In our case, we do onesy and twosy pieces - thus, we must have 100% of our parts made right. Thus, this issue of accuracy of parts per million is a sheer falacy to/for us. We inspect 100% before we ship and RARELY scrap out a part - a worker who does is made to realize that his screw up IS/WAS costly. Result:they take pride in NOT screwing up.
We've had the QS9000 and the other questions re: std deviation, this coefficient or that posed to us. When we explain that our procedures don't fit the ppm scenario, and inspect 100% to verify quality, we've never had an issue with the Q/C depts.
Besides, their argument falls appart when you hit them with the "how do you do a size distribution study when you are working with a statistical sample of one???" inquiry.
We don't do aerospace work but NASCAR Nextel stuff is close - failures are QUITE visible and not tolerated. If you are shooting for a +/-0.0001" total tolerance spread, you creep up on the size. Simple thermal expansion can make the difference in a part being in or out of spec.
Thus, if you "hit' size at +0.000060" (6o millionths) of nominal, you stop grinding, read size, lett cool/thermally stabilize and read again, redo/regrind if required.
You also have to condider what your part is being used with. Thus, if you are making an aluminum widget that fits into a steel whatsits, the RELATIVE size at operating temp is more critical than ABSOLUTE size. YOu also have to know if the part is going to be thrown together or custom fit by the installer.
All the stuff we do is custom fit by user. We still shoot for/hit the nominal size when we grind.
When Mr. Honda was supposedly told by a Q/C guy that he was wasting money trying to hold better quality than 2-3ppm, his reply was, "Yes, that is true but the 2 or 3 customers got the bad parts out of the million who bought my product had a 100% defect rated which is unacceptable in any Q/C program...."
I'm sure the discussion will rage on.
BTW, ball bearings are made in the millions and they control internal bearing clearances within a few microns day in and day out.
It is amazing but if you have to hold a particular tolerance and the customer or market demands will pay you to do it, you find a way - or else go out of business because if you don't, someone else will.  |