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Does Translating The Trihedron Ruin "True Zero"?

Bearpierro
Hello All!

I'm having a debate with a fellow programmer today on the effects of moving the trihedron away from the CAD's zero point.

This is in reference to a True Position measurement of a hole located at X0 Y14.5.


So there are two different methods to get our alignments and then dimension the true position:

Method 1
"Locate" The Part, by tapping a point on X, Translating the trihedron to that point, tapping a point on Y, Translating the trihedron to that point, etc for Z.

This obviously moves the trihedron around a bunch, and because of that physical features need to be used on the part to "return" the trihedron back to X0, Y0, Z0 on the drawing for the true position measurement to be taken.

Method 2
Leave the trihedron at X0, Y0, Z0 of the CAD model and NEVER translate it. Instead, tap points around the part and align without ever moving the trihedron.


Now, once dimensioned, both of these methods give me the same Nominal Measurement Values, X0, Y14.5.

Both of these methods use the same features to create their Iterative and Best Fit alignments.

Is there any difference in the end?

Please let me know if that wasn't clear enough, and thanks in advance for any help!
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  • If I shift the CAD model in a direction, after my alignment, then take points on the shifted CAD model I will potentially have erroneous data based on the direction I moved the model. However, once a CAD model is located and translated, there really isn't much reason to translate it again.
    IE shift a cylinder that is running along the Y axis with a Z translation will give you incorrect vectors using auto points, shift the cylinder along the Y axis and your vectors won't likely be incorrect but your linear distance could cause you to probe before/after the area of the cylinder you were attempting to measure.

    Is this what you are referring to?​

    Not quite. So my method for starting a program would be this:

    Manually move my probe to 0,0,0, which is usually the center of a hole on the part. (Close enough by eye)
    Create Readpoint at that position and align to it.
    What I was taught to do during 101 training was to then "Locate" the part by taking a point on top and setting it as my Z Origin, take a point on the "back" as my Y Origin, and on the "left" as my X Origin.
    Then I do my iterative alignment, because these parts are somewhat complex, and best fit, etc.

    My coworker is telling me that I shouldn't be moving the trihedron at all in these alignments, because the origin should be correct at the beginning when it comes from the CAD.
  • in reply to Bearpierro
    I have never seen a probe be placed perfectly to a readpoint position. We have always used multiple translations prior to starting to measure the actual datum features. I'm not sure what your coworker's thought is, but I feel I'm missing something in these explanations.
    At the end of it all, as long as you are aligning to your datums as described by your print, you shouldn't have any issues with your 0 point. And the measured 0 point is more important than the CAD 0 point, imo.
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  • in reply to Bearpierro
    I have never seen a probe be placed perfectly to a readpoint position. We have always used multiple translations prior to starting to measure the actual datum features. I'm not sure what your coworker's thought is, but I feel I'm missing something in these explanations.
    At the end of it all, as long as you are aligning to your datums as described by your print, you shouldn't have any issues with your 0 point. And the measured 0 point is more important than the CAD 0 point, imo.
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