DRF Datum reference frame

How to verify

On a CMM, probe the primary datum feature first (at least three well-spread points if it is a plane), then the secondary and finally the tertiary, letting the software align following that hierarchy: coarse alignment, fine alignment and origin. On the shop floor the physical equivalent is the inspection fixture: face A rests on the surface plate, face B against one stop and face C against another. Never measure positions using the machine table or the vise jaws as a reference: they coincide with the datum reference frame only by chance.

Workshop guideline values

General

Datums with flatness and perpendicularity of 0.05–0.2 mm; sufficient for positions of 0.2 mm and up.

Accuracy

Datums at 0.01–0.03 mm when the frame calls for positions of 0.05–0.1 mm.

High precision

Datums at 0.002–0.01 mm on fixtures and gauges, where the reference frame error must be a small fraction of the tolerance being verified.

Guidance only: the drawing's value always wins.

Common mistakes when interpreting it

• Treating it as if it were the CAD origin or the machine's G54: the frame is derived from real surfaces of the part, not from a theoretical origin.
• Changing the datum order when programming the inspection because it seems to measure the same: |A|B|C| and |B|A|C| lock the degrees of freedom in a different order and give different results.
• Aligning to a single datum when the frame cites three: with only the primary plane the part can still rotate and translate, and the measured position means nothing.