Symmetry of Slots in Reference to ID Cylinder

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  • Symmetry of Slots in Reference to ID Cylinder

    Good morning! I am very green to PC-DMIS and I need help. I have a part that has an ID -A- and ribs on the outside with slots cut into them. The width of the slots must be symmetrical to -A- within .010. I picked 1 point on each side of the slot and measured a cylinder around -A-. It is not working when I pick the two points alone. How do I properly dimension the symmetry callout? The part looks similar to the picture below. Thank you!


  • #2
    The Insert | Dimension | Symmetry menu option calculates the symmetry of a set of points with a datum feature, or two opposing lines with a datum feature.
    • If the first feature is a set, the second feature that is entered is the datum feature and must be either a plane or a line.
    • If the first feature is a line, the second feature must also be a line and the third feature that is entered is the datum feature. In this case the third feature must be a plane or a line. This dimension type is considered one sided, meaning a single positive value tolerance is applied.
    • If the third feature is a plane, the symmetry algorithm attempts to find a line on that plane that represents the datum. To find the correct line, it intersects (or crosses) the chosen plane with the workplane. For this reason, you should ensure that you are using the correct workplane.

    I hope this helps if not just ask for more help...
    So if you are doing just points
    you will want to construct a feature set with the two points then you can dimension the SYMMETRY of the feature set to the datum A
    Michael A Wildschutz Sr
    I Walk on The WildSide
    "To Each is Own"


    • VVPTD
      VVPTD commented
      Editing a comment
      Thank you for the help! I deleted the points on each side and made 1 constructed line on each side instead. Then I made this a constructed set and was able to dimension the symmetry to -A-. The report shows that it measures 0.000. Does this seem correct because it is coming off of a CAD model so theoretically it should be 0?

  • #3
    This symmetry callout is a bit confusing, as are most symmetry callouts. Hence why it was pulled from ASMEY14.5M in 2009 revision. Effectively, all that symmetry can control is perpendicularity of each slot width back to A. Create a mid-line perpendicular to Datum A for each slot. Measure symmetry to each line, back to A Datum Plane.

    You could argue that it's implied to control polar rotation of the "4 places" orthogonality to each other, but that's ambiguous and ultimately up to the beer-holder (beholder).
    If you want to pursue this interpretation, i'd construct two midplanes between the two mid-lines 180° apart, then I would construct an intersect line between the planes. I'd then dimension the perpendicularity of the intersect line to A, and measure angularity to 0.010 between the two planes (with 90° as nominal). Again, this is totally ambiguous interpretation, so maybe get guidance from engineer or customer on interpretation, prior to taking it to this level.


    • VVPTD
      VVPTD commented
      Editing a comment
      Thank you for your help and with explaining how it all works!

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