Experimental and theoretical investigation on critical cutting force in rotary ultrasonic drilling of brittle materials and composites

Rotary ultrasonic drilling (RUD) has been fully proved to be superior in hole-manufacturing of brittle materials and composites with reduced cutting force and improved machining quality. This paper was devoted to the proposition of a novel index for the design and manufacturing of an ultrasonic machine tool (RUMT), called the critical cutting force, to characterize the RUMT maximum processing capacity. When the cutting force exceeded this critical value, the ultrasonic amplitude decreased and the cutting force increased abruptly, resulting in the RUD superiority suppression. The critical cutting force dependency was investigated theoretically and experimentally. Firstly, the effect of cutting force on the ultrasonic amplitude stability was modeled and subsequently verified by experimental results on both quartz glass and C/SiC composites. It was indicated that the effect of material properties on the amplitude stability could be attributed to the cutting force variation no matter the material is composite or not. Following, a critical cutting force model was developed mechanistically with consideration to the interactive action between the cutting force and the ultrasonic vibration. The modeling results demonstrated that the critical cutting force was an inherent property of RUMT dependent on the corresponding excitation level and independent of the processing conditions. The experimental results on quartz glass, sapphire and C/SiC composites verified the model predictive ability and the critical cutting force independency of the processing conditions such as feedrate, spindle speed, material properties and even tool wear.