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?
They gave me a quiz with a drawing very similar to this same sketch. They didn't call that surface out as a Datum, they just called it Surface A. I'm going to tell you their same question and answer, but substitute in my sketch dimensions. The way it's called out on the drawing though is the same.
Question: If the part was produced at 10.4, what is the maximum flatness error of Surface A?
Answer: 0.1 The print says that you can have a flatness tolerance of 0.5, so why is it now less? The answer is that you are allowed
up to 0.5 flatness tolerance. The part is still constrained by Rule#1 and the limits of size. If you only have 0.1 available before you reach the MMC size limit of the part, your max form error can only go to 0.1
If the flatness says 0.5, the callout is flat 0.5, then the tolerance is 0.5.
The suggestion that the flatness is dynamic going from 0 up to a max of .5 dependent on the size is nothing I have ever encountered.
If you have a customer that is telling you this, then you have to do what you are paid to do.
Everyone I have ever worked for, form is contained by limits of size unless otherwise specified, so I can not have any single point higher than 10.5 or lower than 9.5.
Beyond that, most of my customers (aero) have a workmanship document that says when no form is called, things will not have steps greater than 0.127 or half the tolerance, runout on common axis .127 or half the size tolerance, yadda yadda.
No one says I should hold a ±0.5 flat within 0 because I made it high limit.
No one says I should ignore the print dimension because I made it high limit.
The A surface has a flatness callout from the design authority of 0.5, that's the tolerance because that is what the engineer needs.
Tolerance can be less constrictive to facilitate manufacture and control cost just as it can be more constrictive to ensure function and lifespan.
Now, if you want to use this rule #1 to the side OPPOSITE A, then you would not be over-riding the engineer's design. However, as I said, I have never heard of anyone doing any such thing.
If our customers expected us to do that, 1) we'd charge more money and 2) we'd make everything at LMC.
In that event, I'd program output to determine size, do math to afford a tolerance in a variable assignment, and use that variable assignment in my flatness callout.
The closest I've been to this is a customer that gave me a dynamic tolerance at the BOTTOM of drilled connecting holes based on the size of the hole versus the hole depth. The mouth of the holes had a position of 0.375 at MMC, the bottoms of the holes had a position of .375 at MMC + (MMC * depth / 10) or something like that. It was weird.
I did that with variable assignments on the reports I gave to the customer.
I forced the machine shop to just hold the 0.375 position for the full length of the hole.
If the flatness says 0.5, the callout is flat 0.5, then the tolerance is 0.5.
The suggestion that the flatness is dynamic going from 0 up to a max of .5 dependent on the size is nothing I have ever encountered.
If you have a customer that is telling you this, then you have to do what you are paid to do.
Everyone I have ever worked for, form is contained by limits of size unless otherwise specified, so I can not have any single point higher than 10.5 or lower than 9.5.
Beyond that, most of my customers (aero) have a workmanship document that says when no form is called, things will not have steps greater than 0.127 or half the tolerance, runout on common axis .127 or half the size tolerance, yadda yadda.
No one says I should hold a ±0.5 flat within 0 because I made it high limit.
No one says I should ignore the print dimension because I made it high limit.
The A surface has a flatness callout from the design authority of 0.5, that's the tolerance because that is what the engineer needs.
Tolerance can be less constrictive to facilitate manufacture and control cost just as it can be more constrictive to ensure function and lifespan.
Now, if you want to use this rule #1 to the side OPPOSITE A, then you would not be over-riding the engineer's design. However, as I said, I have never heard of anyone doing any such thing.
If our customers expected us to do that, 1) we'd charge more money and 2) we'd make everything at LMC.
In that event, I'd program output to determine size, do math to afford a tolerance in a variable assignment, and use that variable assignment in my flatness callout.
The closest I've been to this is a customer that gave me a dynamic tolerance at the BOTTOM of drilled connecting holes based on the size of the hole versus the hole depth. The mouth of the holes had a position of 0.375 at MMC, the bottoms of the holes had a position of .375 at MMC + (MMC * depth / 10) or something like that. It was weird.
I did that with variable assignments on the reports I gave to the customer.
I forced the machine shop to just hold the 0.375 position for the full length of the hole.
