I've been doing some GD&T training lately and one thing that seems to complicate things for me is Rule #1 (aka the Envelope Principal). It states that at MMC, a feature must have perfect form and as it departs from MMC, a form error is allowed in the amount equal to the deviation from MMC. Unless there is a form callout that further refines and limits it. Say I have to add a flatness dimension for Datum A as shown below. The flatness comes in at 0.08, which is within tolerance. BUT there are at least 6 other features on this part that are dimensioned to Datum A. If the 23 +/- 0.5 for example measures at 23.45, that only allows me a form deviation of 0.05. The flatness is now out of tolerance. But how do I get PC DMIS to account for that? The flatness is still going to report in tolerance at 0.08, even though Rule #1 has now been violated. Worse yet, (and this is just rhetorical) how do I tell the shop supervisor that I have to reject his parts even though the flatness is reporting in tolerance when it's actually not?
I think where you're getting hung up is the mating envelope principle. The mating envelope assures that parts assemble. The low spots on the outer planes have no effect on the mating envelope size. Nothing is normally contacting those surfaces.
If you wanted to control the form of the planes independent of the mating envelope you would use flatness, profile, etc. to control the form of the entire surface, either independently, or constrained to a Datum feature.
I think where you're getting hung up is the mating envelope principle. The mating envelope assures that parts assemble. The low spots on the outer planes have no effect on the mating envelope size. Nothing is normally contacting those surfaces.
If you wanted to control the form of the planes independent of the mating envelope you would use flatness, profile, etc. to control the form of the entire surface, either independently, or constrained to a Datum feature.
