

turn from the basic dimensions.

Table 77.

Tolerances for chech (or reference) gages, national taper pipe threads

Nominal sizes in inches

Total ac- cumulative

tolerance on diameter (see

fig. 36)

Equivalent

longitudi- nal varia- tion (16 X

column 2)

Equivalent

angular

variation expressed

as decimal

part of one

turn

(')

Vs- X-%-

M1-

m

2__ 2V2

3..

4.. 4V2

5.. 6-

7„

8-

9..

lull. 12.

14.

16. 18.

20.

22.

24. 26.

28.

30.

Inch

0. 00020

. 00022

. 00024

. 00026

.00028

.00030

. 00032

. 00034

.00036

. 00038

. 00038

.00041 .00043

'. 00045

.00047

. 00051

. 00055

. 00059

. 00063

. 00066

00070

. 00074

. 00082 .00090

. 00098

. 00106

.00113

.00121 .00129

. 00137

.00144

Inch

0. 0032

.0035

.0038 .0042

.0045

.0048

.0051 .0054 .0058

.0061

.0061 .0066

.0069

.0072

.0075

.0082 .0088

.0094

.0101 .0106

.0112

.0118

.0131 .0144 .0157

.0170

.0181 .0194

.0206

.0219

.0230

0.086

.063

.068 .059

.063

.055 .059

.062

.067

.050

.050

.053 .055

.058

.060

.065 .070

.075

.080

.090

.095

. 105 .115

.125

.135 .145

.155

.165

.175 .185

Inch

0.0068

.0074 .0080

.0088 .0094

.0100

.0106

.0112 .0120

.0126

.0126

.0136 .0142

.0148

.0154

.0168

.0180

.0192 .0206

.0216

.0228 .0240

.0266

.0292 0318

.0344

.0366

.0392

.0416

.0442

.0464

1 Maximum amount it is possible for plug and ring gages to vary from being flush at small end or at gag- ing notch when screwed together tight by hand (2 times column 3+0.0004 inch).
*

1924 REPORT 135

Table 78.Tolerances for inspection and working gages, national (American

Briggs') taper pipe threads

Nominal sizes in incbes

i otai ac- cumulative

tolerance on diameter

(see ng. 6b)

Equivalent longi- tudinal

variation

(16 X

column 2)

(

!) (!

)

1 % 3 4 5 6

Inch Inch Inch Inch H U. UUU4U 0. 0064 0. 172 0. 0138 0. 0103 M flfifXA A

. UUU44 . 0070 . 126 . 0150 . 0112 Yi—

.00048 .0077 .136 .0164 .0122

Vi%

. 00052 .0083 .118 .0176 .0132

. 00056 . 0090 . 126 . 0190 . 0142

l

f\f\C\D{\

. UUUoU . 0096 . 110 . 0202 . 0151

134 . 00064 . 0102 . 118 . 0214 . 0160 \y2 . 00068 .0109 . 124 .0228 .0170

2 . 00072 .0115 .134 .0240 .0180 m— f\f\f\'~rc

. UuU7o . 0122 . 100 . 0254 . 0190

3

f\nr\iet

. UUU/O . 0122 i t\n

. 100 . 0254 . 0190 nnnoo . UUUo-S . 0131 . 105 . 0272 . 0204

4 . 00086 .0138 .110 .0286 .0214 04— .00090 .0144 . 115 .0298 .0223

5 . uou»4 . 0150 . 120 . 0310 . 0232

6 , 00102 . 0163 . 130 . 0336 . 0252

7 . 00110 . 0176 . 140 . 0362 . 0271

8 . 00118 . 0189 . 150 . 0388 . 0290

9 .00126 .0202 .160 .0414 .0310

10 .00132 .0211 . 170 . 0432 . 0324

11 . 00140 .0224 .180 .0458 .0343

12 . 00148 .0237 . 190 .0484 .0362

14 . 00164 .0262 .210 .0534 .0400

16 . 00180 . 02S8 .230 .0586 .0439

18 .00196 .0314 .250 .0638 .0478

20 .00212 .0339 .270 .0688 .0516

22 . 00226 .0362 .290 .0734 .0550

24 . 00242 .0387 .310 .0784 .0588 26 . 00258 .0413 .330 .0836 .0626

28 . 00274 .0438 .350 .0886 .0664 30 . 00288 .0461 .370 .0932 .0698

1 Equivalent angular variation expressed as a decimal part of one turn.

2 Maximum amount it is possible for new working plug and ring gages wbich come within the specified tolerances to vary from being flush at the small end or at the gaging notch when screwed together tight by

hand (2 times column 3 + 0.0010 inch)

3 Maximum amount it is possible for new working plug or ring gages which come within specified toler- ances to vary from being flush at the small end or at the gaging notch when screwed on reference gage tight bvhand J Column 5- Table 77+ column 5, Table 78. \

Table 79.Corrections in diameter for errors in half angle, national (American

Briggs') taper pipe thread gages

Error in half angle of thread in minutes, o'

Correction in diameter, E"

8 threads

per inch

ll*A threads per inch

14 threads per inch18 threads

per inch

27 threads per inch

1 2 3 4 5 6

1

Inch

0. 00006

.00011 .00017

. 00022

.00028

. 00034

. 00039

. 00045

. 00050

. 00056

Inch

0. 00004

. 00008 .00012

.00016

. 00019

. 00023

. 00027

.00031 .00035 .00039

Inch

0. 00003 .00006 .00010

.00013

. 00016

.00019

. 00022

. 00026 .00029

.00032

Inch

0. 00002

. 00005

. 00007 .00010

. 00012

.00015

.00017

. 00020

. 00022

. 00025

Inch

0. 00002

. 00003

.00005

. 00007 .00008

.00010 .00012

.00013 .00015

.00017

2.. 3--

4

5

6--

7-

8-9..

10
»

136 NATIONAL SCREW THREAD COMMISSION

Table 79.Corrections in diameter for errors in half angle, national (American

Briggs') taper -pipe thread gages—Continued

Correction in diameter, E"

Error in half angle of thread in minutes, a' 8 threads 11)4 threads 14 threads 18 threads 27 threads

per inch per inch per inch per inch per inch

n ia

J4

_ CO

Inch Inch Inch Inch Inch

11 0. 00062 0. 00043 0. 00035 0. 00027 0. 00018

12 .00067 . 00047 . 00038 .00030 .00020

13 .00073 .00051 .00042 . 00032 .00022

14. .00078 .00054 . 00045 . 00035 .00023

i f\

. 00084 . uuuos nnnQ7 . UliUo / . 00025

16 . 00089 .00062 . 00051 .00040 . 00027

17 . 00095 .00066 . 00054 . 00042 .00028

18 _ .00101 .00070 .00058 .00045 .00030

19 .00106 .00074 .00061 .00047 .00031 20.. . 00112 . 00078 . 00064 .00050 .00033

21 .00117 .00082 .00087 . 00052 .00035

22 .00123 .00086 .00070 . 00055 .00036

23 .00129 . 00089 .00074 .00057 .00038

24 .00134 . 00093 . 00077 .00060 .00040 25. .00140 .00097 . 00080 .00062 .00041

26. - — .00145 . 00101 . 00083 . 00065 .00043

27 .00151 .00105 .00086 .00067 .00045

28 .00157 .00109 .00089 .00070 .00046 29 _ _ .00162 .00113 . 00093 .00072 .00048 30 .00168 .00117 .00096 .00075 .00050 45— . 00252 . 00175 .00144 .00112 . 00075 60 — — .00336 . 00233 .00192 . 00149 .00099

a' =error in half included angle of thread. E"=correction in diameter.

1.53812^. , E = Xtan a'-

Table 80.Corrections in diameter for errors in lead

Error in Correction in diameter, E'

ieaa in inches, p'

0. 00000 0. 00001 0. 00002 0. 00003 0. 00004 0.00005 0. 00006 0.00007 0. 00008 0. 00009

1 2 3 4 5 G 7 8 9 10 11

Inch Inch Inch Inch Inch Inch Inch Inch Inch Inch

0.00000 0. 00000 0. 00002 0. 00003 0. 00005 0. 00007 0. 00009 0. 00010 0. 00012 0. 00014 0. 00016

.00010 .00017 . 00019 . 00021 .00023 . 00024 . 00026 .00028 . 00029 . 00031 . 00033

.00020 . 00035 . 00036 .00038 . 00040 .00042 .00043 . 00045 .00047 . 00048 .00050

.00030 . 00052 .00054 .00055 . 00057 . 00059 .00061 . 00062 . 00064 .00066 .00068

.00040 . 00069 .00071 . 00073 . 00074 . 00076 . 00078 . 00080 .00081 .00083 . 00085

.00050 . 00087 .00088 .00090 . 00092 . 00094 . 00095 . 00097 . 00099 .00100 .00102

.00060 .00104 .00106 . 00107 . 00109 .00111 .00113 .00114 .00116 .00118 .00120

.00070 .00121 . 00123 . 00125 .00126 . 00128 . 00130 . 00132 . 00133 .00135 . 00137

.00080 . 00139 . 001 10 . 00112 .00144 .00145 . 00147 .00149 .00151 . 00152 . 00154

.00090 .00156 .00158 . 00159 . 00161 . 00163 .00165 .00166 . 00168 .00170 .00171

.00100 .00173 . 00175 . 00177 . 0017S .00180 . 00182 . 00184 . 00185 . 00187 . 00189

.00110 .00191 . 00192 .00194 .00196 .00197 . 00199 .00201 . 00203 . 00204 .00206

.00120 . 00208 .00210 .00211 . 00213 .00215 . 00217 .00218 . 00220 . 00222 . 00223

.00130 . 00225 . 00227 . 00229 . 00230 . 00232 . 00234 . 00236 . 00237 . 00239 . 00241

.00140 .00242 .00244 . 00246 . 00248 . 00249 . 00251 . 00253 . 00255 . 00256 . 00258

.00150 . 00260 . 00262 .00263 .00265 . 00267 . 00268 . 00270 . 00272 . 00274 . 00275

.00160 . 00277 . 00279 . 00281 .00282 . 00284 . 00286 .00288 . 00289 . 00291 . 00293

.00170 .00294 . 00296 . 00298 . 00300 .00301 . 00303 . 00305 . 00307 . 00308 . 00310

.00180 .00312 .00313 .00315 .00317 .00319 . 00320 . 00322 . 00324 .00326 . 00327

.00190 . 00329 . 00331 . 00333 . 00334 . 00336 . 00338 .00339 .00341 . 00343 .00345

.00200 . 00346 . 00348 . 00350 . 00352 . 00353 . 00355 .00357 . 00359 .00360 .00362

p'=error in lead. £'= correction in diameter. £'=1.732p'
1924 REPORT

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138 NATIONAL SCREW THREAD COMMISSION
1924 REPORT 139

SECTION VII. WOOD SCREWS

This specification summarizes the results of the standardization of

wood screws by the manufacturers in cooperation with the Bureau of

Standards and the technical committee on builders' hardware of the

Federal Specifications Board. It has been officially adopted by the

Federal Specifications Board for the use of all departments and independent establishments of the Government in the purchase of

wood screws, and published as Circular No. 140 of the Bureau of

Standards.

