On the existence of mode-coupling chatter in robotic milling based on chatter type indicators extracted by dynamic mode decomposition

In the stability analysis of robotic milling processes, two primary chatter mechanisms have been identified — regenerative chatter and mode-coupling chatter. They are known to result in undesirable outcomes such as poor surface finish, dimensional errors, and reduced lifespan of tools and machines. Recently, there is increasing disagreement regarding the existence of mode-coupling chatter. Specifically, Mode-coupling chatter has been suspected from the perspective of Stability Lobe Diagrams (SLDs) because of the mismatch between experiments with the predicted SLDs obtained by the mode-coupling mechanism, especially the mismatch between the two in terms of spindle speed dependency on stability boundary. Nonetheless, the opinion has not been fully recognized in robotic milling. Therefore, to investigate the existence of mode-coupling chatter further from the perspective of signal processing, this paper proposes a novel method for determining chatter mechanisms based on two chatter type indicators (CTIs) comprised of the number of true modes (NTM) and the distribution of characteristic multipliers (DCM). Through the simulation using Mathieu equation, the NTM and DCM are obtained by dynamic mode decomposition (DMD) and proved to be effective in distinguishing the response signals of mode-coupling chatter from regenerative chatter. Through robotic milling experiments and Spearman correlation analysis, the mode-coupling chatter is deduced not to happen in robotic milling.