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Scan results different from keyed-in points

TheBradish
I have a part with a very tight profile tolerance (form+location) of .0005. I wrote a program with a model and aligned to all 3 datums.

I keyed in gage points and read the vectors from the model. Part is out of tolerance (which I expected) by about .001, the profile being off center.

Next I added a linear TTP scan along the same section where I probed the gage points. The scan shows deviations up to .018!Confused Same program, same part, same alignment.

What could I be doing wrong with my scan to get a result that differs so much from my keyed-in points?
Parents
  • The first is the datum setup on the drawing it is very confusing and in ISO land ilegal, datum ' B ' is about the only one that could be taken as correct, 'A' and 'C' are a joke. You need to have the engineer/customer state what feature are to be used for the datum setup before you can obtain any meaningful results.


    I did talk to engineering about this. The result was that the 1" faces of the right end are the datum -A- feature (I aligned this by measuring planes on both faces and constructing a midplane), similarly the .400 faces are datum -B-, and datum -C- is the small face at the left side.

    You are right about the radius; I made a sloppy assumption about that. The .470 only exists at section A-A. It blends smoothly into the .333 diameter. I confirmed this with a gage.
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  • The first is the datum setup on the drawing it is very confusing and in ISO land ilegal, datum ' B ' is about the only one that could be taken as correct, 'A' and 'C' are a joke. You need to have the engineer/customer state what feature are to be used for the datum setup before you can obtain any meaningful results.


    I did talk to engineering about this. The result was that the 1" faces of the right end are the datum -A- feature (I aligned this by measuring planes on both faces and constructing a midplane), similarly the .400 faces are datum -B-, and datum -C- is the small face at the left side.

    You are right about the radius; I made a sloppy assumption about that. The .470 only exists at section A-A. It blends smoothly into the .333 diameter. I confirmed this with a gage.
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