/

NBSIR 76-979

Intercomparison Procedures

for Gage Blocks Using

Electromechanical Comparators

J. S. Beers

C. D. Tucker

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

January 1976

Final

U S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

NBSIR 76-979

INTER COMPARISON PROCEDURES

FOR GAGE BLOCKS USING

ELECTROMECHANICAL COMPARATORS

J. S. Beers

C. D. Tucker

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

January 1976

Final

U.S. DEPARTMENT OF COMMERCE, Rogers C.B. Morton, Secretary

James A. Baker, III, Under Secretary

Dr. Betsy Ancker-Johnson, Assistant Secretary for Science arid Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

V

CONTENTS

Page

A. Introduction 1

B. The Comparator 1

C . Environment 3

D. Gage Block Requirements 3

E. Temperature Effects and Their Control 4

F. Intercomparison Procedure 5

1. Block Preparation 5

2. Comparator Preparation 5

3. Block Storage 5

4. Thermal Equalization Time 6

5. Temperature Measurement 6

6. Handling Techniques 7

7. Observations 9

8. Temperature and Deformation Corrections 9

G. Computation and Analysis 9

H. Summary 12

References 13

Appendix: Experimental Evidence of Thermal Effects in Long 14

Gage Blocks

INTERCOMPARI SON PROCEDURES FOR GAGE BLOCKS

USING ELECTROMECHANICAL COMPARATORS

by

J. S. Beers and C. D. Tucker

A. Introduction

The techniques described here for intercomparing gage blocks with

electro-mechanical gage block comparators are the results of considerable

experimental work, statistical design and process analysis. Some laboratories will find these techniques ideally suited to their needs while

others may wish to modify them, but in either event the Measurement Assurance Program (MAP) described here, in part, and in other papers [1, 2, 3,

7]* will promote the analysis of measurement requirements and the application of statistical principles.

Length values are assigned to unknown gage blocks by a transfer process from blocks of known length. Briefly, the transfer process is a systematized intercomparison of 4 blocks of the same nominal size, called

a "4-1" series, using an electro-mechanical comparator. Two of the blocks

are unknowns and two are standards. Such a process is ideally suited to

comparing objects with nearly identical characteristics such as gage

blocks and the process provides the redundancy needed for statistical

analysis. Derived corrections can be applied where such characteristics

as coefficient of thermal expansion and resistance to mechanical deformation differ [4]. The method applies to all sizes but special precautions will be described for long gage blocks where temperature effects

predominate

B. The Comparator

There are a number of suitable comparator designs and a typical one

is shown schematically in figure 1. An upper stylus and a lower stylus

contact the gaging faces of a block supported on an anvil. Each stylus

is attached to a differential transformer core. An integrated signal

from these two transducers is displayed on a meter graduated in length

units (usually microinches) . The difference in length between two gage

blocks is obtained by inserting the blocks, one at a time, between the

stylus tips and taking the difference between the two readings.

*Figures in brackets indicate the literature references at the end of this

paper

1
SECONDARY COILAMPLIFIER

TRANSDUCER CORE

PRIMARY COIL

MEASURING STYLUS

TEST BLOCK

ANVIL

MEASURING STYLUS

REED SPRING

ELEMENTS OF A MECHANICAL

COMPARATOR OF LENGTHS,

FIGURE I.

2
An important comparator feature is r.he point to point measurement

along an axis through the gaging point of the block. Other suitable

comparator designs use only one transducer but by various means insure

the point to point measurement

For the most precise measurements, it is important that the defined

gage point of the blocks be contacted by the comparator stylus. A metal

or plastic bar about 1/4 inch thick is fastened to the anvil (with laboratory wax if no other means is provided) behind the stylus and is positioned

to stop the gage blocks so the stylus will contact the gage point. The bar

can be L-shaped to position the gage block both laterally and transversely

if desired and special configurations can be devised for comparing blocks

of different shapes.

C . Environment

A temperature controlled laboratory is necessary for intercomparisons

of the highest precision. The degree of temperature control needed depends

on the length of the blocks being compared, differences in coefficients of

thermal expansion among the blocks, and the limiting uncertainty required

for the unknown blocks. At NBS, blocks in the size range up to 4 inches are

intercompared in a temperature controlled laboratory at 20° + .25°Celsius.

Blocks longer than 4 inches are intercompared in a laboratory at 20° + .05°

Celsius. Relative humidity is held below 50% to prevent corrosion of blocks

and instruments.

