Machinist here… We’ve been having some issues with QC. We’ll get good readings on the trimos, digital micrometers, CNC probes and optical comparator, and they’ll turn around and reject us because their CMM says differently.
Recently, I got all 40 of my parts rejected while the trimos height gauge showed that all of my parts were good. I explained that you could check deltronicse pins and precision gauge blocks and be within a tenth every time, and they were telling me the part was .0004″ larger than what the trimos was showing. So obviously something wasn’t right. They didn’t care. They were quite confident that the CMMs results are a superior form of measurement. I showed them that the mitotoyo digital micrometers showed the parts were good. I had mitotoyo precision gauge blocks with plain jaws attached as a no go and that the parts passed through. When I put the part on it’s side and used a .0001 test dial indicator… I found that the biggest diameter part they accepted was larger than nearly all the parts they rejected for being too big. None of it mattered to them, because the Almighty CMM is most accurate and it’s judgement is final.
I understand that a CMM, in theory, ought to provide the most accurate results. But it’s quite obvious at this point that they’re getting bad results.
The only explanation I can come up with is that they use tape to hold down just about everything, and I’m thinking that it’s causing the parts to deflect. How would you go about diagnosing the issue?
submitted by /u/iamwhiskerbiscuit
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