Investigation on exit burr in robotic rotary ultrasonic drilling of CFRP/aluminum stacks

The stacks of carbon fiber reinforced plastic (CFRP) and aluminum are widely used in aviation industry due to superior mechanical and physical properties. Robotic rotary ultrasonic drilling (RRUD) technology as a promising method is introduced to machining this stack. However, exit burr formation in drilling is a common problem in aircraft assembly operations. It reduces the assembly accuracy and requires additional deburring process. Prediction of exit burr height is the key to obtain the favorable processing conditions for burr reduction. In this study, an investigation on burr formation during RRUD of CFRP/aluminum stacks is conducted and a theoretical burr height model is proposed. Firstly, analysis on thrust force in RRUD is carried out with consideration of certain factors (the mechanical properties of material, uncut chip thickness and tool geometric sizes) and uncertain factors (the stability of RRUD system and cutting temperature). After that, based on this thrust force model and considering workpiece deflection, burr height model is established. Finally, pilot experiments are conducted for the theoretical model verification. Experimental results show that the trends of burr height agree well with the analytical model.