Research On Trochoidal Toolpath Planning And Corner Smooth Optimization For 2.5D Pocket High-speed Machining

With the development of aerospace and mold technology,more and more structural parts with complex pockets and using difficult to cut materials were designed.It was found by research that more than 80% of pocket machining could be performed using 2.5D milling.At the same time,considering the high-speed machining technology has the characteristics of high processing efficiency,low cutting force,low cutting heat,good dynamic characteristic,process intensification and environmental protection,the application of high-speed machining technology to 2.5D milling can effectively improve the efficiency and quality of the pocket machining.In order to give full play to the advantages of high-speed machining technology,advanced toolpaths such as spiral toolpath and trochoidal toolpath have been developed in recent years.Among them,the use of trochoidal milling toolpath for machining pockets with narrow geometries or difficult-to-cut materials has significant advantages.Due to the large number of non-cutting motions and the large fluctuation of the radial cutting width along the toolpath in the existing trochoidal milling toolpath based on circular curve,the cutting force fluctuates greatly and the peak value is high,which seriously restricts the high-speed machining efficiency and tool life.At the same time,the toolpath input to the CNC system after post-processing is a linear path form composed of a large number of tiny straight segments.The tangent discontinuous linear path results in frequent acceleration and deceleration of the tool movement,which also seriously affects the machining efficiency and machining accuracy of high-speed machining.To address the above problems,the dissertion is in-depth research on the high-speed machining process from three aspects: toolpath planning,linear path corner smooth and motion control in order to improve the machining efficiency of difficult-to-cut material pockets.First,a new elliptical trochoidal toolpath based on medial axis is proposed,so that an overall continuous toolpath with reduced path length is obtained.Secondly,a B-spline curve corner smooth algorithm based on seven control points is developed,which generates smoother toolpath and reduce the maximum curvature of the transition spline curve significantly.Finally,a real-time motion control algorithm is compiled to cooperate with the corner smooth algorithm to further improve the processing efficiency.The research contents are summarized as follows:1.A medial axis transformation algorithm is proposed.Based on the analytical expression of the closed B-spline curve,geometric features such as the tangent vector and curvature of the pocket boundary points is calculated.Given any boundary point,the other boundary points are searched to find a local minimum inscribed circle.Through the minimum inscribed circle algorithm,traverse all boundary points and record the collection of information such as the circular center,radius and tangent points of all local minimum inscribed circles,so as to complete the medial axis transformation of abitrary geometric shape pocket.2.An elliptical trochoidal toolpath planning algorithm based on on the medial axis is proposed.Adopted the medial axis information,the circular toolath is converted into an elliptical toolpath with a higher average cutting width.Limited by the maximum radial cutting width of the tool,the maximum cutting width between two adjacent toolpaths is restricted to be close to but less than the maximum cutting width threshold,so as to reduce the number of cutting toolpath and reduce the total length of the cutting toolpath.With the Hermite curve as a non-cutting path,the machining efficiency of the elliptical trochoidal milling toolpath is improved.Experiments show that the proposed toolpath planning algorithm can increase the machining efficiency by 12% without significantly increasing the cutting force on the tool.3.A B-spline corner smooth algorithm based on seven control vertices is proposed.In order to reduce the maximum curvature on the transition curve and improve the efficiency of tool movement,a new B-spline corner smooth strategy is developed in which the transition curve drives inside and outside the corner.By limiting the maximum machining error in the positive and negative directions,and ensuring the length of the straight line,the maximum curvature constraints,the maximum curvature optimization model is established.The influence of distance parameters on the performance of the curvature optimization model is studied.The analytical algorithm of the model is determined to determine the B-spline transition curve of the maximum curvature optimization between any adjacent straight-line segments.Through simulation experiments,it is proved that compared with the traditional Bspline curve corner smooth algorithm,the proposed improved algorithm can effectively reduce the maximum curvature by more than 20%.4.Experimental verification of the combination of corner smooth algorithm and motion control algorithm on the CNC platform.The results of the air running test on the machine tool show that the processing efficiency improvement of the proposed corner smooth algorithm is not obvious.Thus,after completing the motion control module,a comparative experiment of two corner smooth algorithms is carried out on the CNC platform.The results prove that the proposed smooth algorithm combined with the motion control algorithm can effectively increase the machining efficiency by 6%.At the same time,the comparison results of tracking error also show that the proposed smooth algorithm does not affect the machining accuracy.