# Limit theory

Limit plug gauge for holes

Limit gauges (rare caliber teaching or limit snap gauge ) are teachings that embody the allowable maximum and minimum dimensions of a test object. Limit gauges are used to check whether the actual size of a test object is within the tolerance of a specified nominal size . In contrast to the measuring example with a caliper or a micrometer gives no measure in the form of a numerical value , but it only allows state whether the test object true to size and possibly retain its shape is. The big advantage of a limit gauge is that it is cheaper than e.g. B. is a three-point inside micrometer and it is still very possible to find out whether the machined workpiece is "reject", "good" or "rework".

## Similarities of the border gauges

Tool steel is usually used as the material for limit gauges . Tungsten carbide is the preferred material as the test surface because it has a 20 times higher wear resistance than steel. Plastic handles protect the limit gauges from hand heat, which would heat the gauge above the standard temperature of 20 ° C and thus negatively affect the dimensional accuracy due to the thermal expansion . When using the limit gauges, it must also be ensured that the component to be tested should have the same temperature as the limit gauge. When estimating the inaccuracy, a roughly 11 µm per m and K can be assumed.

Limit gauges have a go gauge on one side, which can be paired with the test item, and on the other end a reject gauge, which has a red marking and must not be paired with the test item. The reject side can also be recognized by another feature; it has a short cylinder and the upper limit is on this side. Since the go gauge always fits on a perfect test item, it represents a dimension and form gauge. The reject gauge, on the other hand, never fits on a perfect test item, which is why the shape accuracy cannot be checked. It is a pure measure.

### Taylor's principle

The Taylor principle (William Taylor) refers to the design and application of lessons for testing fitting parts .

The OK gauge , which can be paired with any test item that can be designated as good, must oppose each element of the workpiece surface to be tested with its own surface element. This checks both the shape and the dimensions. The go gauge must therefore be designed in such a way that it checks the shape to be tested in its entirety.
The reject gauge , which can not be paired with a test item that can be designated as good , should, on the other hand, have such small surface elements that, by pairing with very small elements of the workpiece surface to be tested, it indicates that the limit dimensions are not observed. This means that only individual dimensions of the test item are checked.

## Limit gauges for inside dimensions

Thread limit plug gauge (above) and limit plug gauge with exchangeable go and reject side (below)

To drill holes and grooves to consider using plug gauges. If the surface to be tested is flat, special limit flat gauges are also used. The measuring surfaces can be designed differently, e.g. B. as a cylinder, ball, hexagon or square or as a thread. The good side embodies the minimum dimension and must slide into the hole or groove by applying slight pressure (0.85 Nm). The committee side is only allowed to beak. With the thread plug gauge, the reject side has two threads and only checks the pitch diameter . According to the standard ISO 1502: 1996 (metric ISO threads general application) chapter 7.1.9 section c, the reject side of a thread limit plug gauge may be screwed in a maximum of 2 turns. The revolutions are counted when unscrewing. The good side has a much longer cylinder than the reject side and is often equipped with hard metal strips. The limit dimension is engraved at each end. Commercially available limit plug gauges usually show excessive wear on the good side when they are new, so that the printed value "0" does not correspond to the actual value. Example: A limit plug gauge 6 H7 has a dimension of 6.0027 mm on the good side when new, although the tolerance is defined from 6,000 to 6,0129. This excessive wear is only found on the good side.

## Limit gauges for external dimensions

Limit snap gauge for external dimensions

Limit snap gauges and limit ring gauges, also known as gauge nuts for external threads, form the counterpart to limit plug gauges. They are used to check external dimensions on shafts or threads. In contrast to other limit gauges, which have a gauge on both sides, the ring gauge only embodies a limit dimension. You therefore need two ring gauges for each test. The good side embodies the maximum and is thicker than the reject side. Otherwise the same applies as for the limit plug gauge.