1

GAUGE BLOCK CALIBRATIONBY

MECHANICAL COMPARISON(GAUGE BLOCK COMPARATOR)

Arrangement bySaraswanto Abduljabbar,

Valunteer, specialist and advisor Length Metrology Measurement

1. BACK GROUND

International standard ISO 3650 cover the range of accuracy requirements along

the traceability chain and calibration method is selected according to accuracy require-

ments of the standard and the user. In mechanical comparison methods, the similar

nominal size gauge blocks are compared to each other by suitable probing elements.

Since the compared gauge blocks are in the same nominal sizes, the inductive probes

which have a short measurement range with high accuracy is used in the mechanical

comparison technique, figure 2.

Factors influencing the measurement are: the length calibration of the standard, factors

inherent in the comparator equipment used to measure the length difference such as

scale linearity and reading capability, gauge geometry with respect to its effect on

probing the length difference, the temperature and other environmental factor, etc.

2 Basis of measurement, traceability, reference condition1

2.1 Unit of length: metre

The metre is defined as the length of the path travelled by light in vacuum in 1/299 792

458 of a second (17th General Conference of Weights and Measures, 1983). The

definition is realized by working wavelength standards recommended by the Interna-

tional Committee of Weights and Measures (CIPM).

2.2 Traceability of the length of a gauge block

The measured length of a gauge block is traceable to national or international length

standards, if the measurement result can be related by an unbroken chain of compari-

son measurements each with stated uncertainties to a gauge block which has been

calibrated by interferometry/using appropriate wavelength standards.

1EN ISO 3650:1998, Geometrical Product Specification (GPS) - Length Standards Gauge Blocks, Second Edition,

1998-12-15.

2

2.3 Reference temperature and standard pressure.

The nominal length and the measured lengths of a gauge block apply at the reference

temperature of 20 C (see ISO 1) and the standard pressure 101 325 Pa = 1,013 25

bar2.

NOTE: The effect on the length of a gauge block caused by deviations from the standard pressure may

be ignored under normal atmospheric conditions.

2.4 Reference orientation of gauge blocks

The length of a gauge block up to and including 100 mm nominal length refers to the

vertical orientation with the measuring faces horizontal. The length of a gauge block

over 100 mm nominal length refers to the horizontal orientation with the block supported

on one of the narrow side faces, without additional stress, by suitable supports each at

a distance of 0.211 times the nominal length from the ends. When such a gauge block

is measured by interferometry in horizontal orientation, the weight of the auxiliary plate

wrung to one of the measuring faces shall be compensated.

2.5 Dimensional stability

The maximum permissible changes in length per year of gauge blocks are stated in

Tabel 1. They apply when the gauge blocks are not exposed to exceptional tempera-

tures, vibrations, shocks, magnetic fields or mechanical forces.

Tabel 1 Dimensional Stability

Grade Maximum permissible cahnge in length per yearK0

(0.02 + 0.25 10 )

12

(0.05 + 0.5 10 )

NOTE : is expressed in millimeters

Table 2 Flatness3 Tolerances

Nominal Length, mm

Flatness tolerance,

K 0 1 20.5 150 0.05 0.1 0.15 0.25150 500 0.1 0.15 0.18 0.25500 1000 0.15 1.18 0.2 0.25

2ISO 1:1975, Standard reference temperature for industrial length measurements.

3ISO 1101:, Geometrical Product Specifications (GPS) Geometrical tolerancing Generalities,

definitions, symbols, indication on drawings

3

Table 3 Limit deviation , of the length at any point of the measuring face from

and tolerance, , for the variation in length.

