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This study aims to calibrate the posture of a robot-type machine tool comprising parallel and serial links using a kinematics error model and verify the machining performance based on the measurement results of a machined workpiece calibrated with kinematics parameters. A robot-type machine tool (XMINI, Exechon Enterprises LLC) is used in this study. Typically, the performance required of a robot-type machine tool is not only dimensional accuracy but also the contour accuracy of the machined workpiece. Therefore, in this study, we first construct a forward kinematics model of a robot-type machine tool and identify the kinematics parameters used in it via spatial positioning experiments using a coordinate measuring machine. Based on the parameter identification results, we calibrate this robot-type machine tool and evaluate its machining performance in terms of the dimensional accuracy and contour accuracy of the machined workpiece.

Robot-type machine tools are characterized by the ability to change the tool posture and machine itself with a wider motion range than conventional machine tools. The motion of the robot machine tool is realized by simultaneous multi-axis control of link mechanisms. However, when the robot machine tool performs a general milling process, some problems that affect the machining accuracy occur. Moreover, it is difficult to identify the motion errors of each axis, which influence machining accuracy. Thus, it is difficult to adjust the servo gain and alignment error. In addition, the machining performance is unidentified because of the rigidity differences when the posture changes. In this study, the focus was on robot-type machine tools consisting of a serial and a parallel link mechanism. A geometric model is described, and the forward kinematics model is derived based on the geometric model. Machining tests were then carried out to evaluate the machining accuracy by measuring the machined surfaces and the simulated motion of the tool posture based on the proposed forward kinematics model to identify the mechanism that affects the machined surface roughness and surface waviness. As a result, it was shown that the proposed model can separate and reproduce the behavior of each axis of the machine. Finally, it was clarified that the behavior of the second axis has a great influence on the tool posture and machined surface.

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