A COUPLE OF VERY IMPORTANT DECISIONS SHOULD NOT BE LEFT TO CHANCE.
Regular readers of my rants on these pages will know that since I’m a pretty simple guy, it doesn’t take much to get me excited or amazed. This effort is one of those ‘I’m amazed...’ situations most people providing calibration services encounter on a regular basis but really shouldn’t be involved with—in my humble opinion.
With keyboard blazing, I’ll try to stay civil and explain what it’s all about.
It begins with a request that a calibration report show tolerances that are applicable to the items on the report. The question this brings forth is: What tolerances should be stated? The most common customer response is the tolerances they were made to, i.e. the tolerances when they were new. Where fixed limit gages are involved, or even gage blocks for that matter, they are unlikely to meet such specifications once they have worn a bit. This is especially so when gage blocks are involved. In both cases, the tolerances in the related standards apply to new products and belong to the maker of them. They do not include limits for wear but this may change in the future with respect to gages to ASME standards.
This is the reason that proper gage records will note the ‘new’ tolerance but more importantly, will show a ‘re-calibration tolerance.’ If the two values are the same or only the ‘new’ tolerance is used, a lot of perfectly good equipment will be rejected, product may be recalled unnecessarily, and general mayhem can follow.
If there is a difference i.e., the re-calibration tolerance is greater than the ‘new’ tolerance, it means someone has reviewed the tolerances on the products being checked/measured with these items and determined how much wear can be accepted before replacements are indicated.
Obviously, someone who understands your tolerances, quality system and at least basic metrology should be making this decision. Quality and engineering people usually work together to make the call. When non-technical people make the decision, ‘new’ tolerances become the choice of last resort no matter how impractical that might be. It’s an easy number a quality auditor can’t argue with and all too often that becomes the primary reason for choosing it.
Irrespective of who decides what the re-calibration tolerances are, another very critical decision has to be made: How will measurement uncertainty be applied to the calibration data when it comes to the ‘accept/reject’ decision?
Unless otherwise stipulated, most labs will list their uncertainty and leave it out of the loop if asked for an ‘accept/reject’ decision. You may choose to do the same thing if you are making such a call and the uncertainty doesn’t amount to much. But, if the uncertainty is high compared to the tolerance—often the case with fixed limit gages—you may have to rethink this decision.
By now it should be obvious that a couple of very important decisions should not be left to chance or a calibration laboratory that is not intimately familiar with your company policies and practices. Don’t get me wrong, most labs will do whatever you are prepared to pay for assuming it’s legal and does not call their activities into question.
Once your quality system has been through a couple of independent audits the procedures and rules you have established will have been deemed to be suitable for the work you are doing. If you want an outside calibration source making judgment calls you should stipulate this on your order and provide the rules they are to follow in doing so.
When you do this you are downloading the decision-making process and for what purpose? If you decide what the tolerances shall be and how measurement uncertainty is to be applied to the calibration results, there is no need for an outsider to make the call. All you need from them is the data and the uncertainties involved.
Calibration laboratories are in the measurement business. Numbers are what they produce. Ensure their reports are clean, useful and free of opinions or decisions more properly made by you. Your company’s lawyers will appreciate reports of this nature especially if they become evidence during a legal proceeding.
If you don’t believe me, show your lawyers this column and ask for their decision.