GD&T Symbol Guide
Perpendicularity
Controls a surface or axis so it remains square to a datum plane, line, or axis.
Definition
Perpendicularity is the 90 degree member of the orientation family. It can apply to a surface or to an axis.
For surfaces, it controls an envelope of parallel planes. For holes and pins, it controls where the feature axis may lean.
Application
Use it for square faces, mounting bosses, drilled holes, pins, and tabs that must assemble normal to a datum.
Axis perpendicularity with MMC is common because it supports practical functional gaging for assembly clearance.
Hole Axis Kept Square To Datum A
The diameter symbol means the controlled hole axis must stay inside a cylindrical zone perpendicular to datum A.
3D Tolerance Zone
Surface perpendicularity uses two parallel planes at 90 degrees; axis perpendicularity uses a cylindrical zone.
Inspection Method
Surface perpendicularity is often checked against a square datum setup with a height gage or CMM.
For holes and pins at MMC, a functional gage can confirm that size and perpendicularity together still permit assembly.
Worked Check: Mounting Hole
A hole has diameter 0.10 perpendicularity at MMC to datum A. The hole is 0.03 mm larger than MMC and the measured axis error is 0.11 mm.
Base tolerance
0.10 mm
This is the tolerance available at MMC.
Bonus
0.03 mm
Extra hole size adds bonus tolerance.
Available
0.13 mm
0.11 mm measured error is inside the available zone.
The hole passes because the MMC bonus tolerance covers the measured perpendicularity error.
Comparison Table
| Control | Datum | Zone | Best Read |
|---|---|---|---|
| Form | No | Free-floating | Shape of one feature |
| Orientation | Yes | Locked to datum | Angle relationship |
| Location | Usually | Locked to datum frame | Where a feature belongs |
Notes
Perpendicularity does not directly specify an angle tolerance in degrees; the tolerance is a distance.
A diameter symbol in the feature control frame changes the zone from parallel planes to a cylinder.