D. Gage Block Requirements

Nearly all gage blocks are either square or rectangular in cross

section, but a few have a circular cross section. Any of these cross

sections are easily handled in intercomparison procedures in sizes up to

4 inches. From 5 to 20 inches, however, the rectangular cross section

becomes prone to tipping.

Transferring the length unit to a gage block by intercomparison does

not require rigorous constraints on the flatness and parallelism of its

gaging faces because the transfer is made only along a single line through

the block. Gage block use, however, may be more demanding on the geometry

of the gaging faces. A separate paper covers the measurement of flatness

and parallelism [6].

Thermal expansion coefficients are generally taken from bulk values

and these may vary by as much as 10% from actual values for gage blocks.

This problem can be circumvented in the intercomparison process by insuring

that blocks being intercompared are as close as possible to 20°Celsius.

3
If non-standard temperatures are used, the coefficients must be known unless

the attendant systematic errors are tolerable.

E. Temperature Effects and Their Control

A large uncertainty in the comparison process can be introduced by

temperature effects. For example, a temperature difference of ,5°Celsius

between two one-inch steel blocks will cause an error of nearly 6 microinches in the comparison. Two causes of temperature differences between

blocks are noteworthy:

(1) room temperature gradients or nearby heat sources such as electronic equipment can cause significant temperature differences between

blocks even when they are stored relatively close to each other before

comparison and

(2) blocks with different surface finishes on their non-gaging faces

can absorb radiant heat at different rates and reach different equilibrium

temperatures. The magnitude of these effects is proportional to gage block

length. See the appendix at the end of this paper for further discussion

of this subject.

A number of remedies have been developed. For short blocks (through 4

inches) the remedies are quite simple. For example, storing the blocks,

both standards and unknowns, on a thermal equalization plate of smooth surface and good heat conductivity close to the comparator but away from heat

sources. Also, the use of tweezers or tongs to handle the blocks and a systematic, rythmic block handling technique in the comparison procedure

to insure identical thermal environment for each block.

Precautions for long blocks (over 4 inches) are more elaborate. At

NBS the comparator is enclosed in an insulated housing partially open on

the front to allow handling of the blocks with tongs. Blocks are stored on

the comparator anvil and each block is wrapped in 3 layers of Mylar* reflective film to equalize their thermal characteristics. The laboratory lights

remain off except for one lamp well removed from the comparator but giving

enough illumination to work by. Heat sources are kept as far away from

the comparator as possible and the comparator is located away from other

activities where laboratory personnel might congregate. As a further precaution during intercomparisons the operator wears a cape of Mylar reflective film and heavy cotton gloves while handling the blocks with tongs.

Finally, as with short blocks, the handling procedure is systematic, rythmic

and quick but not rushed.

^Certain commercial equipment, instruments, or materials are identified

in this paper in order to adequately specify the experimental procedure.

In no case does such identification imply recommendation or endorsement

by the National Bureau of Standards, nor does it imply that the material

or equipment identified is necessarily the best available for the purpose.

4
Temperature problems can be detected in long block intercomparisons

by reversing the storage positions of the 4 blocks and repeating the intercomparison after suitable equalization time. Temperature gradients in the

storage area will be revealed by a significant change in the differences

between blocks. Still another method is to measure the temperature of each

block with a thermocouple. A temperature measuring system is described in

reference [7] . Where gradients are not a problem, a mercury-in-glass thermometer mounted on the comparator will be adequate.

F. Intercomparison Procedure

1. Block Preparation

The master and test blocks must be thoroughly cleaned, examined,

and deburred using procedures in reference [5] . The identification

numbers are recorded for inclusion in the test report or records.

Gage blocks longer than 4 inches are wrapped in three layers of

Mylar reflective film (available as BSA Rescue Blanket Stock

No. 1090) with the reference edge and identity of the block

marked on the outside of the wrapping.

2. Comparator Preparation

The instrument anvil should be deburred and cleaned . The comparator transducer pressure, magnification and alignment should

be checked [5] . Periodic cleaning of the instrument anvil is

recommended during the work day to help reduce the number of

spurious readings that result from minute particles that can

contaminate the anvil surface.

It is necessary, when making preparations to intercompare long

gage blocks, that after adjusting the comparator for accepting

a particular size gage block that all four blocks in that series

be inserted between the probes to insure that all readings will

be on scale at that magnification when the intercomparison run

is made.

3. Block Storage

Short blocks, up to 4 inches in length, are arranged in groups of

4 (2 standards and 2 unknowns) of the same nominal size on a thermal

equalization plate next to the comparator. From here they can be

moved to the comparator anvil by groups at the time of comparison.

