However, in most cases it is better to avoid using since this is a very difficult callout to measure and can easily be replaced with a Position tolerance. This symbol only has specific uses for mass balance and form distribution. When you want to make sure that the center plane of two symmetric features is always held exactly center AND has even form along the surface of the part. Both symbols focus on the theoretical center datum being constrained within a specific limit to ensure that the entire structure is uniform. While concentricity really is a focus of symmetry around a datum axis, The Symmetry Symbol is a focus on symmetry over a datum plane. Symmetry is the non-circular version of concentricity. This is a complex and sometimes inaccurate method for determining if a part is symmetrical. Usually, to measure symmetry, a CMM is set up to calculate the theoretical midpoint datum plane, measure the surfaces of both required surfaces, and then determine where the midpoints lie in reference to the datum plane. Due to its tolerance zone being constrained to a virtual plane, you cannot have a gauge to properly measure this feature quickly. The median points of the symmetrical surfaces must all lie within this zone.Īs stated before, symmetry is very difficult to measure.
Parallel Planes on equal sides of a central datum plane. Symmetry is not a very common GD&T callout since it has very limited functional uses (centering location is done with Position) and the verification and measurement of symmetry can be difficult (See: Final Notes). If you take all the midpoints of the entire surface, this must lie within the tolerance zone to be in specification. Each set of points on the reference features would have a midpoint that is right between the two. An established “true” central plane is established from the datum and for the symmetry to be in tolerance, the median distance between every point on the two surface features needs to fall near that central plane. GD&T Symmetry is a 3-Dimensional tolerance that is used to ensure that two features on a part are uniform across a datum plane. GD&T Symbol: Relative to Datum: Yes MMC or LMC applicable: No Drawing Callout: Description:
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