Parallelism

1What is parallelism?

Parallelism in GD&T ensures that a feature maintains a consistent, parallel orientation to a specified datum surface or datum axis within a defined 3D tolerance zone. When applied as a surface control, every point on the controlled surface must lie between two parallel planes that are themselves parallel to the datum. Parallelism can also be applied to a feature of size (FOS), where it controls the derived median plane or derived median line of that feature, ensuring its center element stays parallel to the datum, even if the feature’s actual surfaces vary. Parallelism strictly governs orientation, not location or form (though form error must still fit within the tolerance zone).

2What does parallelism look like on a drawing?

In a drawing, parallelism appears as a feature control frame containing the parallelism symbol ( // ), the tolerance value, and the referenced datum. The frame is attached to the surface or feature being controlled, as in the drawing below, showing that the face it points at must remain within two perfectly parallel planes spaced 0.75 apart and oriented parallel to datum A. The nominal geometry defines the shape; the parallelism callout defines how consistently that surface must stay parallel to the datum across its entire area.

3How is parallelism inspected?

Parallelism is inspected by first placing the part on its datum surface, in this case, the flat block labeled datum A, to establish the reference plane. A CMM or indicator then measures points across the controlled surface, creating a map of its high and low spots. The software (or indicator readings) compares those measurements to two perfectly parallel planes spaced by the parallelism tolerance. If every measured point falls within that “parallel window,” the surface is considered parallel to the datum. In simple terms: set the part on its datum, scan the surface, and confirm it stays consistently the same distance from the reference plane.