Parallelism

Controls a surface or axis so it remains parallel to a datum while staying inside a tolerance envelope.

∥

0.10

Parallelism controls the 0 or 180 degree orientation relationship between a feature and a datum.

For a surface, the controlled points must fit between two planes parallel to the datum. For an axis, the zone may be cylindrical.

Use it for opposing faces, bearing seats, gear faces, slide ways, and any features that need consistent spacing in assembly.

It is similar to flatness, but the zone is locked to a datum rather than floating with the surface.

Gear Face Held Parallel To Datum A

The datum face establishes orientation; the opposite face must remain inside the parallel tolerance zone.

\u2225

0.10

Two parallel planes or a cylindrical axis zone oriented parallel to the datum.

Seat the datum feature against the inspection surface, then sweep the controlled surface with an indicator or height gage.

The high-to-low indicator movement represents the parallelism error for the checked surface.

A gear face has parallelism 0.10 mm to datum A. With A on the surface plate, the opposite face reads from 0.004 mm to 0.076 mm.

Indicator spread

0.076 - 0.004 = 0.072 mm

The spread is the distance between the two parallel boundary planes.

Compare

0.072 <= 0.10

The controlled face remains inside the zone.

Functional read

Pass

The face should maintain more even contact in assembly.

The gear face passes parallelism relative to datum A.

Control	Datum	Zone	Best Read
Form	No	Free-floating	Shape of one feature
Orientation	Yes	Locked to datum	Angle relationship
Location	Usually	Locked to datum frame	Where a feature belongs

Parallelism implies a form refinement for the controlled surface because the whole surface must fit between two parallel planes.

Orientation controls require datums.