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Thread: Combining Circularity and Straightness into Cylindricity

  1. #1
    Associate Engineer
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    Oct 2021
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    Combining Circularity and Straightness into Cylindricity

    Hi All, hoping to get some clarity in a particular situation on how the tolerance zones are shifting when combining straightness and circularity controls into a single cylindricity frame.

    I was tasked with going through and cleaning up some old drawings and now the supplier is saying this change creates a much tighter tolerance and want to requote. Obviously my intention wasn't to make the target harder to hit but easier to interpret, measure, and more inline with the design intent of the feature (bearing surface on a shaft).

    Does this greatly change the tightness of the tolerance zones? I figured it would change slightly but I was under the impression that cylindricity is a combination of straightness and circularity.

    There is also the strong possibility the supplier is just making this an issue because they want to requote (they have previously claimed that they don't use GD&T, but instead have their own standards haha).
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  2. #2
    Administrator Kelly Bramble's Avatar
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    Feb 2011
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    Bold Springs, GA
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    Technically yes, the cylindricity of .004 is slightly more restrictive. Note 1 is unknown.

    The straightness tolerance applies to the derived center line of the as-built cylinder allowing the cylinder to curve end to end (sad or happy face shape) not more than .004 in the worst-case orientation. Also the size variations are only controlled by the limits of size +/- .008 since each surface section is controlled independently by the circularity. Therefore there can be large size variations and meet the straightness + circularity. The circularity is applied to the surface limiting the out or roundness at any surface section not greater than .005.

    The cylindricity does combine the straightness and circularity to a surface only control which will limit the straightness + circularity only to not greater than .004 combined. This limits total variation relative to the size where the separate geometric controls will allow more variation in size. With the separate specifications the cylinder can taper .005 end-to-end (straightness) where with the cylindricity not more than .004.
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