NBSIR 74-523

Preparations for Gage Block

Comparison Measurements

C. D. Tucker

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

Interim Report

July 9, 1974

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

NBSIR 74-523

PREPARATIONS FOR GAGE BLOCK

COMPARISON MEASUREMENTS

C. D. Tucker

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

Interim Report

July 9, 1974

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

NATIONAL BUREAU OF STANDARDS. Richard W. Roberts. Director

PREPARATIONS FOR GAGE BLOCK COMPARISON MEASUREMENTS

by

C. D. Tucker

Introduction

The methods described here for the cleaning and deburring of gage

blocks can be useful to those individuals who have not established formal

laboratory procedures for these operations. Many individuals may employ

valid methods that vary somewhat from those employed at the National

Bureau of Standards.

Many of the gage block comparators that are in use have not been

periodically evaluated by the user. It is the intent of this report to

set forth guidelines and procedures that may be used conveniently by

metrologists to aid in obtaining more meaningful comparisons of gage

blocks by the use of probe-type transducers.

I. Gage Block Cleaning and Deburring Procedures

It is essential that gage blocks being calibrated, as well as standards being used, be cleaned and free from burrs and nicks if the calibration is to be valid. Therefore, it is necessary to institute a cleaning and deburring procedure as a part of every calibration.

A. Types of Gage Blocks

For cleaning purposes, gage blocks may be divided into two types.

1. Rectangular

This is a solid block with a width of 3/8 inch and from 1 1/8

to 1 3/8 inches in length.

2. Square (Hoke)

This type of block is 0.95 inch in cross section with a 3/8 inch

hole drilled and chamfered through the center of the gaging

surfaces

.

B. Cleaning Procedures

1. It is recommended that a cleaning bath be used consisting of a
solvent such as Varsol, trichlorethylene , Freon or any of the

commercial cleaning agents recommended by the various gage block

manufacturers. The bottom of the bath should be lined with a mat of neoprene rubber to minimize scratches and burrs to gage

block surfaces caused by grit or metal to metal contact.

2. The rectangular gage block should be immersed in the bath and

a soft bristle brush be used to remove the layer of protective

,^ film from all surfaces. Lint-free towels should be used to dry

the block. A second cleaning of the block using either Freon

or ethyl alcohol is recommended to remove any remaining residue.

Again, wipe with lint-free towels.

3. The exterior surfaces of the square style blocks are cleaned in

the same manner as the rectangular blocks. The hole through the

center of the block must be thoroughly cleaned and dried to prevent "bleeding out" of an oil film onto the contact surfaces and

supporting platen when positioned in the vertical. A .22 caliber

gun cleaning rod makes an ideal instrument for cleaning the bore

of blocks in the size range of 0.4 through 20 inches. One can

, take a solvent moistened patch of line-free paper towel and push

the material through the hole. This should be repeated until

all visible residue is removed.

C. Cleaning Interval

1. After the initial cleaning, it is recommended that gage blocks

be cleaned several hours prior to use because a film sometimes

forms on the contact surfaces from the surrounding atmosphere.

This film affects the smooth insertion of the gage block under

the stylus tip when performing mechanical gaging.

If blocks are to be joined by wringing, recleaning is necessary

prior to introducing a "wringing film" to ensure that no foreign

substances are on the wringing surfaces that could corrode or mar the contact surfaces when using "wrung" combinations.

D. Storage

Cleaned blocks should be stored in trays covered with lint-free

paper or on temperature equalization plates that are clean. A cover of lint-free towels should be used to secure the blocks from

... airborne pollutants that occur in occupied spaces.

Certain commercial equipment, instruments, or materials are identified

in this paper in order to adeqxiately 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.

2
When blocks are not in daily use, they should be coated with any

one of the many gage block preservatives that are recommended by

the gage block manufacturers. It is recommended that gage blocks

be coated when the relative humidity of the work area exceeds 50%.

Foodstuffs should be banned from all calibration areas, as citric

acids and salts will corrode steel gage surfaces. Hands should be

washed to reduce corrosion on blocks when gloves or tongs cannot be

used conveniently.

E. Deburring of Gage Blocks

Gage blocks can often be restored to serviceable condition after

being damaged by using three different optically flat gage block

stones - black granite, natural Arkansas, or sintered aluminum oxide.

1. Black Granite Deburring Stone

The black granite deburring stone is used on steel blocks when

one wants to push a burr of metal back into the area from which

it was gouged. Clean stone and block before and after (per

instructions No. 5). Sweep with camel's hair brush; then slide

the damaged surface of the block onto the stone, taking precautions that no foreign matter is trapped between the two surfaces, and with firm downward pressure the block is moved over

the stone in either a circular or a back and forth motion until

the metal is restored to its original placement. Generally,

there is a lessening of the drag between the two surfaces as the burr is reduced.

