A 1 1 1 0

1

730072

Reference

NBS

Publi • cations

NBSIR 80-2078

The NBS Gage Block Calibration

Process Using A Measurement

Assurance Program

Clyde D. Tucker

Automated Production Technology Division

Center for Mechanical Engineering

and Process Technology

National Engineering Laboratory

National Bureau of Standards

U.S. Department of Commerce

Washington, D.C. 20234

June 1980

U.S. DEPARTMENT OF COMMERCE

100

,1)56

80-2078

1980

NATIONAL BUREAU OF STANDARDS

I AUG 1 3 1980

f

THE NBS GAGE BLOCK CALIBRATION

PROCESS USING A MEASUREMENT

ASSURANCE PROGRAM

NBSIR 80-2078

loo

Clyde D. Tucker

Automated Production Technology Division

Center for Mechanical Engineering

and Process Technology

National Engineering Laboratory

National Bureau of Standards

U.S. Department of Commerce

Washington, D.C. 20234

June 1980

U.S. DEPARTMENT OF COMMERCE, Philip M. Klutznick, Secretary

Luther H. Hodges, Jr., Deputy Secretary

Jordan J. Baruch, Assistant Secretary for Productivity, Technology, and Innovation

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

THE NBS GAGE BLOCK CALIBRATION PROCESS

USING A MEASUREMENT ASSURANCE PROGRAM

INTRODUCTION

This is a report covering the sequence of operations used to assign length

values to the blocks identified on the title page of the NBS Gage Block Calibration

Report. It includes a brief description of the measurement methods and procedures,

including process control, a list of publications pertaining to gage blocks, and

a discussion of the systematic and random components of error affecting the

measuring process.

The blocks have been divided into groups, to facilitate ease of handling, as follows:

Group I Less Than 0.100 in

Group II 0.100 to 0.107 in

Group III 0.108 to 0.126 in

Group IV 0.127 to 0.146 in

Group V 0.147 to 0.500 in

Group VI 0.550 to 4.000 in

Group VII 5.000 to 20.000 in

In the calibration report the assigned deviations from the nominal lengths,

their associated uncertainties which are a combination of the systematic and

random errors of the process, and the coefficients of thermal expansion (assumed or measured as noted) are presented. In the appendix of the calibration report,

the statistical information shown becomes a part of the collection of data used

to characterize the NBS measurement process. Such a collection has been used to

establish the control limits for surveillance of the measurement process and to

give assurance of validity of statements about these measurements. These collections

are open for inspection at our facility. It is presumed that these blocks will

be used in a similarly well-characterized measurement process so that the statistical

parameters of both processes can be combined to provide a realistic estimate of

the uncertainty of the length unit as actually realized in another facility. A

comprehensive service directed toward such an evaluation is part of a Length

Measurement Assurance Program of the National Bureau of Standards.

LENGTH MEASUREMENT

The blocks covered by this report were cleaned and lightly "stoned" to remove

surface imperfections which might interfere with the measurement. Since "gaging"

surfaces are neither flat nor parallel, it is felt that the most important

attribute of the block is the ability to be made to adhere closely to appropriate

surfaces (i.e., wring). Where it is felt that quantitative estimates of the

degree of "flatness and parallelism" are required, it is suggested that acceptance

tests be performed at the point of usage [1].

- 1 -
The length values assigned to the blocks in this report are with reference to

the values assigned to master gage blocks of NBS. The reference values have

been established by an interferometric measurement process and are the lengths

of a line normal to an auxiliary plane in close proximity of the surface to a defined "gaging” point "X" on the opposite surface. For location of the point,

X, relative to the nominal size marking see the accompanying figure.

INTERCOMPARISON DESIGN

A procedure that is statistically designed to eliminate the effects of short

term thermal and electronic drift is used with four blocks of the same nominal

size, two reference and two "unknowns"

Observation

Y(l)

Y(2)

Y(3)

Y (4)

Y (5)

Y (6)

Y(7)

Y (8)

according to the following design [2]:

Difference Measured

S . — S.

.

Y - S. X - Y

S.. - X

S.. - Y

Y - S.

S. - X

X - S..

