Carl Stearns 0000-00-00 00:00:00
GAGE BLOCKS A measurement system invented more than a century ago, gage blocks are still the primary standard for manufacturing traceability throughout the world. You might think that gage blocks, a measurement system invented more than a century ago, would have been superseded by one of the numerous electromechanical or vision-based measurement marvels that are all the rage today. Nothing could be further from the truth.What they lack in sophistication, gage blocks make up for with their exceptional stability and precision.That is why gage blocks are the primary standard for manufacturing traceability throughout the world. As the name implies, gage blocks are blocks of metal or ceramic that have been manufactured to precise dimensions so that they can be used individually or in combination as a comparative standard for the measurement or calibration of Manufactured parts, machine tools or measurement tools, including other gage blocks. The sides of gage blocks are exceptionally flat and parallel and they have been ground and lapped to an ultra-high finish that allows multiple blocks to be “wrung” (i.e. combined in a stack).This allows one to build other gage sizes with similar accuracy to that of a single gage block. Gage blocks are available in sets ranging from 9 to more than 90 blocks. Having large numbers of blocks in a set allows for more combinations to reach longer lengths in smaller increments.Individual blocks may be purchased to replace damaged ones.Ultra-hard wear blocks can be used at the ends of a wrung combination to minimize wear. Wear Blocks help protect the other gages in the set and are discarded when they become too worn or damaged. Ceramic blocks are a good choice for metrology labs where temperatures are constant. Steel is often better suited to the shop floor applications because steel shrinks and expands quickly as temperatures change. Since most parts are made of steel, using steel gage blocks makes it more likely that both the part and the standard will be in a comparable thermal state during the measurement process, thus minimizing the potential for gaging error caused by ambient temperature changes. Depending on the grade selected, gage blocks used in industry provide comparative standards that typically range in accuracy from ±o.oo1” to ±0.0001.” These users typically have their lab-grade gage blocks calibrated at 1 to 3 year intervals depending on usage factors. The master gage blocks used by some commercial calibration labs are even more accurate and these are calibrated by NIST using blocks which are routinely evaluated using interferometry, for which the basis of comparison is a wavelength of light. As building blocks that comprise the very foundation of manufacturing traceability, gage blocks deserve both understanding and respect. For understanding, users can rely on the gage block manufacturers who have a wealth of information that they are eager to share. As for respect, the following are some basic care and usage tips that will help you avoid most of the problems that can arise using gage blocks: 1. After every use wipe the gage blocks all over with a chamois or soft cloth, holding the gages in such a manner that the hands and fingers never come in contact with the blocks. 2. If the gages are going to be stored overnight or be taken out of use, wipe them all over using a chamois dampened with rust-preventative oil. Do not touch the blocks during handling or when placing them in their case. 3. Before use, clean gage blocks with mineral spirits, which leaves a very slight oily film for corrosion protection while the blocks are in use.Cleaning with alcohol is acceptable, but may leave the blocks so clean that they may be susceptible to corrosion. 4. Before wringing gage blocks together, check the blocks for nicks and burrs. Do no leave blocks wrung together overnight. 5. Do not handle, clean, use, or wring blocks above an open case. Blocks falling onto other blocks will not only damage the dropped block, but will likely damage other blocks in the case. 6. Be sure there are no nicks or burrs on the equipment or instruments you are you are going to check. These Can damage the gage blocks, and give erroneous readings during the check. 7. In packing gage blocks to ship for calibration, use generous cushioning material, and tape the case closed by wrapping fiber tape around the entire case so that they can not be jarred open to spill and damage their contents in transit. 8. Use “wear” blocks if blocks are going to be placed into situations Where they might become scratched or nicked. 9. Handle gage blocks as you would any other precision instruments in that they are precision instruments.Blocks cannot be tossed around, used as wedges or hammered and then be expected to retain their accuracy. Carl Stearns is Quality Assurance Manager for Webber Gage Division, L.S. Starrett Company. QUICK GUIDE » After every use and for overnight storage, wipe gage blocks with a chamois or soft cloth. » Be sure there are no nicks or burrs on the equipment or instruments you are you are going to check. In packing gage blocks to ship for calibration, use generous cushioning material. » Use “wear” blocks if blocks are going to be placed into situations where they might become scratched or nicked. » Handle gage blocks as you would any other precision instruments in that they are precision instruments.
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