Nominallength,

Calibration grade K Grade 0 Grade 1 Grade 2limit

deviationof length

at anypointfrom

nominallength

tolerancefor the

variationin length

limitdeviationof length

at anypointfrom

nominallength

tolerancefor the

variationin length

limitdeviationof length

at anypointfrom

nominallength

tolerancefor the

variationin length

limitdeviationof length

at anypointfrom

nominallength

tolerancefor the

variationin length

0.5 1010 2525 5050 75

0.20.30.40.5

0.050.050.060.06

0.120.140.2

0.25

0.10.10.1

0.12

0.20.30.40.5

0.160.160.180.18

0.450.60.81

0.30.30.3

0.35

75 100100 150150 200

0.60.81

0.070.080.09

0.30.40.5

0.120.140.16

0.60.81

0.20.2

0.25

1.21.62

0.350.40.4

200 250250 300300 400

1.21.41.8

0.10.1

0.12

0.60.70.9

0.160.180.2

1.21.41.8

0.250.250.3

2.42.83.6

0.450.50.5

400 500500 600600 700

2.22.63

0.140.160.18

1.11.31.5

0.250.25

3

2.22.63

0.350.4

0.45

4.456

0.60.70.7

700 800800 900

900 1000

3.43.84.2

0.20.2

0.25

1.71.92

0.30.350.4

3.43.84.2

0.50.50.6

6.57.58

0.80.91

3. Measurement by comparison

3.1 Principle of measurement

In order to determine the length of a gauge block by compa-

rison, the difference of its central length from that of a reference

gauge block is measured and applied algebraically to the length

of the reference. For the probing, the measuring faces of each

gauge are touched from opposite directions as shown in Figure

1, and the length difference is measured by a high resolution

length indicator.

algebraically difference (see arrow point 1) = = Measured length

= Nominal length

Fig. 1 Measurement of central length by comparison taking the perpendicular

distance from the centre of a measuring face to the opposite one

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3.2 Central length

A measurement by comparison transfers the central length of a standard gauge block to

a gauge block under test. The reference gauge block may either directly be measured

by interferometry or related through one or several stages by comparison to a reference

gauge measured by interferometry.

NOTE:

The effect of one wringing, which is included in the length of the reference gauge block

measured by interferometry, is transferred by the comparison measurement.

3.3 Method of determining length by comparison

The relatively small difference in central length between a reference gauge block of

known central length and another gauge of unknown central length is measured by a

high resolution length indicator. (see Figure 2).

Figure 2 Measurement of central length by comparison taking the perpendicular distance from the

centre of a measuring face to the opposite one. Schematic diagram of a gauge block comparator with

Measured Length Difference

Reading ()

Calibration ()

Drift ()

Temperature

Reading ()

Calibration ()

Drift ()

= 20

PenetrationFlatness

= = +

InductiveProbe 1

InductiveProbe 2

Parallelism

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opposing styli and digital readout. The diagram depicts the influence factors in the measurement, and the

mathematical symbols which will represent them in the text.

Figure 2 shows the reference gauge block in the reading position between an upper

contact and one beneath. The anvils are retractable and the weight of the block is

supported independently. The connection line between the two anvils is perpendicular

to the measuring faces. A reading of the indicator is taken at the centre of the reference

gauge block which is then replaced by the gauge block to be measured and a central

reading is taken on it. The vertical position is used for comparison of gauge blocks of

nominal lengths up to 100 mm; Gauge blocks of nominal lengths longer than 100 mm

can also be measured by comparison with a reference gauge block in the horizontal

position. If a vertical orientation as specified in fig.2 is used, the supports are adjusted

horizontally and vertically so that one anvil of the comparator contacts the centre of one

measuring face of the gauge block and the second anvil is moved over the second face

until the minimum reading is obtained.

4. HOW CHOOSING THE RIGHT GAUGE BLOCK COMPARATOR

For mainly economic reasons, most of customers of accredited laboratories prefer their

standards being calibrated by mechanical comparison because of lower costs and

shorter calibration time. In the market there are many products of the Gauge Block

comparators. All of them of course have advantages and disadvantages. This white

paper will guide you how to excecute the right Gauge Block Comparators.

As you know many comparisons, especially those in dimensional metrology, cannot be

done simultaneously. Using a gage block comparator, the standard, control (check

standard) and test block are moved one at a time under the measurement stylus. For

those comparisons each measurement is made at different time. The term calibration

design can be applied to experiments where only differences between nominally equal

objects or groups of objects can be measured.

Ordinarily the order in which these measurements are made is of no consequence. The

usefulness of drift eliminating designs depends on:

1. The stability of the thermal environment, accuracy required in the calibration. The

environment has to be stable enough that the drift is linear.

