CCT-based mode coupling chatter avoidance in robotic milling

Currently, large aerospace structures are machined using large multi-axis CNC machining centers. In comparison, milling with a multiple degree-of-freedom (dof) articulated robotic arm has several advantages due to its lower cost and versatility. The low stiffness of an articulated arm robot, however, gives rise to severe low frequency mode coupling chatter during machining. Previous studies have shown that such chatter can be suppressed by minimizing the angle between the average resultant cutting force direction and the direction of maximum principal stiffness of the robot. This approach limits the range of permissible robot motion, and therefore its flexibility of use. This paper presents a new method for avoiding mode coupling chatter in robotic milling using the Conservative Congruence Transformation (CCT) stiffness model, which does not require changing the tool feed direction or the workpiece orientation. Robotic milling experiments show that mode coupling chatter is significantly reduced when using this approach.