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PC-DMIS 3D Line Straightness

Rondog
PC-DMIS appears to have issues calculating the proper straightness of a 3D line. The definition of straightness from the PC-DMIS online help is: This is the diameter of a cylinder that contains all the input points. A best-fitting routine determines the axis of the cylinder. If this is actually what PC-DMIS is using then the results are wrong using any method available in PC-DMIS. 

; test points and resulting line
PNT1 =FEAT/POINT,CARTESIAN,NO
THEO/<432.03501,-1213.3127,50.2008>,<0,0,1>
ACTL/<432.03501,-1213.3127,50.2008>,<0,0,1>
CONSTR/POINT,CAST,PT_46_1
PNT2 =FEAT/POINT,CARTESIAN,NO
THEO/<455.70566,-1213.79075,42.17132>,<0,0,1>
ACTL/<455.70566,-1213.79075,42.17132>,<0,0,1>
CONSTR/POINT,CAST,PT_46_2
PNT3 =FEAT/POINT,CARTESIAN,NO
THEO/<479.38146,-1214.19232,34.15272>,<0,0,1>
ACTL/<479.38146,-1214.19232,34.15272>,<0,0,1>
CONSTR/POINT,CAST,PT_46_3
PNT4 =FEAT/POINT,CARTESIAN,NO
THEO/<503.0449,-1214.60886,26.09845>,<0,0,1>
ACTL/<503.0449,-1214.60886,26.09845>,<0,0,1>
CONSTR/POINT,CAST,PT_46_4
PNT5 =FEAT/POINT,CARTESIAN,NO
THEO/<526.72087,-1215.06874,18.08364>,<0,0,1>
ACTL/<526.72087,-1215.06874,18.08364>,<0,0,1>
CONSTR/POINT,CAST,PT_46_5
LIN1 =FEAT/LINE,CARTESIAN,UNBOUNDED,NO
THEO/<432.0354,-1213.32863,50.20282>,<0.9468433,-0.0173208,-0.3212286>
ACTL/<432.0354,-1213.32863,50.20282>,<0.9468433,-0.0173208,-0.3212286>
CONSTR/LINE,BF,3D,PNT1,PNT2,PNT3,PNT4,PNT5,,
OUTLIER_REMOVAL/OFF,3
FILTER/OFF,WAVELENGTH=0

​; straightness results
DIM STRA3= STRAIGHTNESS OF LINE LIN1 UNITS=MM ,$
GRAPH=OFF TEXT=OFF MULT=10.00 ARROWDENSITY=100 OUTPUT=BOTH
AX NOMINAL +TOL -TOL MEAS DEV OUTTOL
M 0.00000 0.01000 0.00000 0.04133 0.04133 0.03133 -------->

​FCFSTRA2 =GEOMETRIC_TOLERANCE/STANDARD=ASME Y14.5,SHOWEXPANDED=YES,
UNITS=MM,OUTPUT=BOTH,ARROWDENSITY=100,
SEGMENT_1,STRAIGHTNESS,DIAMETER,0.01,TOL_ZONE_MATH =DEFAULT,
TEXT=OFF,CADGRAPH=OFF,REPORTGRAPH=OFF,MULT=10,
MEASURED:
LIN1:0.04623,
ADD
FEATURES/LIN1,,

​FCFSTRA3 =GEOMETRIC_TOLERANCE/STANDARD=ASME Y14.5,SHOWEXPANDED=YES,
UNITS=MM,OUTPUT=BOTH,ARROWDENSITY=100,
SEGMENT_1,STRAIGHTNESS,DIAMETER,0.01,TOL_ZONE_MATH =LSQ,
TEXT=OFF,CADGRAPH=OFF,REPORTGRAPH=OFF,MULT=10,
MEASURED:
LIN1:0.05824,
ADD
FEATURES/LIN1,,​


First off, why three different (and wrong) answers for the same thing?

