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Tool calibration

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Tweety

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I've been seeing some chatter on other sites about the need for "calibrated" tools for serving Part 91 aircraft. Having been responsible for a manufacturing operation that required calibrated tools and gauges, and which was subject to audits by government and customer auditors, I can tell you it's not a simple thing.

First, there is the question of the required calibration accuracy. If the gauge or tool maker specifies that, it's simple. Otherwise, not so much. It can depend on the ability to read-out the result, e.g. if it reads out to unit ft/lbs, then one could infer an accuracy requirement of +/- 0.499999... ft-lbs. Or, the application could determine the requirement. If the manufacturer specified 18.0 ft-lbs, then the torque wrench accuracy must be +/- 0.04999999... ft/lbs, etc.

Another wrinkle is the accuracy of the standard being used for the calibration. If it's a secondary standard that itself is calibrated by NIST, then the accuracy of the secondary standard needs to be factored into the working gauge/tool calibration criteria. You better hope the facility doing the calibration has good records.

The other big "gotcha" is what do you do when a gauge/tool is found out-of-calibration? Since everything that was done with that gauge/tool might have had the incorrect calibration applied, sometimes a recall of items would be required. So records of what was being done with each calibrated tool/gauge are vital.

You might be able to enlist engineering to provide information on the amount of "safety factor" they used to create the limits...if you are lucky. Otherwise, have fun convincing an auditor that everything was really OK when they review your calibration records.

Fortunately there are usually generous allowances for gauge/tool tolerances. Otherwise the people responsible wouldn't be able to sleep well at night.
 

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