Slip Gauges (Metrology)

Slip gauges are often called Johannsen gauges also, as Johannsen originated them. These
are rectangular blocks of steel having a cross-section of about 30 by 10 mm. These are first hardened
to resist wear and carefully stabilised so that they are independent of any subsequent variation in
size or shape. The longer gauges in the set and length bars are hardened only locally at their
measuring ends. After being hardened, blocks are carefully finished on the measuring faces to such
a fine degree of finish, flatness and accuracy that any two such faces when perfectly clean may be
‘wrung’ together. This is accomplished by pressing the faces into contact (keeping them perpen-
dicular) and then imparting a small twisting motion whilst maintaining the contact pressure. The
contact pressure is just sufficient in order to hold the two slip gauges in contact and no additional
intentional pressure. It is found that phenomenon of wringing occurs due to molecular adhesion
between a liquid film (whose thickness may be between 6 to 7 x 10-6 mm) and the mating surfaces.
When two gauges are wrung together and the overall dimension of a pile made of two or more blocks
so joined is exactly the sum of the constituent gauges. It is on the property of wringing units together
for building up combinations that the success of system depends, since by combining gauges selected
from a suitably arranged combination, almost any dimension may be built-up.
These may be used as reference standards for transferring the dimensions of the unit of
length from the primary standard to gauge blocks of lower accuracy and for the verification and
graduation of measuring apparatus, and length measures for the regulation and adjustment of
indicating measuring apparatus and for direct measurement of linear dimensions of industrial
Slip gauges are classified according to their guaranteed accuracy :
AA for master slip gauges, A for reference purpose, and B for working slip gauges.
Johannsen AA slip gauges are accurate to plus or minus two microns per metre. Type A is
guaranteed accurate up to plus or minus four microns per metre, while type ‘B’ for plus or minus
eight microns per metre. The guaranteed error is not divided for a block which is less than
25 mm ; such a slip gauge has same tolerance as 25 mm gauge. The workshop type, i.e., B type
gauges are finished on their measuring faces approximately to within 250 um for flatness and
parallelism. The corresponding figures for types A and AA are 125 and 75 u.m respectively.
As regards grades or classes of slip gauges, these could also be designed in five grades as
Grade 2. This is the workshop grade. Typical uses include setting up machine tools,
positioning milling cutters and checking mechanical widths.
Grade 1. Used for more precise work, such as that carried out in a good-class toolroom. Typical
uses include setting up sine bars and sine tables, checking gap gauges and setting dial test indicators
to zero.
Grade 0. This is more commonly known as the Inspection grade, and its use is confined to
toolroom or machine shop inspection. This means that it is the Inspection Department only who
have access to this grade of slips. In this way it is not possible for these slip gauges to be damaged
or abused by the rough usage to be expected on the shop floor.
Grade 00. This grade would be kept in the Standard Room and would be kept for work of the
highest precision only. A typical example would be the determination of any errors present in the
workshop or Grade 2 slips, occasioned by rough or continual usage.
Calibration grade. This is a special grade, with the actual sizes of the slips stated or calibrated
on a special chart supplied with the set. This chart must be consulted when making up a dimension,
and because these slips are not made to specific or set tolerances, they are not as expensive as the
Grade 00. It must be remembered that a slip gauge, like any other engineering component, cannot
be made to an exact size. All slip gauges must have tolerances on the length, flatness and parallelism
of measuring faces.
Except for the calibration grade, all slip gauge sets are manufactured to within specified
limits ; the closer the limits the more expensive the slip gauges, but in the case of the calibration
grade, greater tolerances on length are permissible. Because the actual lengths are known or
recorded in the calibration chart, due allowance can be made when the slips are used.
Slip gauges are available in sets both in inch units and in metric units.
The five most usual sets available in inch units contain 81, 49, 41, 35 and 25 pieces
respectively, e.g. in the 81-piece set, the slip gauges are arranged in the following order :
9 pieces from 0.1001″, to 0.1009″ in steps of 0.0001″.
49 pieces from 0.101″ to 0.149″ in steps of 0.001″
19 pieces from 0.050″ to 0.950″ in steps of 0.050″.
4 pieces of 1.000″, 2.000″, 3000″, 4.000″.
In metric units, sets of 103, 76,48 and 31 pieces are available. Metric unit sets of 103 pieces
are made up as follows :
49 pieces with a range of 1.01 mm to 1.49 mm in steps of 0.01 mm.
49 pieces with a range of 0.50 to 24.50 mm in steps of 0.50 mm.
4 pieces of 25, 50, 75 and 1000 mm respectively and 1 piece extra of 1.005 mm.
Slip gauge set of 56 slips is made up as shown below :
9 slips 1.001 to 1.009 in steps of 0.001 mm
9 slips 1.01 to 1.09 in steps of 0.01 mm
9 slips 1.0 to 1.9 in steps of 0.1 mm
25 slips 1 to 25 in steps of 1.0 mm
3 slips 25 to 75 in steps of 25 mm.
Slip of 1.0005 mm.
Many times to reduce wear on inspection workshop grades a pair of protector gauge blocks
(1 to 1.5 or 2 mm length) are also supplied and these are wrung to end of slip gauge combination
block. As these are made of tungsten carbide or similar wear resisting material these do not wear
out and protect the slip gauges from wear. These are marked with letter ‘P on one measuring face.
According to OIML, gauge blocks with rectangular section are length measures in the form
of rectangular plates or rectangular parallelopiped blocks consisting of two opposite flat faces
parallel to each other, called measuring faces, the surface quality of which is such that they have
the property of wringing to the corresponding faces of other gauges blocks or the reference face of
datum surfaces.
Gauge blocks are meant to be used as :
(a) reference standards for transferring the dimension of the unit of length from the primary
standard to gauge blocks of lower accuracy and for the verification and graduation of measuring
(b) length measures for the regulation and adjustment of indicating measuring apparatus
and for the direct measurement of linear dimensions of industrial components.

Basic Forms of Slip Gauges.

Slip gauges with three basic forms are commonly
found. These are rectangular, square with centre hole, and square without centre hole.
Rectangular form is the more widely used because rectangular blocks are less expensive to
manufacture, and adapt themselves better to applications where space is restricted or excess weight
is to be avoided.
For certain applications, square slip gauges, though expensive, are preferred. Due to their
large surface area, they wear longer and adhere better to each other when wrung to high stacks.
Square blocks with centre holes are used to permit the use of tie rods as an added assurance
against the wrung stocks falling apart while handling.
The greatest emphasis in the slip gauges is laid on the two gauging surfaces on opposite
sides of the blocks. The length between measuring surfaces, flatness and surface conditions of
measuring surfaces are the most important features of slip gauges which have to be controlled
within desired limits.

