18.1 General

The following deals with generalities on the inspection of geometrical deviations. A synopsis on inspection methods is given in ISO TR 5460. More detailed descriptions of inspection methods are given in the former East German Standards TGL 39 092 to TGL 39 098 and TGL 43041 to 43045, TGL 43 529 and TGL 43 530.

Geometrical tolerances are geometrically exactly defined. They determine geometrical zones within which the surface of the feature must be contained.

There are several methods for inspecting whether the geometrical tolerances have been respected. These methods are more or less precise. The drawing indications according to ISO 1101 do not prescribe a particular (certain) inspection method. According to ISO TR 14 253-2 the PUMA method should be used in order to optimize manufacturing and inspection (see 18.11).

In industrial practice the inspection of geometrical deviations is often only economical with less precise inspection methods. Less precise inspection methods are normally less time consuming and less costly. In contrast, more precise inspection methods are normally more time consuming and more costly. Therefore it is often advisable to start the inspection with a cheap but less precise method and to switch to a more precise (and more expensive) method only in cases where the measurement result is near the limit given by the tolerance (see, e.g., 18.7.9.3.6).

A prerequisite is that the possible errors of the inspection methods are known. Information on systematic errors of inspection methods is given in ISO TR 14 253-2 (see 18.11)...