A progression will have a uniform distribution, it shouldn’t have a Normal distribution at all (Normal is a proper name, not a description). THINK about it: the concentration of a plating bath will STEADILY decrease as the metal is ‘deposited/transferred/plated onto the objects in the bath. So it starts out high and then decreases a bit at a time. And more importantly the physics of concentration is SUPPOSED to start at the high spec and then decrease to the low spec; you want to use the entire range of concentration for economic reasons. The range of concentration is determined by the economics (time and money) of the process of plating - a very high or a very low concentration within the determined range doesn’t mean that final plated coating is out of spec. Ppk/Cpk were intended for actual finished characteristics whose spec limits are determined based on the PRODUCT being defective…Have you read Bob Doering’s resources?
Your knowledge of quality engineering is very sparse - and it is to your credit that you are here asking questions to learn as this stuff is rarely taught in university. Quality engineering is a complex and deep engineering discipline - it cannot be learned and applied with a few google or AI searches and then copy-pasted. If you wander around here a lot, check out the resources section and ask questions you’ll start to learn. I also highly recommend you visit SPCPRESS.com and read everything that is there.
nickelinstitute.org
