ISO/R 1938:1971

ISO system of limits and fits -- Part II : Inspection of plain workpieces

Specifies the interpretation to be given to the limits of dimensions to be inspected, and gives the essential details concerning limit gauges and indicating measuring instruments necessary for the inspection of tolerances of the ISO system. It includes 11 figures and 5 tables.

Système ISO de tolérances et d'ajustements -- Partie II : Vérification des pièces lisses

General Information

• Status: Withdrawn
• Publication Date: 31-Mar-1971
• Withdrawal Date: 31-Mar-1971
• ICS: 17.040.10 - Limits and fits
• Technical Committee: ISO/TC 213 - Dimensional and geometrical product specifications and verification
• Drafting Committee: ISO/TC 213 - Dimensional and geometrical product specifications and verification
• Current Stage: 6060 - International Standard published
• Completion Date: 01-Apr-1971

RELATIONS

• Revised: ISO 1938-1:2015 - Geometrical product specifications (GPS) -- Dimensional measuring equipment
• Effective Date: 28-Feb-2009

Standards Content (sample)

Ref. No. : NO/R 19384971 (E)
UDC 621.753
IS0
INTERNATIONAL ORGANIZATION FOR STANDARDIZATION
IS0 RECOMMENDATION
R 1938
IS0 SYSTEM OF LIMITS AND FITS
PART II : INSPECTION OF PLAIN WORKPIECES
1st EDITION
April 1971
COPYRIGHT RESERVED
The copyright of IS0 Recommendations and IS0 Standards
belongs to IS0 Member Bodies. Reproduction of these
documents, in any country, may be authorized therefore only
by the national standards organization of that country, being
a member of ISO.

For each individual country the only valid standard is the national standard of that country.

Printed in Switzerland
Copies to be obtained through the national standards organizations.
Also issued in French and Russian.
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BRIEF HISTORY

The IS0 Recommendation R 1938, IS0 system of limits and fits - Part II : Inspection of plain workpieces, was

drawn up by Technical Committee ISO/TC 3, Limits and fits, the Secretariat of which is held by the Association

Francaise de Normalisation (AFNOR).

Work on this question led to the adoption of Draft IS0 Recommendation No. 1938, which was circulated to

all the IS0 Member Bodies for enquiry in February 1970. It was approved, subject to a few modifications of an editorial

nature, by the following Member Bodies :
Austria Ireland Romania
Belgium Israel South Africa, Rep. of
Chile Italy Switzerland
Czechoslovakia Japan Thailand
France Korea, Rep. of Turkey
Germany New Zealand U.A.R.
Greece Norway United Kingdom
Hungary Poland U.S.S.R.
India Portugal
The following Member Bodies opposed the approval of the Draft :
Australia
Canada
Sweden

This Draft IS0 Recommendation was then submitted by correspondence to the IS0 Council, which decided to

accept it as an IS0 RECOMMENDATION.
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CONTENTS
Page
...............................
Introduction
................................
l.Scope
.........................
2. General rules of inspection.
..................
2.1 Reference temperature and measuring force
.................
2.2 Interpretation of size limits (Taylor principle)
..........................
2.3 Exceeding the limits
.......................
2.4 Choice of inspection method
..............................
3. Limit gauges
............................ 8
3.1 Gauge types
.....................
3.2 Application of the Taylor principle
................. 8
3.3 Allowable deviation from the Taylor principle
.............. 9
3.4 Field of utilisation of the various types of limit gauges
....................
3.5 General design features of limit gauges
.................... 10
3.6 Materials and further details of gauges
................... 10
3.7 Definitions and inspection of gauge sizes
........................ 10
3.7.1 Cylindrical plug gauges
.................... 10
3.7.2 Spherical plug, disk and rod gauges
.........................
3.7.3 Cylindrical ring gauges
........................... 11
3.7.4 Gap gauges.
........................ 11
3.8 Method ofuse of gauges
.......................... 11
3.8.1 Gaugesforholes
.......................... 12
3.8.2 Gauges for shafts
.............
3.9 Manufacturing tolerances and permissible wear of gauges.
............................ 12
3.9.1 Symbols.
........................... 13
3.9.2 Limit gauges
.... 13

