Bi-rotary milling head is one of the core components of five-axis machining center and its dynamic characteristics directly affect the machining stability and accuracy. During the five-axis machining, the milling head would change the posture continuously. To facilitate rapid evaluation and optimization of the dynamic behavior of the bi-rotary milling head within the whole workspace, a multi-rigid-body dynamic model considering the flexible joint is established. The varying stiffness of the flexible joints affected by gravity and cutting force at different swing angles is analyzed. A parametric dynamic equation with posture parameters and physical parameters is finally deduced. The sensitive structural parameters on natural frequency and its fluctuation within the workspace are obtained by the sensitivity analysis. The theoretical and experimental results show that the dynamics of the bi-rotary milling head behave pose-dependent and the varying dynamics can be positive controlled by modifying sensitive structural parameters at the design stage.

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