# Development of Dual-Axis MEMS Accelerometers for Machine Tools Vibration Monitoring
## 4 Results and Discussion of the Cutting Experiment  
To verify the performance of the MEMS accelerometers, in addition to calibration methods, a three-axis milling machine was used to perform the cutting experiment for comparing the differences between the MEMS- and PZT-type accelerometers. The cutting tool was a four-flute tungsten steel end mill with diameter of 16 mm. The workpiece was comprised of aluminum alloy 6061. The packaged MEMS and PZT (PCB 353B15) accelerometers were pasted to the spindle. The output signals were recorded in the time and frequency domains by using LABVIEW programs and a data acquisition card (NI 9234) with a sample rate of 20 kHz. The arrangement of the cutting experiment is shown in Figure 8. In the cutting experiment, two conditions were tested: steady cutting, and chatter-generated cutting. Figure 6: The linearity character and cross-axis sensitivity of MEMS accelerometers at 500 Hz. Figure 7: The frequency response of the MEMS accelerometer. The conditions of steady cutting were as follows: rotational speed of 6100 rpm, feed rate of 2400 mm/min, cutting depth of 10 mm and tooth-passing frequency (TPF) of 406.67 Hz. The result of steady cutting is shown in Figure 9. In the time domain, the real cutting processes started with 5.1 s and stopped with 11.2 s. The acceleration of the PZT and MEMS accelerometers were shown in Figure 9a,b, the average amplitudes of acceleration were approximately 8 g. The results of the PZT and MEMS accelerometers in the frequency domain were shown in Figure 9c,d. In order to facilitate observed characteristics, the \(X\)-axes tickets and grids were presented with tooth-pass frequencies. The PZT and MEMS accelerometers produced the same peak values with matching the tooth-pass frequencies. Figure 8: The arrangement of cutting experiment, including the dual-axis MEMS accelerometer, two PZT (PCB 353B15) accelerometers, the data acquisition card (NI 9234) and the Labview programs for signal process. Figure 9: The results of steady cutting process by using PZT and MEMS accelerometers. (**a**) Time domain response of PZT type; (**b**) Time domain response of MEMS type; (**c**) Frequency response of PZT type; (**d**) Frequency response of MEMS type. The conditions of chatter-generated cutting were as followed: rotational speed of 5700 rpm, feed rate of 2280 mm/min, cutting depth of 10 mm and tooth passing frequency of 380 Hz. The result of chatter-generated cutting is shown in Figure 10. In the time domain, the real cutting processes started with 8 s, the chatter-generated with 8.2 s, and finished with 14.3 s. The acceleration of the PZT and MEMS accelerometers was shown in Figure 10a,b. The average amplitudes of acceleration during chatter processes were approximately 19 g, higher than that of the steady cutting. The results of the PZT and MEMS accelerometers in the frequency domain were shown in Figure 10c,d; the \(X\)-axes tickets and grids were also presented with tooth-pass frequencies. In the frequency domain, the MEMS and PZT accelerometers produced the same peak frequencies. Besides tooth-pass frequencies, chatter frequencies were also monitored. Notably, in the process of chatter-generated cutting, chatter frequencies increased as the vibration was raised, not the tooth-pass frequencies. In addition, in this case, the detected major frequency of chatter was approximately 2890 Hz. Table 1 is shown to compare the measurements of the cutting experiment according to time and frequency domain between the PTZ and MEMS accelerometers. As shown in Table 1, the measurement values in the time domain of the MEMS accelerometers are larger than those of PZT accelerometers with a variation of less than 6%. This variation may result from the different attachment location of the spindle. Nonetheless, the peak values in the frequency domain of these two accelerometers are the same. In the above cutting experiment, the performance levels of the MEMS and PZT (PCB 353B15) accelerometers were highly similar in time and frequency response, under steady cutting or chatter.