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Parallelism
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Orientation Tolerances Do Not Locate.
I keep running into people who are convinced that parallelism locates surfaces.
Not True! On this drawing there is a 22mm 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 0.2mm 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 22mm. The parallelism tolerance floats inside the profile tolerance.
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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 22mm basic dimension which provides location. Without the profile tolerance the drawing would be incomplete. |
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