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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?
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  • 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.

    To be honest I dont understand what you mean by the .400 faces. From the looks of it -A- is a cylindrical feature, yet you are Leveling to the Face? If thats the case, why are you creating a mid plane? Unless the engineer changed the datums completely. The print is not very good, but it seems that -A- is (Ø.093) is the center of the part, -B- is the OD ( Also a center of the part), -C- is the right face. So If it was me, would measure the ID .093, Level to it in Z+, Set my XY orgin, then use the tapered face for Z orgin.


    On one side that GP reads: <0.6858,0.2245,-0.6521><1.0000,-0.0000,-0.0000> DEV -0.0002 <---- on the print at .652 the radius is .166, and you are getting .2245? Also, your vector dont seem to be correct, the part is not flat there for it should not be 1 ,0,0
    The opposite side: <0.3525,0.2201,-0.6521><-1.0000,0.0000,-0.0000> DEV .0006
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  • 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.

    To be honest I dont understand what you mean by the .400 faces. From the looks of it -A- is a cylindrical feature, yet you are Leveling to the Face? If thats the case, why are you creating a mid plane? Unless the engineer changed the datums completely. The print is not very good, but it seems that -A- is (Ø.093) is the center of the part, -B- is the OD ( Also a center of the part), -C- is the right face. So If it was me, would measure the ID .093, Level to it in Z+, Set my XY orgin, then use the tapered face for Z orgin.


    On one side that GP reads: <0.6858,0.2245,-0.6521><1.0000,-0.0000,-0.0000> DEV -0.0002 <---- on the print at .652 the radius is .166, and you are getting .2245? Also, your vector dont seem to be correct, the part is not flat there for it should not be 1 ,0,0
    The opposite side: <0.3525,0.2201,-0.6521><-1.0000,0.0000,-0.0000> DEV .0006
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