<p>A pressing scientific and applied problem has been solved: the mechanisms of premature brittle fracture of cubic boron nitride (CBN) cutting inserts during the machining of high-manganese steels (equivalent to ISO GX120Mn13) have been elucidated. A comprehensive approach was applied, combining the vibroacoustic concept of tool condition diagnosis with the methodological foundations of Griffiths’ fracture theory, and based on spectral analysis of tool vibrations and an assessment of spectral component energy using Parseval’s theorem. Laboratory experiments have established that up to 80% of the volumetric damage to the inserts is caused by the energy of high-frequency mechanical vibrations of the cutting edge, which is significantly greater than that from cutting forces. Using SolidWorks Simulation modeling, it has been demonstrated that the critical accumulation of fracture energy according to Griffith’s theory, and consequently the fracture of the tool material, occurs due to the resonance phenomenon formed when the frequencies of forced vibrations caused by cutting forces and the natural frequencies of the tool system approach each other. For the first time, the nature of the critical load has been established as the result of the combined action of the first harmonic and subharmonics of the tool system’s natural frequency. It was found that under certain tuning conditions, a nonlinear increase in the amplitudes of these components is observed, which explains material degradation at stable average cutting forces. The most significant result is the justification for controlling the tool’s service life by changing its geometric stiffness. It has been established that reducing the cutter projection increases the natural frequency by a factor of 9, creating the necessary frequency gap relative to the forced vibrations. This enables minimizing subharmonic amplitudes, eliminating conditions for fatigue microcrack initiation, and increasing the stability of CBN operation under difficult conditions.</p>