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Acoustic Emission Sensing Improves Productivity and Prolongs Grinding Wheel Life By Mark Astor, Contributor I n production of precision metal parts such as automotive or aerospace components, grinding to precise tolerances is of utmost importance. To maintain a high degree of precision throughout a production run, grinding wheel dressing is a necessary operation to ensure that the wheel retains the optimal shape. But grinding wheels, particu- larly the superabrasive ones, can be very expensive, and dressing too much can be wasteful and impact produc- tivity. Also, manual dressing operations may fail to identify emerging structural problems with the wheel. Ideally, a wheel should be dressed only when its shape falls out of spec, and no more material should be removed from the wheel than is absolutely necessary to restore it to its desired shape. Te same need for precision often applies to positioning the grinding wheel against the workpiece. If this opera- tion is not done carefully, too much material could be removed from the production unit, or the wheel itself could be damaged. On the other hand, if the grinding wheel is moved very slowly to avoid this problem, time can :: Figure 1. A high-pressure stream of fuid (blue liquid in this photo) is typically used to cool the grinding wheel during a grinding or wheel dressing operation. An acoustic sensor (visible as the light-colored metal box) attached to the coolant spray nozzle can pick up the sound of the grinding or dressing operation through the fuid. be wasted, and the productivity of the machine will be less than optimal. While grinding machine builders agree about the need for precision and maximum productivity, some people may argue that technology for optimizing the dressing and wheel positioning operations needs further improve- ment. One of the issues that makes monitoring the profle of a grinding wheel during the dressing operation difcult is that the operation typically takes place beneath a high volume stream of cooling fuid (Figure 1). Hence, visual techniques for observing the wheel profle are inefec- tive. In the absence of an easy way to monitor wheel shape, many machine operators run extra dressing cycles (beyond the dressing cycle that would typically do the job) to make sure the desired shape is attained. Sensing Technology For Schmitt Industries of Portland, Oregon, the solution to these problems comes through creative application of a sensing technology that was initially developed for dry grinding applications: acoustic emission (AE) sensing. AE sensing relies on the fact that when a grinding wheel touches a workpiece or a dressing wheel, even the slightest contact produces a sound that can be detected by sensitive instrumentation. Traditionally, AE sensors have relied on vibration transmitted through the frame of the grinding machine, but it turns out that the coolant fow itself can be used to transmit the sound of the grinding wheel and workpiece (or dressing wheel) coming together. In operation, acoustic signals coming from the workpiece travel up the coolant stream to the sensor module, which is otherwise isolated from its mounting attachment to the machine via rubber O-rings that surround it. As long as the coolant stream is uninterrupted, the sensors can pick up even the slightest noise from contact with the grinding wheel. Acoustic emission technology can be so sensitive that it takes only a small number of grains of the wheels coming together to register a touch. By incorporating AE sensors that are supported by automated controls, a CNC machine can automatically perform functions such as grind detection, crash protection, gap elimination and dressing control. TECH BRIEF 28 PRODUCTION MACHINING :: DECEMBER 2014