Checking Aids
Recently I had a corrective action for parallelism. When checked on three points in one direction, it was well within specification. However, the customer was rotating the part 180 degrees on the 3 pts. The concave nature of the blanked steel part caused us to believe we were making a good part, but when turned in the other direction it was out of spec. I am told this is called the crowning effect due to rotation about the x-axis. From this point on we either ask for datum targets to be added to the drawing for new programs, or if the customer engineers just don't get around to addressing the
APQP concerns, I include the checking fixture on the PSW with a copy of the drawing in the PPAP and reference the fixture on the layout. Of course the customer still could argue that the parallelism still must be in spec no matter the orientation off the datum surface. But at least it was on the PSW that showed how we were measuring the part and that they gave approval. It is in your interest to provide this information for fixturing and gaging if you have experienced a lot of problems with your zealous SQA who questionably rejects product. To be honest, freshman, smartass SQAs who believe the customer is always right no matter how stupid he/she is, will approve the PPAP and not know why you are providing this information beyond the fact that it is a blank space on the warrant
and that you are complying with their demands. I had this customer complain to me over dinner about a fellow SQA co-worker because he asked for so many documents beyond the PPAP requirements, held up PPAPs for misspellings, and jokingly said that suppliers offered 10% price reductions just to get rid of him. He has since signed many beautiful PPAPs in which every effort was made to accomodate his desire for information.
Chemlab