Some of the data I’ve noted from the AMTMA studies make it very clear that measurement disputes will continue. When the range of readings is close to or exceeds the tolerances of the gages being calibrated, it is a certainty. Further problems will occur due to unrealistic expectations by all the parties involved in a dispute.
Gage users often believe that the gage tolerances in the standards are a simple matter, easily achieved when test after test shows this is not the case. All too often, gage users specify higher levels of accuracy than required for an application for a little extra insurance. In practical terms, this higher level may never be achieved but the gage user usually has no way of knowing this. And if it was achieved, the environment in which a gage is used may be such that the extra precision expected of the gaging process goes out the window.
Of course, you can’t blame the gage user for believing extra-close tolerances are realistic when they are listed in so many gage manufacturer’s catalogs as being obtainable for just a few dollars more.
Unrealistic expectations on the part of some gage makers and calibration facilities include the belief that they can measure gages to these tolerances with little difficulty. A few still believe that if they can repeat a reading of size to a few millionths of an inch, that’s what their measurement uncertainty is. There are reputable gage makers who understand these problems but competitive pressures keep the beliefs alive. All it takes is one or more companies making such claims—and gage users believing them—to cause others to do the same to keep their share of the market.
GETTING THINGS RIGHT
The first step to putting gage tolerances and measurement expectations into their proper perspectives is to ensure everyone is singing from the same song-sheet. This means the right standard is being followed for the item being calibrated so that elements that contribute to the uncertainty of the process are dealt with. This is particularly important with thread gage calibration where there are a number of factors to be dealt with when determining what the actual pitch diameter is.
In the past, some organizations tried to get around calibration and thread measuring problems by devising a new standard when metric threads started to become more popular in North America. Their proposals would have simplified many aspects of the processes. But in the end it faded away as the gage details were such that compatibility with existing metric systems was not always there.
Somewhere in the universe there is always someone who believes they have designed a better device to calibrate gages compared to the usual equipment found in calibration laboratories all over the world. Some are quite clever and may even speed up the process, but co-relation with existing systems is rarely there as the actual uncertainty using them is not always known to the developer. In some cases this is due to the designers not understanding the many details of specifications for thread gages. Meanwhile, traditional equipment used in this work is being upgraded from time to time to improve performance. This path of development ensures that time tested principles of metrology are not neglected such that the ‘new, improved’ version is just that.
From a gage user point of view, a review of how the gage tolerances they specify are determined might reveal they are ordering something they don’t need and if they got it, would be of little benefit. For them, it all starts with product tolerances and as every gage maker knows, too often newly acquired gages show that the product tolerances they thought they were achieving were no more realistic than the gage tolerances they thought they were getting.
The AMTMA booklet “Searching for Zero” will be of help for gage users and those who provide calibration services to understand the problems encountered in such work and how they can be reduced. Go to www.amtma.com for details. The ASME standard B1.25 released this year can also help to explain why measurements on thread gages between different parties can yield wide variations.
Communication at the technical level between gage makers, calibration sources and gage users is the only way out of these problems which will not go away on their own anytime soon.