PPAP is a risk-reduction activity. I will talk about machined steel parts because I have more experience with those. There are websites which provide cross-reference tables based on alloy composition in various national steel specifications,. But chemical composition is only a portion of material selection consideration. Many components or features in an assembly are not in a demanding role, but some are, and the level of technical review should correspond to the level of technical risk. If you are less than thorough in your PPAP review, you may be left with a lot of risk, more than you should knowingly accept. I recommend a PPAP review team of subject matter experts who collectively know a lot about material properties and fabrication technology and the particular design application.
I had PPAP review responsibility a few years back, when the final product assembly was out-sourced, and the contract manufacturer wanted to source many parts and raw materials from the other side of the world. I remember a debate over the PPAP for a half-inch hex shaft in a wear application. The original part design called for cold-drawn half-inch hex bar, but inch sizes are impossible to source locally in Asian countries, so the potential supplier submitted PPAP on a hex shaft turned from a round-bar. The machined hex shaft PPAP satisfied all print dimensions, although the print did not include tolerances for hex angles and edge conditions between faces, because those reference dimensions are defined in steel bar standard handbooks. Because the shaft was used in a wear application, and knowing that a machined shaft would not have the work hardening of a cold-drawn bar, the team decided the machined alternative shaft would have to be re-validated for durability in the assembly. There was no appetite for the cost or lead-time to life-test the shaft in the assembly, so the machined alternative shaft was rejected as not-conforming to the part print. The moral of the story is, the part design print defines more than just the material callout and dimensions on the print.