2. Each measurement must be made in the same amount of time so that the

measurements are made at fairly regular intervals. Finally, the measurements of

each block are spread evenly as possible across the design. The designs are

constructed to:

Be immune to linear drift.

Minimize the standard deviations for test blocks (as much as possible)

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Spread the measurements on each block throughout the design

Be completed in 5-10 minutes to keep the drift at the 5 nm level

4.1 Critical contributions to the uncertainty

Critical contributions, which could be dimi-

nished, are temperature deviations, calibra-

tion of the comparator and calibration of

reference gauge blocks. Temperature devi-

ations are influenced by the air temperature

deviations and by radiation of illuminating

bodies and of the operator.

Temperature can not be measured

on the gauge block and therefore table

temperature is assumed to be the tempe-

rature of the gauge block. However, devia-

tions between these two temperatures can differ for up to 0.05C. Additional problems

appear by longer gauge blocks (near 100 mm) because temperature is not constant

along the height. Beside that, thermal expansion coefficient of gauge blocks is usually

not exactly known. It may differ for 1 10. Thermal expansion correction is

therefore not accurate.

These problem happened because difficult to know what is excactly the CTE4 of

the material gauge block. According to Ted Doiron and John Beers 5, The uncertainty in

the expansion coefficient of the gauge block is more difficult to estimate.

Table 4: shows that as the blocks get longer, the thermal expansion coefficient becomes systematically

smaller. It also shows that the differences between blocks of the same size can be as large as a few

percent. Because of these variations, it is important to use long length standards as near to 20 C as

possible to eliminate uncertainties due to the variation in the expansion coefficient.

4CTE (Coefficient Thermal Expansion).

5Gage Block Hand Book, Dimensional Metrology Group Precision Engineering Division, National Institute of

Standards and Technology (NIST-USA).

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4.2 UNCERTAINTY BUDGET AND CRITICAL CONTRIBUTIONS

In order to establish proper environmental conditions, special microclimatic chamber

with own precise air conditioning system was built in the laboratory. Temperature

deviations in the measuring space of gauge block comparator are limited to 0,1C.

Temperature is the most important influence factor on measuring uncertainty in

calibration beside the accuracy of calibration of reference gauge blocks. The expanded

uncertainty of the calibration of gauge blocks by mechanical comparison in the machine

is currently (0.03 + L/3000) m, L in mm*. Using coverage factor k = 2. It was

calculated according to guide ISO GUM 2 edition, 1995 and comprises all important

components. Some of them were evaluated statistically by repeated measurements,

some of them were evaluated by experience and some of them on the basis of different

data from manuals and calibration certificates. All the measurements including tempera-

ture measurements were performed with traceable equipment and measuring uncer-

tainties were evaluated for all measurements.

5. STRATEGY FOR DECREASING CURRENT LEVEL OF

UNCERTAINTY 6

5.1 Study of temperature influences

The following temperature parameters have been analysed:

change of gauge block temperature after putting it on the comparator,

temperature difference between the gauge blocks and the comparator table,

temperature influence of the illumination and the operator, and

thermal expansion coefficients of gauge blocks.

All the above parameters were detected as critical and are

therefore subject of optimisation. Research was focused into

experimental simulations of different materials and colours of

these materials. It was found out that the radiation of different

materials is quite different and that calibration results are critically

influenced by radiation. The first resource of radiation are lights

producing constant radiation that can be eliminated from calibra-

tion result by calculation. The second source of radiation is the

operator. This source is not constant and is quite critical because

the gauge block that is closer to the operator is heated stronger

that the gauge block standing behind the first one. Shield that is

put between the operator and the comparator is made of

transparent plastics in order to enable the operator to look into measuring area.

6B. Acko, A. Sostar and A. Gusel, REDUCTION OF UNCERTAINTY IN CALIBRATION OF GAUGE BLOCKS

Laboratory for Production Measurement, Faculty of Mechanical Engineering, University of Maribor, 2000, Maribor,Slovenia.

8

However, the isolating properties of this material are not good enough to stop the

radiation of a human body.

