Bestfit Vs Feature Alignments

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  • Bestfit Vs Feature Alignments


    I just started doing programming and I have a part I been struggling doing dcc alignments. I have 2 main questions:
    Insight: I am aligning to 2 primary datums (cylinders) which are perpendicular to each other

    1- What is the difference of using bestfit to align the part vs the measured features? I see a difference in the results

    2- How do you prove a program offline and make sure you have a reliable program?

    Thanks for any help.

  • #2
    In your case, cylinders are only theoritically perpendicular.
    If you use best fit with both cylinders, PC-DMIS ajust all the hits to minimize a parameter (depends on LS, LS vector, minmax...).
    Then, no cylinder axis should fit exactly to an alignment axis.
    If you construct the alignment, you will use a cylinder as primary (level, origin 2 axes), and the second as a rotation. The second axis will be perp to the first.
    The alignment axis and the first cylinder axis will be the same.
    Hope it's clear...


    • #3
      #2 ?? Reliable program as in " it wont crash at proveout" ? Look at your pathlines and use collision detection ? There are no guarantees and only ways to minimize it. I use 99% clearplanes, in combination with avoidance moves within auto features with a single movepoint at the end of the program. Programming OFFLINE is 100% the way to go. Way faster and your nominals are as perfect as the CAD is. I think I have done 1 program on the CMM in 15 years and the rest have all been done offline.


      • #4
        Thank you JEFMAN, this helps. In order words

        The alignment : level Cyl1 to Y-axis , translate X and Z to Cyl1, rotate Cyl2 to Z-axis Y to Cyl2. When I create a dimension after the alignment, Cyl1 is showing 0 deviation for x and z-axis, while Cyl2 is showing .0003in deviation in Y-axis.
        To prove out a program shouldn't the deviation be .0000in for x, y, and z?
        Also, I'm programming offline.


        • JEFMAN
          JEFMAN commented
          Editing a comment
          Create an intersect point between axes, and dimension the distance between point and axes, to check if axes are concurrent.

      • #5
        Create your alignment but recall" STARTUP" and not another alignment. Make it all fully defined. LEVEL / ROTATE and then lock down x-y-z origin. The x-y-z order doesn't matter but alway LEVEL and then ROTATE in that order. Look at your TRIHEDRON. Is everything locked down ? If everything looks good then you can switch your alignment back to re-calling whatever alignment you want.


        • #6
          OK. Thanks for the help guys


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