Vibration Analysis of Robotic Milling Tasks

Conventional material removal systems, like CNC milling machines, have proven to be very effective and accurate. Their major drawbacks are related to machine cost, restricted working area and limitations on the allowed workpiece geometry. In principle, industrial robots (IRs) could be an excellent alternative for machining, being both flexible and cheap (if compared to a CNC machine). On the contrary, IRs do not offer sufficient positioning accuracy and they are prone to vibration onset due to a low capacity of disturbance rejection. These critical aspects currently limit the use of robots in typical machining applications. In this paper, the cutting capability of a serial IR, for light milling operation on aluminum alloy, has been analyzed with respect to the multiple poses allowed by its kinematics redundancy. Firstly, an experimental modal analysis has allowed to identify the variability of the robot static and dynamic compliance over different poses, which influence both tool deflection and vibration onset during milling. The static stiffness of the robot end-effector (EE), which causes tool deflection, has been analyzed by means of an elastic model with lumped parameters. It is shown that the usual static compliance criterion for the optimal pose selection does not guarantee the best performance in terms of vibration level and, thus, surface roughness. This claim has been supported by a set of cutting tests in two significant cases. The results are explained considering the pose-dependent static and the dynamics properties.