5.2 Surfaces for thermal stabilisation of the gauge blocks

Surfaces for thermal stabilisation of gauge blocks have a great influence on

thermal behaviour of gauge blocks during the calibration. If these surfaces get other

temperature than the air and the comparator table, the gauge block temperature

changes rapidly. Our analyses have shown that an aluminium plate with bright shining

or light grey surface gives the best results. Long term temperature measurements are

shown in Figure 3 as an example. The white plastic surfaces used before these studies

are always warmer than the air and cause additional heating of gauge blocks. The

same properties have steel surfaces. The stone that has been used in the past has the

worst characteristics and is not recommended at all.

In this research four materials (aluminium, steel, stone and plastic) in four different

colours (black, white, grey and shining) were tested. Temperature was measured in the

air, on the tested surface and on the gauge blocks, that were put on these surfaces.

Long term and short term deviations were measured with the light turned on and with

the light turned off in the measurement room. When the light was turned off, the

differences were not so critical because the radiation was very low. In contrary, when

the lights were turned on, big differences in temperature changes were observed. The

worst behaviour was detected at dark steel and stone surfaces.

Figure 3. Temperature comparison between the aluminium base plate and the gauge

block.

5.3 Material of the comparator table

Some simulations with different materials have shown the importance of the

material and the colour of the table. The existing tables (tested on Mahr and Cary

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comparators) do not have ideal properties because of dark colours. The temperature is

constantly above the air temperature. When gauge block is put on the table, the

temperature starts to grow and the length of the gauge block is changing.

Typical temperature behaviour after putting gauge block on the table is shown in figure

3. In this case gauge block was put on the comparator table at 10:41 and at 10:50

calibration of this gauge block was finished. From this case we can learn, that the

gauge block should not be measured immediately after putting it on the table. It should

be stabilised for approximately three minutes. Probe 1 in Figure 3 was measuring the

reference gauge block temperature and probe 2 was measuring the temperature of

measured gauge block. We can see that the temperatures of both gauge blocks after

stabilisation are not equal.

5.4 Thermal expansion coefficient of the gauge blocks

Thermal expansion coefficient must be determined with standard uncertainty

0.3 10. For this we shall determine thermal expansion coefficient within the

limits of 0.5 10. Usually this is not a problem, but sometimes we get gauge

blocks without any data about . In such cases we can not be sure that was

determined with proper accuracy. Temperature deviations must therefore be dimini-

shed to minimum possible values.

Another problem is the difference between thermal expansion coefficients of the

reference and measured gauge blocks. Uncertainty contribution of his difference in mm

is expressed by the equation:

= ()

Where:

- uncertainty contribution in mm() - uncertainty of the difference of thermal expansion coefficients ( ( - nominal length of gauge blocks - maximum possible temperature deviation

If uncertainty of the difference between thermal expansion coefficients is 0.6 10

and maximum possible temperature deviation is 0.1C, than the uncertainty contribution

in calibration of 100 mm gauge block is 0.006 m what is the maximum possible limit for

our demands. If temperature deviation is 0.3C, we get the value of 0.018 m, what is

far too much.

From the above example we can conclude that temperature deviations and the

limits of determination of thermal expansion coefficient must be within very low values.

These two components are in strong correlation and influence the length dependent

part of the combined uncertainty.

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Figure 4. Thermal behaviour of gauge blocks after putting them on the comparator

table

5.5 Indentation of different gauge blocks by the applied measuring force

Research in this field is made in order to be able to calibrate gauge blocks of

different materials. The indentation is calculated by the following (Roark's) equation:

= 1.4

=1

+1

Where:

- indentation - probing force - probing ball diameter - Poisson number of the probing ball material - Poisson number of the gauge block material - elasticity module of the probing ball material - elasticity module of the gauge block material

If probing force is 0,1 N, probing ball of d = 2 mm is made of hart metal and gauge block

is made of steel, than the indentation is 0.116 m. For the gauge block made of hart

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metal this value would be 0.081 m. The difference of the both indentations is 0.035

m. The greatest problem in this calculations is to get right values for and . Every

deviation of these values causes uncertainty of indentation calculation. The second

problem is evaluation of probing force, which shall be measured very precisely. There-

fore, some experimental work has already been performed in order to evaluate uncer-

tainty of indentation. It was found out that the standard uncertainty can be held in the

limits of 0.008 m.

