Orientation Tolerances Do Not Locate (#181)
(In accordance with Y14.5M-1994 standard)
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I keep running into people who are convinced that parallelism locates surfaces. Not True! On this drawing there is a 22 mm basic dimension that locates the considered surface relative to datum A. The parallelism tolerance has nothing to do with this dimension. All three illustrations show a parallelism tolerance zone. The zone is a pair of parallel planes 0.2 mm apart which remain parallel to the datum simulator (the surface plate) while floating to "find" the surface. That’s it. The profile of a surface tolerance is what controls the location of the surface. The 0.5 tolerance is centered on the basic location of 22 mm. The parallelism tolerance floats inside the profile tolerance.
Inspection must "find" the surface to determine if the surface is parallel to the datum simulator regardless of where the surface is. Only the profile of a surface tolerance uses the 22 mm basic dimension which provides location. Without the profile tolerance the drawing would be incomplete.