2. Natural Arkansas Deburring Stone

This white fine-grained stone is used on steel blocks to remove burrs and nicks caused by everyday misuse. It is

especially effective on damaged edges of gaging faces. If

a burr is removed on the contact surface of a block, the

gouge in the surface may not be bridged by the measuring

probe and erroneous readings will result.

3. Sintered Aluminum Oxide

A block of this material is used to treat either steel or carbide gage blocks. This stone is used in the same manner

as the Arkansas stone. It is most effective on the hard

materials such as tungsten and chromium carbide.

4. Checking the Stoned Surface

After stoning a block, one can wring the restored surface

3
to a fused silica optical flat. Looking through the backside of the flat, the interface between the block and flat

should appear uniformly grey when it reflects white light.

Any light areas or color in the interface indicates poor

\^n:inging caused by a burr that has not been fully reduced

to the normal gage surface.

5. Cleaning Gage Block Stones

The gage block stones should be cleaned with Freon or ethyl

alcohol prior to use. Periodically, depending on use, the

stone should be cleaned with a stiff natural fiber brush,

soap and water to remove embedded pieces of metal and other

foreign matter that contaminates the surface with use. It

is advisable to allow any moisture that may be absorbed by

the stone surface to evaporate before using the stone after

cleaning with water.

II. Instrument Preparation

The typical electro-mechanical gage block comparator consists of a rigid frame with a gage block support anvil or platen and one or more linear

variable differential transformers (LVDT) commonly known as probe type transducers. The gaging pressure, magnification and linearity of the transducer

should be checked and adjusted to make valid comparisons of gage blocks.

A. Fundamental Principle of Operation of a Linear Variable Differential

Transformer

The LVDT transducer is a simple voltage transformer whose current

output varies as an iron rod is passed between the primary and secondary coils of the transformer. A stylus attached to one end of the

iron rod is used to probe the gage block surface and the difference

between two nominally equal blocks is measured by the rod wit^ respect

to the position of the core when gaging the initial block [1] . See

Figure 1.

B. Linearity Test

This check should be made initially when the instrument is acquired,

when a malfunction is suspected, and when repairs are made on the

transducer or accompanying amplifier. There are two different methods

that may be employed: (1) the weight method and (2) the gage block

method

.

-•Figures in brackets indicate the literature references at the end of

this paper.

4
1. The weight method

(a) The complete instrument must be placed on a vibration free

table in the gaging area. The instrument should be turned on for a minimum of 30 minutes to assure the instrument is at operating temperature.

(b) The platen or anvil used to hold the gage block is removed

from the instrument. The stylus tip is removed from the transducer core. A piece of shim stock (0.015 inch thick and about one inch square with a hole the diameter of the threaded part of the stylus tip) is placed on the core and then the stylus tip, acting as a retainer for the shim stock,

is replaced. The shim stock is used as a receiving pan for small weights.

See Figure 2.

(c) Place enough weight on the pan, usually several ounces, to

bring the amplifier meter needle on scale at the left end of the meter.

One then commences to load the pan with equal increments of weight, e.g.

50 milligrams, noting the scale reading for each successive weight on the

pan. When the needle approaches full deflection (all the way to the right

hand side of the meter) , one then removes each weight successively, again

noting the scale reading and load until all weights are removed. Do this

operation several times and average all of the load data separately from

the unload data. The difference, if any, between the load and unload will

indicate any hysteresis present in the transducer.

(d) The data points are plotted on graph paper using weight versus meter readings. The plotted data points should fall in a straight

line if the transducer is linear. Acceptable disparities depend upon users'

needs. See Figure 3.

2. The gage block method

(a) The alternate test for linearity, the gage block method, is

not as discriminating as the weight method. This test can be performed

quickly with two gage blocks differing nominally by 10 microinches.

The smallest block is inserted under the probe and the meter is adjusted to zero by means of the control knob. Reposition the block several

times under the stylus and note the average reading.

The longer block is then inserted under the stylus several times and

the average reading recorded. One can check each 10 microinch segment of

the meter in this fashion.

The difference between these two blocks for each scale segment should

be very similar if the transducer is linear.

5
C. Alignment

Proper alignment of the probes on opposed contacting type transducers

is necessary to ensure the best accuracy in the comparison measurement.

Misalignment can add errors of several microinches in some cases.

1. One of the two probes on the instrument should have some form of

alignment adjustment, usually a 3-point adjustment consisting of

3 set screws. The probe tips must have some sphericity to enable

adjustment by this method. The upper probe is lowered to contact

the bottom probe with enough contact pressure to move the meter

- needle to approximately mid-scale.

2. The set screws are then gently turned until the most positive

reading on the scale is attained. This reading indicates that

the upper and Iovjbx styli are properly aligned, that is, touching

at their highest points.

D. Adjusting and Calibrating the Scale Magnification

The scale magnification of the gage block comparator should be checked

daily until one has a history of the stability of the instrument, then

a weekly check may be sufficient.