There is some thermal advantage for sizes from about 0.3 inch to

5
4 inches to leaving the blocks on the plate at all times except

when the block is being inserted in the comparator for measurement. Alternatively, a group can be moved to the comparator

anvil and allowed to equalize there for an appropriate period

(see section F4)

The four long gage blocks in a group are stored on the comparator

anvil after preparation. Additional long blocks that are to be

measured during the day are prepared and placed on a thermal equalization plate next to the instrument. As comparisons of one

group of 4 are completed a new size group can be moved from the

plate to the anvil and allowed to equalize before comparison. All

gage blocks are oriented on the comparator with the top surface

uppermost

4. Thermal Equalization Time

Equalization time varies with block size, treatment and allowable

measurement uncertainty. Blocks prepared in advance and kept in

the gaging area are placed on the equalization plate, or in the

case of long blocks, moved from the equalization plate to the

comparator anvil. They may then be intercompared using the following table as a first approximation:

Experiments establishing optimum equalization times should be conducted in your own laboratory because of the many variables involved

and differing measurement uncertainty requirements.

5. Temperature Measurement

Temperature measurements are made with a calibrated mercury-inglass thermometer. The thermometer is mounted on the block storage

plate in the case of short blocks and on the instrument anvil for

the long blocks. Apply the test described in the last paragraph of

Section E to reveal significant temperature differences among the

blocks.

In cases where thermocouple measurement of temperature in long

Block Size

(inches)

Equalization Time

(minutes)

0.1 to 0.250

0.300 to 1.000

2.000 to 20.000

30

60

90

6
blocks is necessary because of incurable gradients or exceptionally high accuracy requirements, the thermocouple leads should

be attached to the instrument column to preclude their interfering

with vital parts of the comparator but with enough slack to permit

the blocks to be moved on the anvil. The leads are labeled before

insertion into the blocks. The thermocouple is inserted halfway

down the hole in the block and the hole is plugged at each end

with a small piece of polyurethane . A rubber band placed about

one-half inch below the top of the block keeps the thermocouple

wire in place. Be sure the thermocouple junctions are insulated

electrically from the block. Transparent mending tape is sufficient insulation.

Handling Techniques

The comparison sequence is described in Section F7 but the success of this "4-1" intercomparison is largely dependent upon

block handling techniques. These techniques include the insertion of all blocks between the styli in a like manner. The

operator should develop a rhythm, after he acquires some experience with the process, that will ensure that each pair of blocks

is handled for approximately the same length of time as all other

pairs in the series. The time interval to make the sixteen observations should be approximately 2 to 4 minutes for an experienced operator with automatic data recording or an assistant

doing the recording.

A camel's hair brush or an air bulb is useful for sweeping or

blowing dust particles from the blocks and the anvil just before

insertion

The short blocks are moved about by grasping them with rubber

tipped 10-inch tweezers. When handling square style blocks, the

tips of a pair of tweezers may be bent to accommodate this configuration.

Gloves may be worn in addition to using tweezers when handling

blocks from 0.400 inch through 4 inches. The blocks above the

4-inch size are preferably intercompared on a different instrument situated in a more precisely controlled temperature environment and the operator may wear a Mylar reflective cape as well as

gloves

A special pair of tongs can be made to facilitate handling (see

fig. 2) when sliding long blocks (5 to 20 inches) about the an-

vil and between the measuring probes. With these tongs, long

blocks should be grasped about one-half inch above the bottom

to effect the surest, safest method of moving the blocks.

The anvil stop plays an important part in the seating of the

probe on the block surface as well as the positioning of the

gage point between the probes. The block is moved tangentially

toward the stop allowing one corner to touch the stop and then

with an angular motion (which seats the probe on the block surface) proceed to abut the entire edge of the block to the stop.

7 . Observations

The sequence of observations is from a design series developed to

compensate instrument and temperature drifts [3] . The series of

readings for each size utilizes two calibrated master blocks,

and S^, and two unknown blocks, X and Y. The order of observations is as follows:

8. Temperature and Deformation Corrections

Deformation corrections for various stylus radii and pressures [4]

can be applied to the observations when comparing blocks of different materials if the correction is of sufficient magnitude to

be significant.

Temperature corrections are applied to all blocks above 0.350 inch

in size when the blocks are of different materials. This size limit

can be raised if larger uncertainties are acceptable.

G. Computation and Analysis

A computer program and discussion of data processing is in reference 3. Figures 3 and 4 show the file format, input and output.