The former difference of 0.013165 inch in diameter, used as a basis

for the arithmetical progression of the numbering system, has been

discarded in favor of a difference of 0.013 inch (even) as established

by the commission for machine screws. This provides interchangeability for the numbered sizes of machine screws and wood screws

in connection with articles that may be fastened either to metal or wood.

Flat, round, and oval head types are covered in the specification.

The numbered sizes of wood screws run consecutively from No.

(0.060 inch, or 1.5 mm) to No. 24 (0.372 inch, or 9.4 mm), omitting

Nos. 13, 15, 17, 19, 21, 22, and 23.

The number of sizes of brass and steel screws manufactured as standard have been reduced from 555 to 291, a reduction of 47 per

cent.

1. GENERAL SPECIFICATIONS

(a) Material and Workmanship.—Screws shall be made of steel or brass, as specified, and shall be free from any defects which would

affect their serviceability.

(&) Points.—Standard screws shall be furnished with gimlet points.

Cone and diamond pointed screws are special.

(c) Types.—Screws shall be furnished in flat, round or oval

heads as ordered. (See fig. 37.)

(d) Measurements of Lengths.—The length of all screws shall

be measured from the largest diameter of bearing surface of the head

to the extreme end of the point measured parallel to the axis of the

screw.

(e) Threaded Length.—Screws shall be threaded approximately

two-thirds of the nominal length.

(/) Measurement of Diameters.—The diameter shall be measured on the body of the screw under the head.

ig) Tolerance on Diameter.—The maximum variation on

diameter permitted is +0.004 and —0.007 inch, or +0.1 and —0.2

mm.
140 NATIONAL SCREW THREAD COMMISSION"

(h) Tolerance on Number of Threads per Inch.—The

maximum variation in the number of threads per inch permitted is plus or minus 10 per cent.

HEAD DIAMETER : MA X. B * Z.OAA- .0 03

THICKNESS OF HEAD : MAX.C-.SI9A-.OOt

WIDTH OF SLOT i MAX.D = .I8£A t.020

DEPTH OF SLOT : MAX.E. C ,Z.S6A -.OOZ.

HEAD DIAMETER: MA*.. 6* 2. 04- A. . OOS

THICKNESS OF HEAD: MAX. C >= .619 A - .OOl

WIDTH OF SLOT: MAX.D = .1 S2.A +.0"2.0 DEPTH OF SLOT: MAX.E = .592. A -v.OOl HEIGHT OF OVAL. ". MAX.H = 304A -t.OOS

f MAX ,C OF FLAT HEAD

HEIGHT OF HEAD: MAXF'W.MAX.H

y.SZ^ A-KOOI

Fig.

HEAD DIAMETER •• MAX .B * 1-667 A

THICKNESS OF HEAO*. MAX .C - ,6 3SA* .OlS

WIDTH OF SLOT : MAX.De.16'2.A f-O-ZO

DEPTH OF SLOT: MAX.E - .3<oZA* .0»7 5HAPE OF HEAD : SEMI -ELLIPSE.

37.

Wood screws

(i) Included Angle.—The included angle of the head on flat and oval head screws shall be 82°.
1924 REPORT 141

2. THREAD SERIES

The numbered screw sizes, basic and limiting diameters, and

threads per inch given in Table 82 are standard.

Table 82.

National wood screw standard, size numbers, diameters, and pitches

±\ LI 111 iJ\T'L \J I 1 < >s Threads

per inch

Diameter

Basic Maximum Minimum Basic Maximum Minimum

1 1 3 1 5 6 7 8

Inch Inch Inch mm mm mm

32 0. 060 0. 064 0. 053 1.5 1.6 1.3

1 28 .073 .077 .066 1.9 2.0 1.7 2 _ 26 .086 .090 .079 2.2 2.3 2.0 3.,. 24 .099 .103 .092 2.5 2. 6 2.3

4 22 .112 .116 . 105 2.8 2.9 2.6

5- 20 .125 .129 .118 3.2 3.3 3.0

6 18 .138 . 142 .131 3.5 3.6 3.3

7 _ 16 .151 .155 . 144 3.8 3.9 3.6

8 15 .164 .168 .157 4.2 4.3 4.0

9 14 .177 .181 .170 4.5 4.6 4.3

10 13 .190 .194 .183 4.8 4.9 4.6

11 12 .203 .207 .196 5.2 5.3 5.0

12 _ 11 .216 .220 .209 5.5 5.6 5.3

14 10 .242 .246 .235 6.6.2 5.9

16 9 .268 .272 .261 6.8 6.9 6.6

18 8 .294 .298 .287 7.5 7.6 7.3

20 8 .320 .324 .313 8.1 8.2 7.9

24 7 .372 .376 .365 9.4 9.5 9.2

3. TOLERANCES ON LENGTH

(a) Flat and Oval Head Screws.—The maximum variations

permitted in the length of flat and oval head screws are given in

Table 83.

(&) Round Head Screws.—The maximum variations permitted

in the length of round head screws are given in Table 84.

Table 83.

Tolerances on length of flat and oval head screws 1

Nominal length in inches Minus

tolerance Nominal length in inches Minus

tolerance

1 2 1 2

K

Inch

0.031 .033

.035

.037

.039

.041

.043 .048

.052

.056

2 .'. Inch

0.060

.064 .068

.072

.076

.084

.092

. 101

. 109

'A- - —

V2 lYi —

% 2%

>A - - — 3

Vs 3H-— - 1 4 VA 4'A VA - - 5 I'A

1 Plus tolerance=0. 16802°—25t 10
142 NATIONAL, SCREW THREAD COMMISSION

Table 84.—Tolerances on length of round head screws 1

Nominal length in inches

Screw numbers

1 2 3 4 5 6 7 8

Inch

0. 064 .065

Inch

0. 071 .073

.075

Inch

0. 077 .079

.081 .083 .085

Inch

0. 084 .086

.088

.090

.092

.094 .096

Inch

0. 090

.092

.094 .096

.098

.100

. 102

. 106

. 110

Inch Inch Inch Inch

*A-- - 0.099

.101 .103

. 105

. 107

. 109 .113

.117

6. Vol .107

. 109

. Ill

.113

.115

.119

. 123

. 127 .131

. 135 .139

0. 112

. 114 .116

.118

. 120

. 122

. 126 .130

.134

. 138

. 142 .146

0. 118

. 120

. 122 .124

. 126

. 128 .132

. 136

. 140

. 144 .148 .152

Yl% —-

SA

Vs- -

11M - m— - m —

22M-- — —

2y2 -—-

Nominal length in inches

Screw numbers

9 10 11 12 14 16 18 20 24

k- — Inch

0. 127

. 129 .131

. 133 .135

. 139

.143

. 147

. 151

.155

. 159

. 163 .167

Inch

0. 133

. 135 .137

. 139 .141

. 145

. 149

. 153 .157

. 161

. 165

. 169

. 173

. 181

Inch Inch Inch Inch Inch Inch Inch

Vs- - 0. 142

. 144

". 146

. 148

.152

. 156

. 160

. 164

.168

. 172

. 176 .180

.188

0. 148

. 150

. 152 .154

. 158

. 162

. 166 .170

. 174 .178

.182

. 186

.194 .202

- 0. 163

. 165

. 167

. 171 .175

.179

.183

.187

. 191

. 195 .199

.207

.215

.223

.231

Vs--

l-_ 0. 180

.184 .188

. 192

. 196

.200

.204 .208

.212

.220

.228

.236

.244

M— 0. 198 .202

.206 .210

.214 .218

.222

.226

.234

.242

.250

.258

Wv-- - - 0.215

.219

.223

.227

. 231 .235 .239

.247

.255

.263

.271

XH- - 2--

2H- — 2

3A--

3._ 0. 265

.273

.281 .289

.297

3H4..

5_ _•_

1 Plus tolerance=0. Minus tolerances as given in body of table.

4. STANDARD SIZES OF WOOD SCREWS

(a) Steel Screws.—The standard sizes of steel screws are given

in Table 85.

(6) Brass Screws.—The standard sizes of brass screws are given

in Table 86.
1924 REPORT

Table 85.Standard sizes of steel screws

143

Lengths

No. 0, diameter, 0.060 inch,

1.5 mm

No. 1, diameter, 0.073 inch,

1.9 mm

No. 2, diameter, 0.086 inch,

2.2 mm

No. 3, diameter, 0.099 inch,

2.5 mm

No. 4, diameter, 0.112 inch,

2.8 mm

No. 5, diameter, 0.125 inch,

3.2 mm

No. 6, diameter, 0.138 inch,

3.5 mm

No. 7, diameter, 0.151 inch,

3.8 mm

No. 8, diameter, 0.164 inch,

4.2 mm

Inches

V*

ViA

%

%

Vs

1V4, m

m

2m

3

771771 6.4

9.5 12.7

15.9

19.1

22.2

25.4 31.8

38.1 44.5

50.8

57.2

63.5

69.9

76.2

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

x

X

X

X X

X X

V V

X

X

X

X

X

X

X

X • X

X

X

X

X

X

X

X

X

X

X

X

Lengths

No. 9, diameter, 0.177 inch,

4.5 mm

No. 10, diameter, 0.190 inch,

4.8 mm

No. 11, diameter, 0.203 inch,

5.2 mm

No. 12, diameter, 0.216 inch,

5.5 mm

No. 14, diameter, 0.242 inch,

6.1 mm

No. 16, diam

eter,

0.268. inch,

6.8 mm

No. 18, diameter, 0.294 inch,

7.5 mm

No. 20, diameter, 0.320 inch,

8.1 mm

No. 24, diameter, 0.372 inch,

9.4 mm

Inches %

%

1

V/t m

V4. VA

2%

3VA

i

5

77177112.7 15.9

19.1 22.2

25.4

31.8

38.1 44.5

50.8

57.2

63.5 69.9

76.2

88.9

101. 6 114.3 127.0

X

X • X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X
144 NATIONAL SCREW THREAD COMMISSION

Table 86.Standard sizes of brass screws

Lengths

Inches %

A

%

im

VA

mm

6.4

9. 5

12. 7

15. 9 19.1

22.2

25. 4

31. 8

38.

