C8- Gauge Design

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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 GONOT 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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coupled with stability, and at the same time it can be readily machined and brought to a high degree of surface finish.

Heat Treatment of Limit Gauges High carbon steel is fully hardened by heating to 730°C and quenching in water. This will give a hardness of approximately 64 on the Rockwell “C” scale, but it will also make the steel extremely brittle. It is necessary to temper the gauge to reduce the brittleness, but not to make it so soft as to reduce its wear resistance. At the same time the tempering treatment can be used to stabilize the material and relieve any internal stresses which may distort it over a period of time. A tempering temperature of 200°C will reduce the brittleness so that the gauge is not likely to chip and the hardness value will be Rockwell “C” 58. If this temperature of 200°C is maintained over a period of 8 to 10 hours it will also make the gauge extremely stable. Screw thread gauges are particularly fragile and prone to damage if roughly handled, and these gauge should be “let down” further at a temperature of 240°C to give a Rockwell hardness of “C” 52.

Surface Finish For Gauges Much can be done to reduce the initial wear rate of a gauge if its surface finish is good. A poor finish with a small number of high peaks is prone to more rapid wear than a finish having a large number of very small peaks, giving a large contact area. Thus a gauge should be finished by high quality grinding or lapping to give a C.L.A value of not more than 0.10µm.

REFERENCES: 1. S. Kalpakjian and S R. Schmid, Manufacturing Engineering & Technology, 4th Edn., Prentice Hall, 2001. 2. CV Collett & AD Hope, Engineering Measurements, 2nd edition, Pitman Pubr., 1987. 3. FET ONLINE Archive, 2002/2003.