NBSIR 73-239

Gage Block Flatness and Parallelism

Measurement

J. S. Beers and C. D. Tucker

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

August 1973

Final

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

NBSIR 73-239

GAGE BLOCK FLATNESS AND PARALLELISM

MEASUREMENT

J. S. Beers and C. D. Tucker

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

August 1973

Final

U. S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts, Director

GAGE BLOCK FLATNESS AND PARALLELISM MEASURE'lENT

Introduction

The conventional methods described here for raeasurinf»

flatness and parallelism can be useful in evaluating ga^e

block geometry. It is important to realize, however, tliat

limitations are impor;ed by the relatively small amount of

data taken to describe a frequentlv complex geometric shape.

Further limitations are imposed bv the oualit'" and precision

of the measuring devices and the observer's carefulness.

Photographing fringe patterns in the interferomrtric methods

is a useful technique vhere more complete and obiective

measurements are needed or vrhere permanent records are

desired. Results from these methods should bo iudped in the

light of the user's measurement prolyl em. Flatness Measurement

Various forms of an interferometer are applicable to

measuring gage block flatness. All produce interference

fringe patterns formed with monochromatic light bv the gage

block face and a reference optical flat of known planarity.

Flatness is measured by evaluating these patterns.

The reference flat must be sufficientlv plane in an area at

least the size of the gage blocb surface so that fringe

curvature can be attributed to lack of planarity in the

gaging face. Typical commercially available reference

flats, plane to within 1 or 2 microinches over a 2.5 inch

diameter, are usuallv adeouate.

Gage blocl:s 0.1 inch and greater can be measured in a free

state, that is, un wrung, v.'hile gage blocls less than 0,1

inch nominal size should be ^.Tung to an optical flat.

Bloc lis less than 0.1 inch are generally flexible and have

warped surfaces and there is no meaningful way of evaluating

the flatness except by "wringing" tlie block to a fused

silica flat. The flat should be of the sane nuality as the

reference flat described above. The gage bloci is wrung to

the flat using a v.'ringing agent, that is, an oil film, and

the x;rung surface mav be viewed througli the back surface of

- 1 -
the flat. The interface between block and flat should be a uniform grey color. Any light or colored areas indicate

poor wringing contact which can cause erroneous flatness

measurement. After satisfactory wringing is achieved, the

upper (unwrung) face is measured for flatness. This process

is then repeated for the remaining surface of the block.

Figures 1 and 2 illustrate typical fringe patterns. The

angle between reference flat and gage block is adjusted to

orient the fringes across the width of the face as in figure

1 or along the length of the face as in figure 2 and also to

control the number of fringes, preferably 4 or 5 across and

2 or 3 along. Four fringes in each direction are

satisfactory for square blocks.

Fringe patterns under these circumstances can be interpreted

as contour maps. Points along a fringe are points of equal

elevation and the amount of fringe curvature is thus a measure of planarity. Curvature can be measured as shown in

the figures.

Curvature = -r- in fringes

b

For example, a/b is about .4 fringe in figure 1 and .9

fringe in figure 2. Conversion to length units is

accomplished by multiplying the measured fringe fraction by

the half wavelength of the interferometer light source.

Light in the green range is approximately 10 mlcroinches per

fringe (H wavelength), therefore the two illustrations

indicate flatness deviations of 4 and 9 mlcroinches

respectively for green light.

Another common fringe configuration is shown in figure 3,

This indicates a twisted gaging face. It can be evaluated

by orienting the uppermost fringe parallel to the upper gage

block edge and then measuring "a" and "b" in the two bottom

fringes. The magnitude of the twist is a/b.

In manufacturing gage blocks, the gaging face edges are

slightly bevelled or rounded to eliminate damaging burrs and

sharpness. Allowance should be made for this in flatness

measurements by excluding the fringe tips where they drop

off sharply at the edge. Allowances vary but ,02 inch is a reasonable bevel width to allow.

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PARALLELISM MEASUREMENT

Parallelism betvjeen the faces of a gape block can be

measured in two ways; v/ith interferometrv or with an

e 1 e c t r o -me c h an ic al gape block comparator.

Interf erometr ic Technique

In this procedure, blocks are I'rung to an ontical flat and

an evaluation is made of the frinee pattern formed with

monochromatic light by the gage blocl:'s unper face, the

optical flat and the interferometer's reference flat.

Figures 4 and 5 illustrate tvpical fringe patterns. The

angle between reference flat and gage bloc I: is adjusted to

orient the fringes across the width of the face as in figure

4 or along the length of the face as in figure 5 and also to

control the number of fringes, preferablv 4 or 5 across and

2 or 3 along. Four fringes in each direction are

satisfactory for square blocks.

