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?
Flatness is measured independent of the size. You will be measuring the mating envelope of the sides. If it's an outside feature, you need to assume that something will be mating on the outside of the part. So you need to take your extreme points (3 point plane) for Datum A and the other side. The "dips" inside that mating envelope are calculated into your flatness, not the width. You don't lose width tolerance because the flatness.
Yes he does but not directly. if its flatness is already at the limit of the tolerance given for width, he cannot have the material at MMC thus violating rule #1
Yes he does but not directly. if its flatness is already at the limit of the tolerance given for width, he cannot have the material at MMC thus violating rule #1
