Study on dynamic drilling forces of CFRP/Ti stacks during longitudinal-torsional ultrasonic vibration drilling

LTUV (longitudinal-torsional ultrasonic vibration) makes cutting speed change rapidly; it is obvious that the rapid change makes the cutting edge become sharp and drilling force macroscopically reduce. Most experiment study has indicated that comparing with conventional drilling CFRP/Ti-alloy stacks, the hole quality of LTUV drilling is superior. For in-depth theoretical study of drilling forces of LTUV drilling, the drill bit is regarded as a continuous elastic body under the action of foundation vibration in this paper, its dynamic model is established, and the dynamic component prediction models of both thrust force and torque are derived. The reliability of the prediction model is verified by drilling experiment. This research shows that the ultrasonic vibration of the drill bit is the main factor to improve the cutting condition. The magnitude of both dynamic component of thrust force produced by longitudinal ultrasonic vibration and the torque dynamic component generated by the torsional vibration mainly depends on the vibration parameters (amplitude and frequency) and the drill bit geometric parameters. The drilling parameters (including spindle rotation speed and feed rate) nearly have no effect. When the drilling parameters are certain, the equivalent viscous damping coefficient is related to the geometric parameters of the drill bit, vibration parameters, tool materials, and processed materials. For different materials, the equivalent viscous damping coefficient is different. To conduct a small number of experiments, the damping coefficient of one drilling system can be calculated by measuring the difference of thrust force or torque between the LTUV drilling and the conventional drilling under the same drilling parameters. This work provides a reference for predicting the thrust force and torque of the LTUV drilling, rationally choosing the tool and determining the drilling parameters.