Circular target lines and cylindrical target areas may be used to establish a datum axis on rotating parts. See Fig. 4-36.
4.6.5 Secondary Datum Axis. For a secondary datum feature (RFS), a set of thicc equally spaccd targets may be used to establish a datum axis. See Fig. 4-37. The centering device used to establish the datum axis has a set of three equally spaced contacting features capable of moving radially at an equal rate from a common axis that is perpendicular to the primary datum plane. In this example, the datum targets and the contacting features are oriented relative to a tertiary datum feature.
4.6.6 Equalizing Datums. Where a part configuration is such that rounded features on opposite ends are used to establish datums, pairs of datum target points or lines are indicated on these surfaces, as in Fig. 4-38. Equalizing pin locations arc intended where target points are coordinately dimensioned. V-type equalizers are intended where target points are defined by angles shown tangent to the surface. Where target lines are defined by a dimension from another datum plane, as in Fig. 4-38 for lines B1 and B2, knife edge V-type equalizers are intended, whereas V-type planes may be indicated by only showing the lines in the top view. Equalizing datums may be applied to other suitable part configurations. It is permissible, in such a case, to use the datum feature symbol to identify the equalized theoretical planes of the datum reference frame. It should be noted however, that this is an exception, and is to be
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