True Position Fit To Datums

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  • True Position Fit To Datums

    Hello. I would like to ask a question about TP fit to datums. I before didn't make much attention to it because I wasn't getting interesting results. Currently I had some parts TP good, some totally off. I looked at the CAD and it didn't look that off. So I started investigating TP and found this Fit To Datums function. I turned it off and TP drastically changed (from 0.421 with fit to datums to 0.073 without fit to datums). My datums are: plane, hole and point. As I read on forums, all cases say that it makes the result better not worse, because it fits datums. In my case it's totally different. Maybe someone can explain how it works and why this can be happening? I would like to give the right answer to people and say why I got such results and now I am just confused.

  • #2
    Need more information. Are you reporting in Legacy or XactMeasure? If using Legacy it will be dependent on the active alignment and that might change things.
    Time for the Trolls to leave.

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    • #3
      Originally posted by William Johnson View Post
      Need more information. Are you reporting in Legacy or XactMeasure? If using Legacy it will be dependent on the active alignment and that might change things.
      I am using XactMeasure.

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      • #4
        Selecting "fit to datums" creates an alignment within XactMeasure using the datums and reports the position relative to that alignment. If your alignment is correct to the datums and you're not using MMB on the datums, then you shouldn't see a difference. If you are seeing it worse when you use "fit to datums" then the alignment you're in isn't set up correctly to the datums in the FCF.
        PC-DMIS 2016.0 SP8

        Jeff

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        • #5
          Version?

          Also, make sure you have "perp to cl" enabled.
          PC-DMIS CAD++ 2o19 R1 SP10

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          • #6
            Originally posted by vpt.se View Post
            Version?

            Also, make sure you have "perp to cl" enabled.
            It's enabled. PC DMIS Version: 2018 R1 SP8.

            I try to explain all my alignment:
            1) I collect 4 points for plane and level it to YMinus.
            2) I collect Cylinder.
            3) I collect two circles and then create midpoint between them (it;s middle of my needed element).
            4). I create line from Midpoint to Cylinder created in 3rd step.
            5) I rotate X Plus axis to line created in 4th step about Y Minus and also I apply rotate offset of 9.749 because those two parts make this angle to the wished X axis.
            6) I set Cylinder as XZ origin and I am finished with my alignment.

            Then I define datums:
            Datum A - Plane created in 1st step.
            Datum B - Cylinder created in 2nd step.
            Datum C - Midpoint created in 3rd step.

            After that I use FCF to calculate position of Midpoint created in 4th step compared to datum B only.
            And here is when I get very strange results if Fit To Datums is checked.
            I hope this is clear enough, if need more info tell me. I hope this helps to find what I am doing wrong or why position is calculated so wrong using Fit To Datums.
            Last edited by Mindaugas; 06-14-2019, 01:44 AM.

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            • JacobCheverie
              JacobCheverie commented
              Editing a comment
              It sounds like you don't have all 6 DOF locked down (Translation in Z). In which direction are you seeing issues with the Position (x, y, or z)?

            • Mindaugas
              Mindaugas commented
              Editing a comment
              Oh sorry, my mistake in step 6. I set origin here XZ, not XY. Issue is in X direction. Nominal is 3.9 and it shows measured as 4.1, but if i check XZ location of my part using Location it's not same numbers and the X direction is close to nominal.
              Last edited by Mindaugas; 06-14-2019, 05:12 AM.

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