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Thread: GD&T Is the flatness error of the datum feature irrelevant when checking parallelism?

  1. #1
    Associate Engineer
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    Mar 2021
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    GD&T Is the flatness error of the datum feature irrelevant when checking parallelism?

    Hi,

    Assume a scenario where the parallelism of a component is being checked on it's top surface. The bottom surface of this part is on a surface plate. The top surface has a .002" parallelism requirement in respect to the datum feature on the plate (bottom of part). Neither surface has a flatness requirement, but more notably: the bottom surface especially does not have a flatness requirement.

    The part is stable or otherwise does not move, rock, or twist during measurement. The TIR measurement for parallelism is less than .002" on surface A. The flatness measured on the bottom surface exceeds the parallelism tolerance. The bottom surface has a flatness error of .004", however the corresponding feature of size between these two surfaces is within specification. The part is stable during measurment.

    Does this part meet the spec of surface A (Top) being parallel within .002" to surface B, despite the datum feature having a flatness error that exceeds the corresponding surface's parallelism requirement?


    Best regards,

  2. #2
    Administrator Kelly Bramble's Avatar
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    Feb 2011
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    Bold Springs, GA
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    Parallelism is measured from the specified datum which is simulated at the tangent plane of that datum feature. One might place that datum surface against a surface plate thus contacting the three outer most surface locations. The parallelism is measured from the surface plate which simulates the datum specified. The relative flatness of the datum feature is meaningless when measuring the parallelism.

    Flatness is a separate requirement and is measured independently of the parallelism.
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

  3. #3
    Associate Engineer
    Join Date
    Mar 2021
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    2
    Kelly,

    Thank you for your reply. I suspected this was the case.


    Regards


    -Brian

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