Studies on the critical cutting force in rotary ultrasonic drilling of brittle materials

Unique chemical and mechanical properties of hard and brittle materials make them viable for various industrial applications. Due to their hardness and toughness properties, brittle and hard materials, however, are most difficult to machine materials through conventional processes. Although various unconventional processes can be used for machining such materials, rotary ultrasonic drilling with utilization of tool’s ultrasonic vibration has been proved as a superior method for machining such materials. Ultrasonic vibration assisted machining is a method that can be applied in different fields that deals with processes involving hard to cut materials.

The use of rotary ultrasonic drilling technique has been effectively applied in hole-manufacturing of hard and brittle materials. It is beneficial for reducing the cutting force required for the process, preventing subsurface damage, increasing tool life and enhancing the quality of the drilled hole. The tools used in the rotary ultrasonic drilling can also be used for other applications apart from drilling such as face milling.

However, due to the availability of different materials with varying degrees of hardness and brittleness, there is need to experimentally and theoretically investigate the validity and effectiveness of rotary ultrasonic machine tools. This will enable proper design and manufacture of such machine tools to favor various areas of applications.

Researchers led by Professor Pingfa Feng from Tsinghua University in China, and in collaboration with their colleague Professor Ping Guo at The Chinese University of Hong Kong conducted a study on machining hard and brittle materials through the rotary ultrasonic drilling technique. Their main aim was developing a model for critical cutting force that guarantees the effectiveness of rotary ultrasonic machine tools. Their work is published in the journal, International Journal of Mechanical Sciences.

From the conducted experiments, the authors observed that an unstable decrease in the ultrasonic amplitude resulted in a corresponding unexpected increase in the cutting force. This leaded to the severe suppression of the processing superiority of rotary ultrasonic machining in terms of cutting force reduction. According to the authors, cutting forces was the chief contributor to the instability of the ultrasonic amplitude. A critical value of cutting force is found. When the cutting force exceeded a critical value, it would increase abruptly. This was observed in both carbon composite and quartz glass materials.

From the theoretical and experimental results, the authors concluded that critical cutting force was an inherent property of rotary ultrasonic machine tools depending on the corresponding excitation level, being independent of the processing conditions. The authors proposed critical cutting force as an ideal index that should be taken in consideration during the design and manufacturing of rotary ultrasonic machine tools.