What’s a datum reference frame?
As discussed in workshop No. 5, datum reference frames (DRF) are coordinate systems, and preferably—at least to start with—Cartesian coordinate systems.

As we know, coordinate systems serve to orient and locate objects, and in the world of GD&T in particular, to orient and locate tolerance zones.

Focusing on the encircled feature control frame in figure 1, below, it’s common to think of the cylindrical tolerance zone defined by the position tool as being oriented and located relative to datums A, B, and C. That’s sort of right, but in fact, datums A, B, and C don’t act directly, and instead serve to establish a DRF ABC, which in turn serves to orient and locate the tolerance zone. Our purpose in this workshop is to present the orderly process for establishing DRFs.

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