Other Dimensions: Set Your Masters
A customer recently called and wanted a setting ring made to within one millionth of an inch of the stated size. This request was frightening enough until I subsequently received a request for gage pins that had zero errors. In terms of accomplishing either task, in the first request at least that customer gave us a tolerance!
I recall a gage maker claiming he had a setting ring that was a "golden" master. That made me green with envy because mine were plain steel and were calibrated by the folks at the National Institute of Standards and Technology (NIST). I wondered, out loud, as to what entity in the heavens or elsewhere had calibrated this sacred, golden master. When I found out what he meant by "golden," I quickly lost the faith.
What this gage maker meant was that many of the people he thought were good at calibration had all agreed, within a small margin, as to actual size of his ring. Then, I think, he took the average of the readings and "gilded" the ring.
I didn't get into a discussion on measurement uncertainty and why it's difficult for others to do as well, let alone better, than NIST when it comes to calibration. However, many will claim they can do better.
These experiences have brought me to realize that people won't tolerate variation on a master ring or gage; they want perfection. Of course, we all want perfection but that's not possible and that's why we have tolerances. But no one wants the gage maker to have any tolerance on setting masters.
People spend a great amount of money to make a digital display read zero at the push of a button. To do so, they buy expensive masters to tolerances that often can't be proven-even by NIST. The cheaper way is to select a coarser tolerance and get the master calibrated to the number of needed decimal places. Set this "actual" value on the equipment and it's ready to be used.
This works well provided there are one or more witness lines on the master so the settings are at the same place the master was calibrated. If this is not done, geometry could invalidate the calibrated value. Similarly, be cautious when using a ring to set three-point bore micrometers or similar devices. The ring will have been calibrated across two points only.
I've mentioned before that NIST is the only facility I know of where the measurement uncertainty is low enough to verify that a setting ring meets required tolerances. The same applies to required setting discs. There are independent calibration services who claim they can meet the same uncertainty levels as NIST, but most of them base their claims on the repeatability of their measurements or the resolution of the instrument they're using, which while important, are only part of what makes a reading equal to a measurement.
Many who use masters are concerned over the cost of them, particularly when they are not made from off-the-shelf blanks. This is because the user wants the master to be identical to the work being inspected, which is good practice, but only to a point. Incorporating features in a master that are not being checked by the gage or fixture used every day can increase costs.
One way to save wear and tear on setting masters in a production environment is through the use of one or two secondary masters. Secondary masters are components that are carefully checked at the beginning of a shift. Their key dimensions are recorded and they are measured by the gage or fixture, that is used every day, at appointed intervals throughout the shift. Readings from the secondary masters will vary but should remain within limits prescribed by the quality engineer. These masters are shipped with the rest of the product at the end of the shift.
Secondary masters are usually not hardened and stabilized like primary masters, thus they have a short, useful life. However, because they are like the product in almost every respect, they will react to changes in the environment, clamping forces and all other external causes for uncertainty, just as the product being measured. They might be better for setting up gaging equipment in some cases than actual masters.
While millionths of an inch or tenths of a micron are easy numbers to write, it is much harder to wrought those measurements from a piece of steel or carbide, and be able to prove it.