A high-power-density compact giant magnetostrictive rotary ultrasonic machining system for driving large-diameter tools

Rotary ultrasonic-assisted machining (RUM) has been regarded as a superior processing method for difficult-to-machine materials. However, the insufficient driving capability for large-diameter tool in existing RUM systems creates a bottleneck for further improvements in machining efficiency. To address this issue, this study develops a novel rotary ultrasonic machining system for large-diameter tools (LDT-RUM), which enables long-term stable amplitude output while maintaining a compact structural design. The proposed system fully exploits the high energy density of giant magnetostrictive materials (GMM) while overcoming the inherent limitations of low structural stiffness and poor thermal stability in conventional designs. Specifically, the optimized magnetic circuit improves energy transmission efficiency, while the redesigned load-transfer path isolates the GMM from cutting loads and enhances the overall structural stiffness. Furthermore, the dual-vibration-node configuration promotes a more uniform vibration distribution, thereby reducing the detrimental effects of ultrasonic vibration on the machine tool spindle. An air-cooling system is additionally integrated to maintain thermal stability during prolonged ultrasonic operation. Based on the proposed design, a prototype system was fabricated, followed by vibration characteristics tests, thermal stability tests, and machining performance evaluations. The vibration test results demonstrate that the developed LDT-RUM system can effectively drive milling tools with diameters of 14 mm and 20 mm, achieving an ultrasonic driving capability index more than twice that of conventional RUM systems. Thermal stability tests indicate that the LDT-RUM system maintains low-temperature operation and stable amplitude output over extended periods under air-cooling conditions. Machining experiments revealed the existence of an optimal ultrasonic amplitude in ultrasonic-assisted machining. Below the optimal amplitude, the cutting force and surface roughness decreased with increasing amplitude, whereas both increased once the amplitude exceeded the optimal value. At the optimal amplitude, the mean cutting force and surface roughness were reduced by up to 23.15% and 48.79%, respectively, compared to the conventional machining. In addition, the theoretical analysis indicates that a larger tool diameter and a higher spindle speed correspond to a greater optimal ultrasonic amplitude. These findings highlight the critical role of ultrasonic vibration in large-diameter tool machining and demonstrate that the proposed LDT-RUM system effectively extends the application range of RUM.