The degree of parallelism can be measured as shown in the

figures.

a-a ' slope = —r— in fringes

b

For example, a-a'/b in both figure 4 and 5 is about .3

fringe. Conversion to length units isthe same as described

under flatness

.

Points along a fringe are points of enual elevation and the

slope of the block fringes relative to the platen fringes is

thus a measure of the parallelism of the tv/o gaping faces.

This method depends on the v;ringing characteristics of the

block. If the wringing is such that the platen indeed

represents an extension of the block's lower gaging face,

then the procedure is reliable. If, however, a burr on the

block or the platen or even a particle of dust between the

surfaces permits only imperfect wringing, then the method is

not reliable. Eliminating burrs and dust during wringing is

essential

.

Imperfect wringing is often difficult to detect by "feel",

but wringing can be qualitatively -judged if a fused silica

optical flat is used as the nlaten. Once the block is wrung

- 3 -
to the fused silica flat, the interface betv^een block and

platen can be seen through the bacl: of the platen. The

interface should appear uniformlv prev v^hen it reflects a plain light colored wall or ceiling. Any light areas or

color in the interface indicates poor vr ringing.

Gage Block Comparator Techninue

Electro-mechanical gage blocl: comnarators with npnosing

measuring stvli can be used to measure parallelism. A gage

block is inserted in the comparator as shoT'n in figure 6

after sufficient temperature stabilization has occurred to

insure that the block is not distorted by internal temperature gradients. Variations in the block's thickness from

edge to edge in both directions (across tlie width and along

the length of the gaping face) are measured. Tongs are

recommended for handling the blocks to minimize temperature

effects during the measuring nrocedure.

Figures 7 and 8 show locations of noints to be measured with

the comparator on the t'.'O principle styles of gage blocks.

The points designated a, b, c, and d are midway along the

edges and in from the edge about .02 inch to allow for

normal rounding.

A consistent procedure is recommended for making the

measurements

:

(1) Face t he side of th e bloc k a s s n c i a t n d T .'i t h point

"a" t owa r d the comra r a t o r mea s u r i n p tins, push

t he bloc k in until upper t ip c on t a c t s point It „ II rec o r d me t or reading and v i thd raw th e block.

(2) Rotate bl ock 18'^° so sidn as s o C i a t e d w i t h point

"b" fac e s the measuring tips, PU s h bl ocl: in until

upper ti p contacts point "b M e c o r d meter

reading an d v^ithdraw the block

(3) Rotate bl ock 90° so side as s o C i a t e d v i t h point

c r a c e s the tips and procee d a s in previous

steps.

(A) Finally rotate block 180 o and f o 1 1 O T- the

p r o c ed u re to measure at point "d II

•

- 4 -
The estimates of parallelism are then computed from tlie

reading? as folio vs:

Parallelism across vidtli of block = a-b

Parallelism alonp lengtli of block = c-d

uscom:i-nbs-dc

FIGURE 4 FIGURE 5

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X = DEFINED GAGE POINT

UPPER GAGING FACE

FIGURE 7

oC

b a

o X o

UPPER GAGING FACE

FIGURE 8

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FORM NBS-1 14A 1-7!

,

U.S. DLP~ . or COMM. BISLICGRAPHIC DATA

SHEET

1. l-'l'hl.K A ITOX OR REl'OK 1 \0.

NBSIR 73-239

No.

3. R(H 1 [Ji'i ni' s Actcssioti No.

4. Tll LE -AND SUH 1 n i.K

GAGE BLOCK FLATNESS AND PARALLELISM MEASUREMENT

5. I^al)licati(iii Djtc

Auqiist 13, 1973

6. Perlorniiin; L>r gaiii?;it i on Code

J. S. Beers and C. D. Tucker

8. Performing; Org.in izjtion

NBSIR 73-239

PbKr OR.MIN'G OROANI/.ATION N.A.Mh AND .ADDRESS

NATIONAL BURl.AU 01- STANDARDS

DEPARTMENT Ol COMMERCE

VIASHINGTON, D.C. 20234

10. l'roi<-( t/ 1 .isk. 'Vi, ork 1 mit No.

11. ( ontracr/Gr.int No.

12, Sp on s or III g Or an i z.ir ion Name and Address SAME

13. Type of Rrport & Pcrlo.i Covered

Final

14. Spotisorinp A^^.cncy <h1c

15. SUPPLEMEN 1 AKV .\01i:S

16. ABS1R.A( 1 (A 200-\\ord or less facruil sujiiin.iry of mosr si^nifjeant inform.it ion. If il.icument includes a significant

bibi io^T aph) or liter, iturc survey, mention it here.)

Geometric properties of gage blocks are important

in many length measurement applications. Methods

are described for measuring the flatness of gaging

faces and the parallel ism bet^oeen opposing g,agirig

faces. These racthods, used for many years, cnploy

interferometers and electro-mechanical gage block

comparators

.

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