Engineering · GD&T
Symmetry (removed in Y14.5-2018).
A location tolerance for non-cylindrical features whose median points should be symmetric to a datum plane. Like concentricity, it was removed from ASME Y14.5-2018 (still seen on legacy drawings) in favor of position. If your drawing has symmetry, here’s what it means and what to use instead.
01 · What symmetry tried to do
Median plane within a planar zone.
Symmetry was the non-round counterpart to concentricity. It controlled the median points of a feature (typically a slot, keyway, or rectangular opening) to lie within a tolerance zone symmetric to a datum plane. The tolerance zone was two parallel planes 0.5× the tolerance away from the datum on each side.
The same problem as concentricity: median-point evaluation is computationally awkward and inspection in practice usually defaulted to position-of-feature anyway. The symbol was removed from ASME Y14.5-2018 (still seen on legacy drawings).
02 · What to use instead
Position of feature.
For symmetric features (slots, keyways, rectangular cutouts), position applied to the feature’s median plane gives equivalent functional control with universal CMM measurability. The basic dimension is implicit (centered on the datum), and MMC modifier is available for clearance applications.
Inspection is straightforward: probe both sides of the feature, calculate the median plane, measure offset from the datum-derived center. The result is reported as a single position value rather than a dual-side symmetry deviation.
03 · Legacy drawings
Same handling as concentricity.
When we see symmetry on production drawings, we verify with position-of-feature or paired offset measurement on a CMM. The result is documented per the drawing intent and reported on the first-article inspection report. For drawings under your control, the replacement is universal: position-of-feature, optionally with MMC modifier for clearance fits.
Keep exploring
Related tools & references
Full feature-control-frame symbol reference.
The preferred control for centered features in current practice.
The coaxial counterpart to symmetry — also legacy.
Check median-plane location across mating parts.
Why symmetry is rarely the right callout.
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