3.9.2.1 Positions of tolerance zones and wear limits in relation to workpiece limits.

................. 13
3.9.2.2 Tolerances on size of working gauges
.................
3.9.2.3 Tolerances on form of working gauges
................
3.9.2.4 Adjustable gauges of sizes above 180 mm
..................... 14
3.9.3 Reference disks for gap gauges
........ 14
3.9.3.1 Positions of tolerance zones with respect to the workpiece limits
.................
3.9.3.2 Tolerances on size of reference disks
................. 14
3.9.3.3 Tolerances on form of reference disks
.......
3.9.3.4 Relation between tolerances on gap gauges and their reference disks 14
.......... 14
3.9.4 Reference ring and plug gauges for setting measuring instruments
........................ 14
3.9.5 Surface finish of gauges
.........................
3.10 Settlement of disputes 14
.....................
3.10.1 Inspection by the manufacturer 15
......................
3.10.2 Inspection by the purchaser 15
..................... 15
3.11 Marking and designation of gauges
.......................
4. Indicating measuring instruments 15
....................
4.1 Definitions relating to measurements 15
.........................
4.1.1 True size
........................
4.1.2 Error of measurement 15
.......................... 16
4.1.3 Systematic error
4.1.4 Random error ........................... 16
4.1.5 Average ............................. 16
.......................... 16
4.1.6 Standard deviation
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Page
......................
4.2 Types of measuring instruments 16
...................
4.3 Measuring uncertainty of the instrument 16
.......................
4.4 Total measuring uncertainty 17
...........................
4.5 Inspection limits 17
......................
4.6 Influenceofformerrors.. 17
Figures

Fig. 1 - Extreme errors of form of hole allowed by the recommended interpretation of the limits

ofsize . . . . . . . . . . . . . . . . . . . . . . . . . l l . .

Fig. 2 - Extreme errors of form of shaft allowed by the recommended interpretation of the limits

ofsize . . . . . . . . . . . . . . . . . . . . . . . . l . . . .

Fig. 3 - Recommended types of gauges for holes . . . . . . . . . . . . . l . l . .

Fig. 4 - Recommended types of gauges for shafts . . . . . . . . . . . . . . . . . .

Fig. 5 - Types of gauges used to check holes, in order of preference . . . . . . . . . . . .

Fig. 6 - Types of gauges used to check shafts, in order of preference. . . . . . . . . l . .

Fig. 7 - Location of points where forces counterbalancing part of the weight of the gauge should be

applied. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . l . 22

Fig. 8 - Relation between manufacturing tolerances of gap gauges and of reference disks .

.............. 23
Fig. 9 - Tolerance zones of limit gauges and reference disks
...............
Fig. 10 - Normal distribution curve of instrument readings 24
............. 24
Fig. 11 - Relation between “test limits” and the “limits of size”
Tables

Table 1 - Manufacturing tolerances for gauges . . . . . . . . . . . l l . . . . l l 25

Table 2 - Location of the gauge tolerances and the limit of maximum permissible gauge wear in relation

to the nominal limit of the workpiece, for grades 6 to 16 (for D < 500 mm) . . . . . l 26

Table 2 A - Location of the gauge tolerances and the limit of maximum permissible gauge wear in

relation to the nominal limit of the workpiece, for grades 6 to 16 (for D > 500 mm) . . 27

Table 3
- ValuesofsM(forD<500mm). . . . . . . . . . . . . . . . . . . . . 28
Table 3 A - ValuesofsM(forD>500mm). . . . . . . . . . . . . . . . . l . . 28
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ISO/R 19384971 (E)
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April 1971
IS0 Recommendation R 1938
IS0 SYSTEM OF LIMITS AND FITS
PART II : INSPECTION OF PLAIN WORKPIECES
INTRODUCTION

This IS0 Recommendation is a continuation of IS0 Recommendation R 286, IS0 system of limits and fits -

Part I : General, tolerances and deviations.

The information concerning the indicating measuring instruments is new whereas that relating to the choice of gauges

to be used, including their tolerances and wear margins, is practically the same as in the old ISA System (ISA Bulletin

25 of January 1941).

In particular accepted trespassing of the limits for practical reasons (y or y1 margin) in the case of grade 8 and lower

still remains the rule, but might possibly be reduced or omitted in the future. Present values for tolerances and wear

of gauges should be considered as maximum values.

From now on this IS0 Recommendation allows the use of new tolerance grades experimentally; these are named grades

6N, 7N, 8N to distinguish them from the preceding 6, 7 and 8 and differ from the latter only by a much lower wear

of gauges enabling any margin to be dispensed with 101 or y 1 = 0).

