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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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  • I think there's a miscommunication here. I believe what I'm doing is correct, using features to create alignments should give me the proper XYZ coordinates to the drawing. What my coworker is telling me is that the XYZ origin (the original trihedron position set in the CAD) should be set when the CAD is created and not be moved. He claims that by moving it I am never going to have it accurately placed to the drawing. Is that right?

    I appreciate you going to this length by the way. It's a lot of info to take in.


    that is NOT correct. the CAD zero can and will often be meaningless to the datum structure. or how you want to mount the part on the CMM and set up the part. I rarely (read NEVER) leave the CAD trihedron in it's original origin. Most of the design engineers I encounter have NO regard for how the part is to be manufactured or inspected. If you have not run into this, it is probably only because you have not been doing this long enough. Do not concern yourself with moving it, if you need to, do it. Don't let the less informed convince you otherwise. Feel free to point your "co-worker" to this forum and have him ask about his concerns of moving the zero point of a CAD model
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  • I think there's a miscommunication here. I believe what I'm doing is correct, using features to create alignments should give me the proper XYZ coordinates to the drawing. What my coworker is telling me is that the XYZ origin (the original trihedron position set in the CAD) should be set when the CAD is created and not be moved. He claims that by moving it I am never going to have it accurately placed to the drawing. Is that right?

    I appreciate you going to this length by the way. It's a lot of info to take in.


    that is NOT correct. the CAD zero can and will often be meaningless to the datum structure. or how you want to mount the part on the CMM and set up the part. I rarely (read NEVER) leave the CAD trihedron in it's original origin. Most of the design engineers I encounter have NO regard for how the part is to be manufactured or inspected. If you have not run into this, it is probably only because you have not been doing this long enough. Do not concern yourself with moving it, if you need to, do it. Don't let the less informed convince you otherwise. Feel free to point your "co-worker" to this forum and have him ask about his concerns of moving the zero point of a CAD model
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  • in reply to rikracer
    I agree with everything you're saying 100%. My coworker's argument is that if the trihedron doesn't move from the zero point on the drawing when it is imported, and he just creates features and Best Fit aligns to them then that origin point will be "perfect". My argument would be that it's a theoretical origin point with no real tie to the physical part itself.
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