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GD&T Symbol - Symmetry Prior to 2018

See Symmetry (2018)

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Y14.5M-1994 [5.14], Y14.5-2009 [7.7.2]

GD&T Symbol - Symmetry

Symmetry is one of those tolerances that sounds like a good idea but usually isn’t. There are a lot of times where features need to be symmetric. A symmetrical relationship may be controlled with position, profile of a surface or symmetry. The least functional and often the most time consuming to inspect is symmetry; it requires deriving median points.

GD&T Symbol - Symmetry
GD&T Symbol - Symmetry

Symmetry is a control that may be used to locate features of size. Symmetry was removed from the Standard in 1982. The control has remained in the ISO standard. Mainly for that reason, symmetry was put back in the Y14.5 standard. Unfortunately, the definition in Y14.5 does not agree with ISO. Inspecting symmetry, according to Y14.5, requires deriving the feature's median plane. This is a very time consuming exercise which in nearly all instances is not needed. In ISO, symmetry is identical to position at RFS.