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The intersting part is that this is a visual inspection of the process and not the product
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I just wanted to see what some others thought of this type of visual inspection as I cannot fault their thought process and that it is not a visual inspection of the product.
In my opinion it is correct to visually inspect for cause (process)
The detection controls are related to failure mode detection (usually a product defect) or detection of cause (process).
AIAG
FMEA manual, page 95 says:
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"Detection:
Identify (detect) the cause of failure or the failure mode, leading
to the development of associated corrective action(s) or countermeasures.
"
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Regarding visual inspection ranking = 4:
If typical AIAG FMEA rev.4 ranking scale is used then the detection should be 7 (visual inspection at source) not 4.
Det. ranking 4 means automatic detection of failure mode and part lock to prevent further processing (post processing, next op).
I believe that visual inspection detection should NOT be related to "how easy" is to see the problem. I saw several complaints where inspectors/operators inspected products for "small defects" and accidentally omitted "easy to see" defects. I also saw complaints where several layers of visual inspection were used and the defect still passed through. That's why the AIAG FMEA rev.4 Detection tables does not consider "multiple visual detection" as better control than single visual inspection.