A Postprocessing Strategy of a 3-DOF Parallel Tool Head Based on Velocity Control and Coarse Interpolation

It is a trend to develop and use the parallel tool heads with two rotational degrees of freedom and one translational degree of freedom in the aviation industry. Postprocessing is one of the most important components of a numerical control machining process, especially for the parallel tool heads with a closed-loop structure. Due to the nonlinear kinematic transmission characteristics of a parallel tool head, there would exist an obvious velocity fluctuation problem in task space; in addition, the position accuracy of the tool-axis vector cannot be ensured well by the traditional linear interpolation method; these problems should be solved through an appropriate postprocessing strategy. In this paper, by taking a typical 3-DOF parallel tool head as an object of study, a postprocessing strategy based on velocity control and coarse interpolation is investigated. First, the nonlinear kinematic transmission characteristics are analyzed, and the velocity transmission coefficient and coupling velocity transmission coefficient are proposed to evaluate the parameter-varying and coupling velocity transmission characteristics. Next, the velocity control is achieved through feed-rate optimization; meanwhile, a coarse interpolation method is used to ensure the position accuracy of the tool-axis vector. Finally, some experiments are performed, including a model validation experiment, a velocity fluctuation experiment, and a comprehensive cutting experiment. The results show that the velocity transmission model is correct, the velocity fluctuation is almost eliminated, and four typical aircraft structural parts with thin walls are qualified, which prove the effectiveness of the proposed postprocessing strategy.