The present study introduces a novel tool orientation angle optimization method for improving the machining accuracy of robotic milling systems. The proposed approach considers the intrinsic properties of serial mechanisms and their relationship with robotic stiffness to select optimal robot postures in the generation of tool orientation angle for finish cut. The evaluation of the robotic stiffness is carried out with two performance indices presented in this study: the volumetric stiffness performance index, which measures the overall robot stiffness, and the unidirectional stiffness performance index, which measures the robotic stiffness along a specific direction. As machining errors are reduced by optimally selecting the tool orientation angle without modifications to the tool path itself, the proposed method is significantly less convoluted than conventional optimization methods. The efficacy of the proposed method is validated experimentally using a purpose-designed multi-axis milling robot. Experimental results show that the robotic milling system is capable of machining three-dimensional shapes with a fine surface, and reducing the twist caused by the displacement of the cutting tool towards the direction of lowest robotic stiffness.

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