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Thread: Parallelism between an axis and a plane

  1. #1
    Associate Engineer
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    Aug 2014
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    Parallelism between an axis and a plane

    Hello all.

    I have a part, for which Datum A is specified as the axis of a constructed cylinder between two ID features which are 23.75mm apart. This is our basic dimension for length of the axis.
    Datum B is a plane constructed on two small pads, with the datum target points specified such that the constructed plane, for measurement purposes, is 10.5mm in lenght and 2.5mm in width.
    The parallelism callout is for -B- to be parallel to -A- within .050mm.

    One of our engineers says that since Datum A is X amount longer than Datum B, we must multiply the actual measured value of the parallelism by the ratio of -B-s length to -A-'s length, which is something like 2.25.

    Does anyone else apply this type of factor when measauring parallelism of two features which are different lengths?

    Thanks in advance.

  2. #2
    Administrator Kelly Bramble's Avatar
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    Quote Originally Posted by shays View Post
    The parallelism callout is for -B- to be parallel to -A- within .050 mm.
    The parallelism tolerance boundaries will be two planes at least as wide and long as the two plane features (datum B). The tolerance boundaries will be parallel to each other and separated by .050 mm. The tolerance boundaries are oriented parallel to the datum reference axis A. The two parallelism tolerance boundaries will be contained within any limits of size or other higher order GD&t tolerance boundaries specified.

    All surface elements of the two referenced surface datum features "B" are required to fall within these two tolerance boundaries to be an acceptable as-built feature - no exceptions.

    Does anyone else apply this type of factor when measuring parallelism of two features which are different lengths?
    No...

  3. #3
    Associate Engineer
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    Aug 2014
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    Thank you for your quick reply. I have never encountered this type of request before.

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