There are two methods by which the magnification may be checked: (1) a cursory method that employs 2 calibrated gage blocks and (2) a least

squares method that uses A calibrated gage blocks.

1. The cursory method is a very simple test of the magnification.

The smaller of two blocks, with a nominal difference between them

approximately equal to the range of the meter scale, is inserted

under the probe and a zero setting is obtained by adjusting the

control knob on the amplifier. Reseat the gage block under the

probe several times to secure a reliable zero reference. Then, one places the longer block under the probe, reseating as before,

until repeated readings are obtained.

The indicated difference should be equal to the calibrated difference between the two blocks. If not, adjust the instrument

magnification with the potientiometer screw indicated on the amp-

' lifier and repeat the readings as in the preceding paragraph. The

allowable error for the full scale reading is +1 per cent for the

highest order of calibration uncertainty.

2. The least squares method of checking the scale magnification involves checking two intermediate points as well as the full range

of the scale. This method gives a measure of the random error in

6
the observations as well as additional checks on the instrument

magnification. The observations are taken in the sequence as outlined in a "4-1" series design {1] . After the observed data

has been reduced, the observed difference versus the actual difference is plotted. The resultant plot should be a straight line

45° above the horizontal (X) axis or within +1 per cent for highest order calibration uncertainty.

This least squares method will indicate the most linear portion

of the meter scale thus enabling the obs erver to use that portion

of the scale for his comparison measurements.

E. Transducer Pressure

The pressure exerted by the stylus probe on the surface of the gage

block must be greater for the upper probe than the lower probe. This

differential pressure keeps the gage block resting on the anvil or platen. A higher pressure on the lower anvil would raise the block

off the anvil and make accurate reading impossible. A differential

pressure of 1 or 2 ounces is accepted practice.

The stylus pressures must be known when comparing blocks of dissimilar

materials and to guard against damaging gage blocks with unnecessarily

high probe forces.

Most transducer probes are spring loaded and by turning a threaded

cup, rod or knurled adjustment wheel can be regulated to the desired

pressure within the manufacturer's design limitations. A dynamometer

is used to check for the correct adjustment. This force measurement

should be correct to + 1/4 ounce. The pressure check should be done

at the magnification that you are using. Many manufacturers of com- parators suggest using this test on low magnification which may be

entirely satisfactory except for those users that are striving for

the ultimate accuracy in comparison measurements.

The pressures should be checked weekly, making a record of such

readings, until one has confidence in the stability of the pressures,

and then a bimonthly check may be appropriate.

III. Conclusions

The linear variable differential transformer provides one of the

most convient and precise means for obtaining high accuracy comparative

measurements

.

7
The observer must be aware of such refinements as comparator warm-up,

deburrlng of the instrument anvil, cleanliness of instrument probes and

block storage areas, and the use of heat sinks as well as the above guidelines before making the first comparison measurement.

The author has noted from his conversations with many metrologists

that not enough time has been spent in instrument preparation before making

hopefully, meaningful comparison measurements.

8
References

Meyerson, M. R. , Young, T. R. , and Ney, W. R. , Gage Blocks of Superior

Stability: Initial Developments in Materials and Measurement, J. Res.

Nat. Bur. Stand. (U.S.), 64C^ (Eng. and Instr.), No. 3, (1960).

Pontius, P. E., The Measurement Assurance Program - A case study in

length measurements, NBS Monograph (to be published in 1974).

SECONDARY COIL

AMPLIFIER

CORE

FIGURE I.

SCHEMATIC OF LINEAR VARIABLE

DIFFERENTIAL TRANSFORMER,
FIGURE 2.
100

AMPLIFIER SCALE READINGS AS

A FUNCTION OF STYLUS PRESSURE

FIGURE 3

N8S.1 14A (REV. ;-73)

U -S. OEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBSIR 74-5232. Gov't Accession

No.

3. RL-cipicnt's Accession No.

4. TITLE AND SUBTITLE

PREPARATIONS FOR GAGE BLOCK COMPARISON MEASUREMENTS

5. Publication Date

.Till \r Q 1 oni

6. Performing Organization Code

7. AUTHOR(S)

C. D. Tucker

8. Performing Organ. Report No. NBSIR 74-523

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Tasic/Work Unit No. 2321125

11. Contract/Grant No.

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

ms

13. Type of Report & Period Covered

Interim

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less tactual summary of most si^ificant information. If document includes a significant

bibliography or literature survey, mention it here.)

The methods described here for the cleaning and debarring of gage blocks can be

useful to those individuals who have not established formal laboratory procedures for

these operations. Many individuals may employ valid methods that vary somewhat from

those employed at the National Bureau of Standards.

Many of the gage block comparators that are in use have not been periodically

evaluated by the user. It is the intent of this report to set forth guidelines and

procedures that may be used conveniently by metrologists to aid in obtaining more

Tneaningful comparisons of gage blocks by the use of probe-type transducers

.

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)

Deburring; gage blocks, gage block comparator; linear variable differential transformer.

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