∥ Parallelism

How to verify

Rest the part on the datum (a surface plate or table that simulates it) and sweep a dial indicator across the entire surface being controlled: the difference between the maximum and minimum reading is the parallelism error. Hold the part without forcing it against the datum — excessive clamping distorts the measurement. On a CMM, the datum is constructed as the reference feature and the parallelism of the toleranced feature is evaluated relative to that plane or axis, not relative to the machine coordinate system.

Workshop guideline values

General

0.05–0.15 mm on opposite faces in general machining.

Accuracy

0.01–0.03 mm on parallel guideways or close-fit parts.

High precision

0.005–0.01 mm on columns, surface plates or precision parts.

Guidance only: the drawing's value always wins.

Common mistakes when interpreting it

• Confusing it with flatness: parallelism compares the face against an external datum, not against itself; a part can be parallel on average and still not be flat.
• Measuring with the part resting on the face being controlled instead of on the datum: the datum must be established as the physical reference before sweeping the opposite face.