No. 0, diameter,

0.060

inch, 1.5 mm

X

X

No. 1, diameter,

0.073

inch, 1.9 mm

X

X

X

No. 2, diameter,

0.086

inch, 2.2

X

X

X

X

X

No. 3, diameter,

0.099

inch, 2.5 mm

X

X

X

X

X

No. 4, diameter,

0.112 inch, 2.8 mm

X

X

X

X

X

X

X

No. 5, diameter,

0.125

inch, 3.2 mm

X

X

X

X

X

X

No. 6, diameter,

0.138

inch, 3.5 mm

X

X

X

X

X

X

X

X

No. 7, diameter,

0.151 inch, 3.8 mm

X

X

X

X

X

X

X

Lengths

No. 8, diameter,

0.164

inch, 4.2 mm

Inches A

%

H

%

lIX

w2

2J4 2A

3

mm

12.7

15. 9

19.

22. 2 25.4

31.8

38.

44. 5 50.8

57.2

63. 5

76. 2 88.9

X

X

X

X

X

X

X

X

X

No. 9, diameter,

0.177

inch, 4.5

X

X

X

X

X

X

X

X

No. 10, diameter,

0.190

inch, 4.8 mm

X

X

X

X

X

X

X

X

X

X

No. 11, diameter.

0.203 inch, 5.2 mm

X

X

X

X

X

X

X

X

X

No. 12, diameter

0.216 inch, 5.5 mm

X

X

X

X

X

X

X

X

X

X

X

No. 14, diameter,

0.242

inch, 6.1

X

X

X

X

X

X

X

X

No. 16, diameter,

0.268

inch, 6.8

X

X

X

X

X

No. 18, diameter,

0.294

inch, 7.5 mm

X

X

X

X

X
APPENDIX 1. DERIVATION OF TOLEPvANCES

1. PITCH DIAMETER TOLERANCES

(a) Tolerances for Fastening Screws.—The tolerances for fastening screws specified in Section III were arrived at by combining two factors, known

as the net pitch diameter tolerance and the gage tolerance. The theoretical

net tolerances for all screws and nuts of a given class of fit bear a definite mathematical relationship to each other, and it was intended that these should in no way be reduced by permissible manufacturing tolerances for master gages; that

is, gages within class X tolerances. Consequently the net tolerances were

increased by the equivalent diametrical space required to provide for the class X

tolerances on diameter, lead, and angle, to produce the extreme tolerances

specified for the product. In practice, the actual net tolerances will depend

upon the method of gaging and upon the accuracy of the gages used.

1. Basis of net tolerances.—The net pitch diameter tolerances for the various

classes of fit are based on the following series for a pitch of fa inch:

Pitch diameter tolerances for pitches finer than -fa inch are to each other

and to the tolerance for -fa inch as the 0.6th power of their respective pitches.

Pitch diameter tolerances for pitches coarser than fa inch are to each other

and to the tolerance for -fa inch as the 0.9th power of their respective pitches.

The exponent 0.6 was chosen for pitches finer than fa inch because the resulting tolerances, except in two instances, do not vary more than 0.0001 inch from

the pitch diameter tolerances specified in the A. S. M. E. Machine Screw Standard.

2. Gage tolerance.—The gage tolerance to be added to the net tolerance to

obtain the extreme tolerance, which determines the absolute limits within which

all variations of the work must be kept, is determined as follows:

Add together the following:

Pitch diameter tolerance of "go" gage, Diametrical equivalent of lead tolerance of "go" gage, Diametrical equivalent of angle tolerance of "go" gage, Pitch diameter tolerance of "not go" gage. Then subtract the following from the above sum:

One-half diametrical equivalent of lead tolerance of "not go" gage. Diametrical equivalent of angle tolerance of "not go" gage.

(b) Tolerances for Screw Threads of Special Diameters, Pitches,

and Lengths of Engagement.—As stated in Section IV, the pitch diameter

tolerances for special sizes of threads of national form are obtained by adding

three values, or increments; one dependent upon the basic major diameter,

another upon the length of engagement, and the third upon the pitch. The

formulas from which increments are derived are such that if used to derive

tolerances for threads having a length of engagement equal to one diameter

and falling within the regular series of sizes and classification of fits given in

Section III, the resulting values will closely approximate the extreme tolerances

specified in Section III. These formulas are summarized in Table 87.

Class 3, medium fit Class 4, close fit

Class 1, loose fit Class 2, free fit_

Inch

0. 0045

. 0030

. 0020

. 0010

145
146 NATIONAL SCEEW THREAD COMMISSION

Table 87.

Schedule of tolerance increments for special threads

Special classification of fit Corresponding regular classification Diameter

increment

Length of engagement

increment

Pitch in- crement

1 2 3 1 5

Class A Class 1, loose fit 0.002-vTd .002-/D

.002V-D

.002-Vo .001V0

0. 002 Q

.002Q

.002Q

.002Q

.001Q

0.020 4t>

.020 Vp

.010 Vp

.005 V V .0025Vp

Class B Class 1, loose fit (without allowance).-

Class C Class 2, free fit Class T> Class 3, medium fit-- Class E Class 4, close fit

2. RELATION OF LEAD AND ANGLE ERRORS TO PITCH DIAMETER

TOLERANCES

It has been stated in various sections of the report that the tolerances specified

for pitch diameter include all errors of pitch diameter, lead, and angle. Also,

there were tabulated the errors in lead and angle, each of which could be compensated for by one-half of the specified pitch diameter tolerances. These

equivalents were derived from definite mathematical relations which are given

below. A rigorous mathematical analysis upon which these formulas are based

is presented in Appendix 3 of Letter Circular No. 23, issued by the Bureau of

Standards.

(a) Diameter Equivalent of Lead Error.—The formula expressing the

relation between lead error between any two threads within the length of engagement and its diameter equivalent is as follows:

S' = (±p') cot a, in which

E' =pitch diameter increment due to lead error,

p' = the maximum lead error between any two of the threads engaged,

a =half-angle of thread.

The quantity E' is always added to the measured pitch diameter in the case of

external thread, and it is always subtracted in the case of an internal thread, re- gardless of the sign introduced by the lead error p'.

For threads of national form, the above formula reduces to:

£' = 1.7321 p'

(6) Diameter Equivalent of Angle Error.—The general formula expressing

the relation between error in the half angle of thread and its diameter equivalent;

that is, the amount of the pitch diameter tolerance absorbed by such an error is:

cot a'- h

E" sin a cos a ±cot a

in which

E" =pitch diameter increment due to error in half-angle,

h = basic thread depth, a =basic half-angle of thread,

a' =error in half-angle of thread.

The sign of cot a is plus when the half angle of thread is less than nominal, and

minus when the half angle is greater than nominal. By omitting ± cot a from

the formula an approximate mean value for a' or E" is obtained which differs

»
1924 REPOKT

» 147

very little from either extreme value. The commission has, therefore, adopted

for general use the formula:

. , h cot a =-577—= E sin a cos a

For threads of national form this formula reduces to:

or

cot a =2^r,

3 E"=~2 V *an «'

For the form of thread recommended for pipe-thread gages the formula becomes:

,_1.53812 p W

COt CLOT „„ 1.53812 E = tan a

APPENDIX 2. WIRE METHODS OF MEASUREMENT OF

PITCH DIAMETER

Throughout this report emphasis has been placed on pitch-diameter tolerances

and limits, as upon these the fit of a screw thread largely depends. The maintenance of these tolerances and limits requires the use of limit thread gages, and

these, in turn, depend upon the absolute values or measurements of master

gages. The measurement of pitch diameter presents certain difficulties which

may result in an uncertainty as to its true value. The adoption of a uniform

practice in making such measurement is, therefore, desirable. The so-called

"three-wire method" of measuring pitch diameter, as here outlined, has been

found to be the most accurate and satisfactory when properly carried out, and

is recommended for universal use in the direct measurement of thread plug gages.

1. SIZE OF WIRES

In the three-wire method of measuring pitch diameter small hardened steel cylinders or wires of correct size are placed in the thread groove, two on one

side of the screw and one on the opposite side, as shown in Figure 38. The

contact face of the micrometer anvil or spindle over the two wires must be

sufficiently large in diameter to touch both wires; that is, it must be equal to or greater than the pitch of the thread. It is best to select wires of such a size that they touch the sides of the thread at the mid slope, for the reason that the

measurement of pitch diameter is least affected by any error in thread angle

which may be present when such size is used. The size of wire which touches

exactly at the mid slope of a perfect thread of a given pitch is termed the "bestsize" wire for that pitch. Any size, however, may be used which will permit

the wires to rest on the sides of the thread and also project above the top of

the thread.

The depth at which a wire of given diameter will rest in a thread groove depends

primarily on the pitch and included angle of the thread; and secondarily, on the

angle made by the helix, at the point of contact of the wire and the thread, with

a plane perpendicular to the axis of the screw. Inasmuch as variation in the

helix angle has a very small effect in determining the diameter of the wire which

touches at the mid slope of the thread, and as it is desirable to use one size of
148 NATIONAL SCREW THREAD COMMISSION

wire to measure all threads of a given pitch and included angle, the best size wire is taken as that size which will touch at the mid slope of a groove cut

around a cylinder perpendicular to the axis of the cylinder, and of the same

angle and depth as the thread of the given pitch. This is equivalent to a thread

of zero helix angle. The size of wire touching at the mid slope, or "best-size"

wire, is given by the formula:

r

flficeoniEree SptNDt.£ »

YAltCRomiT£R Anvil'

Fig. 38.—Three-wire method of measuring pitch diameter of thread plug gages

in which

This formula reduces to:

G = 2

sec a

G=diameter of wire

p =pitch a = % included angle of thread.

G=0.57735 Xp, for 60° threads.

It is frequently desirable, as, for example, when a best-size wire is not available, to measure pitch diameter by means of wires of other than the best size. The minimum size which may be used is limited to that permitting the wire to project above the crest of the thread, and the maximum to that permitting the

wire to rest on the sides of the thread just below the crest, and not ride on the

crest of the thread. The diameters of the best size, maximum, and minimum

wires for "national coarse " and "national fine" threads are given in Tables 88

and 89.
1924 REPORT 149

2. SPECIFICATION FOR WIRES

A suitable specification for wires is as follows

1. The wires should be cylinders of steel with working surfaces glass hard and

accurately finished.

2. The working surface should be about 1 inch in length, and the wire should

have a suitable handle which is provided at one end with an eye or other suitable means of suspension.

3. One side of the handle, which should be flattened, should be marked with

the pitch for which the wire is the best size, and with the diameter of the working

part of the wire.

4. The wire should be round within 0.00002 inch and should be straight to 0.00002 inch over any quarter-inch interval.

5. One set of wires should consist of three wires which should have the same

diameter within 0.00003 inch, and this common diameter should be within

0.0001 inch of that corresponding to the best size for the pitch for which the wire

is to be used.

3. METHODS OF MEASURING AND USING WIRES

In order to measure the pitch diameter of a screw-thread gage to an accuracy

of 0.0001 inch by means of wires, it is necessary to know the wire diameters to 0.00003 inch. The micrometer to be used for measuring wires should be one which is graduated to ten-thousandths of an inch and upon which hundredthousandths of an inch can be estimated. Such micrometers are available in various forms of precision bench micrometers, and measuring machines. Care

should be taken to make sure that the measuring faces of the micrometer are

flat and parallel to within 0.00002 inch. The taper of wires can best be determined by measuring between a flat micrometer contact and a cylindrical anvil.