Numerical values as given in this IS0 Recommendation are expressed in terms of the various grades provided for in

IS0 Recommendation R 286, and are therefore valid only for the IS0 system of tolerances or by comparison with

this system; all other details of a more general nature may still be applied as a rule to any system of limits for plain

workpieces.

As in IS0 Recommendation R 286, these numerical values essentially concern diameters not greater than 500 mm.

However, values applicable to the limits of workpieces of diameters greater than 500 mm, such as those appearing

provisionally in section 3 of IS0 Recommendation R 286, are also given here as a supplement and are intended

mainly for experimental purposes.
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ISO/R 19384971 (E)
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1. SCOPE

This IS0 Recommendation relates to the inspection of plain workpieces. It specifies the interpretation to be

given to the limits of dimensions to be inspected, and gives the essential details concerning limit gauges and

indicating measuring instruments necessary for the inspection of tolerances of the IS0 system.

2. GENERAL RULES OF INSPECTION
2.1 Reference temperature and measuring force

IS0 Recommendation R 1, Standard reference temperature for industrial length measurements fixes

this temperature at
20 “c

This is the temperature at which dimensions specified for workpieces and their inspection instruments

are defined and at which the inspection should normally be carried out.

In addition all measuring operations provided for in this IS0 Recommendation are understood as referred

to a zero measuring force.

If the measurement is carried out with a measuring force different from zero its result should be corrected

accordingly. This correction however is not required for comparative measurements carried out with the

same comparing means and the same comparing force between similar elements of identical material and

identical surface roughness.
2.2 Interpretation of size limits (Taylor principle)

In order to guarantee, so far as is practicable, that the functional requirements of the IS0 system of

limits and fits are interpreted in the following within the prescribed
attained, the limits of size should be
way
length.

For holes, the diameter of the largest perfect imaginary cylinder which can be inscribed within the hole so

that it just contacts the highest points of the surface should not be a diameter smaller than the GO limit

. of size. In addition the maximum diameter at any position in the hole must not exceed the NOT GO limit

of size.

For shafts, the diameter of the smallest perfect imaginary cylinder which can be circumscribed about the

shaft so that it,just contacts the highest points of the surface should not be a diameter larger than the GO

limit of size. In addition the minimum diameter at any position on the shaft must not be less than the

NOT GO limit of size.

The above interpretation means that if the size of the hole or shaft is everywhere at its GO limit then the

* hole*or shaft should be perfectly round and straight.

Unless otherwise specified, and subject to the above requirements, departures from true roundness and

straightness may reach the full value of the diametral tolerance specified. Typical extreme errors of form

permitted by this interpretation are illustrated in Figures 1 and 2. Such extreme errors are unlikely to

arise in practice.

The above interpretation of the size limits results from the “Taylor principle”, named after the late

W. TAYLOR who first laid it down in 1905. It is based on the use of a correct system of limit gauges to

inspect shafts and holes. According to this principle a hole should completely assemble with a GO cylindrical

plug gauge made to the specified GO limit of the hole, having a length at least equal to the length of

engagement of the hole and shaft. In addition the hole is measured or gaugedto check that its maximum

diameter is not larger than the NOT GO limit. The shaft should assemble completely with a ring gauge

made to the specified GO limit of the shaft and of a length at least equal to that of the length of engagement

of the shaft and hole. Finally the shaft is measured or gauged to check that its minimum diameter is not

smaller than the NOT GO limit.

In special cases the maximum errors of form permitted by the above interpretation may be too large to

allow satisfactory functioning of the assembled parts; in such cases separate tolerances should be given for

the form, e.g. separate tolerances on circularity or straightness, according to IS0 Recommendation R 1101,

Technical drawings - Tolerances of form and of position - Part I : Generalities, symbols, indications on

drawings.
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2.3 Exceeding the limits

The above mentioned size limits are those specified in IS0 Recommendation R 286, IS0 system of limits

and fits - Part I : General, tolerances and deviations. However, in order to take account of the existing

manufacturing techniques of gauges, tolerances of manufacture and wear of limit gauges are such that

the limits of specified dimensions for grades 6 to 8 may be exceeded in some cases 01 or y1 margin, see

clause 3.9.2.1). In this case if workpieces are inspected during the manufacture by means of indicating

measuring instruments instead of limit gauges, the manufacturer may also take into account the same

y or y 1 margin as for gauges, in order to establish a uniform acceptance principle.

