Experimental investigation on the effects of torsional-to-longitudinal ratio on the machining performance in ultrasonic grinding of SiCf/SiC composites

SiC f /SiC composites are regarded as one of the most promising materials for modern ultra-high-speed engines. However, machining this material is plagued by issues such as low efficiency, poor surface quality, and severe tool wear. Longitudinal-torsional coupled ultrasonic grinding (LTUG) has been proven to mitigate tool wear and enhance machining quality, but the impact of variations in the abrasive vibration trajectory on machining performance remains unclear, especially under high spindle speeds. To fill the above knowledge gap, this study first elaborates on the concept of the torsional-to-longitudinal (T/L) ratio of the ultrasonic vibration. It clearly distinguishes the meanings of positive and negative T/L ratios and feasible control approaches. Based on this theoretical framework, mechanism exploration and high-speed continuous machining experiments were conducted. In the mechanistic exploration experiments, under low spindle speed conditions, the grinding force obtained with a negative T/L ratio is approximately 25% higher than that with a positive T/L ratio, indicating that the influence of abrasive vibration trajectories on grinding performance cannot be neglected. Moreover, this trajectory-related effect gradually diminishes as the spindle speed increases. In the high-speed continuous grinding experiments, a systematic comparison was conducted among conventional grinding (CG) and ultrasonic grinding with different T/L ratios in terms of grinding force, surface quality, tool wear, and chip adhesion. The results demonstrate that the introduction of longitudinal ultrasonic vibration reduces the grinding force by 44% compared with CG and decreases tool wear by more than 25%. Furthermore, longitudinal-torsional ultrasonic vibration with a positive T/L ratio leads to additional reductions in grinding force and tool wear, thereby further enhancing grinding performance, with more pronounced improvements observed at higher T/L ratios. In addition, by integrating the analysis of chip adhesion rate with that of other key performance indicators, this study elucidates a synergistic mechanism operating at high spindle speeds. Specifically, the combined effects of vibration trajectory variation and tangential acceleration, both of which are dictated by the torsional ultrasonic vibration, jointly dominate the grinding performance. This finding provides a robust theoretical basis for process optimization toward the low-damage machining of SiCf/SiC composites. Additionally, comparative titanium alloy experiments in the appendix demonstrated the differential effects of T/L ratio on brittle and ductile materials, providing guidance for the machining of composites with varying properties.