True Position Calculation

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  • True Position Calculation

    Program is reporting incorrect True Position calculation. The problem measurements are for CIR2 and CIR3. When calculating the True Position with the location values and diameter, engineering gets .0711 and 0 respectively. PC-DMIS is outputting 0.437 and 0.190. I have dropped the code below. It is not letting me upload images, I made the feature control frame match the drawing. Please help. My understanding of XactMeasure GD&T is limited.

    The code for true position:
    SH2 A2 =POSITION : CIR2
    FEATCTRLFRAME/SHOWNOMS=NO,SHOWPARAMS=YES,SHOWEXPANDED=YES,
    CADGRAPH=OFF,REPORTGRAPH=OFF,TEXT=OFF,MULT=10.00,A RROWDENSITY=100,OUTPUT=BOTH,UNITS=MM,
    COMPOSITE=NO,FIT TO DATUMS=YES,DEV PERPEN CENTERLINE=ON,OUTPUT ALIGNMENT=Datum Reference Frame
    CUSTOMIZED DRF=NO
    STANDARDTYPE=ASME_Y14_5
    SIZE TOLERANCES/1,DIAMETER,7.05,0.025,0.025
    PRIMARY DIMENSION/POSITION,DIAMETER,0.1,MMC,A,C,B
    SECONDARY DIMENSION/,<tol>,<MC>,<dat>,<dat>,<dat>
    NOTE/SH2 A2
    FEATURES/CIR2,,
    DIM LOC31= LOCATION OF CIRCLE CIR2 UNITS=MM ,$
    GRAPH=OFF TEXT=OFF MULT=10.00 OUTPUT=BOTH HALF ANGLE=NO
    AX MEAS NOMINAL +TOL -TOL DEV OUTTOL
    Y -15.199 -15.200 0.100 0.100 0.001 0.000 -------->
    Z -14.537 -14.500 0.100 0.100 -0.037 0.000 ---#-----
    D 7.054 7.050 0.025 0.025 0.004 0.000 ----#----
    END OF DIMENSION LOC31
    SH3A7 =POSITION : CIR3
    FEATCTRLFRAME/SHOWNOMS=NO,SHOWPARAMS=YES,SHOWEXPANDED=YES,
    CADGRAPH=OFF,REPORTGRAPH=OFF,TEXT=OFF,MULT=10.00,A RROWDENSITY=100,OUTPUT=BOTH,UNITS=MM,
    COMPOSITE=NO,FIT TO DATUMS=YES,DEV PERPEN CENTERLINE=ON,OUTPUT ALIGNMENT=Datum Reference Frame
    CUSTOMIZED DRF=NO
    STANDARDTYPE=ASME_Y14_5
    SIZE TOLERANCES/1,DIAMETER,6,.1,.1
    PRIMARY DIMENSION/POSITION,DIAMETER,0.1,MMC,A,C,B
    SECONDARY DIMENSION/,<tol>,<MC>,A,C,<dat>
    NOTE/SH3A7
    FEATURES/CIR3,,
    DIM LOC32= LOCATION OF CIRCLE CIR3 UNITS=MM ,$
    GRAPH=OFF TEXT=OFF MULT=10.00 OUTPUT=BOTH HALF ANGLE=NO
    AX MEAS NOMINAL +TOL -TOL DEV OUTTOL
    Y -15.195 -15.200 0.100 0.100 0.005 0.000 --#------
    Z -14.512 -14.500 0.100 0.100 -0.012 0.000 ----#----
    D 5.988 5.960 0.030 0.030 0.028 0.000 -------->
    END OF DIMENSION LOC32

  • #2
    What feature type and orientations are your A, C, and B datums?

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    • #3
      A is a circle. C and B are Planes. Feature is parallel with plane B. Perpendicular with A and C

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      • #4
        How can a primary gives enough information if it's a circle ?
        Could you measure it as a cylinder, it would make sense...

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        • #5
          When PC DMIS calculates a true position using Xact Measure....It looks at the datums that you defined, then does it's best to create the alignment you set up in Xact's feature control frame builder, then uses that alignment to calculate the true position through. All of this happens in the background thanks to Xact and no actual alignment is made in your code. The answer you come up will only be as sound and repeatable as the datum structure that is being used. Junk on your print = junk output from the CMM. If everything is kosher, this is will save you some programming time by cutting down the amount of alignments that you need to make.

          Personally, I NEVER use Xact measure because I don't trust the software to do things in the background for me. I use legacy. If my print says T.P. w/in 0.005 to -A-, -B-, -C-, I make an ABC alignment and then recall that alignment when I am doing my true position dimensioning.

          Legacy has never let me down.. I always get dead balls correlation between my CMM and the surface plate/other gages.

          All of the above being said, without seeing the code for the rest of your program, I suspect PC DMIS is getting confused with the datum structure and then making an alignment that is different from how your part will be held in real life when it is physically being measured, yielding results that do not correlate. Those deviations are huge, doesn't even seem like they're coming from the same X0Y0Z0.



          **Please note**

          Legacy dimensioning DOES NOT TAKE DATUM SHIFT/ROTATION INTO ACCOUNT AS REQUIRED BY Y-14.5M. It just does straight up t.p. math and allows you to use material modifies for potential bonus.
          Last edited by DAN_M; 04-09-2020, 03:52 PM.
          SF7107(PCD), SF454(PCD), 152614(Quindos), 9159(Quindos), 7107(Quindos), B&S Manual, M&M Gear Checker

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          • #6
            DAN_M +1
            JEFMAN is on to something as well. Primary datum as a circle just doesn’t work. Your primary datum should set some sort of orientation for your DRF/coordinate system/trihedron and a 2D feature (circle) cannot do that.

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