Research On Algorithms Of Tool Path Generation Of Complex Surface Machining Based On The Hilbert Curve In Fractal Theory

NC technology, a fundamental and necessary part in manufacturing industry, determines thenational industrial level. NC programming, as one of the core technologies, plays an important role inCAD/CAM, especially in machining automation, and processing efficiency, improvement of themachining precision and quality, shortening of product-develop cycles.To improve the machining efficiency of complex surface, the key technologies in tool-pathplanning of finishing machining and pencil-cutting machining of sculptured surface are studiedsystematically in this dissertation. The research works is divided into two stages——one is efficientsurface machining using the larger cutter while skiping the local interference, and another ispencil-cutting machining as supplement using smaller cutter.Firstly, a method of tool path generation is proposed to complete the curve in one process based onthe analysis of5-axis NC machining principle and the Hilbert curve of fractal theory. Thishigh-quality, consistent tool path speeds up the entire manufacturing process while ensuring theminimum value of weight synthetical function, preventing frequent cutter lifting. Being applied tomachine complex surfaces with islands and grooves, the proposed algorithm can improve themachining efficiency and surface quality by generating continuous cutting-tool trajectory andavoiding cutter lifting in the position of islands and grooves. In order to improve the cuttingperformance of NC machining, the algorithm is applied to complex surfaces machining. The differentregions are machined separately with different tool path pattern generation methods, and finally thetool path with good cutting performance is generated.Secondly, regions with large chord errors caused by choosing a large cutter radius during finishmachining process are processed by pencil-cutting machining method as supplement. The keytechnologies of pencil-cutting machining are the identification of machining region and the generationof tool path. The pencil-cutting machining technologies are systematically studied.An approach for free-form pencil-cutting machining based on arc approximation and quad treemethod is proposed. The improved quad tree method can subdivide and approximate free-formsurface by grids, which satisfies the shape deviation accuracy. The arc radius was used toapproximately compute the curvature radius to judge the regional interference and to identify thepencil-cutting regions boundary exactly by binary tree. A lot of differential calculation is reduced andthe search efficiency is improved. The pencil-cutting path is the interpolation curve composed by a series of discrete points, so the tool path is unsmooth. An optimization algorithm based on cubicB-spline curve approximation pencil-cutting path is proposed, the smoothness of the pencil-cuttingtool path is improved within the tolerance. The amount of NC code of the pencil-cutting path isreduced.In view of machining complex polyhedral models, since the scallop height is different according tothe angle of adjacent surface patches, a new method to estimate interference condition using the angleof norm vector of adjacent surface patches is proposed. The pencil-cutting region is calculatedaccording to the effective contact length between the cutter and the machined surface by inverseinference. The pencil-cutting area of the complex polyhedral model can be determined rapidly andaccurately. Then the tool path is generated by offsetting method, and optimized and sorted byimproved genetic algorithm. Finally, the shorter auxiliary path is generated.The key technologies for tool path optimization are systematically studied. The points ofintersection were calculated between the tool path and pencil-cutting regions avoiding localinterference and ignoring the tool path in the interference regions. Then the coordinate systemtransformation theory is applied to global interference detection and the least square method is alsoused to determine a minimum envelope for adjustment of the position of the cutter to avoid newinterference. The implementation has proved that the proposed method has a high efficiency. Thesimulation of the tool path processing is carried out in Vericut. The results demonstrate the feasibilityand the validity of the algorithms.