9
PRELMY FILE FORMAT

SAME AS PRELMX FILE FORMAT, BUT NOMINAL SIZES MUST AGREE LINE FOR LINE

LINE 0010

LINE 0020

LINE 0030

LINE 0040

LINE 0050

LINE 0060

LINE 0070

LINE 0080

LINE 0090

LINE 0100

LINE 0110

LINE 0120

LINE 0130

LINE 0140

LINE 0150

LINE 0160

LINE 0170

LINE 0180

LINE 0190

LINE 0200

LINE 0210

LINE 0220

LINE 0230

REPORT FILE FORMAT (OUTPUT FOR 1108)

DATE OF OBSERVATIONS

TWO INITIALS OF OBSERVER

INSTRUMENT USED, FEDERAL i\'k OR FEDERAL //2

TEST NUMBER FOR "X" SET OF GAGE BLOCKS

SUBMITTED BY "X" CUSTOMER

ADDRESS OF "X"

ADDRESS OF "X"

SERIAL NUMBER OF SET "X"

MAKER OF SET "X"

GRADE OF GAGE BLOCKS IN SET "X"

TEST NUMBER FOR "Y" SET OF GAGE BLOCKS

SUBMITTED BY "Y" CUSTOMER ' ADDRESS OF "Y"

ADDRESS OF "Y"

SERIAL NUMBER OF SET "Y"

MAKER OF SET "Y"

GRADE OF GAGE BLOCKS IN SET "Y"

NOM,SER(X) ,COR(X) ,COEF(X) ,PEN(X) , SER(Y) , COR(Y) ,COEF(Y) ,PEN(Y)

(REPEATS THE ABOVE LINE FOR EACH GAGE BLOCK)

COLUMN SUMMATION CHECKS VALUES

NOM .SIZE, TEMP , WITHIN , TOTAL , DIFF , T-TEST , STD.DEV. , F-TEST

(REPEATS THE ABOVE LINE FOR EACH GAGE BLOCK)

COLUMN SUMMATION CHECK VALUES

Key to abreviations :

NOM = nominal size

SER = serial no.

COR = correction (deviation

from nominal size)

COEF = coefficient of

expansion

PEN = penetration correction

TEMP = temperature

WITHIN = within-group standard

deviation

DIFF

T-TEST

STD.DEV.

TOTAL

F-TEST

process standard deviation (long term)

measured difference

between the two masters

statistical t-test

standard deviation of

this run

statistical f-test.

FIGURE 3

10
XBASIC GAGE6 Computer input data

RUN

STARTING WITH NOM. SIZE ! 0.130000

ENTER NO. OF GAGE BLOCKS !1

WRITE IN DATE W/O COMMA !MAY 15 1974

TWO INITIALS OF OBSERVER !GC

INSTRUMENT Fl, F2, OR F3 !F2

(1) KEYBOARD OR (2)DATAFILE!

SIZE .13 INCH X = 111 Y = GDW

s. - s.. !53.2,51.5

Y - s. 152.6,52.3

X ~ Y 150.1,51.8

S • e - X !50.9,50.0

S.

- Y 150.8,50.9

Y - S. 150.8,51.2

S. - X 151.3,49.2

X - S.. !49.3,50.0

ALL SYSTEMS GO

Observations

(in microinches)

THIS CALIBRATION IS NOW COMPLETE

NUMBER OF REPEATS = 0

TURN ON PUNCH AND TYPE G.

?LIST REPORT

Print-out of results

0010*MAY 15 1974

~

0020 GC

0030 F2

0040 XX123

0050 C.E. JOHANNSON CO.

0060

0070

0080 515

0090 CEJ

0100 GRADE A

0110 XY123

0120 FORD MOTOR CO.

0130 DEARBORN MICH.

0140

0150 132

0160 WEBBER

0170 GRADE AA

0180 .130000 111 -1,315 11.5 ,000 GDW ,335 11.5 .000

0190 .130000 -1.315 11.5 .000 .335 11.5 .000

0200 1 0

0210 .130000 20.000 .340 .550 1.150 -.167 .527 2.401

0220 ,130000 20.000 .340 .550 1.150 -.167 .527 2.401

* See line numbers in Figure 3 for explanation of print-out. Nominal size

is in inches. Corrections (deviations from nominal size) and standard

deviations are in microinches. Coefficients of expansion are x 10

-

^ per

degree Celsius.

FIGURE 4

11
The results of the 4-1 series intercomparison are explained as to

statistical handling of data, history of master blocks and predicted

lengths in all size ranges in references 1 and 2.

