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Easy Guide to GD&T: Parallelism [ Symbol, Tolerance, Measurement ]
GD&T Parallelism Symbol
Definition of Parallelism
In GD&T, Parallelism can refer to either Surface Parallelism or Axis Parallelism.
Surface Parallelism is a tolerance that controls Parallelism between two surfaces or other features. The feature is controlled by two parallel planes acting as the tolerance zone, similar to to Flatness. With that said, it is important not to confuse the function of Parallelism and Flatness. For starters, Flatness does not reference a datum while Parallelism must. Put another way, Flatness is a feature compared to itself, while parallelism is a feature compared to a datum. This should make the difference quite clear.
Axis Parallelism is a tolerance that controls how Parallel a specific central axis needs to be to a datum plane or axis. It is controlled by a cylindrical tolerance zone around a theoretically perfectly parallel axis. Surface Parallelism is much more commonly used than Axis Parallelism, but be sure to be on the lookout for cases when you have axis parallelism because it’s different. See our page on Perpendicularity to understand better how an axis may be controlled by a datum.
Parallelism Callout on Drawings
Surface Parallelism Callout…
The Surface Parallelism Callout above shows how Surface Parallelism can be called out on a drawing. Parallelism is relative to the “A” datum in the callout.
Parallelism Tolerance Zone
The Tolerance Zone for Surface Parallelism looks like two planes that are parallel to the datum and separated by the tolerance. Every point on the controlled surface must fall between the planes.
Gaging and Measurement of Parallelism
Refer to the Tolerance Zone diagrams above and you can tell pretty quickly how to sweep to measure Parallelism. Most of the time it’s easy just to place the datum on a surface plate and sweep the top with an indicator at constant height to measure Parallelism.