GD&T symbol · Orientation tolerance

Angularity — Orientation Tolerance

Controls the angle of a surface or axis relative to a datum (other than 90°).

Definition

Angularity controls the angle of a surface or an axis relative to a datum — at any angle other than 0° or 90°, which belong to parallelism and perpendicularity respectively. It is an Orientation control, and orientation controls always require a datum reference frame; there is no self-referencing orientation. This is the cleanest contrast in the library: Form controls (straightness, flatness, circularity, cylindricity) never use a datum, Profile can be specified either way, and Orientation always does. The datum is what the angle is measured from — without it, "this surface at 30°" has no meaning, because 30° from what?

The zone: two parallel planes at the basic angle

The angularity tolerance zone is the space between two parallel planes separated by the tolerance t, held at the basic angle relative to the datum surface. The entire actual surface must lie between the planes. Unlike parallelism (0°, implied) and perpendicularity (90°, implied), the angle is not built into the symbol — angularity is the general case, so the basic angle must always be stated as a dimension in the drawing. A surface that is too steep, too shallow, or warped across the band fails; a uniformly tilted surface that stays inside the band passes, because angularity limits the angle, not the location. For an axis, the zone is a cylinder of diameter Øt around the true axis, and the axis itself is a derived feature rather than a surface.

Side view of a surface at 30° basic to datum A: the zone is two parallel planes 0.2 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.

One family, three symbols

Perpendicularity, parallelism, and angularity are not three unrelated controls — they are one orientation family, and the difference between them is only the angle. Perpendicularity is angularity at exactly 90°, and parallelism is angularity at exactly 0°; both angles are implied by the symbol itself, so neither frame needs an angle dimension. Angularity is the general case, used for any angle other than 0° or 90°, and it is the only one of the three that requires a basic angle dimension, because nothing about the symbol tells the reader what the angle is. When in doubt, pick the specific member: if the requirement is literally "square to A", write perpendicularity, not angularity with a 90° dimension — the symbol alone already says it.

Parallelism — angularity at exactly 0°: the zone is parallel to the datum, and the frame needs no angle dimension.
Perpendicularity — angularity at exactly 90°: the zone is square to the datum, and the frame needs no angle dimension.
Angularity — the general case: any other angle, and the basic angle dimension is mandatory because the symbol cannot imply it.

Reading a feature control frame

∠ 0.2 A reads as: an angularity tolerance of 0.2 zone width, referenced to primary datum A. The 30° basic angle does not live in the frame — it is a dimension on the drawing, because the frame carries the tolerance and the datum, not the angle itself. Compare the position frame ⌖ ⌀0.5 Ⓜ A B C: position locates the feature and may carry material modifiers, while angularity only orients it and never carries a material condition — a surface angularity has no MMC.

The feature control frame, compartment by compartment. The basic angle (here 30°) is dimensioned on the drawing, not written in the frame — the frame carries the tolerance and the datum. Orientation controls always require the datum reference; unlike Form controls, there is no self-referencing angularity.

Measurement

Sine bar or angle plate + height gauge — set the part on a sine bar at the basic angle, sweep an indicator over the controlled face, and read the total range; it must stay within t. The classic production method for wedge faces and chamfered seats.

CMM point sampling — probe a grid over the surface and fit the tightest pair of parallel planes at the nominal angle that bounds the points. Used for complex parts and whenever the measured angle feeds a statistical process.

When to use it

Angled mating faces — wedge seats, dovetail faces, chamfered register surfaces — where the angle itself is the functional requirement
Angled guide or slide surfaces that must hold a consistent angle to a datum for the life of the assembly
Angled mounting faces on cast or machined housings, where a loose angle tolerance would let the assembled unit sit visibly off
Any orientation requirement at a non-90° angle — if the angle is 0° or 90°, parallelism or perpendicularity is the more direct symbol