Is there a CAD tolerance that impacts accuracy?

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  • Is there a CAD tolerance that impacts accuracy?

    We recently performed a test in which we cut a hole in a square block and measured the hole as a circle and with vector points (4points at 0, 90, 180, and 270). The center location of the measured circle would always measure very close to 0, within .00005". However, the vector points would always indicate a shift in the X-axis of .00010"; the vector point at 0 would show +.00010", the point at 180 would show -.00010". I could rotate the block 90° and remeasure, but instead of the shift now showing in the Y-axis, it would still be in the X (the circle would still be within .00005).

    - after a series of tests to eliminate the fixturing, alignment method, machines, etc, I tried something else

    When I key-in the vector points (not click on the CAD model), the shift would match exactly how the circle measured. However, the points that I had clicked directly off the CAD model were slightly different than what they should be.

    This made me wonder if there was a CAD tolerance somewhere that was impacting our measurement practice slightly. When I checked in the Help file, the only thing I found was under EDIT|Graphics Display Window|Cad Tolerance. But from what I can tell this is determined by a default setting in the software, and then over-written by the CAD model when it is imported.

    We measure almost everything with Vector Points and the machine operators adjust their cutter comp and Z-heights based on what they get out of the CMM's. We typically only use IGES files exported from Creo, but we can export any file type that would improve this "issue". .0001" won't hurt us on most jobs, but on some features it's literally half the tolerance.

    Thanks in advance all. I tested this on an old pfx uha machine with 2019 R1 and a TP200, and a Global Image with an SP25 on 2017 R2.
    Global Image PH10M, SP25, FCR25-L6 CAD++ 2019R1SP10
    Global Image PH10M+, SP25, FCR25-L6 CAD++ 2019R1SP10
    MicroExel pfx uha PH6, TP200 CAD++2019R1 SP10

  • #2
    Check it in the CAD software out to 6 decimal and see if it is rounding at the 3rd/4th decimal place.

    Then check in PC-DMIS out to 6-decimal and see if it matches (not measured, just CAD nominal).
    "This is my word... and as such is beyond contestation."

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    • NMC_CMM
      NMC_CMM commented
      Editing a comment
      Thanks, as a diameter, it's .750000, point-to-point across the diameter it's .750014. But is that enough to show a difference?

  • #3
    When you import the IGES file, do you fix the CAD? I had a similar problem but it was coming from a STEP model. When I went with the IGES file, my issues went away. That’s my 2 pennies worth.
    Darroll
    2018R2

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    • NMC_CMM
      NMC_CMM commented
      Editing a comment
      What do you mean by "fix the CAD"?

  • #4
    When you import an IGES model, there’s a pop up window. There should be a box where you can click to fix the CAD. I’m working off memory. Not at my computer.
    Darroll
    2018R2

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    • #5
      The precision on the CAD Tolerances dialog could affect CAD nominal data. However, this usually only affects geometry that doesn't have exact solutions for point projection. For example, a b-spline surface. For circles and cylinders, we compute an exact project so there shouldn't be any loss of accuracy. However, it would hurt to try adjusting the precision value to a lower value and doing your test again.

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