Any pits or worn spots on the wires can be detected with the same arrangement.

Variations in roundness and straightness are usually determined by rotating the

wire between flat contacts one-fourth inch in diameter. However, one form of

variation in roundness can only be detected by rotating the wire in a V groove

against a flat micrometer contact. The V groove may be the thread space in a hardened and well-finished thread plug gage.

The contact pressure used in making measurements is also an important

factor, since the wires, when in use, rest on the sides of the thread, and a given

pressure exerted on the top of the thread has a magnified effect in distorting the

wire and causing the measurement of the pitch diameter to be slightly less than

it should be. In making measurements over the wires inserted in the thread

groove, it has been common shop practice to hold the wires down into the thread

by means of elastic bands. This has a tendency to prevent the wires from

adjusting themselves to the proper position in the thread grooves; thus a false measurement is obtained. In some cases it has also been the practice to support

the screw being measured on two wires, which are in turn supported on a horizontal surface, and measuring from this surface to the top of a wire placed in a

thread over the gage. If the screw is of large diameter, its weight causes a

distortion of the wires and an inaccurate reading is obtained. For these reasons these practices should be avoided and subsidiary apparatus for supporting the

wires and micrometer should be used.

A contact pressure of 2 pounds is recommended in making wire measurements

of screw-thread gages. A skilled inspector naturally uses a light pressure or "feel" in making wire measurements by means of a micrometer caliper. The

actual contact pressure used by a group of inspectors was found to vary from

X to 3 pounds. The effect of variation in contact pressure in measuring threads
150 NATIONAL SCBEW THREAD COMMISSION

of fine pitches is indicated by the difference in readings obtained with 2 and 5 pounds pressure on a 24-pitch thread plug gage. The reading over the wires

with 5 pounds pressure was 0.00013 inch less than with 2 pounds pressure. Measurements of a thread plug gage made in accordance with these instructions, with wires which conform to the above specifications, should be accurate

to 0.0001 inch. If the diameters of the wires are known only to an accuracy of

0.0001 inch, an accuracy better than 0.0003 inch in the measurement of pitch

diameter can not be expected.

4. MEASUREMENT OF PITCH DIAMETER OF NATIONAL STRAIGHT

THREADS

The general formula for determining the pitch diameter of any thread whose

sides are symmetrical with respect to a line drawn through the vertex and per- pendicular to the axis of the thread, in which the very slight effect of helix angle is not taken into account, is

15E=M+^^-G (1 + cosec a)

in which

E=pitch diameter

M=measurement over wires

a =one-half included angle of thread n =number of threads per inch

G= diameter of wires.

This formula differs from those given in most engineering handbooks in that the

latter, as generally given, yield a result which should check with the major

diameter of the screw measured, while the pitch diameter itself is not mentioned.

For a 60° thread of correct angle and thread form this formula simplifies to

£=M+0^60_3_3(? n

For a given set of best-size wires

E=M-X

when

cot a X= G (1 + cosec a) - 2n

The quantity X is a constant for a given thread angle, and, when the wires are used for measuring threads of the pitch and angle for which they are the best size,

15 The general formula, in which the helix angle is taken into account, is:

cot a S2 E=M-\—^—G (1+cosec a+y cos a cot a)

in which S=tangent of the helix angle. The value of S, the tangent of the helix angle, is given by the formula:

s L 1 3.1416 £ 3.1416 NE

in which

i=lead

iV=number of turns per inch .E=nominal pitch diameter.

In commercial practice the term ( -~- cos a cot a ) is neglected, as its value is small, being in all cases less

than 0.00015 inch for standard fastening screws when the best-size wire is used, and the above formula takes the simplified form given above. This practice is permissible provided that it is uniformly followed, and in order to maintain uniformity of practice, and thus avoid confusion, the Bureau of Standards uses the latter (GS2 \

cos o cot a jexceeds 0.00015 inch, as in the case of Acme

and multiple threads, or other threads having exceptionally large helix angles.
1924 REPORT 151

the pitch diameter is obtained by the simple operation of subtracting this constant

or factor from the measurement taken over the wires. In fact, when best-size

wires are used, this factor is changed very little by a moderate variation or error

in the angle of the thread. Consequently, the factors for the various sets of wires

in use may be tabulated, thus saving a considerable amount of time in the inspection of gages. However, when wires of other than the best size are used, this

factor changes quite appreciably with a variation in the angle of the thread.

It has been shown that, with the exception of coarse pitch screws, variation in angle from nominal value causes no appreciable change in the quantity X for the

best-size wires. On the other hand, when a wire near the maximum or minimum

allowable size is used, a considerable change occurs, and the values of the cotangent

and cosecant of the actual measured half angle are to be used. It is apparent,

therefore, that there is a great advantage in using wires very closely approximating the best size. For convenience in carrying out computations, the values of

for standard pitches are given in Table 88.

5. MEASUREMENT OF PITCH DIAMETER OF NATIONAL TAPER

THREADS

The pitch diameter of a taper thread plug gage is measured in much the same manner as that of a straight thread gage, except that a definite position at which

the measurement is to be made must be located. A point at a known distance

L from the end of the gage is located by means of a combination of precision gage

blocks and the cone point furnished as an accessory with these blocks, as shown in

Figure 39 at A. The gage is set vertically on a surface plate, the cone point is placed with its axis horizontal at the desired height, and the plug is turned until

the point fits accurately into the thread. The position of this point is marked by

placing a bit of Prussian blue or wax immediately above it. Measurement is made over the wires in the usual manner, but care must be taken that the contact

surfaces of the micrometer make contact with all three wires, since the micrometer

is not perpendicular to the axis of the screw when there is proper contact. See

Figure 39. On account of this inclination, the measurement over the wires must

be multiplied by the secant of the half angle of the taper of the thread. The

formula for the pitch diameter of any taper thread plug gage, the threads of which

are symmetrical with respect to a line perpendicular to the axis, then has the

form: 16

E=M sec y+

C 2^a ~£r (1+cosee a)

in which

E=pitch diameter

M=measurement over wires

y—half angle of taper of thread n =number of threads per inch=l/p

a

half angle of thread

G=diameter of wires.

Thus the pitch diameter of a "national (American Briggs') standard pipe thread

gage" having correct angle (60°) and taper (M inch per foot) is then given

by the formula:

#=1.00048 M+0.86603 p-3G

16 See footnote 15.
152 NATIONAL SCREW THREAD COMMISSION

The pitch diameter at any other point along the thread, as at the gaging

notch, is obtained by multiplying the distance parallel to the axis of the thread,

between this point and the point at which the measurement was taken, by the

taper per inch, then adding the product to or subtracting it from the measured

pitch diameter according to the direction in which the second point is located

with respect to the first. The following method, illustrated in Figure 40, has a theroretical advantage

over the first method in that it is independent of the taper of the thread, and,

therefore, requires less computation; or if the taper is not measured but assumed

to be correct, it is more accurate. The axis of the gage and the line of

Fig. 39.

Measurement of pitch diameter of taper thread gages by the three-wire

method

measurement are constrained perpendicular to each other. This is easily done

on a measuring machine if the gage is supported on centers mounted on a slide

whose ways are perpendicular to the line of measurement. If a micrometer

caliper is used, its spindle is constrained perpendicular to the axis of the screw. One method is to place the gage on a surface plate with its axis vertical, and

support the micrometer in a horizontal position with its anvil and spindle resting on two equal combinations of gage blocks as shown in Figure 40 at A. A single

wire is inserted in the thread at the point located as in the previous method, and

one other wire is placed in the upper thread on the opposite side. A measure-
J*

1924 REPORT 153

ment is taken over the two wires; the second wire is then moved to the thread

immediately below, and a second reading is taken. The mean of these two

readings is substituted in any of the above formulas.

6. MEASUREMENT OF PITCH DIAMETER OF THREAD RING GAGES

The application of direct methods of measurement to determining the pitch

diameter of thread ring gages presents serious difficulties, particularly in securing

Fig. 40.

Measurement of pitch diameter of taper thread gages by the two-wire

method

proper contact pressure when a high degree of precision is required. The usual

practice is to fit the ring gage to a master setting plug. When the thread ring

gage is of correct lead, angle, and thread form, within close limits, this method is quite satisfactory and represents standard American practice. It is the only

method available for small sizes of threads. For the larger sizes, various more

or less satisfactory methods have been devised, but none of these have found

wide application.
154 NATIONAL SCREW THREAD COMMISSION

Table 88.

Wire sizes and constants, national coarse and fine threads, and

national (American Briggs') pipe threads

Wire sizes * Threads

Pitch Pitch Depth of 2 V thread

per inch 1 cot 30° Best Maximum Minimum u I?= 1 A KTTQ Cf\/r\ U.O/ /doUp i.uwoodp U.oOolo^p 2 TM27» 2n

1 % 3 5 j

Inch Inch Inch Inch Inch

00722 0. 01263 0. 00631 80 0. 01250 0. 00625 0. 01083

. 00802 . 01403 . 00702 72 . 01389 .00694 . 01203

AAOAO A1 K"7C1

. 00789 64 . 01562 . 00781 . 01353

. 01031 . 01804 . 00902 56 . 01786 .00893 .01546

. 01203 . 02105 . 01052 48 .02083 . 01042 .01804

. 01312 . 02296 .01148 44 . 02273 .01136 . 01968

. 01443 . 02526 . 01263 40 .02500 . 01250 . 02165

. 01604 . 02807 . 01403 36 . 02778 . 01389 . 02406

. 01804 . 03157 . 01579 32 . 03125 . 01562 . 02706

. 02062 . 03608 .01804 28 . 03571 . 01786 . 03093

. 02138 . 03742 . 01871 27 .03704 . 01852 .03208

.02406 .04210 . 02105 24 .04167 . 02083 .036P8

. 05052 . 02526 OA . 05000 . 02500 . 04330

. 03208 . 05613 .02807 18 . 05556 . 02778 .04811

. 03608 . 06315 . 03157 16 . 06250 .03125 . 05413

.04124 . 07217 . 03608 14 . 07143 . 03571 . 06186

.04441 . 07772 . 03886 13 . 7692 . 03846 . 06662

.04811 . 08420 . 04210 12 .08333 .04167 . 07217

. 05020 . 08786 . 04393 11.5 . 08696 .04348 . 07531

. 05249 . 09185 . 04593 11 09091 .04545 . 07873

. 05773 .10104 . 05052 10 .10000 .05000 . 08660 .06415 . 11226 . 05613 9 .11111 . 05556 .09623

. 07217 . 12630 .06315 8 .12500 .06250 . 10825

. 08248 . 14434 . 07217 7 .14286 . 07143 . 12372 .09623 . 16839 . 08420 6 . 16667 .08333 .14434

.11547 . 20207 . 10104 5 .20000 . 10000 . 17321

. 12830 . 22453 . 11226 4.5 . 22222 .11111 .19245 .14434 . 25259 . 12630 4 .25000 . 12500 . 21651

1 These wire sizes are based on zero helix angle. Also maximum and minimum sizes are based on a width

of flat at the crest equal to % X p. The width of flat of "national pipe thread gages " is slightly less than

this, so that the minimum size listed is slightly too small for such gages. In any case, the use of wires of either extreme size is to be avoided.
1924 REPORT 155

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156 NATIONAL SCREW THREAD COMMISSION

APPENDIX 3. CONTROL OF ACCURACY OF THREAD ELEMENTS IN THE PRODUCTION OF THREADED PRODUCT

In order to maintain the dimensions of threaded product within the limiting

sizes specified, it is essential that the tools used and the processes applied be

suitable for the particular requirements. An analysis of the various factors

controlling the accuracy of the individual thread elements is here presented. In

this analysis, the fundamental factors controlling the accuracy of the elements of a screw thread are stated, and are followed by a brief discussion of the relationship

of these factors to each of the prevailing commercial methods of producing screw

threads. It is recognized, however, that certain varying factors are involved,

such as lubrication, method of holding the work or tool, sharpness of cutting

edges, etc., so that it is not always possible to predetermine the exact sizes of

the tools required to accomplish the desired results. Screw threads are usually produced either by cutting or rolling. Five general

methods of cutting, two of rolling, and two of finishing screw threads are in com- mon use.

