Cylinder true position is correct but the perp. is way out

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  • Cylinder true position is correct but the perp. is way out

    So I'm using PCDMIS 2020 R1, I have a 2 cylinders that have true position and perp. call outs. This seem redundant because the primary dat is the same on both. But any way. I get a true position of .0015 then the perp is .330 on the same cylinder. Any Ideas

  • #2
    How many levels and how many hits you using ? 4 hits on 2 levels means your cylinder could solve 3 different way ( 6 if you want to include vector directions )
    Last edited by Schlag; 04-16-2021, 04:50 PM.

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    • #3
      Consider the depth of your cylinder. If you are measuring at two levels that are very close together, the best fit algorithm can really get messed up if there is any form issue on the feature. I recently had a large diameter cylindrical feature that was very shallow, like .15” long. The part was out of round .050”. The best fit algorithm saw the shallow oval shape and best fit it by constructing the cylinder at a very severe angle. Seems kinda dumb, but that’s just how the math works out, if you cut a cylinder at an angle you will get an oval. If you measure a very shallow cylinder, the levels are so close together that it is essentially no different than measuring them at the same level, which doesn’t help to construct the true vector of the cylinder.

      As Schlag suggests, more hits and more levels can help a lot. I’ll add that separating the levels as much as possible will help too. On the part I mentioned above, I had to measure a number of circles normal to the datum being compared to and constructed a 3D line between them. Then I reported the perpendicularity of the 3D line. There may be more elegant solutions, but it was what I could come up with on a time crunch.
      Last edited by Cris_C; 04-17-2021, 11:28 AM.

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      • #4
        I am doing 5 hits on 3 levels. the hole is .376 diameter x .330 deep

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        • #5
          Originally posted by ScottP815 View Post
          I am doing 5 hits on 3 levels. the hole is .376 diameter x .330 deep
          What are your datums? Maybe post some code or a snippet of your drawing?

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          • #6
            How are you reporting this?

            Legacy, Xact, GeoTol?

            Perhaps you're reporting position in one location of the Cyl - i.e. Start, Axis Average or End - and it's in Position at that location but still out on the Perp?

            It's times like this where Graphical Analysis can really let you know what's going on.

            Automettech - Automated Metrology Technology

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            • ScottP815
              ScottP815 commented
              Editing a comment
              can you get a graphical image on PCDMIS . I have used Polyworks and that show you what's going on. I have only seen graphic issues if the aligments are not working on PCDMIS

          • #7
            in the picture datum cylinders E +D are the ones in question Dat A is the surface perp to D+E Dat B is the bottom surface dat C is the far side
            Attached Files

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            • #8
              Looks like to me that Datum A is Parallel to D and E or am I seeing this incorrectly? Your A symbol looks like it's on the bottom surface of that picture.
              Last edited by KIRBSTER269; 04-19-2021, 09:12 AM.
              (In Memory of my Loving wife, "Ronda" who I lost March 7, 2016. I love you baby.)
              They say "Nobody's Perfect." I must be Nobody.

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              • ScottP815
                ScottP815 commented
                Editing a comment
                no A is the surface the cyls are on

            • #9
              Kirbster269 you are correct

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              • KIRBSTER269
                KIRBSTER269 commented
                Editing a comment
                Well then you have to check Parallelism not Perpendicularity. Perpendicularity would be back to B

              • KIRBSTER269
                KIRBSTER269 commented
                Editing a comment
                If your creating a Composite then your build would be B|A|C position and B for perpendicular, what are you calling out?

              • ScottP815
                ScottP815 commented
                Editing a comment
                i was wrong A is perp to the cylinders

            • #10
              Do me a favor Post your code, because what you showed in the picture looks like A is parallel to your holes, B is perpendicular, and your Trihedron isn't even corresponding to what your asking. also what is your callout. A|B|C Position and A Perpendicular?

              Capture.JPG
              Last edited by KIRBSTER269; 04-19-2021, 09:43 AM.
              (In Memory of my Loving wife, "Ronda" who I lost March 7, 2016. I love you baby.)
              They say "Nobody's Perfect." I must be Nobody.

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              • ScottP815
                ScottP815 commented
                Editing a comment
                you were right i go my datums mixed up working on fixing

            • #11
              Ok Thanks All I am apparently still asleep my datums were mixed up thanks to Kirbster269 for noticing.

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