Other Dimensions: Wringing Up Your Masters

May 1, 2005
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Making assumptions about dimension can cause problems when it comes to gage calibration.

Most gage calibration work involves the use of masters, the most versatile of which are gage blocks. Aside from a calibration report showing deviations from nominal, most block users ignore other characteristics of the blocks and the skills of those using them. In short, it is assumed that the actual dimension of a stack of blocks is the sum of the errors from that report.

This assumption may be workable for most measurements but when it comes to gage calibration, particularly when readings are close to limits, significant problems may be encountered.

There are two sources that contribute to this problem: human and hardware.

On the human side, few laboratories test personnel to see how well they can repeatedly wring up a stack of blocks as long as the stack stays together when they do it. If the master is not wrung together correctly, the fanciest of instruments is rendered useless. One gage manufacturer I visited had one or two people whose job was to look after several sets of gage blocks and provide build-

ups for the gage makers.

Their skills-finely honed through considerable experience-reduced the variations in wringing blocks together.

NIST has a procedure for wringing blocks to improve consistency, but the time involved is such that most laboratories at the industrial level won't take that time. "Clean them and then wring them" is the process most often used.

When it comes to the hardware-the blocks themselves-there is not much you can do except know what you're working with. Geometry of the blocks is the culprit-flatness and parallelism. Two blocks wrung together in different positions can produce variations because of these problems. The problem is magnified when more blocks are used, which usually is the case.

The best a block user can do is determine the effects these factors will add up to under everyday working conditions. Assuming the blocks are in good condition and free of burrs, and that you have a high-resolution comparator to work with, the following tests can help you understand what you might have to allow for.

Test the human factor under controlled conditions. Four blocks are wrung together to produce a 1-inch or 25-millimeter nominal dimension. The comparator is set using the 1-inch block and the buildup size is checked. Offsets from the calibration report are applied to both the setting and reading to arrive at the difference between the two.

Using a nonpermanent felt-tipped marker, draw a diagonal line down the side of the stack from the top left corner to the lower right corner of the buildup. Break down the buildup and wring it up again using the diagonal line to ensure each block goes back into the new buildup in the same position as the original.

Repeat this step several times with the same person doing the wringing and allow 15 to 20 minutes or more for the stack to cool after wringing. Rest the blocks on a common steel surface so they reach the same temperature. Variations observed when the buildup is measured in the same place each time are an indication of the variations in the wringing ability of the person who did it.

To see the effects, if any, of block geometry, repeat the test with a single technician but put the stack back together in a different way each time. This means you will only see that diagonal line on the side of the buildup once. Variations noticed under these conditions will include contributions by both block geometry and wringing skill.

When the regular staff uses the blocks they work with daily for these tests, you will have an indicator of master reproducibility. If the finest resolution comparator you have shows little or no variations, you would not appear to have anything to worry about, and will have your employee tests and blocks to prove it. You also might find that some people in your facility should not be using gage blocks at all.

In this digital age, metrology skills still make a difference. As these tests show, they will not be improved using adhesives.

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