Engineering · GD&T
Parallelism.
An orientation tolerance that controls a surface or axis to be parallel to a referenced datum within a specified zone. The tolerance zone is two parallel planes (or a cylinder) parallel to the datum, separated by the tolerance value.
01 · Definition
Flatness with a datum reference.
Parallelism is essentially flatness with a datum reference. The toleranced surface or axis must lie within a tolerance zone that’s parallel to the referenced datum, with the zone bounded by two parallel planes (or a cylinder for axis applications).
A surface that’s perfectly flat but tilted relative to its datum fails parallelism. A surface that’s parallel to its datum but bowed in the middle also fails parallelism (and flatness). Parallelism inherently controls form because the bounded tolerance zone is flat.
02 · FCF and example
Top face parallel to datum A within 0.0005.
A precision optical mounting plate with a top face that must be parallel to the bottom face (datum A) within 0.0005″. The two faces define the optical alignment plane, so any tilt translates directly to misalignment downstream.
Inspection: place datum A on a granite surface plate, sweep the top face with a height gauge or CMM probe. The high-low difference is the actual parallelism error. For tight parallelism, use a CMM with profile-best-fit to the datum-derived surface.
Production strategy: machine both faces in one chucking on a precision machine, in stress-free material (MIC-6). Parallelism degrades on wrought 6061 over time.
03 · Axis parallelism
For round features.
For a hole or shaft that must be parallel to a reference axis, use the diameter symbol. The tolerance zone becomes a cylinder of the specified diameter, oriented parallel to the datum. Common in dual-bore housings and shaft-line tooling.
Related
- Perpendicularity — same idea at 90°
- Angularity — same idea at any specified angle
- Flatness — surface form without datum reference
- Full GD&T symbols reference
Keep exploring
Related tools & references
Full feature-control-frame symbol reference.
The 90° orientation control — parallelism's sibling.
Form control often paired with parallelism on faces.
See how orientation tolerances stack across features.
How tight to call out parallelism.
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