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The nature of quality control measurement is continually changing in response to developments in coordinate measuring machines (CMMs) and other technology-laden metrology instruments. Nevertheless, the precision and repeatability of handheld dimensional measuring tools-calipers, micrometers and gages-are still heavily relied on throughout most of manufacturing. And as tolerances of manufactured parts become ever tighter, it is even more important that the accuracy of handheld measuring tools be maintained, requiring the tools themselves to be cared for properly.
There are two main categories of maintenance for hand measuring tools. The first is in response to everyday use and handling. This assumes that the correct tool is selected in the first place, for example, making sure the IP or Ingress Protection Rating is suitable.
The second type of maintenance is specified by formal, periodic and documented inspection and calibration routines. Calibration is most commonly performed in-house, but many quality programs specify additional calibration at accredited labs. These labs provide calibration traceable to final standards such as NIST (National Institute of Standards and Technology). Related to calibration is use of gage blocks-extremely precise artifacts with a care regimen all their own.
CalibrationCalibration establishes the relationship between the measured value indicated by a measuring tool and the corresponding value for that same measurement as set forth by accepted standards. The results of calibration permit adjustment of the measurement tool so that it performs within a desired limit of accuracy.
In-house calibration may be performed on newly purchased tools, or as a result of re-adjustment needed for tools that may have been dropped or otherwise shocked, or in fulfillment of a calibration schedule that may be specified by a company’s own standards and/or as stipulated by external standards such as ISO.
Gage blocksIn-house calibration usually employs gage blocks, precisely manufactured of steel or ceramic, with dimensional tolerances that fall within known standards. A tool being calibrated is used to measure a gage block then the obtained value is compared to the known value of the block.
Gage blocks come in a variety of sizes and in sets ranging from half-a-dozen to more than a hundred. Different size blocks are stacked together to create the exact dimension required. When stacked, the blocks’ optically flat surfaces are rubbed or “wrung” together to eliminate any foreign matter, even air, which may lie between them.
As a result of their extreme flatness, wrung gage blocks can get so close together that the molecules from one surface interact with molecules from the other, essentially welding the blocks together. Rarely, gage blocks left wrung together for extended periods are impossible to separate.
Care issues for gage blocks include wear, burring and corrosion. Wear is caused by foreign matter contamination; some is airborne but most is deposited from the operator’s hands in combination with oil from the fingers. Wearing white gloves counters this problem.
Burring and scratches are caused when an edge from one block being wrung slips over and cuts into the other block. Care should be taken to avoid burring; damaged blocks should be removed and replaced as soon as possible.
Corrosion can easily result from oil deposited from fingers; again, use of white gloves provides the solution. Additionally, corrosion can be prevented by ensuring that gage blocks are always stored in their cases-at room temperature and humidity-and never stored wrung together.
The correct care and maintenance of handheld measuring tools is well worth the effort. Properly cared for tools are more accurate, easier to use and offer extended service life-all factors that impact quality, productivity and the bottom line.