If inspection should exceptionally be carried out without any margin 0 or y 1 = 0) in grades 6 to 8 which

normally require it, this should be explicitly specified by writing the letter N following the grade number.*

2.4 Choice of inspection method

Workpieces may be inspected either by means of fixed limit gauges or by means of indicating measuring

instruments

Both methods have advantages and drawbacks of their own which it is important to know before selecting

one of them.

A system of limit gauges designed in strict conformity with the Taylor principle has the advantage of

checking the geometry as well as the sizes of workpieces. However, for practical reasons departures from

this principle may be made as stated in clause 3.3 and so the inspection is possibly not so satisfactory as

might be expected theoretically.

Furthermore gauges themselves have errors of form and size, and their necessary manufactu ring and

tolerances further reduce the amount of tolerance that remains available on the workpiece.

Measuring instruments give the workpiece size in the measuring position only and do not check the

geometry, which requires separate measurements the result of which, in theory, should be correlated to

that of the dimensional measurement. This tedious procedure is not necessary provided sufficient reliance

can be placed on the manufacturing accuracy to ensure that, in practice, form errors can be ignored.

On the other hand, contrary to what occurs with limit gauges, the use of such instruments has the advantage,

when workpieces have very small tolerances, of not reducing the amount of tolerance.that remains available

on the workpieces. Finally the use of such instruments allows sampling inspection which gives warning

when the sizes approach one of the limits during a continuous manufacturing process.

In order to avoid disputes it is recommended inspection to be for acceptance be
that the type of
specified on the order.

Unless strictly specified to the contrary a workpiece should be considered as good when the manufacturer

can prove that it was recognized as such by the inspection method he chose in conformity with this IS0

Recommendation.

* The designation o f former grades, with margin, remaining unch it is nevertheless permitted to indicate the distinction more

.anged,
clearly by writing the letter A following their grade number.
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ISO/R 1938-1971 (E)
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3. LIMIT GAUGES
3.1. Gauge types

X1.1 Limit gauges are used to inspect the workpieces. For gauging internal diameters they may be of the

following types :
- full form cylindrical plug gauge;
full form spherical plug or disk gauge;
segmental cylindrical bar gauge;
segmental spherical plug gauge;
segmental cylindrical bar gauge with reduced measuring faces;
rod gauge with spherical ends.
For gauging external diameters the following gauges may be used :
- full form cylindrical ring gauge;
- gap gauge.

3.1.2 Reference gauges or block gauges may be used to inspect or adjust limit gauges :

(a) reference gauges are either reference disks intended for setting gap gauges, or cylindrical ring or

plug gauges used for calibrating gauges or indicating measuring instruments;

(b) block gauges are standards of length having parallel plane end surfaces which are used for calibrating

gauges or indicating measuring instruments.
3.2 Application of the Taylor principle

Except for allowable deviations (see clause 3.3) strict application of the Taylor principle leads to using

for checking the GO limit of the workpiece:
work-

a plug gauge or a ring gauge having exactly the GO 1 .irnit diam .eter and a length equal to the

piece length (or the engagement length of the fit to be made)
- for checking the NOT GO limit :

a gauge contacting the workpiece surface only at two diametrically opposite points and having

exactly the NOT GO limit diameter.

The GO gauge should perfectly assemble with the workpiece to be inspected and the NOT GO gauge should

not be able to pass over or in the workpiece in any consecutive position in the various diametrical directions

on the workpiece length.
3.3 Allowable deviation from the Taylor principle

As the application of principle is not always strictly compulsory or comes up against difficulties

the Taylor
in the convenient use certain deviations may be allowed (see clau se 3.4).
of gauges,

At the GO limit a full form gauge is not always necessary or used, as for instance in the following cases :

The length of a GO cylindrical plug or ring gauge may be less than the length of engagement of

the mating workpieces if it is known that with the manufacturing process used the error of

straightness of the hole or shaft is so small that it does not affect the character of fit of the

assembled workpieces. This deviation from the ideal facilitates the use of standard gauge blanks.

For gauging a large hole a GO cylindrical plug gauge may be too heavy for convenient use, and it is

permissible to use a segmental cylindrical bar or spherical gauge if it is known that with the manu-

facturing process used the error of roundness or straightness of the hole is so small that it does not

affect the character of fit of the assembled workpieces.
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ISO/R 19384971 (E)
-9-

A GO cylindrical ring gauge is often inconvenient for gauging shafts and may be replaced by a gap

gauge if it is known that with the manufacturing process used the errors of roundness (especially

lobing) and straightness of the shaft are so small that they do not affect the character of fit of the

assembled workpieces. The straightness of long shafts which have a small diameter should be

checked separately.