6. DRIFT ELIMINATING DESIGN

The sources of variation in measurements are numerous. Measurements on gauge

blocks are subject to drift from heat built-up in the comparator. This effect cannot be

minimized by additional measurement because it is not generally pseudorandom, but a

nearly monotonic shift in the readings. In dimensional work the most important cause of

drift is thermal changes in the equipment during the test.

The purpose of drift eliminating design is remove the effects of linear instrumental drift,

but also alowing the measurement of the linear drift itself. This measured drift can be

used as a process control parameter. For small drift rates an assumption of linear drift

will certainly be adequate. But, for high drift rates or long measurements time the

assumption of linear drift may not be true.

Measurements on gauge blocks are subject to drift from heat build-up in the compa-

rator. This drift must be accounted for in the calibration experiment or the lengths

assigned to the blocks will be contaminated by the drift term. The designs are

constructed so that the solutions are immune to linear drift if the measurements are

equally spaced over time. The size of the drift is the average of the difference

measurements. Keeping track of drift from design to design is useful because a marked

change from its usual range of values may indicate a problem with the measurement

system.

7. OVERVIEW OF THE GAUGE BLOCK COMPARATOR PRECIMAR 826 VS

TESA UPG.

PRECIMAR 826 TESA UPD TESA UPCDESIGN: rigid cast-iron stand with vertical

guiderigid cast-iron stand with verticalguide

Measuring range 0.5 mm to 170 mm 0.5 mm up to 100 mmWeight 37 Kg (comparator only) 23 kg (comparator only)NOTE:Heavier Unit more steady (immune from vibration)Repeatability (with no influenceof external temperature)

0.01 m 0.04 m

Measuring uncertainty (.+ )/ , = (. + . ( For instance Gauge Block 100 For instance Gauge Block 100

12

mm, = (0.03 + 100/3000) = 0.03 + 0.033

= 0.063

mm, = (0.07 + 0.5 0.1) = 0.07 + 0.05

= .NOTE:Following the hierarchy of calibration, calibrator must be 10% or 20% smaller then tolerance of theworking standard. For example, Gauge Block tolerance 0.12 m (Grade 0, length 100 mm), must becalibrated by Gauge Block Comparator with Uncertainty minimum 0.12 x 0.2 = 0.024 m. Following thisreason, methode direct measurement is not acceptable. In the comparison method the stability of theinstrument is 0.02 + 0.25 10 ( Precimar 826).Repeatability (with no influenceof external temperature)

0.01 m 0.025 m

Measuring uncertainty (.+ )/ , = (. + . ( For instance Gauge Block 100mm, = (0.03 + 100/3000)

= 0.03 + 0.033= 0.063

For instance Gauge Block 100mm, = (0.05 + 0.5 0.1)

= 0.05 + 0.05= .

Repeatability (with no influenceof external temperature)

0.01 m 0.015 m

Measuring uncertainty (.+ )/ , = (. + . ( For instance Gauge Block 100mm, = (0.03 + 100/3000)

= 0.03 + 0.033= .

For instance Gauge Block 100mm, = (0.02 + 0.2 0.1)

= 0.02 + 0.02= .

NOTE:Following the Gauge Block tolerance in the Table 3, UPC and UPD can calibrate Gauge Block Grade 1and 2 only. Precimar 826 can calibrate Gauge Block Grade 0, 1 and 2. Only TESA with= (0.02 + 0.2 ( can be used for Grade 0, but this instrument still struggle to get the uncertaintybecause it will depends on the temperature correction. You know that in the paragraph 4.1 explainedThermal expansion correction is therefore not accurate. So that is why the Comparator need the deviationof the temperatur is 0.03C.Material Housing Diecast Iron Diecast AluminumSensors for length dimensions:

Double Probes Yes Yes Yes

NOTE:The measuring configuration consisting of two probes aligned opposite one another combined with theconcept and quality of the measuring system is the guarantee for an extra low measurement uncertainty.

