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We 100% inspect a part of ours, due to part liability issues and not an out of control process, using hard gages. The 100% inspection is driven by customer requirements and if not then my company would not need to put all that quality time into the parts. One of the print dimensions has a parallelism that has both a statistical tolerance (ST) and a arithmetic tolerance. When building the gages we used the statistical tolerance for this parallelism. We have just recently remade the gages to go back to the arithmetic tolerance. My customer said that since we are not using SPC in process we cannot use the statistical tolerance. I know ASME Y14.5 clearly states that "Features identified as statistically toleranced shall be produced with statistical process controls". My interpretation of the standard was that you can use the statistical tolerances when control is assured? In this case control is assured by the 100% inspection, again driven by part demands and not process instability. Am I wrong when I assumed we were allowed to use the statistical tolerance? Or is there some reason why SPC must be used? If so then my next question is that the standard does not specify what level of SPC you need to be at to use the statistical tolerance. So even if I was running at a .002 Cpk I can use the statistical tolerances as long as I collect the data?
Just wanted to get some expert opinion on this? And if anyone could point me towards literature to support my original claim? I would greatly appreciate it.
Just wanted to get some expert opinion on this? And if anyone could point me towards literature to support my original claim? I would greatly appreciate it.