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?
No, a plane is not a feature of size by itself, but the distance between two opposing planes is. And I get that an MMC modifier can only be applied to flatness of a feature of size, but MMC itself and the Envelope Principal is constantly applied to everything unless an independency modifier is used. If the 23 width from Datum A to that opposing surface is measuring 23.45, then that width is only 0.05 from maximum material condition. There is excess material present at either Datum A or the opposing surface or both that is bringing that width only 0.05 away from its outer envelope of perfect form. Therefore, a measured flatness of 0.08 would bring the width outside of the envelope. If what I'm saying is incorrect, then I'm getting some wrong info from the training provider that I paid a good amount of money to.
No, a plane is not a feature of size by itself, but the distance between two opposing planes is. And I get that an MMC modifier can only be applied to flatness of a feature of size, but MMC itself and the Envelope Principal is constantly applied to everything unless an independency modifier is used. If the 23 width from Datum A to that opposing surface is measuring 23.45, then that width is only 0.05 from maximum material condition. There is excess material present at either Datum A or the opposing surface or both that is bringing that width only 0.05 away from its outer envelope of perfect form. Therefore, a measured flatness of 0.08 would bring the width outside of the envelope. If what I'm saying is incorrect, then I'm getting some wrong info from the training provider that I paid a good amount of money to.
