True Position question.

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  • True Position question.

    Could someone please explain how PCDmis comes up with its True Position numbers? Dmis math and my math don't agree.
    TP QUESTION1.pngTP QUESTION2.png

    According to Dmis, True positions of --E and --F are .0322 and .0350 respectively. However, if you do the math, True Positions come to 0.0442 and 0.0499 respectively. Any reasonable explanation would be appreciated.

  • #2
    More code would be helpful. What are your features? What are your datums? What is your output alignment set to? What is your full FCF look like (First and second segment), etc.

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    • #3
      Originally posted by bfire85 View Post
      More code would be helpful. What are your features? What are your datums? What is your output alignment set to? What is your full FCF look like (First and second segment), etc.
      Features are circles. Datums are mounting pads (-A-) and mounting holes (-B-, -C-) Alignment is set to |A|B|C| Full FCF is True Position Ø0.028 (M) |A|B|C| (Legacy True Position get it right.)

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      • #4
        As we don't know the bonus values, or the datum shift, there's not much we can tell…

        Which version of PC-DMIS? 2019 R2 Release has a problem in the summary table, which is fixed in the just released SP1.
        AndersI
        SW support - Hexagon Metrology Nordic AB

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        • #5
          1) True Position to |A|B|C| has no datum shift. All Datums are fixed. 2) Bonus tolerance is irrelevant. OOT is figured separately from MEAS values. (OOT= MEAS+BONUS-TOL Negative values = Acceptable)

          Now, Given the DEV values for any circle listed, how does one come to the True Position values listed for said hole location.? Or do I need to start calculating all True Positions manually?

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          • #6
            Originally posted by Sammus72461 View Post
            1) True Position to |A|B|C| has no datum shift. All Datums are fixed. 2) Bonus tolerance is irrelevant. OOT is figured separately from MEAS values. (OOT= MEAS+BONUS-TOL Negative values = Acceptable)

            Now, Given the DEV values for any circle listed, how does one come to the True Position values listed for said hole location.? Or do I need to start calculating all True Positions manually?
            FIT TO DATUMS is turned on. If there is bonus, Fit To Datums is affecting the calculated result. Turn this off and you will likely see the results as expected. Set GD&T Standard to Custom and you can turn off Fit To Datums.

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            • #7
              PERP-TO-CL will also effect the results.
              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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              • AndersI
                AndersI commented
                Editing a comment
                (Replied to the wrong post)
                Last edited by AndersI; 09-27-2019, 03:35 AM. Reason: Replied to the wrong post

            • #8
              PCDMIS 2018 R2 will not allow me to turn off FIT TO DATUMS. Also, PERP-TO-CL had no effect when tried.

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              • AndersI
                AndersI commented
                Editing a comment
                As Stuart said above: "Set GD&T Standard to Custom and you can turn off Fit To Datums. "

            • #9
              However, what he is saying is that the X and Z deviations, when mathed out, should equal the TP result. Which it doesn't.

              It says Segment 1, what is Segment 2?

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              • #10
                https://www.engineersedge.com/calcul...sition_pop.htm

                I use the above link to verify my math sometimes. I doesn't add up anyway.
                sigpic

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                • #11
                  Originally posted by bfire85 View Post
                  However, what he is saying is that the X and Z deviations, when mathed out, should equal the TP result. Which it doesn't.

                  It says Segment 1, what is Segment 2?
                  Segment 2 is irrelevant. It is a hole pattern to itself. The math and shifts allowed are different and have no bearing on the question. If it had, I would have included it in the original post.

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                  • AndersI
                    AndersI commented
                    Editing a comment
                    Every bit of information is relevant, until proven not...

                • #12
                  Originally posted by Sammus72461 View Post

                  Segment 2 is irrelevant. It is a hole pattern to itself. The math and shifts allowed are different and have no bearing on the question. If it had, I would have included it in the original post.
                  The reason I ask is to help, not to have a smarta$$ comment shot back at me. We are all here to try and help you since initiated the question. Just because you feel like the question is irrelevant does not mean you can't kindly play along. PC-DMIS does some goofy things, that is why I ask.

                  Now, if you were to delete the second segment from your FCF, does that change the deviations in your first segment? Again, the demon does some funky things sometimes. I am just trying to eliminate all possibilities.

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                  • #13
                    Have you tried deleting and recreating the dimension? This is the most common fix for me when Xact isn't working correctly...

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                    • Sammus72461
                      Sammus72461 commented
                      Editing a comment
                      Yes. Recreated "Segment one" only. No appreciable effect (With tenthousandths of the original Max change 0.0004 TP.)

                  • #14
                    Originally posted by bfire85 View Post

                    PC-DMIS does some goofy things, that is why I ask.
                    I feel like a beta tester more than I did when I was a beta tester

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                    • #15
                      Originally posted by bfire85 View Post

                      Now, if you were to delete the second segment from your FCF, does that change the deviations in your first segment? Again, the demon does some funky things sometimes. I am just trying to eliminate all possibilities.
                      Sorry for the delay, I wasn't ignoring you. (I don't work Fridays.) No, deleting segment two has no effect on segment one.

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