Always having symmetry problems

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  • Always having symmetry problems

    So i deal with alot of symmetries on my parts.
    Alot of parts looks like an " H " where the inner (midplane) is supposed to be symmetric with the outer (midplane).
    I measure all 4 planes. Construct a midplane for the outer planes (in X+ workplane). I define this as reference A
    Then i measure the inner planes and construct a midplane for these and evalute the symmetry against reference A.

    I get around 0.06mm from nominal ( nominal is 0)
    But if i measure the distance between the midplane the result is only 0.003mm from nominal.

    Whys this?

  • #2
    There are lots of things that could be going on. Could you post the code with the measured features, constructed features and dimensions?
    Or you could tell your customers that you no longer recognize symmetry since ASME removed it 2 years ago!! Hahahaha!!!
    153010 Global Advantage w/ LSPX1H_T Analog Probe
    7107 Global Classic TP20
    2019R1 SP1 CAD++


    • #3
      I would be nervous driving it off of the midplane for the feature. It might use measured location of the plane instead of looking at the individual hits of the planes. Your measured symmetry value should be exactly double the deviation of the furthest midpoint from the datum plane, not double the midplane to midplane distance. I believe you can select the opposing planes for the dimension (at least in Xact)- there should be no need for the feature midplane, this could maybe be affecting your reading too? Also, Legacy vs. Xact can make a difference in the calculation, too.


      • #4
        I do Symmetry differently using legacy. I probe my points on one side of Datum, then copy and paste with pattern those points to the opposite side so I know they are perfectly opposed. I do the same thing for the inner surfaces. I create midpoints between each opposing point for Datum A and construct a plane. I construct a scan feature set of the inner surface opposed points selecting in a specific order...Right Point->Left opposing point->Next right point->The next opposing point to that->etc. (Basically alternating order.) Then dimension symmetry of the scan set to the Datum plane.
        That isnt the only way, but it works well for me.


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