Process optimization and machining performance of ultrasonic vibration-assisted milling of hardened internal keyways

Introduction Hardened internal keyways are widely used in aerospace transmission components, where machining quality directly affects assembly accuracy, load-carrying capability, and service reliability. Conventional wire electrical discharge machining (WEDM) can produce recast layers, micro-cracks, and other thermally induced defects, while subsequent manual finishing may compromise dimensional consistency. This study investigates ultrasonic vibration-assisted milling (UVAM) as a non-electrical machining process for precision machining of hardened internal keyways. Methods An axial UVAM system was integrated with a vertical machining center, and a series of experiments were conducted to examine the effects of vibration parameters, cutting parameters, and cooling conditions on machining performance. Single-factor experiments were used to determine suitable parameter ranges, followed by orthogonal optimization and response surface analysis to evaluate the relationships between key process parameters and machining quality. Results A vibration frequency of 30 kHz, vibration amplitude of 2.0 µm, spindle speed of 5000 r/min, feed per tooth of 25 µm/tooth, and oil-mist lubrication provided a favorable combination of surface quality, dimensional accuracy, and tool life. Under these conditions, the machined keyways exhibited a surface roughness of 0.32–0.49 µm, a keyway width of 3.01–3.04 mm, and a twin-slot symmetry error below 0.02 mm. No apparent recast layer or thermally induced surface damage was observed on the machined surfaces. Continuous machining tests showed that abrasive wear was the primary tool wear mechanism, with flank wear increasing gradually from 0.001 to 0.009 mm after six consecutive workpieces. Sensitivity analysis identified feed per tooth and vibration amplitude as the dominant process variables affecting machining quality. A response surface model was developed to predict surface roughness, and the predicted values showed good agreement with the experimental measurements. Discussion The results demonstrate that UVAM enables stable and damage-free machining of hardened internal keyways with high surface quality, consistent dimensional accuracy, and controllable tool wear, providing a practical non-electrical alternative to conventional thermal machining processes for precision transmission components.