Question:

I am having a “discussion” with a customer about a part that was rejected because they measured a feature as out of tolerance that is not dimensioned on the print. The part is symmetrical, and the corresponding feature on the other side of the centerline is dimensioned and within tolerance. There is no indication, notation or definition on the print that states any symmetry requirement.

Is there a GD&T rule that defines or requires symmetrical features to be dimensioned? Is there an implied requirement to assume that any undimensioned feature is the same as its mirror? Would this be equivalent to scaling the drawing? By the way, there are other symmetrical non-dimensioned features on this part that they chose not to measure.

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Reply:

Let me make sure I understand. The part appears symmetrical about a center line, not a symmetry line.  (A symmetry line looks like a centerline with two short parallel lines drawn perpendicular across the centerline at both ends of the centerline.) The feature on one side of this centerline is dimensioned as an individual feature (not as a groups of features such as 2X, 4X, ALL FEATURES MARKED “A”, BETWEEN A AND B, etc.)?

I’ve made an illustration to help readers envision the drawing.

blog_symmetryquestion

If my understanding is correct, then there truly is no specification, and the feature cannot be out of tolerance when no tolerance is indicated. It looks like your customer has let their assumptions get the better of them, a common problem when standards are not carefully followed.

Best of luck with your “discussion.”  Let me know if ETI can be of further assistance.

Michael Adcock
ETI Dimensional Mentor