At the NOT GO limit a two-point checking device is not always necessary or used, as for instance in the

following cases :
- Point to rapid wear, and in most cases may be replaced where appropriate by
contacts are subject
small spherical surfa ce s.
plane, cy lindrical or

For gauging very small holes a two-point checking device is difficult to design and manufacture.

NOT GO plug gauges of full cylindrical form have to be used but the user must be aware that there

is a possibility of accepting workpieces having diameters outside the NOT GO limit.

Non-rigid workpieces may be deformed to an oval by a two-point mechanical contact device

operating under a finite contact force. If it is not possible to reduce the contact force to almost

zero, then it is necessary to use NOT GO ring or plug gauges of full cylindrical form.

Such thin-walled workpieces may be out of round (due to internal stresses or heat treatment). In these cases

the NOT GO limit has the meaning that the circumference of the cylinder corresponding to that limit must

not be transgressed. Therefore NOT GO gauges of full cylindrical form have to be applied with a force that

just suffices to convert the elastic deformation into circularity but does not expand or compress the wall of

the workpiece.

Lastly, the sizes of gauges cannot be made exactly to the appropriate workpiece limit : they have to be made

to specified tolerances.
3.4 Field of utilisation of the various types of limit gauges

Taking account of the above remarks,recommended types of gauges for various ranges of workpiece nominal

dimensions are given in Figures 5 and 6, the meaning of symbols used being given in Figures 3 and 4.

General design features of limit gauges
3.5

No recommendation is given for designs; these are left to the initiative of the gauge

the details of the
makers or the nation al standards organizations.

The various types of gauges are illustrated in Figures 3 and 4. The recommended types of gauges for the

different ranges of nominal size of the workpieces are shown in Figures 5 and 6. The key to the symbols

used in Figures 5 and 6 is given in Figures 3 and 4.

A full form cyZindricaZ plug gauge (Fig. 3 A) has a gauging surface in the form of an external cylinder.

The method of attaching the gauge to the handle should not affect the size and form of the gauge by

producing an undesirable stress.

A small circumferential groove near the leading end of the gauge and a slight reduction in diameter of the

remaining short cylindrical surface at the end are recommended to serve as a pilot to facilitate the insertion

of the gauge into the workpiece hole.

A full form spherical pZug or disk gauge (Fig. 3 B j has a gauging surface in the form of a sphere from

which two equal segments are cut off by planes normal to the axis of the handle.

A segmental cylindrical bar gauge (Fig. 3 C) has a gauging surface in the form of an external cylinder from

which two axial segments are either relieved (Fig. 3 C (i)) or removed (Fig. 3 C (ii)). This gauge may have

reduced measuring faces (Fig. 3 E).

A segmental spherical plug gauge (Fig. 3 D) is similar to the gauge shown in Figure 3 B but has two equal

segments cut off by planes parallel to the axis of the handle in addition to the segments cut off by planes

normal to the axis of the handle.

A segmental cylindrical bar gauge with reduced measuring faces (Fig. 3 E) is similar to the gauge shown in

Figure 3 C but has reduced measuring faces in a plane parallel to the axis of the handle.

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ISO/R 19384971 (E)
- lo-
radius of which should not be
A rod gauge with spherical ends (Fig. 3 F) has spherical end surfaces the
able.
greater than half the length of the gauge. The rod may be fixed or adjust

A fuZZ form CyZindricaZ ring gauge (Fig. 4 A) has a gauging surface in the form of an internal cylinder. The

wall of the ring gauge shall be thick enough to avoid deformation under normal conditions of use.