The symbols (S.) and (S..) indicate reference blocks, listed by serial number in

the body of the report. (X) and (Y) designate "unknown" blocks, one set of

which is covered by this report, and the other covered by test numbers stated in

the appendix to the report.

In such an intercomparison, only differences in length can be measured. Because

of the limited range of present electro-mechanical comparators, [3] all blocks

in a given comparison are of the same nominal value. A redundancy in the number

of measurements (eight measurements to determine four values) provides a means

for checking on the precision of the process by the method of least squares

using the sum of the lengths of the two reference blocks, [ ( S . ) + (S..)], as the

restraint. The least squares estimate of the difference between the reference

blocks for this calibration is an independent estimate of the difference, and serves as a "check standard".

PROCESS CONTROL

The standard deviation, as computed from the least squares solution, provides a check on the short term, or "within-run" process precision. This value is

compared with the long run pooled value of these standard deviations designated

as the accepted within-run standard deviation of the process for the group.

The estimates of the check standards provide a sequence of values that realistically reflect the totality of variations which beset measurements of test

items. The standard deviation of this collection of values is the total process

standard deviation.

2
If the "within-run" standard deviation and the values for the check standard

come from a stable measurement process, and if the statistical tests for these

two parameters from this calibration show them to be in agreement with their

accepted values* then, one takes this as evidence that the process is in a state

of control, and the uncertainty statements are valid.

An intercomparison run is repeated, when the statistical tests for either the

"within-run" standard deviation or the check standard value fails. Should the

repeated run fail again, the gage blocks are re-examined for defects, i.e.

scratches, burrs or unclean surfaces or malfunctioning comparator.

SYSTEMATIC ERRORS IN GAGE BLOCK MEASUREMENT

When using gage blocks in practical measurement, two factors may introduce systematic errors into the results: dissimilarity of material and deviation of

temperature from 20°C.

Length values can be assigned to other like blocks by determining the difference

in length with a contacting comparator. If the blocks are of dissimilar material,

the indicated difference is a function of the force exerted by the comparator

probe on the blocks under comparison as well as the elastic properties and

surface finish of the blocks in the immediate vicinity of the point of contact.

Data adjustment to compensate for these differences is necessary. [9] If the

comparator being used has both a top and bottom contact, the differential penetration

for both contacts must be considered. The uncertainty for the reported values

includes an allowance for the uncertainty of the differential penetration corrections.

When the two blocks are made of the same material and tested by the same probe (s)

at the same contact force, no correction is applied.

All measurements for this report were made in a temperature environment in the

neighborhood of 20°C. For groups V and VI, assigned values have been adjusted

to exactly 20°C (1968 International Practical Temperature Scale) using the

stated, or handbook, values for the coefficient of linear thermal expansion.

In the comparison process, all blocks are at very nearly the same temperature.

Corrections based on differential coefficients of expansion are assumed negligible

for blocks of Groups I through VI.

In order to extend the usefulness of the assigned values over a temperature

range of 20°C to 25°C, it may be necessary to determine a coefficient of thermal

expansion for each block in Groups VI and VII. A procedure has been developed at

NBS to measure these coefficients for gage blocks and a report can be requested

at an additional charge.

UNCERTAINTY

The systematic component of the uncertainty of the values in this report is the

uncertainty of the master block value from the interferometric determination.

The bounds for the effects of random errors in the intercomparisons are taken to

be three times the total process standard deviation.

- 3 -
When the blocks are of different material than the master blocks, a correction

is made for differential penetration. The uncertainty for the penetration

increases the uncertainty of the reported deviation by one micro-inch. This

uncertainty has been determined by statistically controlled experiments at NBS.

The magnitude of systematic errors from sources other than that of the accepted

values for the reference blocks is considered negligible at temperatures very

nearly 20°C. It should be noted that the uncertainty reflects the performance

of the comparator process used to establish the reported deviations and the

uncertainties in the interferometric process.

