**6. Results and Discussion**

This section presents the results obtained from the characterization of the ground ceramic workpieces and the digital processing of the signals from both sensors. The results of the subsurface assessment by confocal microscopy are shown in Figure 3. The mean surface roughness values (*Ra*) measured with the portable roughness tester are presented in Table 2. Figure 4 presents the time–frequency analysis by STFT for both sensors and the seven cutting conditions. After the selection of frequency bands, the ROP parameter is shown in Figure 5. Figure 6 presents the magnitude coherence between the AE sensor and the PZT diaphragm at three cutting conditions. Finally, Figure 7 shows the correlation between the ROP metric and the surface roughness. All results were correlated with the surface quality of the ground workpieces.

(**a**) (**b**)

**Figure 3.** *Cont.*

**Figure 3.** Confocal microscopy: (**a**) Without cutting; (**b**) 25 μm; (**c**) 35 μm; (**d**) 50 μm; (**e**) 105 μm; (**f**) 150 μm; (**g**) 210 μm; and (**h**) 350 μm.

**Table 2.** Mean surface roughness (Ra) measured with a portable roughness tester.

