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Incoming QC acceptance on fasteners 1

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limct

Mechanical
Jan 27, 2003
134
My company is buying millions of screws every year. Occasionally, there were some "rejects" found on the incoming lot. My incoming QA only do samplings of 3 screws per lot and if one is found to be out of specs, the whole lot will be rejected.

For an instance, per EN ISO 2702, ST3.9 tapping screws, specs calls for core hardness 270~390 HV5, and when any one of the three readings is found out of this range, the lot is considered as reject.

I wonder if this is the correct way. Since variability is inevitable (material, operator bias, apparatus calibration and method of testing), should we consider a statistical solution by calculating the average and std deviation and judge whether the probability of failure to happen under this given scenario is acceptable.

Thank you.

Best regards,
ct
 
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Are you familiar with ISO 3269 Fasteners - Acceptance inspection? This standard provides details on AQL (acceptable quality level), LQ (limiting quality), etc. and references information from ISO 2859 Sampling procedures for inspection by attributes - Part 1: Sampling schemes indexed by acceptance quality limit (AQL) for lot-by-lot inspection. I recommend reviewing these standards if you want to implement a statistical acceptance procedure.
 
I am vaguely familiar with AQL etc as TVP recommends. You are well advised to follow his suggestion.

Your sampling rate of 3 screws per lot seems very low unless you are receiving unusually small lot sizes. As I recall, if any samples fail the test criteria then larger sampling is dictated unless you simply reject the lot as you are doing since one failed sample suggests the likelyhood of additional unreliable parts.

As TVP states, his reference will spell out the procdure statistacly.

hardness 270~390 HV5 sounds like a generous window but again maybe the reference given will provide even more generosity?

Jesus is THE life,
Leonard
 
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