H . Summary

The electro-mechanical length comparator can be utilized to transfer

length values very precisely. It is useful to understand the various parameters that affect the comparative process, namely, instrumentation, temperature, block handling techniques and intercomparison design. This paper

should be used in conjunction with the references on page 13.

12
References

Pontius, P. E., The Measurement Assurance Program - A Case

Study, Part I, Short Gage Blocks (0.1 to 4 Inches), NBS Monograph 149 (in process)

Pontius, P. E., The Measurement Assurance Program - A Case

Study, Part II, Long Gage Blocks (5 to 20 inches), will be

NBS Monograph (in process)

Cameron, J. M. & Hailes, G. E., Designs for the Calibration

of Small Groups of Standards in the Presence of Experimental

Drift, NBS Tech Note 844 (1974).

Bowman, H. A., Beers, J. S., Gage Block Deformation from

Electro-Mechanical Comparators, NBSIR (in process).

Tucker, C. D., Preparations for Gage Block Comparison Measurements, NBSIR 74-523, 1974.

Beers, J. S. & Tucker, C. D., Gage Block Flatness and Parallelism Measurement, NBSIR 72-239, August 1973.

Beers, J. S., A Gage Block Measurement Process Using Single

Wavelength Interferometry , NBS Monograph 152, in process.

13
APPENDIX

Experimental Evidence of Thermal Effects in Long Gage Blocks

Figures 5 through 9 summarize the results several thermal experiments on long gage blocks. In all cases temperature was measured with

thermocouples inserted halfway down the hole in the square-type blocks

and the hole was plugged at each end with a small piece of polyurethane

foam.

Figure 5 shows the temperature of two blocks, one in the shadow of

the other, as they were exposed to a 40 watt heat source 18 inches away.

(Note: The average human generates 100 to 125 watts.) As expected, the

temperature of the block in the shadow did not rise as much. Both blocks

had nearly identical ground finishes on their sides.

In figure 6, two blocks side by side were exposed to the heat source.

In this case, one block had a dull finish on its sides and its temperature

rose considerably higher than the brighter finished block. The same two

blocks were then individually wrapped in reflective Mylar film and exposed

to the heat source again. Figure 7 shows the results - almost identical

temperature response and a reduced rise.

Further tests were made of the internal temperatures of four 16-inch

blocks while they were being intercompared in the normal "4-1" procedure.

Each point in figure 8 indicates the maximum temperature rise of any single

block during the 3-1/2 minutes it took to make the intercomparisons . There

was no covering on the blocks for the points in the first group, one layer

of reflective Mylar was on each block during the runs in the second group,

and three layers of Mylar were on each block in the last group of runs. In

figure 9, the temperature differences among the four blocks are plotted.

Maximum temperature difference between any two blocks at the moment of the

first comparison is indicated by a dot for each run and the maximum temperature difference between any two blocks at the moment of the last comparison

is indicated by a plus sign.

Temperature differences among long blocks being intercompared are by

far the largest sources of uncertainty in the process. These experiments

show that temperature stability and equality can be greatly improved by

wrapping the blocks in reflective film.

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U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBSIR 76-979

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

INTERCOMPARISON PROCEDURES FOR GAGE BLOCKS

USING ELECTROMECHANICAL COMPARATORS

5. Publication Date

January 1976

6. Performing Organization Code

7. AUTHOR(S)

T S Rport! flnr) fl T) Tur-lroi8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 2131125

11. Contract/Gf ant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Same

13. Type of Report & Period

Covered

Final

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary oi most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

The widely used procedures for calibrating gage blocks by comparison

with blocks of known length generally lack the redundancy needed to evaluate

measurement uncertainty or the controls needed to monitor the process on a continuing basis. A detailed description is given here for the systematized

intercomparison of groups of four nominally equal gage blocks using an electromechanical comparator. Two of the blocks are unknowns and two are standards.

The process provides the redundancy needed for evaluating uncertainty and for

continuous monitoring. Gage block thermal effects, equalization time, handling

techniques, and observation sequence are described.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Calibration; comparator, gage blocks; length; measurement process.

18. AVAILABILITY [^Unlimited

I | For Official Di stribution. Do Not Release to NTIS

I I Order From Sup. of Doc, U.S. Government Printing Office

Washington, D.C. 20402, SD Cat. No. CI 3

I Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151

19. SECURITY CLASS

(THIS REPORT)

UNCL ASSIFIED

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

21. NO. OF PAGES

22

22. Price

$3-50

USCOMM-DC 29042-P74