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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 checked your print. Normally when I have something like that I just use Surface points. From what I can tell, What you have is Gage points, with a given XZ values. This can be very tricky, depending on your customer, some customers have a spec in which explains which of the BASICS is a set dimension and which can be toleranced. For example in the spec it may say that the distance is the SET dimesion .715, and the R .171 can be dimensioned +/-.00025. If it doesnt, than it can moved in both direction as long as it is with in .0005.

    1. When I do Gage points with a given profile of .0005, I dimension my gage pint X and Z +/-.00025.
    2. The reason why i use surface points with 3 sample hits is to assure the Vector being normal to the surface, regardless of the angle (in your case).
    3. if I see that the actuals of the surface points in one Axis is off by a little, I move around THE TARGET values in both the axis, by maybe a .0001 and see if that helps


    I could be wrong in all this, but this is what I usually do.
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  • I checked your print. Normally when I have something like that I just use Surface points. From what I can tell, What you have is Gage points, with a given XZ values. This can be very tricky, depending on your customer, some customers have a spec in which explains which of the BASICS is a set dimension and which can be toleranced. For example in the spec it may say that the distance is the SET dimesion .715, and the R .171 can be dimensioned +/-.00025. If it doesnt, than it can moved in both direction as long as it is with in .0005.

    1. When I do Gage points with a given profile of .0005, I dimension my gage pint X and Z +/-.00025.
    2. The reason why i use surface points with 3 sample hits is to assure the Vector being normal to the surface, regardless of the angle (in your case).
    3. if I see that the actuals of the surface points in one Axis is off by a little, I move around THE TARGET values in both the axis, by maybe a .0001 and see if that helps


    I could be wrong in all this, but this is what I usually do.
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