Major Requirements for Slip Gauges

. The essential purpose of slip gauges is to
make available end standards of specific lengths by temporarily combining several individual
elements—each representing a standard dimension—into a single gauge bar. The combination of
single blocks must result in a bar of reasonable cohesion, whose actual dimension truly represents,
within specific limits, the nominal dimension sought for a particular application.
This objective is accomplished by making slip gauges in dimensions (arithmetically deter-
mined sizes) needed to achieve any combination of sizes within the designed range and graduation
of the set ; ensuring the accuracy of individual blocks to be within accepted tolerance limits ;
attaching the individual blocks so closely to each other that the resulting bar will have a length
equal to the added sizes of the individual blocks ; further the attachment of the elements to each
other should be firm enough to permit a reasonable amount of handling as a unit, and yet when
taken apart, the individual pieces should be reusable without any harm to their original size or
other essential properties.
Slip gauges are used to provide end standards of specific length by temporarily combining
several individual elements—each representing a standard dimension—into a single gauge bar.
The success of system depends on formation of a bar of reasonable cohesion between individual
elements and its actual dimension truly representing within specific limits, thedesired nominal
dimension. For achieving this, the individual gauges must be available in dimensions needed to
achieve any combination with minimum number of gauges. The accuracy of individual blocks must
be within accepted tolerance limits. The individual blocks must be attached so closely to each other
that length of built-up combination is equal to the added size of the individual blocks of the assembly.
This is achieved by wringing the slip gauges. Further the attachment of individual gauges must be
firm enough to permit a reasonable amount of handling as a unit. It should be possible to detach
all individual elements so that these are re-usable without any harm to their original size or other
essential properties.
For this purpose, control of geometry of form, i.e., flatness and parallelism of faces, square-
ness of the gauging surfaces ; surface condition, appearance, homogeneity of gauging surfaces, etc.
is very essential.
The accuracy of gauges can be affected by the dimensional instability of material, or by wear
in use, or damage during storage and handling. These can be taken care of by periodic calibration.
The health of slip gauges can be very easily checked by checking its wringing quality by wringing
the gauge to be tested with an optical flat and inspecting the wrung surface for colouration (which
indicates an air gap and its area should not be more than 5 per cent of the entire wrung surface).

Indian Standard on Slip Gauge.

According to accuracy, the slip gauges can be
graded into three categories, i.e. Grade 0, Grade I and Grade II. As the high degree of finish and
flatness of the slip gauges makes it possible for a number of gauges to be wrung together to form a
composite size, it is essential that one should know more about the accuracy of length of gauges,
their flatness, parallelism and hardness. Generally the slip gauges are made from high grade steel
with coefficient of thermal expansion (11.5 ± 1.5) x 10-6 per degree Celsius between 10°C and 30°C.
These are hardened throughout and suitably heat-treated in order to stabilise their dimensions or
are provided with hard measuring faces like carbide faces. The hardness of the slip gauges is always
more than 800 HV. The measuring faces are free from any defects and finished by high grade lapping
and have good wringing surfaces. All sharp edges are chamfered or rounded.
According to IS : 2984—1966, the size of a slip gauge is defined as the distance I between
two plane measuring faces, one being constituted by the surface of an auxiliary body with which
one of the slip gauge faces is wrung and the other by the exposed face to the slip gauge. In the
absence of true parallelism, the gauge size counts as the vertical distance from the centre of the
exposed face to the surface of the auxiliary body. The definition is conditional on the assumptions
that the measuring faces of the slip gauge and of the auxiliary body are of the same material and
of identical finish and condition, and that the contacting surface of the slip gauge and auxiliary
body have been cleaned as thoroughly as possible with the agents generally used for this purpose
and are free from any special preparations tending to promote adhesion.
The same definition applies analogously to the overall size of an assembly of slip gauges
when wrung together.
An accepted method of measurement which avoids any stresses tending to modify the length
of the slip gauges (zero measuring force) is based on the interference principle. The desired size
(normal size) of a slip gauge is denoted by /0-
Generally two sets of slip gauges (normal set and specials set) are used.
The normal set is made up of the following blocks :

Range Step (All dimensions in mm)
1.001 to 1.009 0.001 9
1.01 to 10.9 0.01 9
1.1 to 1.9 0.1 9
1 to 9 1 9
10 to 90 10 9
Total 45

The special set is made of the following blocks :

Range Step Pieces
1.001 to 1.009 0.001 9
1.01 to 1.49 0.01 49
0.5 to 9.5 0.5 19
10 to 90 10 9
1.0005 1
Total 85

The most commonly used dimensions of the slip gauges which are always rectangular in
cross-section, i.e. apex angles of these rectangles or angles of lateral faces are right angles correct
to within (± 10 minutes of arc) are :

Normal Size Cross-sectional area
(w x d) in mm (Refer Fig. 2.157)
Upto 10 mm
Above 10 mm
30+ 00 x 9″005
d0-0.3 * 9-0.3
35+00 x 9″005
35- 0.3 X 9- 0.3

The measuring faces are finished to high quality
(only insignificant scratches not affecting the wringing
properties and not influencing the conditions of inter-
ferometric measurements are permitted). The edges and
the angles of the measuring faces are rounded over a
breadth not exceeding 0.3 mm, or chamfered (the cham-
fer being merged with the measuring face by a gentle
rounding) such that both the chamfer and rounding do
not exceed a breadth of 0.3 mm.

Standard Terminology.

