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Don’t Apply Geometric Controls to Basic Dimensions
(In accordance with Y14.5M-1994 standard)

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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 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. It might help to think of the BASIC dimensions as the goals. In other words, these are the dimensions manufacturing is trying to hit in their process. The geometric tolerance is the amount the actual feature(s) of produced parts may deviate from the goal.

Back to Tips Tip added Dec 2000