Measuring complex Position features / position to 3 datums
I am a new QT w/5 years working in inspection, though I am doing QE work. I have no formal training in GD&T, inspection, or engineering. Just OJT & lots of self study. I took some machining classes once upon a time but have never been a machinist. I have basic programming skills in PCDMIS & better than average programming skills on a vision system. My new duties have removed me from furthering my programming skills on either.
I am in an argument w/manufacturing engineers on [B]measuring [/B]position to 3 datums in multiple working planes. I hope my words can paint a decent picture...
I have an object where the position of a bore needs to be calculated using datums that are on different working planes. call out is - position |Ø.010mmc|A|B|C
Datum A is a plane that is co-planar to the axis of the bore (I use this measurement for X)
Datum B is the center of a bore that is through Datum A... After zeroing on the center of Datum B I need to rotate the part 90° to measure to the center of the bore if I am using an electronic height gauge (This is Y)
Datum C is normal to the axis of the bore & would be the plane that the bore is milled into.
My understanding is that position is 2D... the position formula is for x & y. the tertiary datum is used to restrict freedom for fixturing purposes & if it it applies to anything from a measurement standpoint it is asking that the feature be perpendicular to the ref datum (C).
Based on that thinking -
I say I can measure/calculate position in X & Y & the resulting number is a valid position number. I say I can measure this on an electronic height gauge. I can account for the 3d nature of a bore (cylinder) by measuring both ends of the bore via EHG from the same datum ref & if both are w/in the tolerance then it is passing.
I am being challenged that this is wrong because of datum C. therefore it can not be measured via something like an EGH. but only a CMM.
a secondary question in this particular matter is that the CMM program only constructs a circle from the bore - not a cylinder. therefore perpendicularity to C cannot be established & the CMM is doing the same thing as the EHG taking measurements from one end of the bore. so if i am taking 2 measurements on the EHG - it is actually more accurate than the current CMM program in regards to the feature being 3d.
a third consideration is that Datum A is often overbuffed at the machine... changing the vector. the CMM is drawing a line & rotating to this line. this line is projected & the measurement is drawn perpendicular from the bore to the projected intersection of that line & the bore. this can be a vastly different measurement than taking a single point from datum A (as you would on an EHG). & this is actually where the argument is arising. I argue I can take a part that fails this position measurement on the CMM & verify it as good via EHG. Primarily because Datum A is changed & because the EHG allows for 1 point to ref from. (i have suggested that a point on datum a be used to calculate the position with on the cmm as well).
Could someone shed some light on (via answer or link) measuring more complex position call outs please (i.e. more than a hole in a square plate).
Specifics in helping me argue my position -if I'm correct, or explaining why I am not would be welcome.
thank you.