I want help with measuring symmetry.

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  • I want help with measuring symmetry.

    I have a one part. I haven't measuring with PCDMIS(CMM). The result is wrong. Can you help me pls about this issue ?
    Attached Files

  • #2
    Er, the drawing is plain wrong.

    Datum A is shown as a face (on the right hand side), which as drawn is NOT anywhere near symmetrical with the 'features' shown. Symmetry is an odd one any way, it should be applied to two parallel faces, not the midpoint of a pair of holes.

    I assume they want datum A to be the C/L shown on the drawing, but it's not what they've drawn.
    Automettech - Automated Metrology Technology

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    • #3
      Originally posted by NinjaBadger View Post
      Er, the drawing is plain wrong.

      Datum A is shown as a face (on the right hand side), which as drawn is NOT anywhere near symmetrical with the 'features' shown. Symmetry is an odd one any way, it should be applied to two parallel faces, not the midpoint of a pair of holes.

      I assume they want datum A to be the C/L shown on the drawing, but it's not what they've drawn.
      I thought Datum A was an O.D., in which case you would have a C/L, but the OP will need to clarify. Also to say the result is wrong, please help without showing us the code of the program or even the report is asking a bit much.

      I would think for the holes you would create a point midway between the holes and you could call for position of the point selecting just the axis that the holes are spread across.
      PC-DMIS 2016.0 SP8

      Jeff

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      • #4
        Hi,
        Recall the 2 holes in a Constructed Feature set, Recall that set as the to measure symmetry.

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        • #5
          As NinjaBadger says: that drawing is wrong, the datum should be in the symmetry center - a center line or mid plane.

          I don't know enough about ASME, but in the ISO world it is not an error to have symmetry on two holes - see quote from ISO 1101-2017 at the end of this post.

          From the PC-DMIS point of view, FCF symmetry is the same as TP with the datum as the theoretical position. If one feature is dimensioned, it is supposed to lie on the datum, if more features are dimensioned their median point/line/plane is supposed to lie on the datum.

          PC-DMIS legacy symmetry is quite different, and assumes you have measured opposite points on the two symmetrical features (check in the Help).

          Originally posted by ISO 1101-2017, section 17.15.1
          The toleranced feature is either an integral feature or a derived feature. The nature and shape of the
          nominal toleranced feature is a point, a set of points, a straight line, a set of straight lines, or a flat
          surface. If the indicated feature is a nominally flat surface, an intersection plane indicator shall be
          indicated, if the toleranced feature is a set of straight lines in the surface. If the indicated feature is a
          nominally straight line, the specification element ACS shall be indicated, if the toleranced feature is a
          set of points on the line. In this case, the datum for each point is also a point in the same cross-section.
          At least one datum that locks a non-redundant translation of the tolerance zone shall be indicated in the
          tolerance indicator. The angular and linear dimensions that are locked between the nominal toleranced
          feature and the datums are defined by implicit TEDs.

          A symmetry specification can be used in all cases where a position specification can be used, provided
          all the relevant linear TEDs are zero.

          AndersI
          SW support - Hexagon Metrology Nordic AB

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          • #6
            I dont understand guys, I am new on these issues. ı want to a lot of much information this issue. can you help me pls, get more understandable

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            • louisd
              louisd commented
              Editing a comment
              is -A- datum, as defined on the drawing, a cylinder, or a plane?

          • #7
            Originally posted by abdulkerim View Post
            I dont understand guys, I am new on these issues. ı want to a lot of much information this issue. can you help me pls, get more understandable
            What we're saying is that this isn't an issue with PC-Dmis, but with the drawing.

            How symbols are shown on a drawing is VERY important. Being able to read a control frame is VERY important.


            The tolerance zone for symmetry is two parallel planes, which are separated by the value shown in the control frame. In your case your tolerance zone is two parallel planes 0.3mm apart.

            In the case of symmetry this tolerance zone is located equally around the datum (in your case Datum A)

            On your drawing the Datum symbol is located on the face, not inline with the dimension (I assume is not shown on your image).

            See the following...

            https://www.dropbox.com/s/pou0wy9rgw...atums.png?dl=0

            Note the difference in definition between A and E.

            Therefore your tolerance zone is two parallel planes 0.3mm apart centered on that right hand face.

            The designer is asking that the midpoint of each pair of holes or opposing faces falls within that tolerance zone, which they obviously won't.
            Automettech - Automated Metrology Technology

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            • #8
              state original
              Attached Files

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              • #9
                That makes things no clearer. The datum symbol is firmly between two dimensions there. We can take an educated guess at what's intended (that the overall width / the top dimension is meant to be datum A) but that's not what's shown.

                If you measure a plane on each side, the construct a mid-plane and call that datum A you should be able to ask for the symmetry of mid points / mid planes and get some sort of reasonable answer.
                Automettech - Automated Metrology Technology

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                • #10
                  Thanks for your help guys.

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