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'm sorry to be a pain in the butt, but I don't see how this wouldn't be a feature of size. I know it's hard to tell from my snapshot, but I don't think I'm allowed to post the whole thing since it's a controlled document and all. So here's a rough sketch instead. Datum A is on one side of the part, and the 23.5 surface is on the other. The planes are parallel and have opposing vectors as far as I can see. It's the overall thickness of that section of the part. So what am I missing?
I think, for it to be a feature of size, that 23 dimension would need to be called out to BOTH surfaces, like:
|<----23---->|
To me, it is only calling out the non-datum surface, no different really than putting a PROF A 1.0 on the surface.
I think, for it to be a feature of size, that 23 dimension would need to be called out to BOTH surfaces, like:
|<----23---->|
To me, it is only calling out the non-datum surface, no different really than putting a PROF A 1.0 on the surface.
