TABLE OF CONTENTS The function of a feature limits the permissible deviations of size, form, orientation and location. If one of these deviations is too large it affects the functioning of the feature. This applies to each feature and may be illustrated by the following example of a flange shown in Fig. 17.1.
The deviations allowed by the function may sometimes be relatively large, e.g. for D in Fig. 17.1, but there are always limits given by the function. Otherwise the considered feature would not need to exist.
Consequently the tolerance for each characteristic should be given. As there is hardly any communication other than by technical drawings the reliance on undefined workshop accuracy or good workmanship is not sufficient. The international exchange of technical drawings requires that the drawings specify all dimensional and geometrical tolerances necessary to define the shape and size of the part completely.
Regarding geometrical tolerances this requirement has not always been respected. In the past, in most countries there has been a reliance on undefined good workmanship regarding deviations of perpendicularity, coaxiality and symmetry for features that have no corresponding tolerance indication on the drawing.
In some countries - contrary to others - the form deviations have been assumed to be controlled by the dimensional (size) tolerances. This is specified, for example, by Rule #1 of the USA Standard ASME Y14.5M-1994.
This requires that the feature (cylindrical surface or...
