XactMeasure Profile Tolerance Zone Optimization

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  • XactMeasure Profile Tolerance Zone Optimization

    Primary planar datum A with normal along Z+. Secondary cylindrical datum feature B with axis along Z.

    The datum reference frame [A|B] is unconstrained in rotation about Z.

    There are two opposed cylindrical slots (axes along Z), both at a basic distance from datum B.

    There is a profile specification on each slot to [A|B]. When evaluating the slots individually, the results make sense. When evaluating the slots simultaneously, XactMeasure attempts a 90 degree rotation about Z and the results are unrealistically OOT.

    The profile specification is controlling the perpendicularity to datum A, the distances to datum B, and the 180 degree angle between the two slot axes. XactMeasure is attempting to optimize the angle between the slot axes but is doing so incorrectly.

    Is there a way to obtain proper optimization (and thus simultaneity) without using Xact? We are not running 2020 R2+ in our production environment yet, so the new Geometric Tolerance command is not an option.

  • #2
    Here's something interesting. This is what the report looks like when each feature is evaluated individually:


    This is what it looks like when evaluated simultaneously:


    Now, I remove the simultaneous evaluation and I turn on Customized DRF for each dimension. This naturally yields A[z,u,v]|B[x,y]. In the edit window, I add the unconstrained rotation (w) in the <DOF> field of the tertiary datum (which doesn't exist) and PC-DMIS keeps it as shown here:


    Now, when I re-evaluate simultaneously, I receive the following:


    It looks like the measured values have gotten worse, which is expected when evaluating simultaneously. There is no Z rotation shown, however. Is this just a strange bug? Or is the optimization working as desired?
    Attached Files


    • #3

      ​​​​​​​Can you create a Datum C line out of the centerpoints of the cylinders and run the Sim Eval?

      Maybe try create a scan from the numhits of the cylinders?


      • JacobCheverie
        JacobCheverie commented
        Editing a comment
        Creating a scan from numhits doesn't work, I've tried that. Everything works out fine when I have a tertiary datum (there is a .010 profile to [A|B|C] specification that is evaluated simultaneously beforehand)

    • #4
      Hi Jacob,

      a few weeks ago i asked about how to constrain the 6 Deg of freedom of a cylinder. Its when i Learn its is best practice to do so. neil.challinor "pasted a old tech sheet explaining we do need to constrain all 6 DOF. and on how to crating a alignment plane instead of a line would eliminate error for best results. something in those lines.

      "It is not possible to define the rotation by Z with the directly measurable elements. This value must be defined using a theoretical element that includes suitable orientation information. It is preferable to define a theoretical plane when doing this. It is not advisable to use theoretical lines to define the orientation, since calculating the alignment becomes unstable in certain cases."

      maybe you did this already. best of luck.


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