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 understand where you're coming from. This company (aka Big Yellow) uses what I think is called zero plane dimensioning on their prints. I believe it's just a different way of calling out dimensions. Most of the dimensions are all referenced back to a set of three intersecting planes. In this case, plane 01, which also happens to be datum A. But that doesn't change the structure of the part, just the formatting of the dimensions. Or maybe I'm just super confused.
Edit: I guess I need to understand whether using zero plane dimensioning changes anything. It's not talked about much in any training I've seen, but I deal with it in almost every print I see because Big Yellow is dominant where I work.
I understand where you're coming from. This company (aka Big Yellow) uses what I think is called zero plane dimensioning on their prints. I believe it's just a different way of calling out dimensions. Most of the dimensions are all referenced back to a set of three intersecting planes. In this case, plane 01, which also happens to be datum A. But that doesn't change the structure of the part, just the formatting of the dimensions. Or maybe I'm just super confused.
Edit: I guess I need to understand whether using zero plane dimensioning changes anything. It's not talked about much in any training I've seen, but I deal with it in almost every print I see because Big Yellow is dominant where I work.
