Proper Care of Handheld Measuring Tools 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. 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 specifi ed 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 fi nal 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. EVERYDAY CARE • Misuse. The leading cause of damage to hand measuring tools is misuse. Though they may look simple, these tools are precision instruments capable of delivering high accuracy: typical dial and digital calipers can measure to a resolution of 0.0005 inch. Calipers are prone to misuse; they usually posses ID jaws that come to very sharp points—thought by some to be ideal for ripping open boxes. This type of misuse can put burrs on the jaws, maybe too small to see, yet capable of throwing jaws out of stated accuracy. Micrometers are commonly misused in operation. Many micrometers include a ratchet-stop feature to assist in making proper workpiece contact by “slipping” to stop closure of the micrometer faces once proper contact is made. Nevertheless, some users vigorously torque the barrel even after the ratchet starts clicking; this may cause the spindle face to degrade as it is spun or “ground” into the workpiece. As a result, parallelism between spindle and anvil faces may be aversely affected, thus taking the micrometer out of stated accuracy. The most common mishap to affect any precision measuring tool’s integrity is dropping. Any tool dropped to the fl oor—or shocked against hard machine tool or work surfaces—should be re-calibrated before it is used again. • Preventive care/storage. Preventive care begins with an assessment of the working environment. If measurement tools are used in a harsh environment, for example, with coolant, mist, metal chips and debris, preventive care can be as simple as wiping tools clean before storage. Wiping down with Kimwipes impregnated tissues or with a lintfree tissue and denatured alcohol will remove whatever material may be deposited on the tool. This will prevent deposits from solidifying during storage so as to not inhibit free movement at next use. Also, calipers and micrometers should be stored with jaw faces, spindles and anvils open, gapped slightly and not touching. This will prevent distortion that can result from the following phenomena: • Any increase in temperature will cause tools to expand, putting pressure on the touching surfaces. • Even with no rise in temperature, the slight pressure of continual contact itself can cause distortion. It is important to note that to prevent corrosion, measurement tools should be stored at or near room temperature and at relatively low humidity. Gages with electronic display windows should be kept away from direct sunlight that can cause windows to fog. Also, monitoring the charge of batteries used in electronic measuring tools will prevent battery compartment corrosion that can be caused by spent batteries. In typical applications, battery life may be about two years. Removing batteries for extended tool storage— more than a year—is recommended. Even with proper care, normal usage can cause wear, misalignment of moving parts and changes to pre-load tensions such as found in dial caliper gear trains. Because of these factors, periodic calibration of tools is prudent even when no wear is evident. CALIBRATION Calibration 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 fulfi llment of a calibration schedule that may be specifi ed by a company’s own standards and/or as stipulated by external standards such as ISO. GAGE BLOCKS In-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 halfa- dozen to more than a hundred. Different size blocks are stacked together to create the exact dimension required. When stacked, the blocks’ optically fl at surfaces are rubbed or “wrung” together to eliminate any foreign matter, even air, which may lie between them. As a result of their extreme fl atness, 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 fi ngers. 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 fi ngers; 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.
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