In recent years, industrial robots have received extensive attention in manufacturing field due to their high flexibility and great workspace. However, the weak stiffness of industrial robots makes it extremely easy to arouse chatter, which affects machining quality inevitably and generates noise pollution in severe cases. Compared with drilling, the chatter mechanism of robotic countersinking is more complex. The external excitation changes with cutting width and depth in countersinking. This characteristic results in time-varying and nonlinearity of robotic countersinking dynamics. Thus, it is urgent to propose a new method of chatter suppression and provide an accurate stability analysis model. As a new special machining technology, rotary ultrasonic machining has been proved to improve robotic drilling and milling stability effectively. Based on this, robotic rotary ultrasonic countersinking (RRUC) is proposed to improve the robotic countersinking stability in this paper. A three-dimensional stability domain method of RRUC is established. First, the countersinking process was divided into ρ parts. The dynamic model of every unit was constructed based on ultrasonic function angle (γ) and dynamic chip area. Then, the stability region of RRUC is obtained based on the semi-discrete method (SDM). Compared with the robotic conventional countersinking (RCC), RRUC improves the stability by 27%. Finally, the correctness and effectiveness of the stability region model are proved by robotic ultrasonic countersinking experiments.

Carbon fiber reinforced plastic (CFRP) and aluminum stacks are widely used in aviation industry due to light weight and high performance. Millions of rivet holes need to be drilled on body materials, and more than 80% of fatigue cracks occur at the connection holes, so the damage and residual stress of hole surface have crucial effect on the riveting fatigue life of CFRP/aluminum stacks and the flight performance. Recently, robotic rotary ultrasonic drilling (RRUD) technology is a promising method to machine the stacks. However, the hole surface strengthening mechanism in RRUD and the service performance of the riveting joint are not verified. Thus, in this paper, the hole surface strengthening mechanism of RRUD for CFRP/aluminum stacks is investigated, a theoretical residual stress model is established, and the fatigue life experiment of riveted joints is conducted. Firstly, analysis on residual stress in RRUD is carried out with consideration of strengthening force and cutting temperature. Residual stress model is established based on the calculation of elastic stress, plastic stress and stress release. Validation experiment results show that ultrasonic vibration changes residual stress from tensile stress to compressive stress. At the same time, comparative damage analysis of CFRP hole exit and hole surface in robotic conventional drilling (RCD) and RRUD is presented. Finally, fatigue strength experiments of riveted joints are conducted for performance verification. Experimental results indicate that fatigue life of single-hole riveted joints is increased by 68% with ultrasonic vibration, and four-hole riveted joint arranged according to aerospace design standards is increased by more than 86%.