The uncertainty in assigned value contained in the report becomes a systematic

error for the length measurements of the user. In the absence of other significant systematic effects in the user's measurement process (a condition which

must be demonstrated) the uncertainty of the value assigned by the user is an appropriate combination of the systematic error in the standard and the random

component associated with his process. If the measurement processes are in

control and appropriate uncertainties are assigned, the values produced by

different measurement facilities will have overlapping uncertainty bands. One

cannot discuss differences in values for the same object obtained by different

facilities with any degree of seriousness unless each value is accompanied by a realistic uncertainty statement.

The author wishes to acknowledge the contributions of Mr. Joseph M. Cameron and

Mr. Paul E. Pontius in the development of the NBS Gage Block Measurement Assurance

Program. This paper is the result of their original work.

- 4 -
Publications pertaining to gage blocks that are available from the

Superintendent of Documents, U.S. Government Printing Office,

Washington, D.C. 20402 or the National Bureau of Standards, Washington,

D.C. 20234.

1) Gage Block Flatness and Parallelism Measurement, J. S. Beers and

C. D. Tucker, NBSIR 72-239, August 1973. SD Cat. No. C13.

2) Designs for the Calibration of Small Groups of Standards in the

Presence of Experimental Drift. J. M. Cameron and G. E. Hailes,

NBS Tech Note 844, August 1974.

3) Preparations for Gage Block Comparison Measurements, C. D. Tucker,

NBSIR 74-523, July 1974. SD Cat. No. C13

4) A Gage Block Measurement Process Using Single Wavelength Interferometry,

J. S. Beers, NBS Monograph 152, December 1975. SD Cat. No. C13. 44:152.

5) The Measurement Assurance Program - A Case Study in Length Measurements,

Part 1. Long Gage Blocks (5 inch to 20 inch) Paul E. Pontius, NBS

Monograph 149, November 1975. SD Cat. No. C13. 44:149.

6) Intercomparison Procedures for Gage Blocks Using Electromechanical

Comparators, J. S. Beers and C. D. Tucker, NBSIR 76-979, January 1976.

7) Fortran Program to Determine Length of Gage Blocks Using Single

Wavelength Interferometry, R. N. Varner, NBS Tech Note 956, September

1977. SD Cat. C13. 46:956.

8) Measurement Assurance for Gage Blocks, M. Carroll Croarkin, John Beers,

and C. D. Tucker, NBS Monograph 163, February 1979. SD Stock No.

003-003-.

9) Contact Deformation in Gage Block Comparisons, J. S. Beers and

J. E. Taylor, NBS Technical Note 962, May 1978. SD Stock No. 003-003-.

- 5 -
r“ NOMINAL

SIZE

REFERENCE

EDGE

REFERENCE

EDGE

NOMINAL

“"SIZE
HdS-1 l 4A IHC.V. »-/<

U.S. DEPT. OF COMM. 1. PUBLICATION OR REPORT NO. BIBLIOGRAPHIC DATA

SHEET NBSIR 80-2078

Z. Recipient's Accession No.

,

4. TITLE AND SUBTITLE

THE NBS GAGE BLOCK CALIBRATION PROCESS USING A MEASUREMENT

ASSURANCE PROGRAM

5. Publication Date June 1980

7. AUTHOR(S)

Clyde D. Tucker

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

11. Contract/Grant No.

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City. state, ZIP) 13. Type of Report & Period Covered

Agency i

15. SUPPLEMENTARY NOTES

I I Document describes a computer program; SF-185, FIPS Software Summary, is attached.

16. ABSTRACT (A 200-word or teas factual summary of moat significant information. If document includes a significant bibliography or

literature survey, mention it here.) The calibration method for gage blocks, employing a measurement assurance program,

is described for the user of an NBS calibration report. The various parameters

and their significance to the calibration process are discussed.

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)

Check standard, gage blocks, measurement assurance, random errors, standard

deviation, systematic errors, uncertainty.

18. AVAILABILITY [X] Unlimited

d] For Official Distribution. Do Not Release to NTIS

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

20402, SD Stock No. SN003-003-

fXl Order From National Technical Information Service (NTIS), Springfield,

VA, 22161

19. SECURITY CLASS

(THIS REPORT)

UNCLASSIFIED

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

21. NO. OF

PRINTED PAGES

8

22. Price

$5.00

USCOMM-DC

,
,

)

1

-

gi/?<T

-og

aisaN