Dynamic Model of Ultrasonic Milling for Aluminum Alloy with Unequal Helix Angle Milling Cutter and Chatter Suppression

With the global trend towards lightweight new energy vehicles, the processing demand for weak-rigidity components such as vehicle frames has been increasing. Milling, as an indispensable method for machining such thin-walled structural parts, faces a major bottleneck: machining deformation caused by chatter during processing has become a significant challenge. Unequal-helix-angle milling cutters and ultrasonic-assisted machining have been proven effective in suppressing chatter. However, research on the milling dynamics model and chatter suppression based on ultrasonic-assisted unequal-helix-angle milling cutters remains incomplete. Based on this, this study established a dynamic model for milling aluminum alloy 6061 using an ultrasonic unequal-helix-angle milling cutter. Firstly, based on the instantaneous milling force model, the time-varying characteristics of cutting thickness caused by the tool's motion trajectory under ultrasonic vibration and the variation in cutting time difference between adjacent cutting edges due to the unequal-helix-angle cutter were considered. Furthermore, both the workpiece and the tool for machining thin-walled parts were regarded as flexible bodies. Combined with the dynamic response of the machine tool system, a modal parameter-based milling dynamics model was established. Based on the closed-loop control system of milling dynamics, the stability of the milling system was determined by solving eigenvalues, and a solution for calculating the limiting cutting depth and spindle speed was obtained. Modal parameters were acquired through modal hammering experiments. Numerical analysis of the lobe diagram shows that the limiting cutting depth increases with rising spindle speed and ultrasonic frequency. At different spindle speeds, the stability of the lobe diagram for unequal-helix-angle ultrasonic milling is significantly higher than that of both conventional milling and ultrasonic milling. Compared with conventional milling, the limiting cutting depth for unequal-helix-angle ultrasonic milling is 3.2 mm, representing a 113% improvement. Finally, in milling experiments, the stability of the milling process was analyzed from both time-domain and frequency-domain perspectives by performing a fast Fourier transform on the force time-domain signal. This verifies that the coupling effect of ultrasonic vibration and unequal-helix-angle milling cutters can effectively suppress chatter when milling thin-walled workpieces.