October 12, 2022
In the world of custom manufacturing you have to deal with less-than-perfect parts. There is just not enough time or pieces to dial in the production process to the levels achieved in serial production programs. If your life only involves long runs of parts, this may sound like heresy, but you do not stay in business throwing big, expensive pieces of hard-to-obtain material in the scrap hopper and starting over. Job shops hate scrap and rework just as much as a mass producer; no amount of skill, caution, or planning can prevent “stuff” from happening, so the engineers, designers, and quality people have to know how and when to fix things. This requires knowing when the best thing to do is to start over. I draw the line at cracks and grind burns. Some claim they can salvage even these “victims,” but I remain unconvinced. The potential liability is simply too great for me to sleep at night. A few times the shop attempted to regrind flanks and root fillets but ultimately the poor things got tossed. There are also situations where the cost or delay in reworking the part makes starting over a better choice. And just because you can fix something does not mean you should. Over the next few blog postings I will consider the typical defects that occur and offer my thoughts on how to fix them — and when to toss them out. Among the concerns with any rework decision are whether the customer’s approval is needed. There are contractual and ethical considerations to ponder; I prefer to figure out a way to restore the part to full functionality before even beginning the notification process. If you cannot be completely confident in the rework, why risk your firm’s reputation even asking for permission? Not much is published on this topic; it leads some to think no one else ever makes a non-conforming part. I once sat through a long meeting arguing over surface temper etch inspections. My client wanted 100% of his parts etched; the vendor insisted they never cracked or burned a gear and this was a waste of time and money. We were at loggerheads; our host suggested a plant tour. It ended at final inspection where a skid held three pinions that were tagged “scrap due to grind cracks.” Reality bites sometimes.