GD&T Symbol Guide
Symmetry
Controls the median points of opposed surfaces so they remain centered about a datum center plane.
Definition
Symmetry is a 3D control for opposed features. It requires the midpoint between each pair of opposing surface points to lie near a datum center plane.
It is not the casual meaning of symmetric appearance; it is a precise, point-by-point median-plane requirement.
Application
Use symmetry only when balanced distribution about a center plane is truly functional, such as some rotating or load-balanced features.
For most slots, grooves, and tabs, position or profile is easier to inspect and usually communicates the design intent more directly.
Centered Groove In A Rotating Coupling Block
The groove faces are sampled in opposing pairs. Their median points must stay inside the symmetry zone about datum plane A.
3D Tolerance Zone
Two parallel planes equally disposed about the datum center plane.
Inspection Method
A CMM typically samples paired points on the two opposed surfaces, calculates each midpoint, and compares those midpoints to the datum center plane.
Because the controlled median points are theoretical, quick hard-gaging is rarely practical.
Worked Check: Groove Centering
A coupling groove has symmetry 0.10 mm relative to datum plane A. Three measured median points are +0.018, -0.033, and +0.041 mm from the datum center plane.
Zone half-width
0.10 / 2 = +/-0.05 mm
The tolerance value is the full distance between the two symmetry planes.
Worst midpoint
max(|0.018|, |0.033|, |0.041|) = 0.041 mm
Every sampled midpoint must remain inside +/-0.05 mm.
Compare
0.041 <= 0.05
All measured median points are inside the zone.
The groove passes symmetry. If the only need is assembly location, a position callout would usually be simpler to verify.
Comparison Table
| Control | Reference | Measured Element | Typical Use |
|---|---|---|---|
| Symmetry | Datum center plane | Median points | Balanced opposed surfaces |
| Concentricity | Datum axis | Median points | Balanced round features |
| Position | Datum frame | Axis or median plane | Practical feature location |
Notes
Symmetry is related to concentricity, but symmetry works about a plane while concentricity works about an axis.
It does not accept MMC or LMC modifiers in typical ASME usage.