Calibration Results Significantly Out & Product Impact

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#1
I've just been given this as a science project. This related to physical gages (threads, rings, plugs, etc.) The practice has been to allow for internal calibration failures for up to 1.5 times the total gage tolerance without needing to investigate the impact on the product - total gage tolerance is 0.005mm (this is a total of 0.0075mm or 0.0003"). The question was posed "Where did this come from? How is it justified?" At this point in time the response is "we've always done it that way" 25+ years. ;)

At a guess it came from the "old" school since the procedures reference NCLS-150. So instead of using uncertainty we used the 10:1 to develop the gage/method and 4:1 for the calibration accuracy. Am I off base? :bonk:

I definately want to get to an uncertainty budget for our lab but we aren't there yet.
 
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Hershal

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#2
In terms of the tolerance, first find out if there is a customer or similar requirement for the tolerance. If so, question answered. If not, then you may want to do at least a bit of engineering review to see if there is a good engineering reason. An example would be that whatever you are measuring has a tolerance (e.g. a threaded hole) of +/- 0.75mm in which case you are good.

Once you have the rationale, then I always suggest making sure your equipment is calibrated by an accredited lab.

As for the 4:1 rule for cal, that is allowed under ANSI/NCSL Z540-1-1994, however, the default number form calibration uncertainty (a Type B contribution) is 25% of the rated accuracy of the instrument for that measurement. There are of course all the other Type B contributions and the Type A contributions, but that is how the 4:1 rule is addressed for uncertainty calculations.

Hope this helps.

Hershal
 
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