Iterative fixture tolerance

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  • Iterative fixture tolerance

    I have searched this morning and read through 20 or more threads and do not see what I need to know. I am having a hard time with a customer supplied program where an iterantive alignment is used (a 3-2-1 alignment should have been used but that is another story) and I feel it is not correct. I have always been told that you have to a fixture tolerance for the interative alignment to do it's magic. They are telling me this is not what the help files say and reading what is in the help files tells me the same thing but I have told by someone (AE at Hex) that if no tolerance is entered it ignores the iterative best fit to CAD nominals.

    This is from the V3.7MR3 help file and they are running V4.2MR3 and it basically says the same thing.

    The Fixture Tolerance box allows you to type a fitting tolerance value against which PC-DMIS compares feature elements making up the iterative alignment to their theoretical values.

    If after fitting the measurement values to the theoretical values, one or more of the input features have an error along their assigned datum axis that exceeds this tolerance value, PC-DMIS automatically goes to the error label (if one exists). See Error Label.

    If you don't provide an error label, PC-DMIS displays an error message showing the errors along each of the datums. You will have the choice to accept the datum as it is and continue with the rest of the part program, or to cancel the part program execution.

    PC-DMIS can only use the fixture tolerance value if you used more than the minimum number of points needed to create the feature. For example, if you are measuring a plane, the minimum number of points needed for that plane is usually three points. However, if you wanted to use the fixture tolerance value, you would need to measure at least four points. If you use only three points then only one solution exists and PC-DMIS cannot adjust or re-iterate.
    Xcel & MicroVal Pfx & Global 37mr4 thru 2012mr1sp3
    Contura Calypso 5.4

    Lord, keep Your arm around my shoulder and Your hand over my mouth. Amen.

  • #2
    As the help file says,
    If you only use three points for a plane there can only be one solution.
    If you use four or more then flatness can be calculated.
    The fixture tolerance is in effect the flatness tolerance or profile tolerance of the points used in an iterative alignment (if you use more than the required 3-2-1).
    Roses are red
    Violets are glorious
    Don't try to sneak up
    On Oscar Pistorius.

    MIL-TFP-41C

    3.7mr3, 2012mr1, 2013mr1

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    • #3
      That is correct, minimum number, Pcdmis will ALWAYS solve the iterative with zero deviation in the locating axis. So, if you use 3 hits for the plane, 2 hits for the rotate and one hit for the last axis, no tolerance will evner be needed because all 6 hits will always solve perfect in the iterative alignment.
      sigpic
      Originally posted by AndersI
      I've got one from September 2006 (bug ticket) which has finally been fixed in 2013.

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      • #4
        Ok. I learned something today. Is there a star in the eastern sky this morning??
        Xcel & MicroVal Pfx & Global 37mr4 thru 2012mr1sp3
        Contura Calypso 5.4

        Lord, keep Your arm around my shoulder and Your hand over my mouth. Amen.

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        • #5
          Several stars!

          I'm amazed at the wealth of knowledge these fine "peoples" posess. How do they cram so much information into their respective craniums??

          It is a mystery!!

          Just my way of thanking you all for your continuous valuable input.
          sigpic

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