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The major difference between the old coordinate (+/-) tolerancing method and GD&T is that the tolerances no longer apply to the dimensions. A geometric tolerance applies to the surface of a feature (the thing you can actually touch and directly measure) or to the feature’s center point, axis or center plane. A common error found on many drawings is the application of geometric tolerances to basic dimensions. On the drawing shown, for instance, it would be incorrect to place the profile tolerance under or next to the R30 dimension. Actually, the 180, R30 and implied centering of the feature on datum center plane B all work together to locate the profile tolerance zone. The basic dimensions locate the tolerance zones perfectly with respect to the datum reference frame, while the geometric tolerances define how much variation the actual features can have within those tolerance zones.
This tip is in accordance with ASME Y14.5M-1994, Y14.5-2009 and Y14.5-2018.
This tip was originally released in December 2000.