All points that are on the referenced feature must in the tolerance zone. Two parallel planes or lines which are oriented parallel to the datum feature or surface. It is important to determine what the reference feature is (surface or axis) and then what is acting as the datum (surface or axis) to determine how the parallelism is to be controlled.
Note: Parallelism does not control the angle of the referenced feature, but only creates an envelope in which the feature must lie. See the tolerance zone below for more details. The tolerance indirectly controls the 0° angle between the parts by controlling where the surface can lie based on the datum. It can reference a 2D line referenced to another element, but more commonly it relates the orientation of one surface plane parallel to another datum plane in a 3-Dimensional tolerance zone. Parallelism is a fairly common symbol that describes a parallel orientation of one referenced feature to a datum surface or line. GD&T Symbol: Relative to Datum: Yes MMC or LMC applicable: Yes GD&T Drawing Callout: However, be sure to pay attention if it is referencing a central axis because it is different! We will only discuss surface parallelism on this page but be sure to check out our page on Perpendicularity to see how an axis is controlled with GD&T. Parallelism is most commonly called out as surface parallelism. The axis form is controlled by a cylinder around a theoretical perfectly parallel axis. Axis Parallelism is a tolerance that controls how parallel a specific parts central axis needs to be to a datum plane or axis. The surface form is controlled similarly to flatness with two parallel planes acting as its tolerance zone. The normal form or Surface Parallelism is a tolerance that controls parallelism between two surfaces or features. Parallelism actually has two different functions in GD&T depending on which reference feature is called out.
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