## 3 Results of Studies of AE Signals
### Results of Studies of AE Signals during Blade Processing
The insert in Figure 9 shows the change in the K\({}_{\text{f}}\) parameter during turning under the same conditions, but with cutters with different wear. It is essential that the wear was assessed by experimental studies of the surface layer after processing the part [53]. It was not the wear of the cutting edge itself that was plotted horizontally, but the deformation intensity indicator of the surface layer \(\epsilon_{\text{int}}\). To form the K\({}_{\text{f}}\) parameter, the ratios of RMS amplitudes in the frequency ranges of 0.75-1.5 kHz and 4.4-15 kHz were used. The choice of frequency ranges for the formation of K\({}_{\text{f}}\) depends on the natural frequencies of the elastic system of the machine and on the location of the accelerometer and should be based on preliminary experiments. There are no strict boundaries for the selected frequency ranges; they should include areas of natural frequencies with the most remarkable change in amplitudes with increasing wear. The K\({}_{\text{f}}\) parameter depends less on  
Figure 7: Change in the intensity of deformation along the depth of the surface layer at different radii of rounding of the cutting edge, obtained using the Deform program (in the color insert—the field of strain rates during chip formation). processing modes if they do not vary too widely (Figure 10). Figure 10a shows the change in K\({}_{\text{f}}\) with changes in tool wear and cutting speed in a wide range during face turning of NiCr20 TiAl steel [54]. Figure 10b shows the difference in the RMS amplitude of the VA signal in the ranges of 0.5-3 kHz and 6-16 kHz and K\({}_{\text{f}}\) when turning a stepped roller with different cutting depths. It follows from Figures 9 and 10 that the K\({}_{\text{f}}\) parameter is quite informative to tool wear, but it significantly depends on the cutting speed. When the cutting speed varies over a wide range, to monitor the condition of the tool at different cutting speeds, it is necessary to have a set of permissible values K\({}_{\text{f}}\)\(\leq\) [K\({}_{\text{f}}\)] for each speed range. Variations in cutting depth and tool feed affect K\({}_{\text{f}}\) significantly less. The situation is simplified, since on modern  
Figure 8: Experimental data on changes in the intensity of deformations along the depth of the surface layer for replaceable plates with different wear along the back edge: 1—h = 0 mm; 2—h = 0.4 mm; 3—h = 0.68 mm; 4—h = 1.1 mm; 5—the cutting edge is destroyed. Figure 9: Octave spectra of the VA signal during turning: 1—no wear; 2—maximum wear. In the insert, the change in K\({}_{\text{f}}\) depends on the intensity of deformation of the surface layer with different wear levels of five cutters. machines, the cutting speed is maintained in the vicinity of a rational value set in advance. In practice, there should not be such ranges of cutting speed changes as in Figure 10a. Another method of eliminating the impact of a variable cutting speed on the K\({}_{\text{f}}\) parameter used as a failure criterion may be the use not of K\({}_{\text{f}}\) itself, but of its relation to the initial value in the absence of wear. Considering the linear form of the dependencies in Figure 10a, this ratio will have an almost constant value over a wide range of cutting speeds. Another example of changing the K\({}_{\text{f}}\) parameter is shown for a flat-grinding machine (Figure 3). Parts made of the heat-resistant alloy Inconel 718 were ground with a grinding wheel (electrocorundum) with a solid bond with grain F90 (grinding speed 1800 m/min, double stroke feed 10 \(\upmu\)m, processing without lubrication). Figure 11 shows the recording of AE signals during 30 passes. To assess the change in the K\({}_{\text{f}}\) parameter, recordings in two frequency ranges, 2-8 and 23-33 kHz, were considered. Figure 12 shows RMS signal amplitudes and K\({}_{\text{f}}\) for the first and 29 th passes. Figure 11: Recording of the AE signal in the frequency range of 2–8 kHz for 30 passes.