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Thread: Circularity / Roundness

  1. #1
    Associate Engineer
    Join Date
    May 2011

    Circularity / Roundness

    I have a questions in regards to circularity/roundness of a feature. My plants in Korea I think are interpreting the tolerances inappropriately as the go from a radius to a diameter. When the print calls out the circularity on a radius dimension, they double it in the control plan for the diameter and half it when called out in a diameter on the print in the control plan. Shouldn't the geometric tolerance remain the same for a diameter and radius measurement? Can you call out circularity for a radial dimension or should it always be called out on a diameter dimesnion? Please help as I have been tasked with determining if they are correct as doubling our tolerance could have grave consequences. Thank you for your help.

  2. #2
    Lead Engineer RWOLFEJR's Avatar
    Join Date
    Mar 2011
    Rochester Pennsylvania
    The purpose of creating a drawing is to convey to the user of the print exactly what it is that you want and to do it as clearly as possible to avoid any confusion. There is obviously confusion so the surest way to eliminate the confusion would be to change the print to clear things up. I honestly don’t know what the GDT “Rule” is for applying a tolerance to a radius and I can see how easy it would be to have more than one interpretation of the tolerance by the user of the print. Maybe someone else here on this board can let you know which interpretation is correct… half or doubled… but regardless I’d change the print to make sure you don’t have any issues down the road.

  3. #3
    Project Engineer
    Join Date
    May 2011
    Circularity is always a radial measurement. However, it is to be used on a circular feature. A radius is not a circle (only a portion of a circle) so one of the profile symbols might be better.

    Sometimes people see circularity and double that number in order to check the diameter of a part, or take the diameter and halve the tolerance to get the circularity, but neither of those work because circularity is controlling shape not size. See the attached graphic; notice that the diameter tolerance is 1 mm. That doesn't translate to a circularity of 0.5, because it's possible that all of the 1 mm variation is squished to one side. A GD&T callout for circularity could be added with a value of 0.9 mm and be valid.
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