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?
Pasted below is what the ASME Y14.5 says concerning the Envelope Principal (Rule #1). It does say that it doesn't apply if a flatness tolerance is associated with the size dimension. However the flatness tolerance in question here is not associated with size dimension itself, but rather one of the surfaces it's composed of. And as you said, a plane itself is not a feature of size. So Rule #1 still applies. It also says that a variation of form equal to the amount of departure from MMC is allowed. In my theoretical example, the 23 dimension measured 23.45. So I would only be allowed a form variation of 0.05. The flatness callout may be 0.1, but that amount only applies if the departure from MMC is 0.1 or more. Anything less, and Rule #1 would take over and I would only be allowed an amount equal to the departure from MMC. In this case, 0.05. So again my question is how do I get PC DMIS to account for this when reporting flatness? Or is that even possible?
Pasted below is what the ASME Y14.5 says concerning the Envelope Principal (Rule #1). It does say that it doesn't apply if a flatness tolerance is associated with the size dimension. However the flatness tolerance in question here is not associated with size dimension itself, but rather one of the surfaces it's composed of. And as you said, a plane itself is not a feature of size. So Rule #1 still applies. It also says that a variation of form equal to the amount of departure from MMC is allowed. In my theoretical example, the 23 dimension measured 23.45. So I would only be allowed a form variation of 0.05. The flatness callout may be 0.1, but that amount only applies if the departure from MMC is 0.1 or more. Anything less, and Rule #1 would take over and I would only be allowed an amount equal to the departure from MMC. In this case, 0.05. So again my question is how do I get PC DMIS to account for this when reporting flatness? Or is that even possible?
