Always relative, never locating
Perpendicular to what, parallel to what: the characteristic makes no sense without a datum reference, which is why every orientation frame carries one. The zone is held at the stated angle to that datum and is free to translate.
That freedom is the whole point. A face can be perfectly square and still be in the wrong place, and correcting the place is the job of a size dimension, a position tolerance or a profile tolerance.
The refinement pattern
A hole with position diameter 0.50 mm to A, B, C may lean by that whole amount. Adding a second frame with perpendicularity diameter 0.15 mm to A says that however the axis is placed, its lean relative to A must fit a tighter cylinder.
A refinement is always tighter than what it refines and never loosens it. That pattern buys location tolerance where it is cheap and spends it on orientation where the function is sensitive, such as a hole that has to receive a long dowel.
Zone shape and modifiers
A diameter symbol in the tolerance compartment of an orientation frame makes the zone a cylinder, limiting lean equally in every direction. Without it the zone is two parallel planes and the axis is free to tip in the other direction.
Applied to a feature of size, an orientation tolerance may carry a material condition modifier, which grants bonus lean allowance and produces a constant virtual condition. A gauge bushing at that boundary, held square to the datum, then checks size and squareness together.
Form comes along for free
A parallelism tolerance of 0.08 mm places the surface between two planes 0.08 mm apart, so the surface is flat to at least that much by construction. A separate flatness callout is only worth adding when it is tighter.
The tangent plane modifier changes what is judged: a perfect plane resting on the high points rather than every point of the real surface. That matches how a mating face actually sits and stops local waviness from consuming an orientation tolerance that exists to control lean.
Perpendicularity at MMC on a pin
A pin with MMC 10.00 mm carries perpendicularity of diameter 0.06 mm at MMC to datum A.
- MMC size
- 10.000 mm
- Stated perpendicularity tolerance
- 0.060 mm
- Virtual condition boundary
- 10.060 mm
- Bore of a gauge bushing that checks it
- 10.060 mm
The boundary rule does not care which characteristic produced the tolerance. An external feature at MMC has its boundary pushed outward, and the same arithmetic under a position tolerance gives the same number.