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TitleC8- Gauge Design
Tags Metalworking Heat Treating Engineering Tolerance Sheet Metal
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Chapter 10

Gauge Design

Intro
The word gauge has been used to describe the several types of measuring instruments such as a

caliper gauge, depth gauge, telescoping gauge and electronic gauge. Both instruments and gauge

have traditionally been used interchangeably. Figs. below show the fixed type gauge which is the

replica of the shapes of the parts to be measured.





Figure (a) Plug gage for holes, with GO-NOT GO on opposite ends. (b) Plug gage with GO-

NOT GO on one end. (c) Plain ring gages for gaging round rods. Note the difference in knurled

surfaces to identify the two gages. (d) Snap gage with adjustable anvils.


Limit Gauges
Adoption of a system of limits and fits logically leads to the use of limit gauges, with which no

attempt is made to determine the size of a work piece – they are simply used to find whether the

component is within the specified limits of size or not. The simplest forms of limit gauges are

those used for inspecting holes or shafts.



Consider first a hole on which the limits on diameter are specified. It would appear that quite

simply the “GO” gauge is a cylinder whose diameter is equal to the minimum hole size, and that

the “NOT GO” gauge is a similar cylinder equal in diameter to the maximum hole size.

Unfortunately it is not as simple as this, for the same reason that limits of size are required for

the work; nothing can be made to an exact size and this includes gauges. Thus the gauge maker

requires a tolerance to which he may work, and the positioning of this gauge tolerance relative to

the nominal gauge size requires a policy decision. For instance, if the gauge tolerance increases

the size of a “GO” plug gauge, and decreases the size of the: “NOT GO” end, the gauge will tend

to reject good work which is near the upper or lower size limits.



Similarly if the gauge tolerance increases the size of the “NOT GO” plug gauge and decreases

the size of the “GO” end then the gauge will tend to accept work which is just outside the

specified limits.



It follows that a number of questions must be answered in designing a simple limit gauge:

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a) What magnitude of tolerance shall be applied to the gauge?
b) How shall the tolerance zones for the gauge be disposed relative to the tolerance zones for

the work?

c) What allowance shall be made for the gauge to wear?


Gauge Tolerances
It is a general rule in measurement that the precision of the measuring equipment should be at

least ten times as good as that of the workpiece to be measured. Thus, if a workpiece has a

tolerance of 0.08 mm the measuring equipment should be capable of detecting differences of

0.008 mm or less. While it is true of gauging that the gauge tolerance must be less than the work

tolerance. B.S 969 : 1982 recognizes that a hard and fast rule cannot be applied to limit gauges.

The gauge tolerances in B.S. 969: 1982 are therefore arranged to be a reducing percentage of the

work tolerances as the work tolerance increases. They are set out in a table below, along with the

wear allowances. This is a modified form of that in B.S. 969 : 1982 and, for interest, the gauge

tolerance is also shown expressed as a percentage of the mean work tolerance.

Disposition of Gauge Tolerances

Having determined the magnitude of the gauge tolerance it must now be positioned relative to

the work limits in such a manner that the gauge does not tend to accept defective work. In order

to achieve this the gauge tolerance on both the “GO” and “NOT GO” gauges must be within the

tolerance zone of the work. The tolerance on the “GO” gauge is set in from the maximum

material limit by an amount equal to the wear allowance.



The tolerance on the “NOT GO” gauge is set within the tolerance zone of the work, there being

no wear allowance.

These relationships between gauge tolerances and the work tolerance are shown in Fig. below.

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Wear Allowance


An allowance for wear is normally applied only to the “GO” gauge. A “NOT GO” gauge should

rarely be fully engaged with the work and should therefore suffer little wear. The allowance for

wear on new “GO” gauges is therefore made by setting the tolerance zone for the gauge in from

the maximum material limit for the work by an amount equal to the wear allowance. A new

gauge is then made to within the limits specified by the tolerance zone for the gauge in this

position. If the gauge then wears with use it can be allowed to wear until its size coincides with

the maximum material limit for the work.

When the gauge is in use it must be checked regularly and if wear is detected on the “ GO”

gauge then it can still be used as long as its size does not exceed from the nominal size.









Taylor’s Theory of Gauging


This theory is the key to the design of limit gauges, and defines the function, and hence the

form, of most limits gauges. It states:

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The “GO” gauge checks the maximum metal condition and should check as many dimensions as

possible. The “NOT GO” gauge checks the minimum metal condition and should only check one

dimension. Thus a separate “NOT GO” gauge is required for each individual dimension.

Consider a system of limit gauges for a rectangular hole, as shown above.



The “GO” gauge is used to ensure that the maximum metal condition is not exceeded and that

metal does not encroach into the minimum allowable hole space. It should therefore be made to

the maximum allowable metal condition dimensions, due allowance being made for wear and the

gauge tolerance. The Fig. below shows the design size of the gauge is associated with one of the

limit material conditions of the objective feature.








Materials For Gauges
If a material is to be used successfully for gauge manufacture, it must fulfil certain requirements,

either by virtue of its own properties, or by having these properties conferred upon it by

manufacturing or a heat treatment process.

These requirements are:

a) Hardness. To resist wear
b) Stability. Its size and shape must not change over a period of time.
c) Corrosion resistance.
d) Machineability. It must be easily machined into the required shape and to the required degree

of accuracy and surface finish.

e) Low coefficient of linear expansion. A limit gauge is often subject to a considerable amount
of handling compared with the workpiece. For this reason it is desirable to have a low

expansion coefficient but it should be noted that parts of the gauge which are to be held in the

hand should have low thermal conductivity. It is recommended, for example, that plug

gauges consist of steel gauging units held by tapers in ebonite or other plastic handles.



It is perhaps fortunate that a suitable material for gauge manufacture is relatively inexpensive

good quality high carbon steel. Suitable heat treatment can produce a high degree of hardness

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