Other Dimensions: Are You Beating Your Gages to Death?

April 18, 2006
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There are a number of rituals followed in the quality field that drive gage makers crazy. The first one involves gages that are rejecting parts. The ritual: reject the gages. Another one involves gages that appear to wear out too quickly. The ritual: reject the gages.

In both cases, a process of eliminating possible causes might determine that the parts really are rejects in the former case-or that the taps are the wrong size or grade, or the machine set-up has gone wonky. The situation and its causes are relatively easy to sort out. Causes of gage wear are not determined so easily as the gage maker must depend on secondhand or third-hand information.

Assuming that you have a valid concern-meaning that a gage appears to have worn out quicker than it should-here are some of the reasons this may occur:

  • The finish on the gage is not up to a good standard so it will have accelerated wear when first used. Low quality materials or bad heat treating also contribute to accelerated wear.
  • The components being gaged are not cleaned before being checked. Grit on the parts will create scratches on plain gages and score marks on the flanks of thread gages.
  • The material being gaged is highly abrasive such as diecast aluminum, or sand castings have been used and some of the sand remains and acts like a grinding wheel.
  • Poorly made plug and ring gages that use up to half the gage tolerance for roundness-which is allowed-will wear rapidly. On plain gages, shadowy bands along the axis of the gage may show as the high spots or lobes will wear down first, and quickly.
  • Power driven thread gaging systems may have improper clutch settings or no clutch at all. Similarly, automated application of the gage without a floating holder can cause all manner of mayhem. These problems will show up as deformities in the thread form of the gage as it quickly wears out.
  • Last, but not least, gages can show wear because they are forced onto or into parts that should be rejected.

When thread gages are involved, it takes very little wear to have a dramatic effect on their pitch diameter size. About 50 microinches or 1 micron wear per thread flank will affect measured size by nearly four times that amount, ie: 0.0002 inch or 4 microns.

Before blaming gage quality for wear problems, take a good look at them with a magnifying glass. You may answer the "why" question without going further. Dings, chips or material missing from gages indicate abusive treatment-so don't expect them to last long.

Contrary to the beliefs of some, hexagonal handles were not designed for wrenches to be used with thread plug gages, and you can easily see when this has been done. The hole in a taperlock gage handle is not there for use with a tommy bar for more gaging torque. If the ends of the handles of plain gages are burred over, you know someone has been using a hammer as part of the gaging process. Last time I checked, these practices are considered abuse.

Assuming you do have a significant wear problem, there are a number of solutions such as:

  • Buy hard chromed gages and be prepared to accept their higher purchase cost.
  • Alternatively, buy gages made from carbide. Be prepared to pay significantly more and realize that small, long diameters have a high risk of breakage.
  • Talk to your gage maker who may select alternative steels and heat treatment that can make a big difference.
  • Consider using a lubricant made for gaging. It will displace and not interfere with the ‘fit' of the gage in the work.
  • Retrain the gage users, or fire the gage abusers-or do both.

There are no easy answers to the gage wear problem. My experience, and that of many of my colleagues, indicates that gages are usually not the problem. The more information you provide your gage maker, the more chances you will have of obtaining an effective solution.

Whew! I'm all worn out from writing this column. Perhaps a suitable libation is in order. It won't fix being worn out, but then I won't worry about it so much.

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Charles J. Hellier has been active in the technology of nondestructive testing and related quality and inspection fields since 1957. Here he talks with Quality's managing editor, Michelle Bangert, about the importance of training.
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