PC-DMIS 3D Line Straightness

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

Rondog over 1 year ago
JEFMAN The axis that results in the minimum 3D line form error is 0.946782827303 -0.017286046094 -0.321408572591 and when I use this I am able to get the same result as PC-DMIS:

; Create two points along the target axis. I scaled up the second point so it would not be rounded. PNT6 =FEAT/POINT,CARTESIAN,NO THEO/<0,0,0>,<0,0,1> ACTL/<0,0,0>,<0,0,1> CONSTR/POINT,OFFSET,ORIGIN,0,0,0 PNT7 =FEAT/POINT,CARTESIAN,NO THEO/<94678282.7303,-1728604.6094,-32140857.2591>,<0,0,1> ACTL/<94678282.7303,-1728604.6094,-32140857.2591>,<0,0,1> CONSTR/POINT,OFFSET,ORIGIN,94678282.7303,-1728604.6094,-32140857.2591 LIN2 =FEAT/LINE,CARTESIAN,UNBOUNDED,NO THEO/<0,0,0>,<0.9467828,-0.017286,-0.3214086> ACTL/<0,0,0>,<0.9467828,-0.017286,-0.3214086> CONSTR/LINE,BF,3D,PNT6,PNT7,, OUTLIER_REMOVAL/OFF,3 FILTER/OFF,WAVELENGTH=0 A1 =ALIGNMENT/START,RECALL:MC_CSY,LIST=YES ALIGNMENT/LEVEL,ZPLUS,LIN2 ALIGNMENT/TRANS,XAXIS,LIN1 ALIGNMENT/TRANS,YAXIS,LIN1 ALIGNMENT/TRANS,ZAXIS,LIN1 ALIGNMENT/END ; smallest containing circle CIR2 =FEAT/CIRCLE,CARTESIAN,IN,MIN_CIRCSC,NO THEO/<-0.00812,0.00477,0>,<0,0,1>,0.0463 ACTL/<-0.00812,0.00477,0>,<0,0,1>,0.0463 CONSTR/CIRCLE,BF,2D,PNT1,PNT2,PNT3,PNT4,PNT5,, OUTLIER_REMOVAL/OFF,3 FILTER/OFF,UPR=0

3D Line:
Expected: 0.04936
PC-DMIS Legacy: 0.04133 ; result is impossibly small
PC-DMIS GeoTol LSQ: 0.05824 ; result is too large
PC-DMIS GeoTol DEFAULT: 0.04623 ; okay. smallest containing cylinder

I tried the same kind of test with a plane and everything makes sense. Using the four points below the results are as described in the following summary:

-98.349897 372.834009 384.745043 -105.289880 358.827047 403.885249 -107.735745 373.212952 381.762867 -109.651065 362.120718 397.638148

Plane:
Expected: 0.091857
PC-DMIS Legacy: 0.09186 ; okay. range of points following Least Squares fit of plane
PC-DMIS GeoTol LSQ: 0.09186 ; okay. range of points following Least Squares fit of plane
PC-DMIS GeoTol DEFAULT: 0.07966 ; okay. range of points following MinMax fit of plane

@ neil.challinor there is something obviously wrong with the 3D line results (Legacy and GeoTol LSQ). Although the standards define straightness as the smallest containing cylinder (or minimum separation of containing parallel planes for flatness) I don't have a problem with the Least Squares version. You have this an an option so I don't think you see this as a problem either. A CMM is just a big approximation machine to begin with and Least Squares is the preferred fit in most (maybe all) cases as it tends to filter out the filiers. The Least Squares result is usually what I expect to find in any software as the default.
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Rondog over 1 year ago
JEFMAN The axis that results in the minimum 3D line form error is 0.946782827303 -0.017286046094 -0.321408572591 and when I use this I am able to get the same result as PC-DMIS:

; Create two points along the target axis. I scaled up the second point so it would not be rounded. PNT6 =FEAT/POINT,CARTESIAN,NO THEO/<0,0,0>,<0,0,1> ACTL/<0,0,0>,<0,0,1> CONSTR/POINT,OFFSET,ORIGIN,0,0,0 PNT7 =FEAT/POINT,CARTESIAN,NO THEO/<94678282.7303,-1728604.6094,-32140857.2591>,<0,0,1> ACTL/<94678282.7303,-1728604.6094,-32140857.2591>,<0,0,1> CONSTR/POINT,OFFSET,ORIGIN,94678282.7303,-1728604.6094,-32140857.2591 LIN2 =FEAT/LINE,CARTESIAN,UNBOUNDED,NO THEO/<0,0,0>,<0.9467828,-0.017286,-0.3214086> ACTL/<0,0,0>,<0.9467828,-0.017286,-0.3214086> CONSTR/LINE,BF,3D,PNT6,PNT7,, OUTLIER_REMOVAL/OFF,3 FILTER/OFF,WAVELENGTH=0 A1 =ALIGNMENT/START,RECALL:MC_CSY,LIST=YES ALIGNMENT/LEVEL,ZPLUS,LIN2 ALIGNMENT/TRANS,XAXIS,LIN1 ALIGNMENT/TRANS,YAXIS,LIN1 ALIGNMENT/TRANS,ZAXIS,LIN1 ALIGNMENT/END ; smallest containing circle CIR2 =FEAT/CIRCLE,CARTESIAN,IN,MIN_CIRCSC,NO THEO/<-0.00812,0.00477,0>,<0,0,1>,0.0463 ACTL/<-0.00812,0.00477,0>,<0,0,1>,0.0463 CONSTR/CIRCLE,BF,2D,PNT1,PNT2,PNT3,PNT4,PNT5,, OUTLIER_REMOVAL/OFF,3 FILTER/OFF,UPR=0

3D Line:
Expected: 0.04936
PC-DMIS Legacy: 0.04133 ; result is impossibly small
PC-DMIS GeoTol LSQ: 0.05824 ; result is too large
PC-DMIS GeoTol DEFAULT: 0.04623 ; okay. smallest containing cylinder

I tried the same kind of test with a plane and everything makes sense. Using the four points below the results are as described in the following summary:

-98.349897 372.834009 384.745043 -105.289880 358.827047 403.885249 -107.735745 373.212952 381.762867 -109.651065 362.120718 397.638148

Plane:
Expected: 0.091857
PC-DMIS Legacy: 0.09186 ; okay. range of points following Least Squares fit of plane
PC-DMIS GeoTol LSQ: 0.09186 ; okay. range of points following Least Squares fit of plane
PC-DMIS GeoTol DEFAULT: 0.07966 ; okay. range of points following MinMax fit of plane

@ neil.challinor there is something obviously wrong with the 3D line results (Legacy and GeoTol LSQ). Although the standards define straightness as the smallest containing cylinder (or minimum separation of containing parallel planes for flatness) I don't have a problem with the Least Squares version. You have this an an option so I don't think you see this as a problem either. A CMM is just a big approximation machine to begin with and Least Squares is the preferred fit in most (maybe all) cases as it tends to filter out the filiers. The Least Squares result is usually what I expect to find in any software as the default.
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