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 more of an ISO user but obviously we get ASME stuff as well so here's my take:
I think it comes down to how you think about a datum.
A datum is theoretically perfect (in this case it's a perfectly flat plane)
A datum feature is what's on the actual part, and will therefore not be perfect (i.e. have form error)
A datum feature simulator, is what we use to create our working datum from the datum feature, be that a surface table, or a CMM
Obviously on a surface table we have it as close to zero form error as possible, so when we place out part on it we're using an as-near-perfect simulation of the datum (external to material). We would then measure the 23±0.5 and as long as it measures (over the full surface) within those limits we're good.
With the CMM, we measure the plane with a suitable number of points, and from that we construct the appropriate type plane (a primary datum in this case, external to material also) to use in our alignment for the evaluation.
The Flatness of datum A is evaluated independently of this. Again, to use first principals you'd measure this characteristic separately.
I'm more of an ISO user but obviously we get ASME stuff as well so here's my take:
I think it comes down to how you think about a datum.
A datum is theoretically perfect (in this case it's a perfectly flat plane)
A datum feature is what's on the actual part, and will therefore not be perfect (i.e. have form error)
A datum feature simulator, is what we use to create our working datum from the datum feature, be that a surface table, or a CMM
Obviously on a surface table we have it as close to zero form error as possible, so when we place out part on it we're using an as-near-perfect simulation of the datum (external to material). We would then measure the 23±0.5 and as long as it measures (over the full surface) within those limits we're good.
With the CMM, we measure the plane with a suitable number of points, and from that we construct the appropriate type plane (a primary datum in this case, external to material also) to use in our alignment for the evaluation.
The Flatness of datum A is evaluated independently of this. Again, to use first principals you'd measure this characteristic separately.
