Parallelism : GD&T Basics

Parallelism controls parallelism between two lines, surfaces or axis.
In another words, it describes a parallel orientation of one referenced feature to a datum surface or line in 3d tolerance zone.

Parallelism does not controls the angle of the referenced feature. it creates a tolerance zone where all points of the feature must lie. MMC and LMC conditions are also applicable with parallelism tolerance.

Parallelism Tolerance Representation

Tolerance Zone

Tolerance zone will be two parallel planes that are parallel to the datum feature or surface.

Application Of Parallelism

Parallelism ensures two surfaces are in synchronized with each other and constant distance between them is maintained. It also avoids taper in any slot or cylindrical hole.

Parallelism Measurement

Dial Gauge can be used to measure the parallelism of a part with respect to datum plane. 

For measurements, part is constrained w.r.t datum plane and gauge is moved across reference surface.

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