The other ter-
minology in connection with slip gauges as per IS :
2984—1984 is as follows :
Size It, at an Optical Point on the Measuring Face, [lb dbi, ^2)] (Refer Fig. 2.158). It is
the vertical distance between some optional point on measuring face A and the plane surface of a
body B made of the same material and having the same surface finish and condition, such body
being wrung with the other measuring face A of the slip gauge.
Rectangular slip gauge
Fig. 2.157. Rectangular slip gauge.
Size lb.
Fig. 2.158. Size lb.
Mean size lm
Fig. 2.159. Mean size lm.
Means Size, lm (Refer Fig. 2.159). It is the vertical distance between the mid-point of
measuring face A and the plane surface of a body B made of the same material and having the same
surface finish and condition, such body being wrung with the other measuring face A’ of the gauge.
Gauge Error, fb (fbi, fb2) (Refer Fig. 2.160). It is the amount by which the size lD at some
optional point on the measuring face varies from the desired size la.
fb = h~ lo
Error fm in Mean Size (Refer Fig. 2.160). It is the amount by which the mean size lm varies
from the desired size l0 :
fm ~lm~ lo
Measuring Face Error: ff (ffi, ffij) (Refer Fig. 2.160). It is the amount by which the size l(,
at some optional point on the measuring face varies from the mean size lm :
ff=lb~ lm = fb~ fm.
(Note. Symbol “~” denotes : subtract either of the quantities so that the result is positive
Squareness Error (S). It is
amount by which the side face deviates
from a true right angled position relative to
each measuring face. The error must be
determined for all the four side faces and in
relation to each of the measuring faces (Si
to S8).
Wringing and Enforced Ad-
hesion. The term ‘wringing’ refers to the
conditions of intimate and complete contact
and of permanent adhesion between
measuring faces which is brought about by
wringing together the surfaces in question
without application of pressure, assuming
that the surfaces have been thoroughly
cleaned and exhibit a good standard of flat-
ness and smoothness. The wrung gauge can
be handled as a unit, without the need for clamping all the pieces together.
It is believed that the phenomenon of wringing is due
to molecular adhesion between a liquid film and the mating
surfaces of the flat surfaces. In fact, the success of precision
measurement by slip gauges depends on the phenomenon of
wringing. It has been found that the gap between two wrung
flat pieces is of the order of 0.00635 microns from which it
would be very clear that the film thickness in two wrung flat
pieces contributes no effect. It should be remembered that
slip gauges are wrung together by hand through a combined
blinding and twisting motion. First the gauge is oscillated
slightly with very light pressure over other gauge so as to
detect presence of any foreign particles between the sur-
faces. One gauge is then placed perpendicular to other using
standard gauging pressure and rotary motion is then applied until the blocks are lined up. In this
way air is expelled from between the gauge faces causing the two blocks to adhere. This adherence
is caused partly by molecular attraction
and partly by atmospehric pressure (Refer
Fig. 2.162). Similarly for separating two
wrung slip gauges, combined sliding and
twisting motion should be used and no at-
tempt should be made to separate them by
direct pull because considerable load would
have to be applied which may damage the
slip gauges. The overall thickness of the wrung gauges, for most practical purposes, is the sum of
the individual sizes in combination. If during wringing, there is slightest feeling of roughness the
process should be stopped and the surfaces examined for contamination.
Wringing is thus defined as the property of measuring faces of a gauge block of adhering,
by sliding or pressing the gauge against the measuring faces of other gauge blocks or the reference
faces of datum surfaces, without the use of any extraneous means.
The ‘enforced adhesion’ refers to the condition of contact over the entire surface and of
permanent adhesion, which, owing to the lack of flatness or smoothness, can only be brought about
Gauge error, error in Mean Size and measuring face error
Fig. 2.160. Gauge error, error in Mean Size,
and measuring face error.
Squareness Error
Fig. 2.161. Squareness Error.
 Wringing of slip gauges.
Fig. 2.162. Wringing of slip gauges.
by the application of pressure. It may be noted that the use of any extraneous agent to promote
adhesion is not correct and it is sometimes specified that surfaces which are to be wrung together
should be absolutely dry. On the other hand, a truly clean surface, though difficult to attain may
not wring satisfactorily. The usual practice is to use silicone or filtered kerosene as lubricant, apply
a thin coat of same and wipe it as thin as possible. There is as much danger from too little lubricant
as from too much.
Measuring Faces. The top measuring face is that face on which the size is marked. If such
marking is on a side face, then the right hand measuring face is the one which forms the right-hand
boundary of the surface on which the size characters are marked assuming the characters are
upright to the observer.

Permissible Errors.

The various permissible errors for slip gauges of different
grades are as given below :

Grade 0 Grade I Grade II
Permissible Gauge Errors /}, ± (0.1 + 0.0021) nm ± (0.2 + 0.005 /) um ± (0.5 + 0.01 I) um
Permissible measuring face
error ff
±(0.1 + 0.00014 l)\aa ±(0.16 + 0.00025 Z)um ± (0.25 + 0.00045 /) um
Permissible squareness
Error S
± (50 + 0.11) jim ± (60 + 0.12 I) mm ± (75 + 0.2 I) um
Permissible variation of side
face from flatness
± (20 + 0.02 1) nm ± (20 + 0.02 1) um ± (20 + 0.02 /) um
Permissible variation of side
face from true parallelism
(40 + 0.04 /) urn (40 + 0.04 I) um (40 + 0.04 /) um
Permissible variation of side
face from true squareness
with each other
± 100 um over 35 mm
± 90 um over 30 mm
± 60 um over 20 mm
Same as per Grade 0 Same as per Grade 0
Permissible variation in
width and depth
+ 0 mm
- 3.0 mm
+ 0 mm
- 0.3 mm
+ 0 mm
- 0.3 mm
Surface finish of measuring
Satisfy the condition for
wringing and enforced ad-
Same as for Grade 0 Same as for Grade 0
Edge Zone An edge zone of I mm in
width is disregarded on all
Same remarks as for Grade 0

I is the length of slip gauge in mm.
For testing purposes, slip gauges upto 5 mm nominal size are wrung with a body having
plane and parallel faces and a thickness of not more than 12 mm. In case of longer slip gauges the
permissible errors apply with the gauge placed horizontally and supported at the optimum points
of 0.21131 from ends of slip gauge.

Grades of Slip Gauges. IS :

2984—1966, Bureau of Indian Standards for Slip
Gauges specifies three grades of slip gauges : grade 0, Grade I and Grade II. Some examples of the
maximum permissible errors in the mean length of gauges at 20°C are given below :
BIS Specification for Slip Gauges

Nominal Size Grade 0 Grade I Grade II
mm micron micron micron
10 0.02 0.10 0.35
20 0.04 0.15 0.45
50 0.10 0.30 0.75
100 0.20 0.50 1.20

Grade II is intended for use in workshops during the actual production of components, tools
and gauges ; Grade I is of higher accuracy and intended for use in inspection departments ; and
grade 0 is for use in laboratories and standards room. This latter grade serves as standards for
periodically checking the accuracy of grade I and II gauges. The actual sizes of gauges in grade 0
sets should be used only in conjunction with the certificate of sizes obtained from a recognised testing
authority such as the National Physical Laboratory.

Sets of Gauges.

The recommended sets in the metric units are M112, M105, M87,
M50, M33 and M27. The normal set of M112 is made up of blocks as given below :
Normal Set of Ml 12 Gauges

Range mm Steps mm Pieces
1.001 to 1.009 0.0001 9
1.010 to 1.490 0.010 49
0.50 to 24.50 0.050 49
25, 50, 75, 100 25 4
1.0005 1
Total 112


Protector Blocks.

These are two additional 2 mm blocks with a letter P on
measuring faces and are provided with high-grade sets of gauge blocks. These are accommodated
at each end of a combination so that all the wear occurs on them. These are made from tungsten
carbide or other suitable material and take all the wear due to rubbing on surface plates.

Accumulative error of slip gauges.

Though the error by combination of two gauges
in thickness due to separation is negligible (of the order of 6.35 x 1QT6 mm), it is advisable to select
least number of gauges for a given size when combining slip gauge blocks.

Selecting slip gauges for required dimension.

Much time is wasted if the slip
gauge combinations be made by hit and trial. Therefore standard procedure given below must be
Let us say that the dimension to be arranged is 58.975 mm.
Always start with the last decimal place e.g., here it is 0.005 mm and for this 1.005 mm slip
gauge is selected.
Now dimension left is 50.975—1.005 = 57.970 mm.
Take second decimal place ; and for it select 1.47 mm slip gauge.
Therefore, the remainder is 57.970 – 1.47 = 56.500 mm.

