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?
Well it looks like I will have to ask the training providor for clarification because it seems from the general concensus of this thread, they're either giving me some misinformation or I'm just not understanding their lesson. I really want to show you the part that is tripping me up, but I don't want to get in hot water. Maybe I will ask them if I could screeshot and snip just that portion and post it here. In any case, I appreciate all the advice. I just want to make sure I understand this correctly because it should be a fairly basic concept.
Well it looks like I will have to ask the training providor for clarification because it seems from the general concensus of this thread, they're either giving me some misinformation or I'm just not understanding their lesson. I really want to show you the part that is tripping me up, but I don't want to get in hot water. Maybe I will ask them if I could screeshot and snip just that portion and post it here. In any case, I appreciate all the advice. I just want to make sure I understand this correctly because it should be a fairly basic concept.
