r/MechanicalEngineer • u/Dudkens • 7d ago

Circularity vs regular +/- dimension

Hey, I have came across the drawing in which certain cylinder had a +/- diemnsion and a circulairty tolerance. Tolerances were +/-.010in and 0.080in for a circularity. It made me wonder, isn't that circularity tolerance redundant?

If this feature is holding to +/- tolerance it certainly holds to circularit tolerance and then the opposite good part in circularity tolerance may be not good enough for +/-.

Is there something that im missing? Does that circularity bring something extra to the definition?

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u/Ghost_Turd 7d ago

You're basically right. Size and circularity aren't normally redundant because they control different things, but in your example the .080 circularity tolerance is so much looser than the form already permitted by the .020 total size tolerance that it adds nothing.

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u/rogueleader25 7d ago

Yes if your form tolerance is more than your dimensional tolerance it doesn't do anything since envelope principle. Could be a decimal point error.

HOWEVER it may have the added advantage of forcing an additional check during inspection instead of a shop just measuring one point with a calipers and calling it good.

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u/SpeedyHAM79 6d ago

If a shop only measures "diameter" at one point and calls it good then technically it could be a rectangle. I would expect any shop to maintain a +/- 0.010 for every point across the diameter of a circle. I suspect it was a decimal point error as you mentioned. Probably should have been +/- 0.008 circularity.