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Different readings between Location and GD&T

JTM@AlloyP
Hello all,

I am having a strange occurrence happening when I go to do my measurements and I was curious if it was something I was doing, something PC-DMIS is doing, or if I am just misunderstanding how GD&T effects measurements.

I am doing a cylinder scan on a bore =3.506 +/-.001 and when I use location to dimension the tolerance, its giving me .00002 off from nominal but when I try to dimension the circularity of that same cylinder it says the diameter is .00054 away from nominal.

confirming this with other measuring devices (Bore gage) and it seems the Location dimension is more accurate then when it is dimensioned through GD&T.

is PC-DMIS recalculating the diameter based on how out of round the part is and which is more correct? both are in tolerance but this data will be presented to our customer and I just want to have a good answer as to why those numbers don't match up completely.

please help, Thank you,
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  • I am assuming that you are using the Geometric Tolerance command for GD&T.

    The Geometric Tolerance command takes the hits from the feature(s) and re-calculates everything based on your chosen math type settings (Datum Math, Feature Math and Tolerance Zone Math). Legacy Dimensioning (including location) and the older, XactMeasure Feature Control Frame method use pre-resolved data - they take the values directly from the ACTL fields of the measured features. Generally, these will be a least squares (average) value at the centroid of the feature. Another difference is that the Geometric Tolerance command reports a UAME (Unrelated Actual Mating Envelope) as well as optionally including local size (either two-point local size or a local size based on circular elements). If you want to strictly adhere to the standards (ASME or ISO) you should report both UAME and local size because that gives you the "full picture" of what is going on - think of it as similar to reporting the max and min.

    See: PC-DMIS Help Center - 2023.1 (hexagonmi.com)​
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  • I am assuming that you are using the Geometric Tolerance command for GD&T.

    The Geometric Tolerance command takes the hits from the feature(s) and re-calculates everything based on your chosen math type settings (Datum Math, Feature Math and Tolerance Zone Math). Legacy Dimensioning (including location) and the older, XactMeasure Feature Control Frame method use pre-resolved data - they take the values directly from the ACTL fields of the measured features. Generally, these will be a least squares (average) value at the centroid of the feature. Another difference is that the Geometric Tolerance command reports a UAME (Unrelated Actual Mating Envelope) as well as optionally including local size (either two-point local size or a local size based on circular elements). If you want to strictly adhere to the standards (ASME or ISO) you should report both UAME and local size because that gives you the "full picture" of what is going on - think of it as similar to reporting the max and min.

    See: PC-DMIS Help Center - 2023.1 (hexagonmi.com)​
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