[Note :

One could have selected 1.07 mm piece also, but that way we would have been left
with 56.900 and for it we need another 1.4 mm piece. Our aim should be to choose minimum number
of slip gauges for a given dimension.]
Next for 56.500 mm, we choose 6.500 mm piece and finally 50.000 mm piece.
Thus, we have 50.000 + 6.550 + 1.47 + 1.005 = 58.975 mm.
All these four slip gauges are wrung properly to get required dimension.
When the dimension to be measured is unknown and is to be found using slip gauges, then
first it must be determined to the nearest 0.1 mm size with a caliper etc. Let us say that size lies
between 47.0 and 47.4 mm and we have to estimate it correct upto 3 decimal places of mm. It is
now obvious that we require 1.005 mm for 0.005 mm and 1.01 to 1.09 mm for .01 to 0.09 mm. Thus
2 mm must be taken off from 47.3 mm to account for these three decimal figures. Thus first make
combinations of 47.3 – 2.0 = 45.3 mm i.e. 1.3 + 19.0 + 25.00 mm pieces.
Next try with these and 1.005 and all the rest, i.e. 1.01 to 1.09 mm pieces. Then starting
from 1.09 pieces one of these will give large dimension and next one small. Then decide whether
1.005 mm piece is needed or not by taking it out and adjusting the dimension near that value. The
sense of’feel’ is very important in the use of slip gauge and depends upon the skill of the operator.
Since during the combination operation the gauges are in contact with body temperature,
their dimension actually increases. Therefore, heat transferred from the body must first be rejected
to surrounding by leaving the set at room temperature for some time. Other possible errors due to
temperature can be due to heated parts or room temperature being higher or lower than 20°C.
These can be avoided easily by necessary precautions.

Ceramic Slip Gauges (Refer Fig. 2.163)

Salient Features:

Corrosion Resistant
— Unaffected by water, acids and
— Simple maintenance with no re-
quirement of any anti-corrosion
— No adverse effects due to fingering
while using.

Superior Wringability

— The uniform and close grain struc-
ture enables the blocks rigidly
wrung together and easy to
Dents and burrs are not easily
produced on gauge surface.

Resistant to Impact

— Zirconia ceramic blocks are hard and highly tough to withstand the knocks and drops
that occur during use and will not chip or fracture easily.

Resistant to Wear

— About 10 times life or even more compared to steel slip gauges. The natural stability
and durability due to its low friction co-efficient maintains their geometry longer.

Thermal Expansion

— Co-efficient of thermal expansion of ceramic is 10 x lO^fC is very close to that of steel.
— The lowest thermal conductivity makes these gauge blocks easy to use at different
Long Series Slip Gauges and Length Bars

Long Series Slip Gauges and Length Bars

Long Series Slip Gauges (Ref. Fig. 2.164).

Long Series Slip Gauges are made from high
quality steel having cross section (35 mm x 9 mm) with holes for clamping 2 slips together. They
are manufactured to DIN-861 of 1988 specifications in different grades. They are available in the
following grades : ’0′, T and ‘II”. These are available from 100 mm to 500 mm.
Long series slip gauge.
Fig. 2.164. Long series slip gauge.
 Length bars.
Fig. 2.165. Length bars.

Length Bars.

Length Bars are made from High Carbon High Chromium Steel, ensuring
that the gauge faces are hardened to 64 RC (800 HV). The bars have a round section of 30 mm for
greater stability, and ease of handling. Both ends are threaded and recessed, and precision lapped
to meet with the stipulated requirements of finish, flatness, parallelism and gauge length. Length
Bars are available upto 500 mm in the following grades : Calibration CO’), Inspection (T) and
Workshop (‘IF). With 8 pieces set, length bars can be combined upto 2000 mm by using M6 stud.

Adjustable Slip Gauges (Refer Fig. 2.166).

Some firms like Pitter have developed
adjustable slip gauges with little refinement over Engineer parallels desrcibed in Art 2.2.13. These
consist of two hardened, ground and
lapped blocks having one tapered sur-
face. The tapered surfaces are finished to
wringing quality and it is possible to
slide two tapered surfaces alone the
length of each other thereby obtaining
different sizes within a very small range.
The upper block has a rack fixed on bot-
tom tapered surface, and lower block has
receptacles on top tapered surface to receive the teeth in upper block. A scale with divisions
corresponding to rack tooth in upper block is marked in lower block and a reference point is marked
on upper block, so that by keeping upper block with its teeth placed at suitable distance, any
dimension within the range can be obtained between the outer faces of two blocks.

Manufacture of Slip Gauges.

The following sequence of operations is followed in
order that slip gauges possess certain desirable qualities.
(i) Marking the approximate size by preliminary operations.
(ii) A special form of heat treatment to make the blocks hard and wear resistant.
(iii) An artificial and natural seasoning process to ensure stabilising for the whole life of the
blocks. Stabilising is generally carried out by heating and cooling the gauges successively, after
Adjustable slip gauges.
Fig. 2.166. Adjustable slip gauges.
rough grinding. The successive temperatures used in the four stages of stabilising are 40°, 70°, 130°
and 200°C, the gauges being heated in sand and cooled slowly at each stage.
(iv) A final grinding process to reduce the block to the approximate required dimension.
(v) A final-lapping operation to reduce the blocks to exact size and impart a beautiful finish
to the surface. A special lapping machine is used for lapping. Lapping is done in a room maintained
at 20° C and controlled humidity of 50%. The blocks are held in a moving spider between upper and
lower cast iron laps. The spider is imparted both rotating and reciprocating motion to produce a
complicated path of travel for each block and wear is distributed all over uniformly.
(vi) Comparison of finished gauges with grand master sets.

Manufacture and Generation.

Various manufacturing methods have been
devised for the production of slip gauges. The method employed by the National Physical laboratory,
utilises special type of magnetic chuck on which eight similar steel blanks are mounted and spot
ground on each face. A preliminary lapping operation is carried out on this chuck, by which all the
blanks become parallel to about 0.0002 mm and within about 0.002 mm of size. The final lapping
process is carried out on solid steel chuck on which the blocks are wrung as shown in Fig. 2.167 (a).
The lapping operation is continued until they all lie in one true plane. The blanks are next reversed
and the opposite faces lapped again until they also lie in one true plane. After lapping the blanks
in this position, half of the blanks are interchanged and turned end for end as shown in Fig. 2.167
(b). Further lapping produces a very high degree of parallelism and equality of size between the
eight blanks. In between, the eight blanks are wrung together and their overall length compared
with an equivalent size standard gauge. In this way, the individual errors are reduced to one-eighth
of the usual error for a single block. By this method, flat and parallel blanks are produced which
are of equal size to within 0.002 mm.
 First Arrangement (b) Second Arrangement
(a) First Arrangement (b) Second Arrangement
N.P.L. Arrangement of manufacturing slip gauges on lapping chuck
Fig. 2.167


The whole set of gauges is generated by this lapping technique by taking
simultaneously a number of pieces which are of same size starting from the longest member. Hence
eight 100 mm gauges are first lapped simultaneously, the process being stopped at intervals and
any three of them are wrung together and compared with the 300 mm standard. The lapping is
continued in stages until equality is obtained. Each one of the 100 mm gauge is thus correctly
adjusted to size. One of the 100 mm gauge is again used as standard as we had used 300 mm gauge
and 100 mm gauges in turn are used to generate the next lower series till the set is complete.

