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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?


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  • 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.
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  • 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.
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