1. FUNDAMENTAL FACTORS

The accuracy of the individual elements of a thread is controlled mainly as

follows:

Angle by the angle between, and contour of the cutting edges of the tool used

for cutting, or of the sides of the grooves of the die used for rolling.

Lead by the rate of the longitudinal motion of the tool with respect to the rate

of revolution of the part to be threaded.

Major diameter of external thread by the outside diameter of the stock, or by

the forming tool. Minor diameter of internal thread by the diameter of the hole in the work before

threading. In the case of a drilled hole, this depends on the diameter and accuracy

of grinding of the tap drill used.

Pitch diameter by the radial setting of the forming surface of the tool. Threadform by the form and position of the tool, and the conditions under which

it is used.

Inspection of the angle and profile of the thread-forming tool is essential to

control the accuracy of the thread produced. The same means and methods can be applied to such inspection as are applied in the measurement of screw thread

gages and threaded product. Attention is directed to the optical projection

apparatus for measuring angle and lead, and examining profile; the microscope,

which may be readily adapted to shop requirements; and indicating gages, which

may be designed to check the dimensions of threading tools. The sources of lead errors require special consideration and for this purpose the

methods of producing- screw threads may be considered under two headings,

namely, those in which relative longitudinal motion of the tool and product is controlled by means of a lead screw, and those in which the tool is self-leading.

(a) Tool Controlled by Lead Screw.—In cutting a thread on a lathe or other machine embodying a lead screw, using a single point cutting tool or single

milling cutter, progressive lead errors are caused by (1) a progressive lead error

in the lead screw; (2) lack of parallelism of the motion of the cutting tool, the

axis of the lead screw, and the axis of the part to be threaded; and (3) incorrect

ratio of the rate of revolution of the s pindle to that of the lead screw, due to an

incorrect or approximate combination of gears. Local lead errors are caused by (1) local lead errors in the lead screw; (2) lost motion in the action of the lead screw or connecting mechanism; (3) varying

frictional resistance in the mechanism; (4) when a live center is used, irregular
1924 REPORT 157

play of its spindle in the bearings; and (5) variations in the amount of metal

removed by the cutting tool. Periodic lead errors are caused by (1) periodic lead errors in the lead screw;

(2) eccentricity of motion of the lead screw; (3) thrust bearings of spindle or

lead screw running out of true; (4) variations in the spacing of gear teeth, or

eccentric gears or mountings; (5) when a live center is used, eccentricity of motion

of its spindle; and (6) periodic variations in the amount of metal removed, due

to lack of uniformity of the material in diameter, straightness, or physical properties. When a multiple-toothed threading tool is controlled by a lead screw, variations from correct spacing of the teeth of the tool are superimposed on the lead

errors resulting from any of the above causes in that portion of the thread not

passed over by every tooth of the tool. In the portion of the thread completely

passed over by the tool, the effect of the difference in lead between the tool and

lead screw is to produce a thin thread.

The simplest method of inspecting a machine tool to determine whether it will cut a screw thread within satisfactory limits is to cut carefully a sample

screw on the machine and measure the lead errors of the screw. The obvious

remedy for errors from such sources is the careful inspection of the various

elements of the machine, and correction of the errors thus located, either by

improving the design or by carefully refinishing or remaking the parts to a greater

degree of accuracy.

(b) Self-Leading Threading Tool.—When a thread is cut by means of a

tap or die, which, as ordinarily used, is self-leading and not controlled by a lead

screw, lead errors may occur as the result of: (1) Incorrect lead of the tap or die;

(2) too much or too little relief at the throat of the die or on the chamfer at the

end of the tap; (3) the setting of an adjustable die or tap chaser to cut a thread

considerably larger or smaller than that for which the tool was intended; that is, to cut a helix angle considerably different from the helix angle of the chaser;

(4) excessive resistance to longitudinal motion; (5) improper alinement of the

axis of the tap or die with that of the work, etc.; and (6) excessive angle relief. The accuracy of the lead of the tap or of the chasers in the die is the most

difficult of these sources of error to control, and indeed presents serious difficulties. There is, first, the difficulty of cutting a tap or chaser which is free from lead errors resulting from any of the causes outlined above; and second,

the distortion which the steel composing the tap or die undergoes in hardening.

In the inspection of such thread forming tools, practically the same means and

methods can be applied as in the measurement of screw-thread gages. For

checking the lead, indicating gages or some of the usual lead-measuring devices

for screw-thread gages may be used. To measure the lead of a die chaser, the

chaser must be held in a fixture in such a position that the direction of measure- ment corresponds to the direction of longitudinal motion of the chaser threads

when cutting a thread.

2. CUTTING OF SCREW THREADS

(a) Single-Point Tool.—A screw thread may be produced by traversing a

single-point threading tool—shaped to correspond to the shape of the thread

space in an axial plane, and so placed as to cut an angle, equal to the angle of

the top surface of the tool, in correct relation to the axis of the thread—along

the revolving part to be threaded at such a rate as to produce a thread of the

desired lead. This is the common method of cutting screws in an engine lathe; a lead screw driven by gearing being the ftsual means for imparting to the tool

the longitudinal motion at the desired rate. This method is used commercially

16802°—25t 11
158 NATIONAL SCREW THREAD COMMISSION

only when special conditions make it necessary, as when the thread to be cut is not standard, or it is not practicable to apply other methods.

Various forms of single-point cutting tools for cutting threads of national

form are illustrated in Figure 41, at A, B, C, and D. The circular tool shown

at C has the advantage that it can be reground indefinitely without destroying

its correct form. The diagram at D shows the method for calculating the angle x of the cutting tool, having a clearance angle v, in a plane perpendicular to the

edge MN; and the formula for determining the clearance angle v, of a tool for

cutting a thread of helix angle s, is also given. Such tools usually consist of hardened tool steel, ground to the correct form after hardening; special alloys

such as "stellite" are also used for this purpose.

tan DC — fh.n.CL secV

fan V = fay/ S coTau

D

Fig. 41.

Single point and multiple point thread cutting tools

(b) Thread Chaser.—A screw thread may be produced by successively

traversing a multiple-point thread tool, known as a chaser, along the part to be

threaded, each tooth following in the thread in the same manner as a single-point

thread tool. Two forms of chasers are shown in Figure 41 at E and F, the one

at F being especially suitable for cutting fine threads. Chasers are well adapted

to roughing out threads, as they cut rapidly, and may be used for finishing

threads accurately if the teeth are ground after hardening.

(c) Tap or Die.—A screw thread may be produced by using a tap for internal

threads or die for external threads. These tools occur in considerable variety in

their commercial forms, but consist essentially of a number of multiple-point

cutters or chasers, usually four, arranged circumferentially. They may be
1924 REPORT 159

either solid or adjustable, and collapsible or self-opening, respectively, for withdrawing quickly from the work after threading. By their use a thread is generally

finished by one passage of the tool, although a second or finishing cut is sometimes made to secure greater accuracy. Dies are applied, in general, to threading

screws, bolts, and studs; and taps to nuts or other internal threads within the

usual range of sizes. They are also applied to the threading of pipe and pipe

fittings. The rapidity with which threading operations may be performed by

the use of taps and dies, within the limits of accuracy suitable for a large percentage of commercial work, makes them most efficient and widely used threading

tools. It is only in cutting large sizes or coarse pitches, or where a high degree

of .accuracy is desired, that their use may be less economical than other means of

cutting threads.

Aside from lead errors, which have been previously considered, the accuracy

of the thread produced depends on the form of the cutting teeth, character of the

cutting edges, clearance or relief for cutting edges, construction of the tool, and

the conditions under which it is used.

(d) Milling Cutter.—A screw thread may be produced by feeding in to the

depth of the thread and then traversing a rapidly revolving single milling cutter along the slowly revolving part to be threaded at such a rate as to produce a

thread of the desired lead; the profile of the cutting edges of the cutter conforming

approximately to the shape of the thread groove in an axial plane, and the axis

of the cutter being set at an angle to the axis of the thread, in a plane parallel to the axis of the thread, equal to the mean helix angle of the thread cut. The

single-cutter method of thread milling is especially applicable to the cutting of

large threads of coarse pitch, multiple threads, and the heavier classes of work.

When the amount of metal to be removed is large, as compared with the size of

the screw, this method is especially suitable because the torsional strain is much

smaller than that produced by a die, and consequently the accuracy of the screw produced is greater.17

(e) Threading Hob.—A screw thread may be produced by feeding in to the

depth of the thread, and then traversing a rapidly revolving multiple milling

cutter or thread hob, somewhat longer than the length of the thread to be cut

which consists of annular rows of teeth, whose centers lie in planes perpendicular

to the axis of the cutter (in effect a series of single cutters formed into one solid

piece) , and the axis of which is parallel to the axis of the thread—along the slowly

revolving part to be threaded slightly more than either one or two complete

revolutions of the work, at a rate per revolution of the work equal to the pitch

of the thread. The multiple-cutter method of thread milling is used largely for

cutting comparatively short threads, usually of fine or medium pitches, when

smoothness or a considerable degree of accuracy is desired, or when the thread

must maintain a fixed relation with a point or surface on the work.

The error introduced in the form of thread produced by cutter teeth having the same form as that of the intended form of thread, as the result of the axes of cutter and thread being parallel, is usually not serious except when the helix

angle is large. 18

17 For refinements in connection with the determination of the profile of cutting edge of a thread milling

cutter, see "The milling of screw threads and other problems in the theory of screw threads," by H. H.

