Calclating Concentricity

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  • Calclating Concentricity

    My set up guys check concentricity by putting an indicator on a plug, they put that plug into Hole A, then they run the indicator around Hole B. (A is centered in B) Circle B is a half a circle (180 degrees) when they do this they will measure say .001" out of concentricity. But PC-Dmis reports .002" exactly double....EVERY TIME. Why is this?
    CMM Programmer
    Jackson Michigan
    Mistral 7.7.5

  • #2
    From the "Mathematical Definition of Dimensions" Document available at the Wilcox Site:

    If the first feature is a slot, cylinder, cone, or circle, the perpendicular distance from the centroid of the feature to the datum feature’s axis is calculated. The 2D concentricity is twice this value.

    The concentricity calculation is a cylindrical zone, sort of similar to TP.

    This page might help to explain it to others.


    • #3
      the reason is - Pcdmis calculates exactly as it does position. Runout (which they are checking) is going to be half of what pcdmis shows as the value. Reason - pcdmis is using position formula, and that reports a diametrical tolerance zone. It shows you the deviation x's 2 (in a way).
      My question to you is - is it really concentricity that is on your drawin, and does your drawing specify ASME Y14.5M-1994?
      If it does, neither of your inspection techniques is valid for this type of symbol. It is one of the most misunderstood symbols (Symmetry too!) in the entire standard.
      Now, if it is ref'ing an ISO standard, than you are correct in your interpretation, because ISO calls Concentricity what we call coaxiality.
      RFS Means Really Fussy Stuff

      When all you have is a hammer - everything looks like a nail....


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