Mechanical Lapping of Slip Gauges.

This method of finishing slip gauges to size
involves the mechanical lapping of both faces simultaneously. The lapping machine employed for
this purpose consists of three lapping plates which are of annular form. These plates are prepared
Mechanical lapping operation of slip gauages
Fig. 2.168. Mechanical lapping operation of slip gauages.
by lapping operation keeping the plates together in pairs until they fit together interchangeably.
In the machine, these lapping plates are disposed one above the other. The lower lap (lapping plate)
is rigidly secured to the base, the upper most lap is so mounted that it is free to tilt in any direction,
although restrained from rotating. Between these two laps is disposed a-third flat steel plate (spider)
in which a number of holes which are slightly larger in diameter than the distance across the corners
of blocks, are provided for the reception of the gauges to be lapped. The block can thus move freely
and the surfaces which project on their side of the plate can bear evenly on the faces of the upper
and lower laps. The holes in the spider are disposed in a circle. The spider is imparted a planetary
motion in addition to the rotary motion so that each block is caused to slide over every portion of
the surface of both laps as the operation proceeds. The ratio between the rotary and planetary
motions is so selected that the spider and the gauge blocks do not follow the same paths around the
surfaces of the laps during successive turns. This is done in order to evenly distribute the wear on
the laps. The gauge blocks before lapping operation are made flat and parallel by some other
operations and are 0.025 mm over size. Initially the upper lap, which is free to take up any angular
position, rests only on the three high points of the circle of gauge blocks. As the blocks are carried
round between the laps, the area on which the upper lap bears is soon enlarged and finally the
upper surfaces of all the blocks would be flat and lie in one plane and the lower surfaces of the blocks
would be flat and would lie in another plane. It may be noted that there is no guarantee that the
upper and lower planes of the gauge blocks are parallel. In order to achieve this„the alternate blocks
are changed through 180 degrees to a position diametrically opposite to that which it previously
occupied. In Fig. 2.168 (a), 12 blocks 1, 2, 3…..12 are shown disposed in a circle. Actually 24 blocks
are lapped simultaneously. Supposing, block No. 1 is the highest of the set and No. 7 is the lowest,
and if the blocks are now disposed as shown in Fig. 2.168 (b) then as lapping operation is continued
further, the upper lap will initially bear only on No. 1, 2 and 12 and thus the greatest amount of
material will be removed from these. By repeated lapping and interchanging in this manner, a high
degree of parallelism can be obtained.

American Method of Manufacturing slip gauges.

By this method the gauges
are produced to meet the requirements of the US National Bureau of Standards, for size, flatness,
parallelism and surface finish. Other requirements taken care of are uniform hardness, stability
(to prevent shrinkage or growth), good wringing characteristics, and wear and corrosion resistance.
The gauges are made from steel containing 1.45 per cent chromium, 0.35 per cent manganese and
approximately 1 per cent carbon. Tungsten carbide is used for the protective gauges.
The blocks, after being separated from the bar by an abrasive cut-off machine, are rough
ground, except on the end gauging faces. About 0.25 mm of stock is left on all the faces, except the
gauging faces, for final grinding. The blocks are then heat treated and brought to a temperature of
840°C and quenched at 50°C. Next follows tempering by immersion for one hour in an oil bath
maintained at 105°C followed by cooling in still air at room temperature. The minimum hardness
value at this stage is 65 Rockwell C.
The gauge blocks are then finished ground, with the exception of the end gauging faces.
Stabilisation is achieved by alternately cooling and heating the blocks a number of times. In this
process the gauges are placed in pans containing a wax lubricant (to prevent oxidation) and the
pans placed in a dry-ice refrigerating unit, which cools them to a temperature between (— 73 to
- 85°C). The length of the gauge determines the time taken, which varies between six and ten hours.
The blocks are then removed and allowed to attain room temperature. To relieve transformation
stresses, the gauges are tempered by heating to a temperature of 110°C for three house in a gas-fired
furnace. The cooling tempering cycle is repeated about ten times, to increase the martensitic
microstructure from approximately, 93 per cent (after heat^ treatment) to nearly 100 per cent. About
10 per cent of each batch of blocks is subjected to a Bureau of Standards approved stability test.
The blocks concerned are immersed for twenty-four hours in a boiling 0.5 – 1 per cent potassium
dichromate aqueous solution. The dimensions of the blocks must not change more than 0.05
micrometer per 25 mm, otherwise they are rejected.
Batches of gauge blocks are finished processed in quantities of sixteen, or multiple of this
number, for any one size. The end gauging faces are ground to size plus 0.025 mm * Q’QOO • ^NE
finish ground blocks are magnetically inspected for cracks while the blocks remain mounted on the
magnetic chucks of the surface grinding machine. A magnetic comparator is also used for detecting
internal flaws and for checking hardness. The blocks after being demagnetised, are now ready for
The end gauging faces of the blocks are lapped in four stages, namely, rough lapping,
semi-finish lapping, and first and second “polishing” operations. Sixteen blocks are lapped at a time,
on Norton lapping machines. The upper lap floats but does not rotate. The lower lap plate is driven
by a variable-speed motor, the work being held in a spider in the usual way. Variable-speed drives
enable various sized blocks to be lapped and the
different operations to be performed. Roughening
speeds are from 60 to 90 rev./min. whilst 30 rev.
min. is the speed for finishing.
Lapping plates constructed from
Mechanite cast iron are normalised, stabilised and
available in sets of three. Their flatness is checked
by means of optical flats. The work holding spiders
are made of steel plates, each having four con-
centric rows of sixteen rectangular openings as
shown in Fig. 2.169.
About 0.028 mm of stock is removed from
the two end gauging faces of each block in rough
lapping in twelve runs, each of thirty seconds
duration. This brings the gauges to within 0.00045
mm of the required size. The surface finish at this
stage is about 0.0625 nm. r.m.s. After each thirty
second lapping period the positions of the gauge in
the spider are changed to ensure uniform size
 Diagram showing the method of interchanging the blocks in the spider
Fig. 2.169. Diagram showing the method of
interchanging the blocks in the spider.
reduction. Fig. 2.169 shows the method of interchanging the blocks. The blocks are moved to other
radial rows at subsequent changes.
At the end of each fourth lapping period the blocks are removed from the spider and cooled
to room temperature by being placed on a steel block for about thirty minutes per 25 mm of length.
An electric comparator, with 0.002 mm graduations is used to determine the lengths of the blocks.
In semi-finish lapping, 0.003 mm of stock is removed, bringing the blocks to within 0.0018
mm of the finished size, with a surface finish of about 0.0375 \im r.m.s. The procedure resembles
that previously discussed for rough lapping. During the first “polishing” operation about 0.001 \xm
of stock is removed, to bring the blocks to within 0.0007 mm of size with a surface finish of 0.025
um r.m.s. At this stage the size flatness and parallelism of each block are carefully checked. The
second “polishing” operation, using a very fine abrasive, brings the blocks to size.
The National Bureau of Standards permit the following plus or minus tolerances per 25 mm
of block length ; 0.00005 mm for grade AA, 0.0001 mm for grade A and 0.0002 mm for grade B.
Flatness and parallelism of the measuring surfaces must be maintained within 0.00008 mm, and
the surface finish specified is 0.0225 um r.m.s.
The size, flatness and parallelism of the block gauges by comparison with “working masters”,
which, in turn, are recalibrated weekly by comparing them with “grand master”. One set of six of
the latter is calibrated by the National Bureau of standard every three months.
Final inspection is carried out by means of electric comparators and various size checks are
made. Optical flats are used for checking both flatness and parallelism.