The correct result is 0.04936 mm using the above data. I was able to verify this in PC-DMIS by creating an alignment from the best fit line and finding the smallest diameter circle containing all the input points: 

A1 =ALIGNMENT/START,RECALL:MC_CSY,LIST=YES
ALIGNMENT/LEVEL,ZPLUS,LIN1
ALIGNMENT/TRANS,XAXIS,LIN1
ALIGNMENT/TRANS,YAXIS,LIN1
ALIGNMENT/END

CIR1 =FEAT/CIRCLE,CARTESIAN,IN,MIN_CIRCSC],NO
THEO/<-0.00715,0.00477,463.95901>,<0,0,1>,0.04936
ACTL/<-0.00715,0.00477,463.95901>,<0,0,1>,0.04936
CONSTR/CIRCLE,BF,2D,PNT1,PNT2,PNT3,PNT4,PNT5,,
OUTLIER_REMOVAL/OFF,3
FILTER/OFF,UPR=0


I did figure out the method PC-DMIS used to calculate the straightness with the GeoTol command when the TOL_ZONE_MATH is set to DEFAULT or LSQ (Least Squares is not a valid description by the way). Neither method results in the smallest containing cylinder. The legacy method is a mystery and clearly wrong.
​
Parents
  • @ neil.challinor​​​ it is very awesome that PC-DMIS is trying to adhere to the standards. I have been critical of this for a very long time (not just PC-DMIS but any inspection software). The reality is that CMM's only sample points so this is all you have to work with and trying to create something that mimics perfect mating parts, as defined in the standards, could be an unachievable goal at the end of the day. At best, a CMM result would be no better than an approximation as it is unlikely you will touch on the exact key points of the part surfaces.

    The vast majority of CMM's out there use touch trigger probes. I do run into machines with SP25M or XS1 probes from time to time but not that many. At least with an analogue probe you can scan more of the features although not all. If you are using a typical CMM with a touch trigger probe you are dealing with very few points so using the max-min fit of the data is going to give you larger variations than the Least Squares version for sure (the form errors will be smaller but the position and orientation will change a lot more than the Least Squares version).

    What kind of Careful Study would a typical CMM user be able to perform? I think I could count on one hand the number of users who understand the significance of the best fit choices and I don't know of any user who would have access to utilities that would help them to fully understand the impact of method selection. If the user did something like run the same part over and over and compare results for repeatability then the Least Squares version will show better where best possible case would be identical variations between the two methods. What else would a typical user do, or be able to do?

    I really believe the default option should be Least Squares. When you consider what a typical CMM is this would be the more logical default in my opinion. For the minority of machines that have more capable equipment using min-max fits or constrained datum's makes more sense but this is a very small set of users.

    I am not trying to fence with you. It is just philosophical thoughts about CMM's, what they are, and what they are capable of.
Reply
  • @ neil.challinor​​​ it is very awesome that PC-DMIS is trying to adhere to the standards. I have been critical of this for a very long time (not just PC-DMIS but any inspection software). The reality is that CMM's only sample points so this is all you have to work with and trying to create something that mimics perfect mating parts, as defined in the standards, could be an unachievable goal at the end of the day. At best, a CMM result would be no better than an approximation as it is unlikely you will touch on the exact key points of the part surfaces.

    The vast majority of CMM's out there use touch trigger probes. I do run into machines with SP25M or XS1 probes from time to time but not that many. At least with an analogue probe you can scan more of the features although not all. If you are using a typical CMM with a touch trigger probe you are dealing with very few points so using the max-min fit of the data is going to give you larger variations than the Least Squares version for sure (the form errors will be smaller but the position and orientation will change a lot more than the Least Squares version).

    What kind of Careful Study would a typical CMM user be able to perform? I think I could count on one hand the number of users who understand the significance of the best fit choices and I don't know of any user who would have access to utilities that would help them to fully understand the impact of method selection. If the user did something like run the same part over and over and compare results for repeatability then the Least Squares version will show better where best possible case would be identical variations between the two methods. What else would a typical user do, or be able to do?

    I really believe the default option should be Least Squares. When you consider what a typical CMM is this would be the more logical default in my opinion. For the minority of machines that have more capable equipment using min-max fits or constrained datum's makes more sense but this is a very small set of users.

    I am not trying to fence with you. It is just philosophical thoughts about CMM's, what they are, and what they are capable of.
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