GD&T symbol · Orientation tolerance

Perpendicularity — Orientation Tolerance

Controls squareness of a surface, axis or line to a datum.

Definition

Perpendicularity controls how square a surface, axis, or line is relative to a datum. It is an Orientation control, and orientation controls always require a datum reference frame — there is no self-referencing perpendicularity. The angle is exactly 90°, and it is implied by the symbol itself, so no basic angle dimension is needed: the frame carries the tolerance and the datum, and the drawing does not need to state that the requirement is a right angle. Perpendicularity is the 90° member of the orientation family — angularity at any other angle, parallelism at 0° — and it is the one most often confused with position, which is a separate concern: perpendicularity controls orientation only, and says nothing about where the feature is located.

The zone: two parallel planes at 90° to the datum

The perpendicularity tolerance zone is the space between two parallel planes separated by the tolerance t, held at exactly 90° to the datum surface. The entire actual surface must lie between the planes. A face that leans over or under the right angle by more than the zone fails; a face that is square but shifted along the datum passes, because perpendicularity limits the orientation, not the location. For an axis — a bore, a pin, a stud — the zone is a cylinder of diameter Øt around the true axis, and an axis control may carry a material-condition modifier, because a feature of size has a size to depart from. A perpendicularity frame therefore reads differently depending on what it controls: a surface frame has no Ø and no Ⓜ, while an axis frame may have both.

Side view of a surface perpendicular to datum A: the zone is two parallel planes 0.1 apart, held at that exact angle to the datum surface, and the entire actual surface must lie between them. The planes are oriented by the datum — they are not free to float the way a Form-control zone is. Schematic, exaggerated.

Reading a feature control frame

⟂ 0.1 A reads as: a perpendicularity tolerance of 0.1 zone width, referenced to primary datum A. No angle dimension appears — 90° is implied by the symbol. For an axis, the same frame takes a diameter: ⟂ Ø0.1 A, and a feature of size may add a material-condition modifier. Compare the position frame ⌖ ⌀0.5 Ⓜ A B C: position locates the feature with basic dimensions, while perpendicularity only orients it — a bore can be perfectly perpendicular to its mounting face and still be drilled in the wrong place.

The feature control frame, compartment by compartment. No basic angle dimension is needed — 90° is implied by the symbol. Orientation controls always require the datum reference; unlike Form controls, there is no self-referencing perpendicularity.

Measurement

Cylinder square + height gauge — hold a precision square against the datum, sweep an indicator over the controlled face, and read the total range; it must stay within t. The standard shop method for faces and bosses.

CMM point sampling — probe the surface and fit the tightest pair of planes at 90° to the datum that bounds the points. Used for bores, where an internal square cannot reach, and for parts with several perpendicularity callouts at once.

When to use it

Mounting faces that must seat square against a datum — flanges, motor pads, fixture registers — where a lean would concentrate load at one edge
Bores perpendicular to their mounting face: bearing fits and bushing seats, where tilt at depth shows up as apparent location error at the far end of the hole
Pressed or coined holes, where the punch and die naturally produce a lean that must be bounded
Stacked assemblies, where each part's squareness compounds into a visible lean by the top of the stack