LOCATION CONTROL / GD&T SYMBOL GUIDE

SYMMETRY — LEGACY ASME

Legacy ASME symmetry controls the median points of opposed elements relative to a datum plane. It was removed from Y14.5-2018.

Datum required: Yes
MMC / LMC applicable: No — legacy median-plane control
Control type: Location, orientation and form
What it controls: The illustrated slot is intended to straddle datum plane A. The control addresses its derived median plane, not merely the spacing of one edge.
Typical features: Centred slots, forked brackets and opposed tab faces, particularly when interpreting older drawings.

INTERACTIVE APPLICATION

GD&T symmetry example: centred slot

U-shaped part with a symmetry frame of 0.5 relative to datum A, applied to the slot — Drawing callout: symmetry 0.5 relative to datum A.

WITH SYMMETRY

The drawing controls the slot's derived median plane

The feature control frame states symmetry 0.5 relative to datum A.
The leader attaches the control to the slot feature of size.

What to read

The datum feature establishes plane A.
The slot's derived median plane is evaluated across the controlled extent.

01 / PURPOSE

Why is Symmetry used?

A slot or tab can have acceptable width but sit off centre, leaving unequal wall thickness or assembly clearance.

02 / VALUES

Commonly defined variable values

The controlling value is a tolerance-zone width, not a universal process grade. For this symmetry example, the drawing states the controlled feature and applicable extent.

Scroll the table sideways to see every column.

Illustrative symmetry tolerances for early design discussion
Application	Illustrative starting value	Example feature	Notes / caution
Legacy machined slot	0.05–0.20 mm	10 mm slot	Confirm the cited drawing edition.
Forked bracket	0.10–0.30 mm	Two 8 mm arms	Check wall-thickness function.
Precision centred tab	0.02–0.10 mm	20 mm tab	Position/profile may be clearer for new work.

These are planning examples, not universal rules or guaranteed process capabilities. Set the actual tolerance from function, size, length, process capability, inspection method and the standard specified on the drawing.

ASME Y14.5-2018 (R2024) — legacy control

This is a legacy ASME control removed in the 2018 edition. Interpret older drawings using their stated edition; select position, profile or runout for new requirements as appropriate to function.

ASME Y14.5 edition reference. Examples are learning schematics; use the complete requirements on the drawing.

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