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Inspection/Measurement 20 MoldMaking Technology —— FEBRUARY 2018 The relationship of vibration versus time for a normal wear mechanism. This is a sample signal that is generated by a micro electro-mechanical system (MEMS) or a piezoelectric sensor device. data over time, one can track component condition and pre- dict the onset of imminent failure. Figure 1 shows three examples of the vibration-versus-time relationship for a normal wear mechanism. Although it takes time and experience to develop this type of relationship, a well-correlated vibration signature can be a cost-saving alter- native to regular maintenance performed at short cycle times. Using actual vibration observations provides an opportunity to take quick action when warning conditions are detected (red curve), while avoiding premature maintenance on machines that have more life remaining (blue and green curves). A typical application for this tech- nology is placing a sensor on the main bearing casting of a machine tool spindle, since the main bearing is the most common point of failure. The signal that the sensor generates will look something like the idealized trace in Figure 2. It is analyzed using the ISO standard procedure for determining the root mean square (RMS) value of the area under one half of the trace. This provides more useful data than merely measuring the peak-to-peak value and is the procedure used in the algorithms in most monitoring software. Fourier analysis is another stan- dardized procedure used to normalize vibration data. Fourier analysis rep- resents functions as a sum of simpler trigonometric functions. A fast fourier transform (FFT) changes vibration data based on time values to frequency values. It is useful for preventive maintenance applications because wear tends to increase both amplitude and fre- quency, which may not be obvious when comparing time- domained signals. Other Monitoring Technologies Acceleration detection and vibration analysis technologies are the foundation of any practical machine-monitoring application, but they are by no means the only tools available. Anyone machining high-value workpieces on expensive high- precision machine tools ought to seriously consider moving beyond the basics to a more comprehensive suite of technol- ogies. The best place to start is probably with a true power monitoring application that measures the actual energy con- sumption of a motor. Current alone is an unreliable measure of motor performance. A combination of current, voltage and the phase difference between them enables a machinist to accurately calculate true power consumption. The most common application of true power monitoring is on the spindle motor of a machining center. On a large machine, however, this may not be sufficient to provide the necessary protection. Consider, for example, the difference in power consumption between a 4-inch diameter face mill and a 5-millimeter drill on a 50kW spindle. For all practi- cal purposes, the drill will be invisible even to the best true power technology. The answer is to apply true power tech- nology to the axis drive that feeds the drill. That motor will be much smaller, and measuring its power consumption will provide tool condition data and breakage protection on the drill. FIGURE 2 FIGURE 1 Figures courtesy of Marposs.