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Tangent Point - actually Cosine Error from incorrect IJK vector

Tumbleweed
I've a question about how the CMM calculates hits on tangent points. In the attachment I have a picture showing the issue.
I have 2 lines. Y is vectored with straight on hits. X is also vectored with straight on hits.
Since X- is not a 0/45/90 degree angle, but at a 20 degree angle form a straight on hit, will the ball calculate slightly off as it thinks it makes contact at 0 degrees, but actually hits earlier at 20 deg. thus creating the line 0.096mm sooner than it should have?

It obviously figures this out with circles/cones/cyliders, but it doesn't seem to do this with lines and points.
Should this be planned for and the measurement offset in calculation, or can it be corrected with some really specific (and tedious) vectoring so the probe knows where it should be hitting?


Attached Files
  • Your diagram is showing what is usually referred to as "cosine error".

    This will always be present on points that do not vector normal to the surface.

    It should not be an issue for a measured line if the software is working properly. If you measure points and construct a line using BF, the error will be present in the line. If the line is constructed using BFRE, there will be no cosine error in the line.
  • You can also use sample hits and PCDMIS will calculate the angle of the surface and compensate the probe radius accordingly no matter what theo vector is for the feature.
  • Hand correct your vectors for each point and this should take out you error.
  • Using 3D line could also give a good result... DJAMS and Mchough1 solutions should solve the problem to.
  • How would I create Datum B and C in the attached picture? How do I get the points on the radii to establish Datum B? Datum C is even worse as it only shows 1 end of the line the other is off in space. I have a true position of a diameter to B and C.
    Attached Files
  • in reply to Ken8282
    Create an offset line.

    Measure the two radii, then construct on offset line between the two. When you create it, you'll make the offsets equal to the theoretical radius value, but then change the offsets in the code to the actual measured values: 

    CIR1       =FEAT/CONTACT/CIRCLE/DEFAULT,CARTESIAN,IN,LEAST_SQR
            THEO/<0,0,0>,<0,0,1>,0.7
            ACTL/<0,0,0>,<0,0,1>,0.689
            TARG/<0,0,0>,<0,0,1>
            START ANG=0,END ANG=360
            ANGLE VEC=<-1,0,0>
            DIRECTION=CCW
            SHOW FEATURE PARAMETERS=NO
            SHOW CONTACT PARAMETERS=YES
              NUMHITS=9,DEPTH=0,PITCH=0
              SAMPLE METHOD=SAMPLE_HITS
              SAMPLE HITS=0,SPACER=0.1181
              AVOIDANCE MOVE=NO,DISTANCE=1
              FIND HOLE=DISABLED,ONERROR=NO,READ POS=NO
            SHOW HITS=NO
CIR2       =FEAT/CONTACT/CIRCLE/DEFAULT,CARTESIAN,IN,LEAST_SQR
            THEO/<60,0,0>,<0,0,1>,0.7
            ACTL/<59.97,0.009,0>,<0,0,1>,0.689
            TARG/<60,0,0>,<0,0,1>
            START ANG=0,END ANG=360
            ANGLE VEC=<1,0,0>
            DIRECTION=CCW
            SHOW FEATURE PARAMETERS=NO
            SHOW CONTACT PARAMETERS=YES
              NUMHITS=9,DEPTH=0,PITCH=0
              SAMPLE METHOD=SAMPLE_HITS
              SAMPLE HITS=0,SPACER=0.1181
              AVOIDANCE MOVE=NO,DISTANCE=1
              FIND HOLE=DISABLED,ONERROR=NO,READ POS=NO
            SHOW HITS=NO
LIN1       =FEAT/LINE,CARTESIAN,UNBOUNDED,NO
            THEO/<0,0.3445,0>,<1,0,0>
            ACTL/<-0.0001,0.3445,0>,<1,0.0001501,0>
            CONSTR/LINE,OFFSET
            SURFACE NORMAL = <0,0,1>,MULTI POINT
            ID = CIR1,CIR2,,
            OFFSET = CIR1.R,CIR2.R
  • How would I create Datum B and C in the attached picture? How do I get the points on the radii to establish Datum B? Datum C is even worse as it only shows 1 end of the line the other is off in space. I have a true position of a diameter to B and C.

    Build a fixture with certified flat and perpendicular surfaces to contact the part at those tangent high points, measure planes on those surfaces.

    If you can't do that, then you have a long multi-step process ahead of you involving CAD files, Iterative Alignments, and a lot more training than we can do over a forum interface.

    FYI, the old thread you resurrected was misnamed. The Original Poster was really talking about "Cosine Error ", caused by not having the correct IJK vector that the CMM uses to approach the probing point with.
  • I would try measuring it as a torus, then construct datums (but I never had to do it !!!!!!!)
  • I got a CAD and I can do a Iterative alignment but the long and multistep process will not be pleasing to the powers that be. Of course I will hear "Why can't everything be measured in just 3 hits" like the engineers think. I will also try Random's idea. Lucky me more fun thing's to try.

    Thanks
  • You can also use sample hits and PCDMIS will calculate the angle of the surface and compensate the probe radius accordingly no matter what theo vector is for the feature.
    Any explanation from the manual ?
    For example: I have a circle on a plane 40 degrees with horizontal surface on the CAD, if the part is only 39 degrees with base surface, 3 sample hits will compensate this 1 degree cosine error ?
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