Datum Reference Frames, Datum Features, and Datum Selection
From real surface to theoretical datum
A datum feature is the actual surface or feature of size on the part; a datum is the theoretical plane, axis, or point derived from it; a simulator is the physical equipment (surface plate, chuck, mandrel, gauge pin) that establishes it. The three datums form a datum reference frame of three mutually perpendicular planes that removes six degrees of freedom. A primary planar datum removes three (one translation, two rotations), the secondary two, and the tertiary one.
Selection rules: pick features that are functional (the surfaces that actually locate the part in assembly), stable and large enough to repeat contact, and accessible to inspection. The primary should generally be the largest, flattest, most stable surface; the secondary should be oriented perpendicular to the primary; the tertiary should arrest the final rotation. A datum feature of size at RFS derives its axis from the actual mating envelope - smallest circumscribed cylinder for an external feature, largest inscribed cylinder for an internal one. An MMB modifier on a datum feature allows datum shift.
Traps: naming the surface being controlled as its own datum; treating datum precedence as interchangeable (order changes the frame and the inspection result); forgetting that the real surface is not the datum, only the simulator is; using a small or interrupted surface as primary; ignoring datum targets on irregular parts.
Rule of thumb: function first - the datum sequence should mirror the assembly sequence, and each datum in the frame should remove at least one degree of freedom not already removed by its predecessors.