Parallelism

Controls

A surface or axis must lie within a tolerance zone oriented exactly parallel to the datum reference. For a surface, the zone is two parallel planes separated by t and parallel to the datum. For an axis, the zone is a cylinder of diameter t whose centreline is parallel to the datum.

Parallelism controls orientation only — it does not control the distance between the controlled feature and the datum.

Example

The two guideways of a lathe bed must be parallel to each other within 0.01mm per 300mm so the saddle does not rack or bind during traversal. Parallelism is also used on parallel bores in engine blocks — if the bores are not parallel, the crankshaft and connecting rods cannot move freely.

Inspection

• Surface plate (datum) + dial gauge traversed across the controlled surface
• CMM — measure surface normal vectors and compare orientation to datum plane
• Height gauge sweeping across the surface at multiple positions
• For parallel bores: pin gauges in both bores + measurement of centre distance in multiple planes

Datum & Modifiers