Datum using 3 diameters

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  • Datum using 3 diameters

    I got 3 diameters for a datum for a shaft. 2 of the diameters on the back end are the same size and the front is a smaller diameter. Does anyone have an ideas to zero out all 3 diameters? I can see how using two diameters work with a 3d line. Should I be using a cylinder from 3 circles?
    I used to be high on life but I built up a tolerance.

    Brown & Sharpe Global Advantage
    PCDMIS CAD++ v2011mr2
    PH10MQ/SP600M


    sigpic

  • #2
    You know the routine, show a pic. So if I get you right its a cylinder? Then yes create a cylinder and do your alignment using that.
    sigpicSummer Time. Gotta Love it!

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    • #3
      Two diameters are the same size, the 3rd is different, I don't think using a cylinder is a good idea.
      I used to be high on life but I built up a tolerance.

      Brown & Sharpe Global Advantage
      PCDMIS CAD++ v2011mr2
      PH10MQ/SP600M


      sigpic

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      • #4
        Construct points at the center of each circle, then construct a 3-D line using all 3 points.
        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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        • #5
          I guess I can construct a circle from 2 scans of the same two backend diameters and use that constructed circle to create a 3D line to the other diameter?

          whatever I do, I can't zero out all 3. I don't think it's even possible.
          I used to be high on life but I built up a tolerance.

          Brown & Sharpe Global Advantage
          PCDMIS CAD++ v2011mr2
          PH10MQ/SP600M


          sigpic

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          • #6
            Measure the 3 diameters then construct a 3D line using the 3 diameters. Level and set origins on the 3D line. Dim all 3 diameters to ensure they are 0,0. If they are not, repeat using only the two diameters farthest apart for the line, but dim all three and you will see where the 3rd is - this usually means a bent part. HTH
            sigpic"Hated by Many, Loved by Few" _ A.B. - Stone brewery

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            • #7
              I forgot to mention there is a callout of TP 0.000[M][A-B]

              Looks like the two diameters (-B-) are out of tolerance.
              I used to be high on life but I built up a tolerance.

              Brown & Sharpe Global Advantage
              PCDMIS CAD++ v2011mr2
              PH10MQ/SP600M


              sigpic

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              • #8
                Construct a cylinder or 3D line between the two diameters that are farthest apart. Level to that and set origins on it. Remeasure all three features then dim tp 0.00 @ mmc. If the third dia is out, then it is out, but the two you used to create the alignment will be perfect, by definition. HTH
                sigpic"Hated by Many, Loved by Few" _ A.B. - Stone brewery

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                • #9
                  Originally posted by Underspec View Post
                  I guess I can construct a circle from 2 scans of the same two backend diameters and use that constructed circle to create a 3D line to the other diameter?

                  whatever I do, I can't zero out all 3. I don't think it's even possible.
                  It is possible to create a straight line through three points. However it is highly improbable.

                  If B is 2 diameters then measure B as 2 diameters. You need to get a centroid of B

                  Measure A as one diameter and use its centroid

                  Construct line AB through the two centroids B and A. A-B should be a valid feature at this point.Tehey will both be exactly on center. You should then be abel to report to A-B. All three diameterw wil not zero (virtually impossible). However, the 'features' A and B WILL be on zero. he only thing with B is Parallelism to AB. With this typs of Datum Structure the Eng should call out PAR of B to AB and even A to AB if both are cylinders. I have seen this type of Datum Structures on Printing Press Rolls ets that ride on bearing journals. THe Journals were A and B Datums and the AB Datum was the line shooting through A and B. The Parallelism callouts (sometimes profile or runout also) were there to ensure the bearing journals were true to the AB Alignment.
                  Bill Jarrells
                  A lie can travel half way around the world while the truth is putting on its shoes. - Mark Twain

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