Stupid/incorrect position callouts on prints

[[Da...]]

I've been doing manufacturing QC work for about 20 years now, and I've seen a good number of lousy prints; often companies use an intern to create prints, even if they don't have a good grasp on ASME Y14.5. Just this week, I found 2 pretty good ones (well, one that's worse than the other, at least).

The first was a general callout for all the PEM studs on the final assembly to be toleranced at "1.3mm MMC". Not sure if you can eve DO MMC on a threaded PEM stud? I suppose I could break out a pitch micrometer and the Machinery Handbook to find the major Ø and the pitch Ø, then determine bonus tolerance that way, but somehow I believe that's not correct.

The second was REALLY bad, though. A plane/hole/hole alignment, with datums B and C both being toleranced to A|B|C. Ummmm, no, you can't tolerance a hole to itself. I also think that, in the case of datum B, it would need to be defined before datum C, so that would make the callout on datum B wrong on 2 separate levels.....

[[Ti...]]

I get 30 deg arc cutouts all the time with positional callouts.....with MMC...sometime I get the radius at +- tol then the position is obviously called out diametrically.  Our programming dept only gives us the bonus based on the +- or the rad callout when it should be the inferred dia tolerance. I tried once to show then a part interference on this and they still decided to disagree.

Since then, I just take what I'm given and do my best. For a threaded MMC id just use the minor/major dia as the feature, or whatever gives the least amount of bonus or is most stable or whatever you can probe or simulate with a joe plug or gage pin. We use CTs so I'll use the most stable dia. I'll guardband the bonus sometimes.  Also...how are they gonna double check your work???

 

Can't comment on the second rant. I tolerance position using the feature(sl) themselves as a rotation/tertiary datum when that deg of freedom is left open. Sometimes I let the software best fit the rotation/clocking for a best fit but I much prefer to lock all 6 dof down....and this is a must in Zeiss Inspect in order to output xyz coordinates using a datum system.  A plane, hole, hole, is a valid datum system btw.