A gap gauge (Fig. 4 B) has for its working size flat and parallel gauging surfaces (or, alternatively one flat

and one spherical or cylindrical surface, or two cylindrical surfaces being parallel to the axis of the shaft

being checked). The GO and NOT GO gaps may lie on the same side of the gap gauge. The gap gauge may be

fixed or adjustable.
Materials and further details of gauges
3.6

The gauging surfaces shall be of a wear-resistant material such as hardened steel, hard chromium plating of

a thickness at least equal to the wear zone of the gauge, or tungsten carbide.
.s far as possible, against the warmth of the hand of the user,
It is advisable that the gauges be insulate
4 a
measurement.
when this is likely to affect significantly the accuracy of the

NOT GO gap gauges should bear an identification mark such as a groove or a red colour or an easily

visible reduction in the length of the gauging surface. This identification is not necessary if the NOT GO

side is self-evident as with the progressive type GO and NOT GO gap gauge.
3.7 Definitions and inspection of gauge sizes

3.7.1 Cylindrical plug gauges. The gauge diameter should be measured between a plane and a spherically-

ended anvil or between measuring anvils having plane parallel surfaces. The value obtained should be

corrected for deformation of the surfaces in contact caused by the measuring force (i.e. the diameter

of the gauge is the diameter when the measuring force is zero). The diameter should be measured in at

least four positions selected to reveal form errors.

All the measured diameters of the gauge should be on or between the specified limits of size, and the

range (i.e. the difference between the maximum and minimum values) should not exceed the form

tolerance of the gauge (see clause 3.9.2.3). If the presence of lobing is suspected then it may be checked

by a three-point measurement or by a roundness measurement.

3.7.2 Spherical plug, disk and rod gauges. The diameter of the spherical part of the gauge should be measured

between two parallel planes; these planes need only have a small area (for example the diameter of the

plane-ended surface of the anvil of a measuring instrument may only be 5 mm).

The value obtained should be corrected for deformation of the surfaces in contact caused by the

.s zero).

measuring force (i.e. the di .ameter of the gauge is the diameter when the measuring force i

The diameter should be measured in at least four positions, selected to reveal form errors. All the

measured diameters of the gauge should be on or between the specified limits of size, and the range

(i.e. the difference between the maximum and minimum values) should not exceed the form tolerance

of the gauge (see clause 3.9.2.3). If the presence of lobing is suspected then it may be checked by a three-

point measurement or by a roundness measurement.

3.7.3 CyZindricaZ ring gauges. The diameter should be measured by means of two spherically-ended anvils

positioned in a plane normal to the axis of the ring gauge. When moving the measuring instrument in this

plane the. greatest distance apart of the two anvils determines the diameter. The value obtained should be

corrected for deformation of the surfaces in contact caused by the measuring force (i.e. the diameter of

the gauge is the diameter when the measuring force is zero).

The diameter should be measured in at least four positions, selected to reveal form errors. All the

measured diameters of the gauge should be on or between the specified limits of size, and the range

(i.e. the difference between the maximum and minimum values) should not exceed the form tolerance

of the gauge (see clause 3.9.2.3). If the presence of lobing is suspected then it may be checked by a three-

point measurement or by a roundness measurement.
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ISO/R 1938-1971 (E)
- ll-

3.7.4 Gap gauges. The actual size of a gap is defined as the perpendicular distance between the gauging surfaces,

when no force is exerted on the gauge.

The working size of a gap gauge is defined as the diameter of a reference disk over which the gap gauge

just passes in a vertical direction under the working load marked on it, or, if this is not indicated, under

its own weight. Beforehand, the disk should be greased with a thin film of petroleum jelly and then

carefully wiped but not rubbed. The gauging surfaces of the gap gauge should be cleaned. The gap gauge

should slide over the disk after having been brought carefully to rest in contact with the disk and then

released : inertia forces are thus avoided.

For heavier gap gauges it is recommended that the working load should be less than the weight of the

gauge, so that the working size may be determined more accurately. The positions of the places where the

forces counter-balancing part of the weight of the gauge are to be applied (see Fig. 7) should be marked

on gauges of nominal sizes above 100 mm (4 in).

The working size of a gap gauge is not defined with a zero measuring force, as for the other definitions,

because the size of a reference disk is defined with a zero measuring force and the gap gauge may be

regarded as a comparator which is intended to transfer (on the particular limit) the size of the reference

disk to the workpiece.

In practice a reference disk may be used directly to accept a gap gauge in the case where the disk and

the gap gauge are supplied together and the gap gauge has been adjusted to the disk. In other cases the

following two alternative procedures are recommended.

(a) Determine the successive loads under which the gap gauge will pass over two reference disks of

different diameter under the conditions specified in the definition of the working size. The difference

in these two loads is taken as a basis for calculating the working size of the gap gauge at its working

load.

(b) Take a reference disk with a diameter smaller* than the smallest permissible size of the gap gauge.

Place gauge blocks successively on the gauging surfaces of the gap~gauge, if possible

...