Definition
Symmetry is a geometric tolerance that controls how symmetrically two features are positioned relative to a common datum plane or axis.
What it means for the calculation
A tight symmetry requirement usually means both sides of a feature must be machined in the same set-up, or with a process that guarantees alignment to the datum, to avoid an offset that fails the check.
Where it appears on the drawing
In a feature control frame with the ⌯ symbol, a tolerance value, and a datum reference.
Common mistakes
Assuming a feature that looks centred automatically satisfies symmetry, without measuring it against the actual datum. Also confusing symmetry with position, which uses a different tolerance zone shape.
A tight symmetry requirement usually means both sides of a feature must be machined in the same set-up, or with a process that guarantees alignment to the datum, to avoid an offset that fails the check.
See how Blake reviews a partFrequently asked questions
Does symmetry always need a datum?
Yes, symmetry is always defined relative to a datum plane or axis.
Is symmetry the same as position?
No, position controls the location of a single feature; symmetry controls the balance of two features around a datum.
Can a part be visually centred but fail symmetry?
Yes, a small offset invisible to the eye can still exceed a tight symmetry tolerance under measurement.