Study on tool wear in longitudinal-torsional composite ultrasonic vibration–assisted drilling of Ti-6Al-4V alloy

Severe tool wear is always considered as a main negative result in the process of drilling Ti-6Al-4V alloy. Although the idea of employing one-dimensional longitudinal ultrasonic vibration in the drilling process to reduce tool wear has been proposed for many years, studies on longitudinal-torsional composite ultrasonic vibration–assisted drilling (LT-UAD) are seldom reported. LT-UAD is a hybrid machining process in which high-frequency and low-amplitude vibrations are superimposed simultaneously on the rotational direction and the axial direction of the drilling bit respectively. In this study, a hollow vibration converter with thread grooves is presented for LT-UAD purpose. Based on an in-depth kinematic analysis, a condition for separating the tool from the workpiece is derived, indicating that a lower feed rate, a lower spindle speed, and a higher amplitude are more advantageous. Machining experiments are performed, wherein good agreement is found between the experimental and theoretical results. The results show that LT-UAD has obvious advantages over longitudinal ultrasonic vibration–assisted drilling and conventional drilling in reducing tool wear. Additionally, the influences of the vibration amplitude, spindle speed, and feed rate on the flank wear width are analyzed, revealing the influences of the three factors in descending order are feed rate, amplitude, and spindle speed. The optimum machining parameters are obtained as 23 μm of amplitude, 385 r/min of spindle speed, and 0.06 mm/r of feed rate. Furthermore, a reliable regression model of flank wear width is established based on response surface methodology. With this prediction model, the flank wear width of drill bit under different machining parameters can be accurately predicted.