German (Zeiss) Method of manufacturing slip gauges.

By this method, gauges
are made to the highest quality (grade 0) and are held to limits of ± (0.1 + 0.002 x nominal length)
microns. For example, a gauge of 10 mm nominal length is required to be finished to ± 0.12 microns.
Variations from the mean size of a gauge resulting from face errors must be held to 0.10 micron
over the range of sizes upto 200 mm nominal length. These gauge blocks are normally used as
master or reference sets.
The gauges are manufactured from steel containing carbon 1%, chromium 1.8% and man-
ganese 0.4 per cent. The cross-sectional dimensions of gauges of length upto 10 mm, and exceeding
10 mm are 9 by 30 mm and 9 by 35 mm respectively. Blanks are sawn from ground steel bars 0.3
to 0.4 oversize leaving 0.4 to 0.6 mm on the overall length of each block. The blocks are then hardened
to Rockwell 65C to ensure dimensional stability. First, the gauges are heated to 900°C, and
quenched in oil for hardening. Second, an artificial ageing treatment is undertaken. This involves
holding the blanks at 120°C for ninety hours, and then at – 90°C for eight hours. Thin blanks are
then surface ground, leaving about 0.07 mm on the length. Following this the side faces are ground,
using angle blocks to ensure squareness within 0.07 mm of the finished sizes. Longer gauges are
first ground on the side faces, followed by the measuring faces to give same finishing allowance.
Demagnetisation follows, after which the blocks undergo a preliminary lapping operation. All
lapping processes are carried out in fully air conditioned workrooms to permit both temperature
and humidity to be controlled. In the room used for the preliminary lapping operation the
temperature is held at slightly less than 20°C to provide compensation for heat generated in the
gauges during the process.
The laps are made from close-grained cast iron, 450 mm diameter by 75 mm thick; weighing
about 75 kg. Three laps comprise a set, used to condition one another. At the preliminary stage the
laps are charged with a compound of fine emery, with a mixture of oil and petrol. The work carrier
plates have twenty-two apertures each of which holds two thin blanks. Longer blanks are mounted
with a thickening piece to produce a near square section, to ensure greater stability, with less
tendency to tip. Six or seven hours are normally required, for the preliminary operation. The blanks
are changed around during the process to equalise the action, and to bring each block to the overall
length within 0.003 mm of the nominal size for block gauges up to 10 mm, with a slightly larger
allowance on longer blanks. Dimensional checks are carried out with the aid of Zeiss optimeter
Following preliminary lapping the gauge blocks are marked, by etching giving the nominal
size, the grade of accuracy, etc. All the edges of the blanks are then bevelled 0.2 mm, at 45°.
Finish-lapping operations are carried out with corresponding position-changing sequences
for the blanks in the work-carrier holes. The laps employed are specially heat treated to produce
extremely hard iron phosphides in the matrix and cause the ferrite particles to break down slightly,
leaving the phosphides in the matrix and cause the ferrite particles to break down slightly, leaving
the phosphides embedded in the lap surface. No abrasive is used. Petrol is applied to clean the lap
surfaces when required. The first lapping operation is continued until the blanks are within 0.0008
to 0.0009 mm of the nominal dimension.
For the final lapping stage, the gauges are transferred in trays lined with lint-free cloth to
a room where the temperature is controlled within ± 0.5°C. Several hours elapse and further work
is done to ensure that the temperature of the blocks is in equilibrium. The blocks are then ready
for combined sizing and polishing operations. By varying the load on the upper lap by means of
compression springs (to increase the almost non-existent cutting action) the gauges are reduced in
length, leaving 0.0002 mm, on sizes up to 50 mm long, and slightly more for the longer blocks. The
spring pressure is released an lapping continues to give a final polish to the measuring faces with
a very slight amount of metal removal. The surfaces are finished within 0.00003 to 0.00004 mm.
Frequent checks are made during the final lapping operation. When the gauges have been completed
they are stored in the inspection department for several days to allow the temperature to reach
equilibrium, after which the gauges are compared with approved masters. Records are maintained,
of the difference in the length from the nominal dimension, and a month later the gauges are again
checked. Blocks which show no measurable change on this occasion are accepted for service.


Due to handling in the laboratory or inspection room, slip gauges are
liable to show signs of wear appreciable period of use, and, therefore, they should be checked or
recalibrated at regular intervals.
Workshop and inspection grade gauges are calibrated by direct comparison against the
calibration grade gauges in a comparator. A variety of comparators are available which use
mechanical, optical, electrical and pneumatic means of amplification. Eden-Rolt Millionth com-
parator and Brook-level comparator have been specifically designed for slip gauges.

Eden-Rolt Millionth Comparator.

This comparator is capable of calibrating
gauges upto 25 mm. It achieves a very high magnification of about 20,000 partly by mechanical and
partly by optical means. Each division of its scale represents 0.0002 mm so that by estimation it is
Principle of Eden-Rolt millionth comparator
Fig. 2.170. Principle of Eden-Rolt millionth comparator.
possible to read 0.00002 mm. The measuring contacts consist of a steel ball on one side and a group
of three balls in triangular formation on the other which eliminate many difficulties that would
arise if plain parallel faces were used. The principle of the instrument is shown in Fig. 2.170.
The design of this comparator is extremely simple. In the mechanical amplifying system, a
strip of spring steel is attached to the fixed block and a similar strip is fastened to the movable
block. The moving member operates a lever at the end of which is fixed a light metal ring or cursor,
with a piece of spider’s web across it. Thus when a slip gauge is inserted between the contact faces
of the instrument, the movable member is displaced, causing the light lever to twist. The magnifica-
tion depends upon the relation between the length of the lever and the distance between the leaf
springs. As the strip can be made quite small, of the order of 0.5 mm, a mechanical leverage of the
order of 300 to 400 is obtained with a fairly short lever (about 200 mm long). The particular
advantage of the spring-steel hinges is the almost entire elimination of friction and consequent
The layout of the optical system is shown in Fig. 2.171 which further provides an amplifica-
tion of about 60 : 1. A spider’s web is inserted near the end of the lever and movements of this line
(in the spider web) are further magnified by an optical shadow-projection apparatus. The light rays
from a light source pass through a condenser, a prism and are further deflected towards a mirror
(placed at a height of about 750 mm above the comparator) through the spider web and a lens. From
the mirror the rays are reflected on a scale. The measurements are recorded from the position of a
line image of the spider’s web relative to the scale on the base of the machine.
To measure the difference in size between a slip gauge and a corresponding slip gauge of
known size, the adjustable anvil of the comparator is undamped and withdrawn sufficiently to
enable the standard gauge to be inserted between the measuring faces. The anvil is then pressed
forward so as to grip the gauge between the faces, at the same time bringing the image of the
crosswire approximately to the middle of the scale and the reading is noted. Having clamped the
anvil in this position, the trigger handles are pinched together so as to release the gauge, and the
other gauge is inserted and its reading noted. It is usual to take three readings on each gauge, one
at the centre and one each at either end. The difference between the two gauges is given by the
difference between their mean readings.
Optical system.
Fig. 2.171. Optical system.
Brook-level comparator.
Fig. 2.172. Brook-level comparator.