Jeffeott. Proceedings of the Institution of Mechanical Engineers, 1922-1, pp. 515-52S, and discussion pp.

529-562; or Engineering (London) 113, Apr. 7, 1922, pp. 441-442, and discussion pp. 412-411.

18 For formulas which may be applied in such cases to determine and plot the exact contour of the cutting

edges to produce, as nearly as possible, the thread form required, see " Side-cutting of thread milling hobs,"

by Earle Buckingham. Transactions of the American Society of Mechanical Engineers, 42, 1920, pp.

569-593; also the reference cited in foot note 17 for thread milling cutter profile.
160 NATIONAL, SCREW THREAD COMMISSION

3. ROLLING OF SCREW THREADS

The second general process for forming screw threads, namely, that of rolling,

is a cold-forging process. It may be defined as an impression or displacement

method whereby the threads are formed by means of a die or roll having threads

or ridges, which are forced into the material to be threaded, and, by displacing

it, produce a thread of the required form and pitch. In this process no material

is removed, but the metal is displaced from the thread space and forced up on

each side above the original surface of the piece to be threaded. Thus, the major

diameter of a V-shaped 60° thread so produced is found in practice to be greater

than the original diameter of the blank by an amount varying from 65 per cent

of the single-depth of thread for small screws to 85 per cent for large screws. An

approximate formula, based on geometrical considerations only, for the diameter

of a blank to be threaded to national form is as follows:

in which

Di = diameter of blank

D = major diameter of thread

p = pitch of thread. In case the thread required must be accurate within close limits, the exact value

of Di necessary in any given case must be determined experimentally, as its value is affected by the physical properties of the material.19 The thread-rolling process is the most rapid and economical method of forming

screw threads in quantity production, when the part to be threaded is of such

form as to permit its use. It is used only for external threads and is not regarded

as being feasible for internal threads, since the area of contact of the roll in an

internal thread is relatively much larger than on an external thread, and in order

to displace the metal a very heavy pressure is required. It is difficult to support

the work with the necessary rigidity to withstand the heavy pressure, and to provide a bearing for the roll which will withstand the strain.

Screw threads may be rolled by either of two methods, as follows:

(a) Threading Roll.—By forcing a cylindrical disk or roll, having a threaded

periphery and being free to rotate on the pin or bolt on which it is mounted,

against the piece to be threaded while the latter is revolving. The cylindrical

roll is used when the work is in an automatic screw machine or turret lathe and it

is impossible to cut the thread required by means of a thread cutting die, or when

an additional operation would be necessary before cutting the thread. The

thread on the roll corresponds in pitch, and approximately in form, to the thread

to be rolled. The roll may be presented to the work in either a tangential direction as shown at A, Figure 42, or radially as shown at B; a satisfactory thread is formed in either case.

(6) Thread Rolling Dies.—By rolling the blank between dies, which may be

either flat or cylindrical in form, when performed by machines designed exclusively

for this work. When flat dies are used, as shown in Figure 42 at C, one die M

remains stationary, and the other die, N, which is parallel or nearly parallel to M, has a reciprocating movement. The faces of the dies have parallel milled or planed grooves of approximately the same form as that of the required thread which

are set at an angle to the line of motion of the blank equal to the helix angle of

19 This formula is derived in "Size of stock for bolts having rolled threads," by F. Webster. American

Machinist, 30, Oct. 31, 1907, p. 630.
1924 REPORT 161

the thread to be produced. The angles of the grooves and ridges in a plane perpendicular to the direction of the grooves are given by the formula:

Tan ai =tan a cos s in which

ai =half angle of ridge of die a =half angle of thread to be rolled

s = helix angle of thread.

The spacing of the ridges is determined by the formula:

Pi = p cos s in which

Pi = spacing of ridges of die

p = pitch of thread to be rolled

s = helix angle of thread.

The blank is inserted at one end of the stationary die, and rolls between the

die faces until it is ejected at the other; the thread being formed in one passage

of the blank. When cylindrical dies are used, one of the dies, which is a complete

cylinder, revolves continuously in one direction and the other is a stationary

cylindrical segment. This method is used extensively for threading almost all A

Fig. 42.

Methods of rolling screw threads

forms of small and medium sizes of screws and bolts, when required in suffi- ciently large quantities to warrant the use of a thread rolling machine.20

4. FINISHING OF SCREW THREADS

On account of the difficulty of producing an accurately finished thread by means

of a cutting tool, in ordinary gage-making practice the thread is lapped, or ground

and lapped, in order to finish all elements of the thread to correct dimensions.

The process of grinding is applied to hardened screws only, and is intended to

correct any errors present as the result of distortion in the hardening process, as well as those resulting from the cutting operation. Lapping is usually applied

to hardened screws threads, and may be either substituted for grinding, or performed after grinding to remove the marks left by the grinding wheel and to produce a smooth and highly-polished surface. These processes are used largely

in the production of screw-thread gages.

J0 The principles involved in determining the spacing and angle of ridges of flat dies, and position of the

dies, are considered in " Principles of thread rolling and the setting of dies,'' by J. F. Springer, American

Machinist, 33, Apr. 21, 1910, pp. 739-741.
162 NATIONAL SCREW THREAD COMMISSION

(a) Grinding.—The grinding of a thread is similar to the process of milling a thread by the single-cutter method. The profile of the periphery of the grinding wheel is "dressed" by means of a diamond to conform to the shape of the

thread groove in an axial plane, with the axis of the wheel set at an angle to the

axis of the thread, in a plane parallel to the axis of the thread, equal to the helix

angle. In order to produce a thread having straight sides and correct angle, the

periphery of the wheel should be dressed to the required angle after the wheel

has been set to the helix angle, in the plane containing the axis of the thread

and the center of the wheel. The same considerations as to the exact profile of

the periphery of the grinding wheel, to produce a thread of exactly correct form,

apply as for the tooth profile of a single milling cutter set at the helix angle of the

thread. The principal differences between the thread milling and grinding processes are that a large diameter of grinding wheel is desirable, and several light cuts are taken, whereas, a small diameter of milling cutter is desirable and a

single cut is taken.

(b) Lapping.—The lapping of a screw thread may be defined as a process

of abrasion by successively traversing the thread, as it revolves, with a so-called

lap, which consists of an engaging screw thread of softer material, usually fine- grained cast iron, brass, or cold-rolled steel, in which very fine abrasive material

is embedded in the thread surface. For removing considerable material, the

laps are charged with coarser abrasive, and for imparting a fine finish, a finer abrasive; in either case the abrasive used is very fine, and the lap is thoroughly

lubricated. A number of laps may be necessary to finish either an internal or external thread to the required form and dimensions. The necessary laps for reducing the national form of thread are as follows:

(1) For reducing the sides of the thread, and thus correcting the pitch diameter,

the lap consists of a thread, of correct form, slightly large on all dimensions

(about 0.001 inch) to go onto a plug, or small to go into a ring. The thread form

of this lap is shown in Figure 43 at A. It is shorter in length than the thread to be lapped, in order that only local and periodic lead errors will be corrected and

the lead over the entire length of the thread will not be affected.

(2) For reducing the major diameter of an external thread, or increasing the

minor diameter of an internal thread, and correcting the form at the crest, a lap

of full thread form may be used if the thread angle of the lap for an external

thread is large (by about 2°) , or for an internal thread is small, and proper clear- ances are provided on pitch and minor diameters.

(3) If the angle of the thread has been changed by improper lapping, or dis- torted by strains produced in hardening, it may be corrected by means of a lap

of correct angle, with clearances at crest and root, shown at C, Figure 43.

(4) Local or periodic errors in pitch will have been corrected to a considerable extent by the above laps, but a progressive error in lead can only be corrected,

if negative (short), by means of a lap of correct thread form whose lead is long;

and if positive (long) , by means of a lap whose lead is short. In either case the

length of the lap should be about one and one-half times that of the thread to be

lapped.

The order in which these laps are applied will depend on the nature of the er- rors in the work, and the changes which take place in the dimensions of the work

during the operation. Frequent checking of the dimensions of the work is neces- sary to assure good results. For sizing internally threaded laps, a hand tap, having a pilot whose diameter

is equal to the minor diameter of the thread, tapered at the leading end threads,

and accurately made to the dimensions of the finished thread is very convenient.

It should be lubricated with a good grade of lard oil to impart a smooth finish to the threads in the lap.
1924 REPORT 163

APPENDIX 4. DESIGN AND CONSTRUCTION OF GAGES

The following recommendations will be helpful in the design and construction

of gages used in the inspection of threaded product (see also Section III division

6, and fig. 22).

1. MATERIAL

The commission has no recommendations as to material to present. A body

of manufacturers and users of gages, known as the Gage Steel Committee, with

headquarters at the Bureau of Standards, is developing such recommendations.

These will be based on results of an extensive experimental investigation now

being carried on at the Bureau of Standards in cooperation with the Ordnance

Department, United States Army, and various manufacturers.

LAP

A 3

Fig. 43.

Thread form of laps for lapping screw threads

2. PLUG THREAD GAGES

(a) Type.—Thread plug gages inches and less in diameter should be made

with a shank inserted in a handle and securely fastened.

Plug gages of more than iy% inches in diameter, unless otherwise specified,

should have the gaging blank so made as to be reversible. This can be accomplished by having the finished hole in the gage blank fitting a shouldered projection on the end of the handle, the gage blank being keyed on and held with a nut. Inspection and working thread plug gages with chamfered end threads should

be provided with dirt grooves which extend into the gage for a depth of from one

to four threads.

(b) Design of "Co" Thread Plug Gages.—The handle of the "go" thread

plug gage should be chamfered in order to provide a ready means of identification

of "go" gages as distinguished from "not go" gages. The thread should be of

basic form with the root cleared to facilitate grinding and lapping.

The length of thread parallel to the axis of all standard "go" thread plug

gages should be at least equal to the length of engagement of the product.
164 NATIONAL SCREW THREAD COMMISSION"

(c) Design of "Not Go" Thread Plug Gages.—All "not go" thread plug

gages should be made to check the pitch diameter only. This necessitates

removal of the crest of the thread, so that the dimension of the major diameter is never greater than that specified for the "go" gage, and also the removal of the

portion of the thread at the root. • The crest of the thread should be truncated to

to a depth corresponding to a flat equal to p/4 and the groove be widened from a

position one-fourth the depth of the sharp V thread below the pitch diameter line to the bottom of the V-thread groove. This form of thread checks principally the

pitch diameter and has sufficient gaging surface to provide against rapid wear.

3. RING THREAD GAGES

(a) Type.—All ring thread gages should be made adjustable. The thread

should have the root cleared to facilitate grinding and lapping, and the crest truncated to correspond to the minor diameter of the internal thread of the

product.

