18.4 Interchanging the datum feature and toleranced feature

With related geometrical tolerances, the drawing distinguishes between toleranced features and datum features. When, for the inspection, toleranced and datum features are interchanged, completely different values of the geometrical deviation may be obtained from the same workpiece (Fig. 18.9).

Figure 18.9: Change in geometrical deviation caused by interchanging of toleranced and datum features

Therefore, for the inspection, toleranced and datum features must not be interchanged. When the datum feature indicated on the drawing is not suitable for inspection purposes (e.g. because it is too short) a change in the drawing is necessary (Figs 19.13 and 19.14).