Brook-Level Comparator.

On this comparator longer gauges can be compared.
It has an extremely sensitive level mounted on a small aluminium cradle on the underside of which
two steel balls are attached, and the whole unit is mounted on a protection case which can be moved
up and down a fixed pillar. The gauges to be compared are wrung on a circular steel platen in such
a way that the two balls bridge the gauges. The platen can be rotated through 180° to reverse the
position of the gauges under the level.
The position of the end of the bubble is read off a scale which is carried in the level housing
and is graduated so that one division represents a difference in height of the balls of 0.00025 mm.
The level is raised while the platen is rotated through 180°, then lowered again and a second reading
taken. One-half of the difference between the readings obtained is equal to the difference in the
length between the gauges.
In Fig. 2.172, if the first reading is 18.2, the standard being on the left hand side, and the
18 2 — 18 4
second is 18.4 on metric scale, the difference in length is : —’■—-—— = 0.1 x 0.00025 = 0.000025
mm, the gauge being shorter than the standard.
The principle of measurement is illustrated in Fig. 2.172, which shows the two opposite
measuring positions.

Calibration by Interferometry.

In engineering applications, wavelengths of light
are used for extremely accurate measurements of surface flatness, slip gauges and other end gauges.
These light waves have wavelengths of the order of 0.000375 to 0.000675 mm and by estimation of
the widths of light interference bands, measurements to within 0.000025 or 0.00005 mm are
Interference methods are suitable for determining the absolute size of calibration and
reference grade gauges. For this the gauges must satisfy the following basic requirements :
(a) The end faces must be flat and parallel to each other ;
(b) They must have a high degree of surface finish ;
(c) The actual size must agree with its nominal size to within a very small tolerance ; and
(d) The edges must be properly rounded off and deburred.
Various types of interferometers have been designed from time to time over the last 50 years
for the specific purpose of inspecting block gauges. Most prominent among these is the NPL-Hilger
gauge interferometer. The interferometer is suitable for measuring sets of slip gauges up to
100 mm.

NPL-Hilger Gauge Interferometer.

An important feature of the design of this
interferometer is that batches of upto 20 gauges of differing sizes can be set up for measurement
in one operation. The optical scheme is shown diagrammatically in Fig. 2.173.
Light from a cadmium or mercury-198 discharge lamp A is concentrated on a narrow slit B
situated at the focus of a lens C. The parallel beam
from C is directed into a constant deviation prism
D, which disperses the light into its several con-
stituent monochromatic rays. By rotating the
prism, any one colour can be selected and directed
vertically downwards on the optical flat E, which
is supported above and at a slight inclination to
the upper surface of the gauge F wrung on to a
horizontal base plate. The rays are deflated from
two parallel surfaces—the upper surface of gauge
and face of base late—and after retraversing the
prism and colimating lens are brought to a focus
on a small reflecting prism H situated close to the
slip B. An eye placed close to this prism sees two
Optical arrangement of the N.P.L. Hilger gauge interferometer.
Fig. 2.173. Optical arrangement of the N.P.L.
Hilger gauge interferometer.
sets of interference fringes which are formed between the lower semi-reflecting surface of the optical
flat on the one hand, and the faces of the gauge and the base plate on the other. By suitably inclining
the flat, the fringes can be seen as in Fig. 2.173 (b).
Theory. The two sets of fringes seen in the field of view are equispaced parallel to each other
and in general, are displaced parallel to each other relatively by fraction alb of the fringe-spacing
as shown in Fig. 2.173 (b).
The height h of the gauge above the base plate is then equal to a certain whole number n
plus the fraction part alb of the half-wavelength of the light used. If the fringes are observed in
The difference between the fractions in each case is known and it is also known that the
whole numbers – N{), etc. are only a few integers, since the errors of gauges do not usually
exceed a few half-wavelengths.
The exact number of half-wavelengths to be associated with the known fractions to give the
error of the gauge is found from a slide rule, on which various half-wavelengths are set out to scale
from a common zero over a range of few units. (Also refer Ch. 6).

Determination of Length.

The length of a slip gauge is defined as the perpen-
dicular distance between the centre of the top of the gauge and the surface of an auxiliary base
plate upon which the gauge has been wrung. This definition takes into account the thickness of one
wringing film which is always associated with the gauge in its use.
For interferometric determination of its length, the gauge is brought to the centre of the field
of view and an estimation is made of the fraction a expressed as a fraction of the fringe separation
b [Fig. 2.173 (6)] at the centre of the gauge surface. Observations of the fractional displacements of
two sets of fringes are made in four different wavelengths of either cadmium or mercury-198 or
krypton-86. The observed estimated excess fractions are compared with the calculated nominal
fractions. Thus, we get a series of different fractions corresponding to the uncorrected difference of
the observed length of the gauge from its nominal size. The error in the length of the gauge is then
evaluated by means of a special slide rule and is suitably corrected to give error at the standard
temperature of 20°C.
The whole series of observations are subsequently repeated with the gauge wrung down on
the opposite faces.
The overall accuracy of determination of length is one part in a million for lengths upto
25 mm.