(b) Design of "Go" Thread Ring Gages.—The "go" gage should be dis- tinguished from the "not go" gage by having a decided chamfer, and both gages

should have their outside diameters knurled if made circular.

(c) Design of "Not Go" Thread Ring Gages.—All "not go" ring thread

gages should be made to check the pitch diameter only. This necessitates

removal of the crest of the thread so that the dimension of the minor diameter is never less than that specified for the maximum or "go " gage and also the removal

of the portion of the thread at the root of the standard form.

4. PLAIN PLUG GAGES

Plain plug gages of 1*^ inches and less in diameter should be made with a plug

inserted in the handle and securely fastened. Plain plug gages of more than 1}£

inches in diameter should have the gaging blank so made as to be reversible.

This can be accomplished by having a finished hole in the gage-blank fitting a shouldered projection on the end of the handle, the gage blank being held on

with a nut. The "go" plain plug gage should be distinguished from the "not go" gage by

having a decided chamfer on the handle of the "go" gage.

5. PLAIN RING GAGES

Both the "go" and "not go" gages should have their outside diameters knurled

if made circular. The "go" gage should have a decided chamfer in order to provide a ready means

of identification for distinguishing the "go" from the "not go" gage.

6. PLAIN SNAP GAGES

Snap gages may be either adjustable or nonadjustable. It is recommended

that all snap gages up to and including one-eighth inch be of the built-up type.

For larger snap gages, forge blanks, flat plate stock, or other suitable construction may be used.

Sufficient clearance beyond the mouth of the gage should be provided to permit the gaging of cylindrical work.

Snap gages for measuring lengths and diameters may have one gaging dimension only, or may have a maximum and minimum gaging dimension, both on

one end, or on opposite ends of the gage. When the maximum and minimum

gaging dimensions are placed on opposite ends of the gage, the maximum or "go" end of the snap gage should be distinguished from the minimum or "not

to" end by having the corners of the gage on the "go" end decidedly chamfered.
1924 REPORT 165

APPENDIX S. FUTURE WORK OF THE COMMISSION

The following brief summary of the uncompleted projects to which the commission has devoted its attention is presented for the purpose of acquainting the

public with the progress made, and of inviting the cooperation of all interested

parties in bringing as much of this work as possible to a satisfactory conclusion.

While some of these projects are practically completed, others have assumed

considerable proportions, on account of the variety of interests which must be

considered, and some will undoubtedly require much time to complete. It is the intention of the commission to issue supplementary reports from time to

time, as additional standardization projects are completed.

1. SCREW THREADS USED IN ELECTRICAL INDUSTRY

Three different forms of thread are in common use on electrical equipment,

each type finding application on a certain class of work, but much confusion

has existed as to their proper uses and combinations. Specifications have been

formulated for the following types of screw threads: (1) Screw threads for conduit, and conduit couplings and fittings, (2) special straight pipe threads for

fixtures, (3) fine threads for brass tubing, and (4) rolled threads for screw shells of electric sockets and lamp bases.

Data regarding item 2 are given in A. S. M. E. Bulletin No. 1525, report on

"Straight Pipe Threads" of the Committee on Standardization of Special

Threads for Fixtures and Fittings. Data on item 3 are given in a bulletin

issued by the National Council of Lighting Fixture Manufacturers in 1921.

Data on item 4 are given in A. S. M. E. Bulletin No. 1474, report on "Rolled

Threads for Screw Shells of Electric Sockets and Lamp Bases" of the Committee on Standardization of Special Threads for Fixtures and Fittings.

These specifications will be available in mimeographed form pending the

publication of further sections of this report.

2. BOLT AND NUT PROPORTIONS AND WRENCH OPENINGS

Another project to which the commission has given attention is the standardization of bolt and nut proportions, particularly as to the widths across flats, because

of the desirability of reducing the number of sizes of bar stock, dies, and wrenches

necessary.

Several months after the commission undertook this work, the Sectional

Committee on the Standardization of Bolt, Nut, and Rivet Proportions was

organized under the procedure of the American Engineering Standards Committee

with the Society of Automotive Engineers and the American Society of Mechanical Engineers acting as joint sponsors. To avoid duplication of effort and prevent

the promulgation of two conflicting standards, a subcommittee of the commission and a subcommittee of the sectional committee have worked in close cooperation in developing proposed standards for wrench-head bolts and nuts. The results of the work of these subcommittees were published late in 1923 by

the sponsors, as a tentative report of subcommittee No. 2 of the sectional committee entitled "Proposed Standard Sizes of Wrench Head Bolts and Nuts,"

for the purpose of obtaining criticisms and comments of all interested manufacturers and users, before the subcommittee made its final report to the sectional

committee. The proposed sizes are intended to supersede all existing standards

of wrench head bolts and nuts, of which a large number have grown up in various

industries and sections of the country. As the result of the adoption of such

standard series of sizes, there will be a marked saving in the number of sizes of

bar stock and of finished bolts and nuts which the manufacturers and dealers

must carry, and in the number of wrenches which the consumer requires to fit articles produced by the different industries.
166 NATIONAL SCREW THREAD COMMISSION

In fixing tolerances on widths across flats of bolt heads, nuts, and wrenches,

it was intended that a minimum clearance be provided which would assure a fit of the wrench to the bolt head and nut, and a maximum clearance which would

not permit the rounding of the corners of a hexagonal nut or bolt head nor spreading of the wrench jaws.

3. MACHINE SCREW AND STOVE BOLT PROPORTIONS

As in the standardization of bolt and nut proportions and wrench openings, the same subcommittee of the commission and a subcommittee of the Sectional

Committee on the Standardization of Bolt, Nut, and Rivet Proportions have

worked in cooperation to develop standard dimensions for machine-screw heads.

The proposed standard sizes were published in March, 1924, by the sponsor

bodies as a tentative report of subcommittee No. 3 of the sectional committee

entitled, "Proposed Standard Sizes of Slotted Heads for Machine and Wood

Screws." This report is being submitted to all interested manufacturers, dealers,

and users of these products for criticism and comment. This report contains

head sizes for a number of machine screw sizes which are not a part of the national coarse or national fine thread series. The commission recommends that these

sizes be eliminated.

There has been considerable difficulty in arriving at a satisfactory standard

for stove bolt proportions on account of the difference in the types of manufacturing equipment used by Eastern and Western manufacturers. This difficulty

is discussed in the above-mentioned report of subcommittee No. 3. Although

the commission has not come to an agreement on head dimensions for stove

bolts, it recommends that thread diameters and pitches conform to the specifications given in Section III of this report for the national coarse thread series, class 1, loose fit.

4. OIL-WELL CASING THREADS

One of the special problems brought to the attention of the commission was

the great need for standardization of oil-well casing threads. Definite work

toward such standardization was initiated by the Mid-Continent Oil and Gas

Association, in 1921, but this was complicated by a proposal to simplify casing

sizes and weights, and provide new standard sizes of nesting casing required

for the deeper well drilling which is now necessary. Certain manufacturers have

also endeavored to come to an agreement on thread standards, and a further

influence toward that end has been the fostering of standardization of oil-field equipment by the American Petroleum Institute, and by Purchasing Agents'

Associations.

Aside from the variety of casing sizes, further complications arise from the

fact that three tapers and three or four pitches have been in use. Through the

cooperative efforts of the American Petroleum Institute, the Standardization

Committee of the Mid-Continent Oil and Gas Association, and the commission

the standardization of casing threads has progressed to the point where certain

agreements as to pitches and tapers have been effected. On the larger sizes 8 threads per inch, and on the smaller sizes, 10 threads per inch will be standard,

with the dividing line at about 8 inches diamater of casing. The taper on the

larger sizes will be three-fourths inch per foot, and on the smaller sizes threeeighths inch per foot, the change in taper and in pitch taking place at the same

point. The American Petroleum Institute has determined upon a series of

casing sizes which will meet present requirements, and only minor details in the

specifications remain to be developed.

The standardization of oil-well casing and casing threads has an important

bearing on the national defense, and for this reason the commission is very much

gratified with the progress made by the industry in such standardization.
1924 REPORT 167

S. WRENCH FIT OF THREADED STUDS

As announced in the 1921 report, insufficient data were available for the proper

specification of tolerances and allowances (interferences) for class 5, wrench fit. The commission, in cooperation with the Bureau of Standards, has since col- lected and compiled much information as to stud fits from technical literature

and by correspondence with manufacturers of automobiles, trucks, gas and steam

engines, tractors, and other types of machinery in which studs are used. While

the information thus obtained was very complete and valuable, the commission

feels it necessary to obtain additional experimental data as to safe and proper

limits for such interferences, on account of the wide variations in commercial

practice as to maximum and minimum interferences.

The Bureau of Standards is conducting a series of tests, and has already

obtained considerable data, in which the relations between pitch diameter interference, thread form, and assembling torque for representative sizes and pitches

are being determined. The results of a somewhat similar series of tests also have

been submitted by a large automobile manufacturer. As a result of this work

the commission expects to be in a position to make definite recommendations in

the not far distant future.

6. WIRE GAGES, AND STOCK SIZES OF WIRE, METAL SHEET, AND

PLATE

The interest of the commission in sizes of stock for screws, and bolts, and in

tap-drill sizes, led to an investigation of the possibilities for the simplification of

wire gages. The existing confusion in the industries due to the many wire and

sheet metal gages in use is well known. One of the causes of uncertainty arising

from the multiplicity of gages in use is the failure on the part of a purchaser to designate a particular wire gage or to state the decimal size corresponding to the

gage number; also errors frequently occur in transmitting this information. A

further complication arises from the fact that, in common trade practices, dif- ferent gages are used for different materials. The result is that, if, for example, a purchaser who has been accustomed to using a No. 10 steel wire for a certain

purpose should order a No. 10 copper wire for the same purpose, he would receive a wire of a different diameter. Alleged sharp and dishonest practices arising

from this situation have been brought to the attention of the Department of Commerce.

In the course of its investigation, the commission held a hearing on October 8, 1923, in New York, N. Y., to consider the question. From the discussion at this

hearing and the letters received, the commission has arrived at the following

conclusions:

The following gages are in customary use in the United States:

Designation of gage Material on which gage is used American Wire, or Brown & Sharpe.. Copper, brass, bronze, and other nonferrous wires and sheets. Steel Wire, or Washburn & Moen Steel wire (other than music wire and

drill rod)

Birmingham, or Stub's Iron Wire Telegraph and telephone wire, and to some extent thickness of nonferrous

sheets, and wall thickness of tubing.

Twist drill and steel wire Twist drills and drill rods.

