Parallelism is a type of orientation control tolerance. It controls parallelism between two lines, surfaces or axis. It does not controls the angle of the referenced feature. But it creates a tolerance zone where all points of the feature must lie.
Surface or axis datum is required to represent parallelism Tolerance.
MMC and LMC conditions are also applicable with parallelism tolerance.
Tolerance zone will be two parallel planes that are parallel to the datum feature or surface. All points on the controlled surface must lie within these parallel planes.
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.
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.
To sum up, Parallelism is a type of orientation control tolerance.It can be represented with surface or axis as datum.
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