I tried to be deliberately a little vague on this, as I know there are many variations in industry. I did attempt to stick with the central issue, which is assuring that product be tested ONLY with properly calibrated instruments, which are adequate for the intended use.
In the case of marking personally owned gages as "do not use on product", I won't try to adjudicate whether or not that is acceptable, as that's not my place. I think that enters a grey area (depending on the particular company), and on some of the specifics.
There are cases where I think it questionable as to whether that method is prudent, and others where it would not be an issue. If there are personally owned gages that closely replicate calibrated production gages (similar accuracy, range, etc.), and the personally owned gages are used in a production area, it may pass the audit, and it may not. In such a case, one could envision an auditor querying as to why two similar gages are in the production area, and how the factory could guarantee those uncalibrated units would never be used to verify product. In such a circumstance (as an example; or similar), I would recommend against such practice. It would also become confusing, and you may have to show evidence that production pepole are trained as to which ones to use, and when.
The other end of the spectrum would be gages that are self-evident to not apply to product test, and/or which are used in areas where the potential ambiguity does not exist.
And 'ambiguity', I think is the operative word in determining if employee owned gages could be labelled as 'do not use for production' or not. If it creates a reasonable ambiguity, I don't think it wise to place uncalibrated gages in production. I could envision in such an ambiguous circumstance, a diligent auditor putting a great deal of effort into proving one way or the other as to whether evidence exists that those uncalibrated gages could have been used on product or not.
Okay... enough said.
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