U. S. Standard for sheets and plates,. Sheet and plate iron and steel. Music wire Piano and other music wire.

Sheet zinc Zinc sheets.
168 NATIONAL SCREW THREAD COMMISSION

From the testimony given before the commission, it appears that while there is a certain amount of overlapping of the various gages used on various materials

and in different industries and much uncertainty on the part of the public, the

opinion in each industry is that the particular gage used in that industry is rea- sonably satisfactory, and that all other industries should adopt that particular

gage. In view of this attitude on the part of those who use gages, it is the opinion

of the commission that it would be impracticable to attempt to remedy the present

situation by recommending the adoption of any one of the various existing gages

and the elimination of all others. It is further the opinion of the commission

that a more practical solution would be to eliminate all of the arbitrary gage

series and to express all diameters and thicknesses of wires, drill rods, plates,

and sheets in terms of decimal parts of an inch.

The designation of diameters and thicknesses of wires and sheets in decimal

fractions of an inch instead of by gage numbers does not necessarily involve any

change from present practice so far as actual dimensions are concerned. Each

industry would naturally continue the use of present stock sizes. A point which

should not be overlooked, however, is the possibility of selecting stock sizes for

each material or industry such that the stock sizes for all materials and industries

would form a consistent and logical series. A part of this series would be employed in one industry or on one type of material, while other parts would be used on other materials or in other industries. It seems probable that judicious selec- tion of stock sizes in the various materials and industries would make possible a very considerable reduction in the quantity of materials carried in stock, and in the number and variety of dies used in the production of drawn material, by eliminating certain sizes from the various gage series which are unnecessarily close

together. This presents a problem in simplification by elimination, the solution

of which the commission is not in a position to undertake.

7. ACME THREADS

The sizes and pitches of Acme threads in use are so extensive in variety that an

effort to evolve a standard series has been considered futile. The table given

below was compiled from 34 replies to questionaires sent to machinery manufacturers and shows the relative percentage of each pitch used in screws for trans- mitting power, and those diameters corresponding to the various pitches most

often used. Both right and left handed single threads are included. This

table, however, can not be considered as covering the entire field.

Table 90.Diameters and pitches of Acme threads commonly used

Threads per inch Diameters most often used

Percentage of each

pitch

used, all diameters

1 2 3

1

Inches

3 1

16

3

5

38

10

7

1

17

18

2

IX

2... m, 2 m m 3 1X,2X -

4 x, i, iys, ix, \%, m ix, 2, ix —

5 X,h IX— - - - 6 54 v», m~ - 6V2

8 J4 54 X— - - 9

10 i, ix, m, ix

12
1924 REPORT

8. OTHER STANDARDIZATION PROJECTS

(a) Tolerance Specifications for Pipe Threads Subject to High Pressures.—The requirements of taper threads for high-pressure work are not as generally realized as they should be, and attempts are frequently made to use commercial pipe threads, made to the usual tolerance specifications, with very

unsatisfactory results. An investigation as to tolerances on the individual thread

elements of pipe threads for such purposes is proposed.

(6) Instrument Tubing Threads.—The lack of a universal standard for

threads on brass tubing for instrument work is not as important a question

from the standpoint of the general public as standard threads for tubing for

domestic purposes, for which specifications will be given in the section on screw

threads used in electrical industry. Nevertheless, it would be very desirable

to have definite recommended pitches for various sizes of instrument tubing.

^ The French have considered the matter of sufffcient importance to formulate (\ a definite standard for threaded brass tubes for optical instruments, which was

adopted in December, 1916. Very little information is available as to American

practice, but standards are used for certain special purposes, such as threads on miscroscope objectives, which are to a large extent interchangeable.

(c) Threads on Instrument Screws.—Smaller sizes and finer pitches of screws than those provided in the national coarse and national fine thread series,

together with the latter, are extensively used in instrument work. Various

manufacturers' standards for the small sizes are in use but there has been no

concerted effort among American manufacturers to adopt a common standard.

(d) Form of Thread for Valve Stems.—From the points of view of the

threading tool and valve manufacturers, it would be desirable to standardize

the form of thread used on valve stems. A variety of forms are now in use, principally the truncated 60° thread and modified Acme thread.

(e) Threads on Condenser Tube Ferrules.—It has been brought to the attention of the commission that several different pitches are used on the same nominal sizes of condenser tube ferrules. This is a source of inconvenience

and expense in making repairs.

(/) Plumbers' Fine Threads.—Fine pitch taper threads on thin-walled brass

tubing are being produced to various plumbing manufacturers' standards. A

unification of these standards is desirable.

(g) Special Threads.—Other threads in use in the United States, for which

standard specifications may be formulated are the Buttress Thread, Harvey

Grip Thread, Knuckle Thread, Square Thread, etc.

INDEX

Page

Accuracy, control of - 156-163 Allowance (definition) 14 (numerical values).— 24, 28, 32, 33, 92, 112 Angle errors, diameter equivalent of.. 24,

28, 32, 33,92,112,136, 146 Angle of helix (see Helix angle) 14 thread 14 Arrangement of report _ -- H

Authorization by Congress . 5 Axis of screw 14

Base of thread - -- 14 Basic size - 15 Bolt and nut proportions 165 Bolts and nuts, screw threads for 18-52

Chaser, thread 54,158 Class A (fit) 90 Class B 93 Class C 95 Class D - 97 Class E - 99 Class 1, loose fit 21,23,37,41 Class 2, free fit 21,27,38,42 Class 3, medium fit.... 22, 30, 39, 43 Class 4, close fit 22, 32, 40, 44 Class 5, wrench fit 22, 167

Classification of fits 21,23,90 Clearance, crest 14

at major diameter 18 minor diameter 18 Close fit, class 4. 22, 32, 40, 44 Committees, sub 8 Comparators, thread. 71 Core diameter (see Minor diameter) 13 Crest 14 clearance 15 Cutter, thread milling 54, 159

Definitions 13 Depth of engagement 14 thread 14,64 Direction of tolerances on gages 69 hose couplings 110 pipe threads 116 screw and nut 22,89

Engagement, depth of 14 length of 14,22,116

Errors, control of lead 156-157 diameter equivalents of lead and angle.. 24, 28,32, 33, 92, 112, 136, 146 European trip 9, 11

Finish 15 Fit - 1 15 Form of thread:

fire-hose coupling threads 108 hose-coupling threads. 108 national coarse and fine threads 17 pipe threads 115

Page

Form of thread—Continued.

threads of special diameters and pitches. 88 valve stems 169 Free fit, class 2 21, 27, 38, 42 Future work of commission 165-169

Gage classification 68, 73, 103, 128 Gages, classification as to accuracy 73, 103 Class X 73,75, 105 Class Y — 74, 76, 103, 105 Class Z 74,77,103, 106 design and construction of 163, 164 dimensions of 79-87,114,137

direction of tolerances on 69 "go" and "not go" 67 indicating 71 inspection 68, 114, 129, 132, 135 master - 68,128, 130 material for - 163 mechanical - 68 national coarse and fine threads 67-87 national fire-hose coupling threads 113-114 national pipe threads. 128-138 threads of special diameters and pitches. 103-106

optical 68

plain 72,164 plug thread.. 72, 163 ring thread 71,164

setting or check 68, 128, 131, 134

specifications for 73

testing of 72 thread snap 70 wear on 74, 134 working 68,129, 132,135 Gaging, fundamentals of 67-69

object of 67

practices 69-73,76,130 Grinding, thread 162

Hearings, public... 10 Helix angle 14 Historical 4-5 Hob, threading 159 Hobs, tooth outlines of 51

Interchangeability 12 International standardization 11 Lapping of screw threads... 162 Lead (definition) 14

errors, control of 156-157 diameter equivalent of 24,

28, 32, 33, 92, 112, 136, 146 Length of engagement _ 14, 22, 116 pipe thread 116 Limits 15 Locknut threads 122 Long screw joints 120 Loose fit, class 1 21, 23, 37, 41

Machine screw proportions 166 Major diameter 13 171
172 INDEX

Page

Manual on pipe threads 114 Marking of tools, gages, etc 16, 58, 76, 102 Measuring pressure 69, 149 Medium fit, class 3 22, 30, 39, 43 Members of commission 7,

9

Micrometers, thread 70 Minor diameter 13 uniform, of nut _ 22

National coarse-thread series 19, 37-40, 45, 60-62, 65, 79, 84 fine-thread series... 20, 41-44, 49, 61-63, 66, 81, 86

fire-hose coupling threads 106-114 form of thread 18,88,108 hose-coupling threads 106-109, 112 locknut threads ._ 122

straight pipe threads 120-121 taper pipe threads. 116-119,137 Net tolerances, basis of 145 Neutral zone. 15 Nomenclature (see Terminology) 13 Notation 15-17, 115-116, 120, 123 Number of threads 14

Oil-well casing threads 166 Organization of commission 6-11 Outside diameter (see Major diameter) 13

Pipe dimensions, tables of 124-126 thread gages 128-138 manual 114 threads, straight 120-121 taper 116-119,137 Pitch .. 14 diameter 13 measurement of - 147-155 uniform, of minimum nut 22 Procedure of commission 10 Public hearings. 10

Report, arrangement of 11 Roll, threading. 160 Roller dies 53, 160 Root 14

Screw axis 14 thread 13 threads, Acme 168 cutting of 53-67, 157-159

finishing of... 161-162 on oil-well casing 166 on tubing 165, 169 regular. _ _ _ 18-87

rolling of - 160-161

special 88-106

tools for cutting 53-67, 101-103, 157-159 used in electrical industry 165 Screws, instrument 169 machine 18-87 wood 139-144

Page

Side of thread 14 Standard temperature... 69 Stove bolt proportions 166 Straight pipe threads 120-121 Symbols 16-17,116 Tap drill sizes 63-67, 102-103, 126-127 Taps 58-63, 101-103, 126-127 dimensions and tolerances. 60-63, 102-103,126-127 shape of cutting edge for 59 Temperature, standard 69 Terminology 13 Thread (see Screw thread) 13 angle... 14 depth (see Depth of thread) 14

series 19-21, 88, 108-109, 116-123, 141 Threads, internal and external 13 Threading tools, specifications 63-67, 101-103, 108, 126-128 Tolerance (definition) 15 Tolerances, derivation of 145-147

direction on gages 69 hose couplings... 110 pipe threads 116 screw and nut 22, 89 (product specifications) 22-23,

88-89, 109-112, 116, 120, 122, 139-142, 169, gage 73-78,103-106,130-135 numerical values

Class 1, loose fit 24, 45-52 Class 2, free fit 28, 45-52 Class 3, medium fit 32, 45-52 Class 4, close fit 33, 45-52 Class A 92 Class B 94 Class C 96-97 Class D 98-99

Class E 100-101 wood screws 141-142 Tool, single point 54, 157-158

Tools, form of 53-58, 59, 102, 156-162 marking of— 16,58,102 Tubing, screw threads on 165, 169

Uniform minimum nut.

Utility of report

22

11

Wire gages 167-168 methods of measurement 147-155 Wires, measurement of 149-150

sizes of. 147-148,154-155

specification for 149 Wood screws 139-144 Wrench fit, class 5 22, 167 openings 165-166

o


i

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