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 wish I could show you the training material I have that suggests that particular surface doesn't necessarily get evaluated independently when it comes to the flatness tolerance. But I'm sure it's copyrighted. However, it does come from ASME Senior Level certified engineers, so I can't just automatically disregard it. I can probably point them to this thread and ask for their take on it though. Trust me, I want to be able to apply that full 0.1 flatness to that surface and not have to worry about anything else related to it. As an inspector, Rule #1 really complicates things for me because I can't just measure something and see if it's within tolerance. I have to be constantly thinking about the envelope principal.
I wish I could show you the training material I have that suggests that particular surface doesn't necessarily get evaluated independently when it comes to the flatness tolerance. But I'm sure it's copyrighted. However, it does come from ASME Senior Level certified engineers, so I can't just automatically disregard it. I can probably point them to this thread and ask for their take on it though. Trust me, I want to be able to apply that full 0.1 flatness to that surface and not have to worry about anything else related to it. As an inspector, Rule #1 really complicates things for me because I can't just measure something and see if it's within tolerance. I have to be constantly thinking about the envelope principal.
Rule#1 doesn't apply to the example you have given. As
NinjaBadger said, the flatness is associated with datum A which is a plane. Planes are not features of size. The 23 ± 0.05 is a linear dimension to another plane - also not a feature of size. For the 23 ± 0.05 dimension to be a feature of size (a width), the planes would need to be parallel and have opposing vectors.
