⌯ Symmetry

How to verify

Measure the actual width of the feature (slot or tab) and its location relative to the reference faces at several points, calculating the actual median plane and comparing it with the datum median plane. On a CMM this is automated by calculating both median planes directly. ASME Y14.5-2018 recommends replacing symmetry with a position tolerance with a basic dimension of 0, which is easier to verify on a CMM and avoids the ambiguity of the median plane on irregularly shaped parts.

Workshop guideline values

General

0.1–0.2 mm on slots and tabs in general machining.

Accuracy

0.03–0.08 mm on keyways or close fits.

High precision

0.01–0.02 mm on precision parts with critical centered features.

Guidance only: the drawing's value always wins.

Common mistakes when interpreting it

• Relying on visual layout marks or the machine zero without first verifying the actual centering of the part with an indicator or probe.
• Ignoring that ASME Y14.5-2018 replaces it with position using a basic dimension of 0: a new drawing with this symbol may be a direct carry-over from an old, never-updated drawing, so it is worth confirming the intent with engineering.