This isn't a PC-DMIS question but I thought someone here might know the answer. What is the logic behind having a Ø called out at 20.0mm with a -.007 / -.04 tolerance. Why not just make the Ø19.993 +0 / -.033?
I always change that to the NONMINAL size in my report with a note for the operator. No one reads a bilateral tol. or a +/+ or -/- tol correctly. I might use the print tol for a report , but never for production.
I number of years ago I had a similar callout with a +/+ tolerance. The engineers designed it that way because it was a press fit pin that went into the hole. I shook my head and walked away.
Ivy League engineering that's why. We had one engineering intern from Northwestern University. According to her, they have to take 1 GD&T class but 5 humanities classes to graduate.
Changes during design & testing of the part or the mating component can often create unusual tolerances. Instead of changing a feature on a cad that could potentially affect many earlier elements, the adjustment gets pushed to the tolerances. Especially if you're in the final testing phase and found a diameter would fit better if it was .005" larger, then instead of going back into cad and "redesigning" the part which could potentially change any restraints and constructs tied to it, it's less effort to tag the tolerances to account for the change.
That's nice. Now we can stop wondering why airplays are grounded because parts don't fit, automakers recalling millions of cars because parts fail prematurely & appliances fail after 1 year. I will be keeping my 1970s built fridge as long this thing keeps chilling sausages & cheese I keep buying.