Quality Assurance

325Geometric Dimensioning and Tolerancing (GD&T)

At this point, beyond the ofcial denition of a datum, the user of GD&T

must be aware of two more concepts. The rst is the relationship of datum

features and the second is what goes in the datum reference frame (DRF) box.

A datum feature is a physical feature that acts as an acceptable substitute for

a datum. Datum features relate the various features of the part to each other.

Three relationships of the datum feature must be considered. They are

1. Primary datum features establish basic stability for the part with

respect to a functional mating feature and typically control three

degrees of freedom.

2. Secondary datum features establish orientation to the primary

datum features and typi ...