The measured lengths as derived interferometrically in ambient air require
corrections on account of:
(a) Difference in temperature of the gauge from the reference temperature of 20°C ;
(b) Difference between conditions of ambient air and standards air ;
(c) Phase change due to the difference in finish between the exposed surface of gauge and
top surface of base plate ; and
(d) Obliquity errors due to the inclination of incident light with the normal to the surface of
base plate.
The temperature correction is calculated on the basis of an average value for the co-efficient
of thermal expansion of hardened steel of 12 parts in a million per 1°C and is applied as 12 I (20
-1) x 10~6, where I is the observed length of the gauge at the value corresponding to 20°C, and
t = room temperature.
The wavelength correction comprises of two parts, one for barometric pressure and air
temperature, and the other for vapour pressure. This is given by the formula :
The total value of the wavelength correction is obtained by adding algebraically the value of
the two terms and applying the results as a correction to the length of the gauge. Thus, if + x and
+ y are the values of the terms for a particular set of conditions t, h and f, then the correction to a
gauge of nominal length N is given by
The phase correction is zero, if the material and surface finish of the gauges and the base
plate are the same. In case they are different, then it can be determined by taking five gauges from
the set and finding the optical length of each by wringing on a base plate. The optical length of the
combination is also measured on the same base plate, and from the length, the optical length of the
uppermost gauge is subtracted. The resulting difference is the sum of the practical length of the
other gauges. This is compared with the sum of the optical lengths of the same gauges and the
average value of phase corrections is calculated.
For this interferometer, the viewing aperature is a rectangular hole of dimensions I and h,
and obliquity correction for this is given by :
where S is the distance between the centres of the illuminating and viewing apertures situated in
the focal plane of the collimating lens of focal length F, and N is the nominal length of the gauge
being measured.
The total correction factor is the algebraic sum of all the individual corrections. The value
of the correction to be actually applied to the length of a gauge is derived by finding the product of
the length and the total correction factor, taking due account of the sign of the correction.

On line calibration of Slip Gauges.

In calibration of slip gauges by hand, other
than time consuming process, disadvantage is that the accuracy of determination depends on the
subjective judgement of the operator. In manual calibration, the operator will have tendency of play
safe and thus high grade slip gauges may be classed as low grade which can be costly mistake.
Automatic calibrator utilises microprocessor technology and provides an individual calibration and
grading, and print-out of every gauge checked. It may take as small as 10 seconds to calibrate a
gauge. Grading is done by measuring its length, parallelism between measuring faces and flatness
of each face. Since the length tolerance on highest grade gauge is ± 50 mm, it is imperative that
machine be capable of measuring 10% of this value, i.e. ± 5 nm. The handling by machine should
not produce any scratch because a scratch of 1-2 micron width would affect the accuracy of slip
gauge and lead to its rejection. The machine should be able to measure gauges of all sizes. The
machine has to be installed in a tightly temperature controlled room, totally free of dust and totally
vibration free foundation. Such a machine is totally enclosed and is fitted with hinged covers for
access to the working zone and for loading and unloading of gauges. Operation is performed from
a control station and all measuring cycles are performed automatically.
Each set of gauges is loaded into a cassette with a previously calibrated master gauge of the
same nominal size and calibrated by interferometry. The cassettes are then loaded on to one end
of a storage rack and a walking-beam mechanism brings them in turn to the measuring position.
At measuring head, the master gauge is measured first and other gauges in sequence. Measuring
heads consist of specially designed high-sensitivity, capacitive transducer to minimise risk of
damage to the measuring faces. The gauges being measured are precisely positioned between two
measuring heads, each of which incorporates fine probes to take measurements at the corners of
each face. Extra probes are also included to cater for differences in cross-sectional area over the
range of gauges which can be measured.
The probes comprise steel rods inserted into holes in the measuring head but are insulated
from it by a thin film of epoxy resin. The measuring head acts as a guard ring to eliminate fringing
of the electrical field between the gauge of the probes. The raise and lower measuring head also
incorporates three gauge support rods on which the slip gauge rests while being measured. The
heads are moved apart during gauge insertion. The lower measuring head is fixed and upper one
is mounted on a ram which is raised/lowered hydraulically to adjust the separation of the measuring
heads to suit the nominal size of the gauge being measured.
Each step of the control sequence is initiated only when a signal from a sensor has indicated
completion of the previous action. The sequence is as follows : automatic loading of cassettes from
the storage rack on to the beam ; adjustment of ram height to give the correct gap between the
gauge and the upper measuring head, the placement, in turn, of each gauge at the measuring
position ; and unloading the cassette from the beam after all the gauges have been measured.

Care and Use of Slip Gauges for Workshop and Inspection Purposes.

All the
surfaces are protected against climatic conditions by being covered with a high grade lanotin
preparation or petroleum jelly or other suitable anti-corrosive preparation. The joint between the
lid and the case is made with fillet and groove, to prevent the ingress of dust. The slip gauges are
kept in a suitable case in which there is a separate compartment for each gauge. The case is designed
such that the gauge can’t become displaced. The sizes of the gauges are marked on the case,
immediately adjacent to the appropriate compartment. The gauges and their case should be
protected from dust and dirt. When the gauges are not in use they should be kept only in their case
which should be kept closed. The gauges should be used only in air-conditioned rooms free from
dust and maintained at constant temperature. Every care should be taken to protect the gauges
from getting magnetised otherwise they will attract metallic dust. Gauge blocks should be handled
using a piece of chamios leather or perspex tongs. In the case of new gauges, first the protective
coating applied to it should be removed with petrol and finally gauges be wiped with a clean soft
linen cloth. The wiping should preferably be done everytime before using the gauges.
During the actual use, the fingering of the lapped faces should be avoided as far as possible
in order to preclude the risk of tarnishing, and the handling should be as minimum as possible to
avoid transfer of heat from body to the gauges. Handling would also corrode high finish of gauges
due to the natural acid in the skin. If handling is unavoidable, the hands should be washed and
then coated with a film of pure petroleum jelly. If the gauges have been handled for some time, then
they should be allowed to settle down to the prevailing temperature of the room. Actually both the
work to be tested and the gauges wrung together should be allowed to settle down to the prevailing
temperature of the room before doing any test. During actual use the slip gauges with their working
surfaces should never be placed on the surface plates etc. After use, the gauges are wiped and finger
marks removed, and replaced in their proper compartment in the case. For making combination,
the proper gauges should be taken out of the case and case closed. Before starting the operation of
wringing, the faces of the gauges should be wiped clean from the dust. The standard method of
wringing the gauges together is first to bring the faces of the gauges into contact at right angles to
one another and then turn them through 90°. This method causes less rubbing of the surfaces and
for this reason the wringing should not be done by sliding two gauges parallel to each other. Thin
slip gauges should not be wrung together without using a rigid gauge block as a basis for one of
them since there is a danger of deforming thin gauge blocks permanently. The gauges should never
be left wrung together for an unnecessary length of time. This may lead to micro cold welding and
slight pitting of the surface when they are separated. The gauges should always be slid apart and
the wringing joint between them should never be broken. If the gauges are not to be used for long
time, they should be coated with petroleum jelly which should be applied only with a clean piece of
soft linen. A brush should never be used for this purpose, as otherwise the jelly may be aerated and
the moisture in the bubbles so formed may cause rusting of the faces.
When any gauge is knocked or dropped, its edges are most likely to be damaged. Such burrs
may be removed with care by drawing an Arkansas stone lightly across the damaged edge in a
direction away from the measuring face of the gauge and finally gauges washed and cleaned. Gauges
with scratched surface should be returned to the manufacturer for the true surface to be restored.
If during wringing process, any sign of roughness or scratching is felt the process of wringing should
be stopped immediately and faces examined for burrs or scratches.

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