PFC Simulation and Experimental Investigation of Crack Evolution and Surface Morphology in Rehbinder-Effect-Assisted Longitudinal–Torsional Ultrasonic Vibration Milling of Borosilicate Glass

Borosilicate glass is widely used in microfluidic chips, optical components, and precision devices, but it is susceptible to cracking and subsurface damage during machining because of its high hardness and brittleness. Longitudinal–torsional ultrasonic vibration-assisted milling (LTUVAM) can improve material removal stability, whereas the Rehbinder effect can reduce surface energy and weaken local material resistance. In this study, the additional effect of a Rehbinder-active medium under LTUVAM conditions was investigated to reduce crack-related surface damage and improve surface integrity. First, a two-dimensional cutting model of borosilicate glass was established using Particle Flow Code in two dimensions (PFC2D) to simulate crack initiation and propagation under conventional milling (CM) and LTUVAM conditions. The simulation results show that ultrasonic vibration promoted finer and more distributed microcracks and reduced the development of large-scale fractures, thereby modifying the brittle material removal behavior. Subsequently, milling experiments were conducted under LTUVAM conditions with a Rehbinder-active medium supplied to the cutting zone. Compared with LTUVAM without the solution, the proposed process significantly reduced surface roughness, with maximum reductions of 40.7% in Sa and 32.3% in Sq at a cutting depth of 50 μm. Finally, the simulated force variation and crack evolution were qualitatively compared with the experimental roughness trends and reported results, showing consistent mechanism-level trends. This work provides a theoretical and experimental basis for improving the machining quality of hard and brittle materials through the coupling of ultrasonic vibration and surface-active effects.