When doing a dimensional inspection of a machine part, the print often has callouts for position that use three datum, most often datums A, B and C. This ABC datum structure is the datum reference frame to which geometric dimensions and tolerances of the part are defined.
The primary and secondary datums, datum A and datum B, have the most control over the orientation of the datum reference frame while the tertiary constrains the last degree of freedom; typically this is the clocking of the part. Because the tertiary datum only constrains the last degree of freedom, it can look oddly out of place since the primary and secondary datums have already taken effect. Additionally, this can make other features look out of place if they are oriented similarly to the tertiary datum.
In order to have a proper measurement, the datums must be measured prior to measuring the feature that is to be inspected. This is where issues may arise concerning deviations of position tolerances. The slightest difference in orientation of the measured primary and secondary datums force the tertiary out of position causing large deviations of measured features when the expected value should be much closer to the print. Using a best-fit alignment can be used to verify that the part is generally made correctly however the print requires a datum reference frame.
Within our inspection software, Polyworks, we can observe how these GD and T measurements are taken using the feature navigator tool. The tool shows where the measured and nominal primitives are when using the datum reference frame so one can easily see that the reported deviations are in fact true. Another check that we use to ensure the reported values are true, is by editing the measured probed points. The software allows us to see which probed points are deviant and can then be removed from the measurement.
Some measured values may seem incorrect at first glance but error may be attributed to the datum reference frame that was selected.