With analog and digitalindication system,

Yes Digital Only Analog Only

Measuring force activation Measuring bolt retraction

YesBy electrical vacuum pum lifted

YesBy Vacuum

YesBy electro-motorised

Measurement Method Comparative Measurement DirectMeasurement

ComparativeMeasurement

Advantage:Enhances the measuring condi-tions, thus permitting all measure-ments to be taken with loweruncertainty. NO linear error.Disadvantages:Allows gauge blocks of samenominal lengths to be measuredby comparison. Need 122 GaugeBlock set as a standard compa-rator.

Advantage:Permits over 90 % of a 122-pieceset to be checked using the samereference gauge block All nominallengths of the full gauge set beingcontained within 0,5 and 25 mm.Disadvantages:In fact the measuring span istherefore exceeded, caused ofLinear Error of the probes.

NOTE:For the Direct measurement method actually data will found linear error that is not an advantages for themachine due to the machine must immune to linear drift. So the choice of the Comparative methode is thebest solution.

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PRECIMAR 826 TESA UPD TESA UPDComputer-aided calibration Measurement of single gage blocks Measurement of complete gage

block sets Simultaneous calibration of several

equal gage block sets.

YesYes

Yes

YesYes

Yes

Features of the Hardware: Selection and determination of

measuring sequences.Yes Yes

Management of testpiece andstandard gauge block sets.

Yes Yes

Management of individual gaugeblocks

Yes Yes

Measuring program to perform gaugeblock tests

Yes Yes

Control of all operations and inputs Yes Yes Automatically assigning the sequenceof nominal dimensions for set tests

Yes Yes

Organization of the measurementprocess for testing multiple sets

Yes Yes

Printer program for test records andfor the printout of standard gage blocksets

Yes Yes

Data management Yes Yes

ADVANTAGES: Rigid cast-iron stand, making the

unit temperature stableYes Yes

Easily adjustable vertical slide withupper probe

Yes Yes

Fine adjustment via rigidlyConnected Parallelogram springs

Yes No

NOTE:Fine adjustment is carried out by adjusting a rigidly connected spring parallelogram system which isintegrated into the support arm. Electro pneumatic lifting Yes Yes Gentle, precise and extremely smoothmanipulator operation due to slideways,which are impervious to dirt

Yes Yes

NOTE:The foot of the base holds the manipulator for correctly positioning the gage blocks. Smooth movement isensured by high precision ball bush guides. Ergonomical operation due to the

optimally arranged gauge blockmanipulator

Yes Yes

NOTE:Whenever the gage blocks are moved, the inductive probes are lifted by means of an electrical vacuumpump. Measurement not influenced by

manual forceYes Yes

Easy movement of the gage blocksdue to support consisting ofhardened circular guide bars

Yes Yes

No zero setting required, since theset value is automatically related tothe nominal value of the respectivereference gage block

Yes Yes

Very effective protection from heat Yes Yes

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due to an acrylic glass screen alongthe front and both sides of the unit

Penetration correction Yes NONOTE:See paragraph 5.5, the correction of the geometric factor of the probe tip penetration. Correction of differing thermal

expansion coefficientsYes Yes

Computation of mean values Yes Yes Simultaneous calibration of several

sets possibleYes Yes

Inherent stabilization of temperatureand vibration

Yes Yes

CONCLUTION:

All the concept of the Comparator must be supported for the its accuracy, with the

requirement of the uncertainty measurement factors, such as:

1. The design, must be immune from the errors, such as measuring base by refelcted

material to reduce temperature influence.

2. Comparation method, to avoid linear error of the probes.

3. Quick measurement to avoid linear drift cause of time of measurement.

4. Avoid themperature correction due to difference of the CTE, unless the themperature

difference over 0.4C.

5. Correction from the elasticity of the material effect from the contact point.

References:

1. ISO 3650:1999, Geometrical Product Specifications (GPS) Length standards

Gauge blocks.

2. ISO. GUM: 1995, Guide Uncertainty of Measurement.

3. Ted Doiron and John Beers, Gage Block HandBook, Dimensional Metrology Group

Precision Engineering Division National Institute of Standards and Technology.

4. B. Acko, A. Sostar and A. Gusel, reduction of uncertainty in

calibration of gauge blocks, Laboratory for Production Measurement Faculty of

Mechanical Engineering University of Maribor, 2000 Maribor, Slovenia.