Symmetry measurement problem with PCDMIS-2011 MR1

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  • Symmetry measurement problem with PCDMIS-2011 MR1

    I need to measure the symmetry position of a slot which is machined in a cylinder. The reference axis (datum feature) is the centre axis of the cylinder. The external diameter of the cylinder is 44.5 mm and the internal diameter is 34.0 mm.
    I measured a line on each side of the groove and the width of the groove is 20.0 mm. To determine the symmetry position of the groove, I define the axis of the cylinder as datum 'A' and select the two measured lines, enter the tolerance and
    select 'create'. The result is displayed as 122.984 mm. All the measurements are made in the XY plane. Is there a bug with symmetry measurement in this version? PLEASE HELP.
    "Learning is like rowing. Once you stop, you drift back" - B. Britten

  • #2
    Can you post your code?
    Horizontals - Dual Arm 40-17-21 / Single Arm 30-14-16 - PCD 2013MR1 (64bit)

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    • #3
      Originally posted by fvdwalt View Post
      I need to measure the symmetry position of a slot which is machined in a TUBE. The reference axis (datum feature) is the centre axis of the TUBE. The external diameter of the TUBE is 44.5 mm and the internal diameter is 34.0 mm.
      I measured a line on each side of the groove and the width of the groove is 20.0 mm. To determine the symmetry position of the groove, I define the axis of the TUBE as datum 'A' and select the two measured lines, enter the tolerance and
      select 'create'. The result is displayed as 122.984 mm. All the measurements are made in the XY plane. Is there a bug with symmetry measurement in this version? PLEASE HELP.
      My changes in RED are just so I can visualise it better.........if I am so far out of kilter then file it for later.
      If I can assume the slot is round ended we datum up on the TUBE as per normal Cmm practice to establish datum 'A'.
      Measure the circle @ each end of the slot & connect their centre's to create a line down the middle of your 20mm slot.
      Construct a mid point on that line & also on the already established datum 'A'.
      Connect those 2 midpoints with a line & rotate the alignment to it.
      So now your TUBE diameter centres read Zero @ each end & the slot end circle centres also read zero @ each end in the same axis.
      From this set-up I can now pick points anywhere on each side of the 20mm slot & 'see' where it lies in relation to the alignment.
      I am expecting to see all measuring 10.00mm down one side in your example & then -10.00mm up the other.(Allowing for the symmetry & slot size tolerances)

      However if the slot is not round ended, but flat for instance, then I would still generate a centre point @ each end of the slot & go from there.
      In lots of my cases the slots are not parallel to the TUBE's axis & are on an angle (the inner cone for example in my siggy pic has 8 slots for gas transfer down the hollow connecting legs) but the basic principle to prove the symmetry, is correct & still valid.
      Poleaxe

      Beware the environment you live in for it will shape you
      Be careful the friends you choose for you will become like them.

      sigpic

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      • #4
        Thanks, I agree with your method of determining symmetry but I need to use the GD&T function. Here is a part of the program:

        LIN10 is measured on the lefthand side of the slot and LIN11 is measured on the righthand side of the slot. LIN 12 is the mid liel constructed from LIN10 and LIN11. Datum 'A' is the centre line of the tube.

        LIN10 =FEAT/LINE,CARTESIAN,UNBOUNDED,NO
        THEO/<-10.04,34.222,19.87>,<0.0003157,0.9999999,0.0001961 >
        ACTL/<-10.036,34.223,19.864>,<0.0002983,0.9999999,0.00021 4>
        CONSTR/LINE,BF,3D,PNT5,PNT6,,
        OUTLIER_REMOVAL/OFF,3
        FILTER/OFF,WAVELENGTH=0
        LIN11 =FEAT/LINE,CARTESIAN,UNBOUNDED,NO
        THEO/<10.063,31.832,19.874>,<-0.000627,0.9999998,0>
        ACTL/<10.067,31.835,19.87>,<-0.0007285,0.9999997,0>
        CONSTR/LINE,BF,2D,PNT7,PNT8,,
        OUTLIER_REMOVAL/OFF,3
        FILTER/OFF,WAVELENGTH=0
        LIN12 =FEAT/LINE,CARTESIAN,UNBOUNDED,NO
        THEO/<0.011,33.027,19.872>,<-0.0001557,1,0.0000981>
        ACTL/<0.015,33.029,19.867>,<-0.0002151,1,0.000107>
        CONSTR/LINE,MID,LIN10,LIN11
        FORMAT/TEXT,OPTIONS,ID,HEADINGS,SYMBOLS, ;MEAS,NOM,DEV,TOL,OUTTOL, ,
        DIM DIST1= 2D DISTANCE FROM LINE LIN10 TO LINE LIN11 PAR TO XAXIS,NO_RADIUS UNITS=MM,$
        GRAPH=OFF TEXT=OFF MULT=1000.00 OUTPUT=BOTH
        AX MEAS NOMINAL DEV +TOL -TOL OUTTOL
        M 20.088 20.020 0.068 0.100 0.000 0.000 #DATDEF/FEATURE=LIN3_CENTRE,A
        FCFSYM1 =SYMMETRY OF LIN12
        FEATCTRLFRAME/SHOWPARAMS=YES,SHOWEXPANDED=YES
        CADGRAPH=OFF,REPORTGRAPH=OFF,TEXT=OFF,MULT=1000.00 ,ARROWDENSITY=100,OUTPUT=BOTH,UNITS=MM
        CUSTOMIZED DRF=NO
        DIMENSION/SYMMETRY,0.25,A,<dat>
        NOTE/FCFSYM1
        FEATURES/LIN12,,
        "Learning is like rowing. Once you stop, you drift back" - B. Britten

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        • #5
          Symmetry is actually true position but is only measured and calculated in one direction. There must be some sort of glitch because when I tried symmetry the answer that the software spits out I cannot duplicate on a surface plate so I always use true position in place of symmetry.

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          • #6
            Symmetry function in PC-DMIS is limited due to way the coders interpret GD&T.
            Datum must a plane or a line, cylinder axis is not a line. Symmetry is the distance between the datum and median point of two opposing points. When PC-DMIS evaluating symmetry of two lines, it does not see them as lines but the actual points in those lines. Therefore, the hits along the axis of LIN11 must match LIN10.

            I would level and origin to tube axis, rotate to LIN12 (if clocking is not req'd in FCF), and create a generic line at center of the tube, paralleled to LIN12.

            Open symmetry dialog box, select LIN10, LIN11, and then Generic line, in that sequence…

            Comment


            • #7
              Symmetry

              Originally posted by fvdwalt View Post
              I need to measure the symmetry position of a slot which is machined in a cylinder. The reference axis (datum feature) is the centre axis of the cylinder. The external diameter of the cylinder is 44.5 mm and the internal diameter is 34.0 mm.
              I measured a line on each side of the groove and the width of the groove is 20.0 mm. To determine the symmetry position of the groove, I define the axis of the cylinder as datum 'A' and select the two measured lines, enter the tolerance and
              select 'create'. The result is displayed as 122.984 mm. All the measurements are made in the XY plane. Is there a bug with symmetry measurement in this version? PLEASE HELP.

              To get symmetry

              1) Construct reverse feature of datum i.e.cylinder

              2) Construct mid line of two measured lines of slot

              3) Create scan of original datum & reverse datum using option 'Construct feature set'

              4) Call symmetry of scan & mid line

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