Doubt about an evaluation

[[Al...]]

Hello all,
I describe my doubt: i have 3 datums, two circles and one plane (ABC). if i measure the elements regarding this datums with a secondary geometrical best fit allignment i have too much difference if i measure and set the datums individually. Why i have these difference, can both methods be trusted? I hope the pictures can explain better my question.

[[Cl...]]

I would use the CAD evaluation to show me where the plus and minus material conditions are for that surface.
Then I would pick space points on the model in those areas and measure them. That should verify which result
is correct.

[[Je...]]

Not quite enough information to make a complete assessment but I'm willing to bet that your DOF constraints aren't sufficient in the characteristic built DRF. A circle does not constrain any rotation and only 2 degrees of translation. The secondary alignment is cascading the base alignment in to apply full constraint of 6 DOF

Try measuring Datum A as a cylinder instead of a circle which will constrain 4 DOF. Datum B may need adjustment in the same manner as well but its difficult to see based on the provided info.