Symmetry — Location Tolerance

Controls balance of median points of a feature about a datum center plane.

Definition

Symmetry controls the median points of opposed surface elements of a feature relative to a datum center plane. It is a Location control, so it always requires a datum reference. The zone is the space between two parallel planes separated by the tolerance t, centered on the datum center plane, and every median point must fall between them. Symmetry is the plane-based sibling of concentricity — same derived-median-point mechanics, same inspection cost, same professional reluctance — with the axis replaced by a center plane. Where concentricity asks "are the median points coaxial with the datum axis?", symmetry asks "are the median points evenly disposed about the datum plane?".

Why the professional consensus is to avoid it

Everything expensive about concentricity is expensive here too, and the specifics are the same specifics: the median points are computed, not measured — each one is the midpoint of two opposed surface points — so inspection is a many-point CMM analysis with no possible functional gauge, and the result shifts with point density. The control is also insensitive to exactly the errors that matter functionally: opposed surfaces can both be lobed, so long as their midpoints sit near the datum plane, and symmetry will pass a feature that visibly rocks in assembly. The ISO world removed the symbol entirely — ISO 1101 deleted symmetry as a separate characteristic — and ASME Y14.5 has flagged it for future withdrawal; new drawings should not be introducing it.

The modern replacement

The functional intent behind nearly every symmetry callout is served by position with the tolerance applied to a width — position controls the feature's center plane to the datum plane with a zone between two parallel planes, is gaugeable, accepts material modifiers where the feature is one of size, and is the callout the standards community itself points to as the replacement. Where the requirement is really about the surfaces' orientation rather than their balance, perpendicularity or profile of a surface to the datum plane is the honest control. The one genuinely irreplaceable use — true mass balance about a plane, for spinning or oscillating parts — is better handled by a balance specification than by a geometric tolerance.

• Center plane located to the datum plane: position on the feature's width — gaugeable, MMC-capable, the standard community's designated replacement
• Surfaces must sit square to the datum plane: perpendicularity — orientation, not balance
• Surfaces must conform about the plane: profile of a surface referenced to the datum — controls the surfaces themselves

Reading a feature control frame

⌯ 0.1 A reads as: a symmetry tolerance of 0.1 zone width, referenced to datum center plane A. No diameter symbol appears — the zone is a width between planes, not a cylinder. And like concentricity, symmetry never carries a material-condition modifier: it controls derived median points, not a feature of size, so a frame pairing ⌯ with Ⓜ is not a valid callout. The datum reference is mandatory — there is no self-referencing symmetry, because “evenly disposed” is meaningless without something to be even about.

Measurement

CMM median-point analysis — probe opposed point pairs across the feature's surfaces, compute each pair's midpoint, and fit the tightest pair of planes about the datum center plane that contains them. The same expensive, point-density-sensitive analysis concentricity requires, rotated 90°.

There is no functional gauge — as with concentricity, a physical gauge cannot reach derived midpoints. Any shop "checking symmetry with a gauge" is measuring position or profile and calling it symmetry.

When to use it

Almost never on new work. The legitimate remaining cases are narrow: legacy drawings that must be interpreted as written, and the rare balance-driven specification where median-point distribution about a plane is explicitly the requirement and a balance spec is not available. For anything else — a slot that must sit centered on a part, a tab that must align with a groove — position on the width is the correct, gaugeable, modern callout, and the standards bodies themselves have signaled that this symbol is on its way out. If a drawing in front of you specifies symmetry, the useful question is not "how do I inspect this?" but "which control did the designer actually mean?" — and the answer is almost always position.