Chatter stability in robotic milling

Industrial robots are desired to be used in milling light but large aerospace parts due to easier set-up and portability than large machine tools. However, robots are significantly less stiff than the machine tools, hence they cannot be used in all machining applications. This paper presents dynamics of robotic milling. The structural dynamics of an articulated manipulator with a spindle and a tool are modeled. The dynamic milling forces are applied on the robot structure which has strong cross coupling terms. The stability of the resulting system is analyzed using semi-discrete time and frequency domain methods. The predicted stability charts are experimentally validated in milling of Aluminum and Titanium parts. It is shown that the pose-dependent modes of the robot structure are all at low frequencies, and they are damped out by the machining process at high spindle speeds. Only the pose independent spindle modes cause chatter in high-speed milling, hence high material removal rates can be achieved by selecting analytically predicted stable depth of cuts and spindle speeds in robotic milling of Aluminum parts. In low speed milling of Titanium parts however, the pose dependent low frequency robot modes chatter.