OBRABOTKAMETALLOV TECHNOLOGY Vol. 25 No. 1 2023 Fig. 6. Frequency response of harmonic sound waves obtained by milling with an inclination angle of 40° (Formulated by the authors) Fig. 7. Filtered acoustic signal (Formulated by the authors) The frequency spectra considered in Fig. 5 in a detailed analysis, are consistent with the vibration diagnostics signals, however, it is advisable to analyze the acoustic signal within the cutting frequency range. To extract a narrow band of the sound wave (Fig. 6), FFT filter was used, which used a fast Fourier transform (FFT), the FFT size corresponded to a value of 4,096. The frequency ω1 corresponds to the cutting frequency (see Fig. 6), and according to the harmonic law, the resonant frequencies are found as the product of ω1 and an integer (ω1 = 50 Hz, 2ω1 = 100 Hz, etc.). The frequencies are calculated up to the fourth order, since with an increase in the order the intensity of the mode frequency decreases markedly and at higher orders it has practically no effect on the overall sound picture. It can be seen from the records (Fig. 7) that the acoustic signal is modulated by the tool revolutions, and when cutting with a double-edge cutting tool, the signal amplitude changes when the tool inclination angle changes. When studying the spectrum of acoustic diagnostic signals, a uniform phase alternation and the absence of a chaotic regime are established. After denoising, filtering and normalizing the signal amplitude, a limited number of bifurcation points are determined when the tool inclination angle is changed during machining. These facts allow concluding that the method of acoustic monitoring has informative diagnostic features.
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