Every gage manufacturer receives a number of rejects in the course of a year—it goes with the territory. Some are justified, but many are not and a lot of time and money go out the window trying to sort out the situation. Proving the gage is not a reject without offending a customer calls for the greatest of diplomacy. When a stocked gage is involved some will forego the costs and discussions and just send the customer another one from stock knowing the odds are it will be found to be okay.
All that this method of solving the problem does is imply that the gage was not correct. So those who do this can expect more of the same. When the gage in question has been specially made this cost saving option is not available so the gage maker has to defend his measurements.
I should make clear that even gage makers can make mistakes producing or measuring gages. But the users of the gages who reject some of them must realize that they too are not perfect. Sadly, some will not accept this only to be embarrassed later when they are proven incorrect in their rejection. The purpose of this column is to save you from making mistakes that will put you in this situation if you believe you have a gage that is not accurate.
The first element to be considered before rejecting a gage is to ensure everyone is singing from the same song sheet. This involves confirming the nominal dimensions the gage was to be made to, along with the tolerances involved and the direction they have been applied. There’s nothing more embarrassing than discovering your rejection was based on the wrong dimensions.
When the rejection is based on your measurements of the gage vs. those of the gage supplier or a laboratory that may have calibrated it, the question you must objectively answer is: can I measure it with the required level of precision for the tolerance involved? Sometimes this is a no brainer because the variation is so great. Other times, the variation is so great due to the fact that you do not have the required equipment to do the job.
What’s the “right” kind of equipment needed for calibrating the gage? Obviously it will vary with the type of gage in question and the tolerances involved. Practical advice on this subject is available in the booklet “Searching for Zero” available from the American Measuring Tool Manufacturers Association. (Visit www.amtma.com.)
Assuming you’re confident regarding the equipment, environment and personnel that produced your readings on the gage, we get down to the difference between your “readings” and measurements: the uncertainty of the process you have used. If you’re not familiar with measurement uncertainty you’ll find an overview on the subject in the booklet I mentioned. One thing to keep in mind is that being able to repeat your measured values to a fine degree is important, but it is not the same as the uncertainty attached to the process.
The final step before you reject a gage is to evaluate your measured values compared to the required values by applying your measurement uncertainty. Your “readings” of a size may indicate a gage is out of limit but when your uncertainty is applied, it could mean the gage is just as likely to be inside the limit. If this is the case, you don’t have a sound technical basis to reject the gage.
Sometimes you can follow these steps and the numbers indicate you have a bona fide reject. What may not be obvious is the significance of the amount by which the gage is considered a reject.
For example, a thread gage with a 0.0003 inch limit that would be rejected because it is ten millionths of an inch (0.00001 inch) outside that limit should cause you to consider that maybe your uncertainty value needs to be looked at. Your uncertainty budget that generated the value used may not be realistic.
Is this a practical possibility? The short answer is yes. In fact, some ISO standards dealing with the subject require you to provide your budget to the person you are rejecting the gage on in the event this is the case.
What started out as a battle over dimensions could end up as hand-to-hand combat over uncertainty budgets.
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Hill Cox is the chairman for the technical committee for the American Measuring Tool Manufacturers Association and president of Frank Cox Metrology Ltd. He can be reached at firstname.lastname@example.org or at 905-457-9190.