No I didn't mis-read...the statement about the centerline of the holes was an addition. The centerline between the holes is what I was saying was probably not necessary (so I'm agreeing with you) but I called it the centerline of the part, which makes your point. What I was saying otherwise is that if there was another feature (let's say a hole) on the part and there was a dimension to it from centerline, and the FCF specified the hole WRT to A, B, and C, then it would be measured from the center of the OD of the part, not from the center of the hole pattern. Believe me, I am not from the school of "Centerlines on Everything".
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The centerlines in the Tec-Ease Nov'08 tip are used to graphically show symmetry; really they don't achieve anything else. What I've seen too often is that people will assume that the centerline (regardless of where it is on the drawing) represents the datum plane, and that the datum plane is thus in the symmetrical center of the workpiece. I've tried to illustrate the issue in the following graphic (yes, I know that the workpiece geometry is not the same as we have been discussing in the OP, but I had something similar on my mind for a while, so here it is):
I'm sure that this will raise questions and comments that the misinterpretation is blatantly not correct, however this is what trainers deal with on a regular basis. I once spent over an hour in one class converting the masses to acceptance that the centerlines are only graphics with no physical meaning ... long hour. Some still argued that graphically represented centerlines were absolutely critical to convey information ... I asked where the centerline was drawn in the CAD model and on the actual workpiece ... gave them pause for thought.
My favorite is a hinged glass assembly drawing that came across my desk.
The previous design was orthogonal...so the datums were the hinge surfaces primary and the pattern slots in the hinges secondary. I didn't think the old drawing was toleranced correctly, but at least I think I could make an educated guess on intent.
In the new design the Hinge Slots were not orthogonal...but, the part seemed to have been dimensioned orthogonally anyway.
![[rofl]](/data/assets/smilies/rofl.gif "[rofl] [rofl]")
