⌖⌖ Composite position tolerance

How to verify

On a CMM: first establish the complete datum reference frame (A, B, C) and evaluate the whole pattern against it for the upper row; then, without changing frames, evaluate each feature relative to the theoretical position of the others for the lower row, ignoring the angular orientation of the tertiary datum if the frame so indicates. Metrology software with native GD&T support (not just coordinates) is practically mandatory to avoid mixing up the two evaluations.

Workshop guideline values

General

Upper row 0.2–0.5 mm / lower row 0.05–0.1 mm on multi-hole plates in general machining.

Accuracy

Upper row 0.08–0.15 mm / lower row 0.02–0.04 mm on tight fastener patterns.

High precision

Upper row 0.03–0.06 mm / lower row 0.005–0.015 mm on precision patterns (fixtures, templates).

Guidance only: the drawing's value always wins.

Common mistakes when interpreting it

• Inspecting each row as if they were two independent position tolerances: the lower-row zone must fall within the upper-row zone for each feature, not be evaluated separately with no relationship.
• Assuming both rows give each datum the same weight: it is common for the lower row to ignore the angular orientation of the tertiary datum if the frame so indicates, which changes what each row actually controls.