摘要
数控技术作为制造业的基础技术和关键技术，已经成为衡量国家工业技术水平的重要标志。数控编程技术是数控加工技术的关键技术之一，不但在CAD/CAM系统中发挥着巨大作用，而且在实现加工自动化、提高加工精度和表面质量、提高加工效率、缩短产品研制周期等方面也发挥着重要作用。
为了提高复杂曲面的加工效率，对刀具轨迹规划的相关理论及关键技术进行了系统的研究。针对复杂曲面提出了分阶段加工的方法：第一阶段使用半径较大的刀具进行高效加工，跳过加工过程中的局部干涉区域；第二阶段利用小半径刀具进行笔式精加工。
第一阶段：在分析五轴数控加工原理和特点的基础上，提出了一种基于分形理论中二维Hilbert曲线的轨迹生成算法，能生成权值综合函数值小且连续的刀具轨迹，避免了加工过程的多次抬刀，提高了表面质量。将此算法应用于带有岛屿或凹槽的复杂曲面加工中，能生成连续的刀具轨迹且避免了岛屿或凹槽位置处的抬刀，提高了整体加工效率和表面质量。通过轨迹规划算法实现复杂曲面的分区域加工，对不同区域采用不同的走刀方式，最终生成加工质量相对较高的刀具轨迹。
第二阶段：对于精加工过程中由于选择的刀具半径过大所产生的弓高误差偏大的加工区域，通过笔式加工的方法进行补充加工。笔式加工技术的关键是笔式加工区域的确定和笔式加工轨迹的生成。本文对此进行了系统深入的研究，主要成果有：
针对自由曲面的笔式加工，提出了一种基于圆弧逼近的曲面曲率半径计算算法，能快速地找到第一阶段中的局部干涉位置。首先利用改进的四叉树算法对自由曲面进行分割逼近生成面片网格，从而准确地完成曲面分割。基于圆弧逼曲面的曲率半径计算方法能够快速检测出发生曲面局部干涉的大体位置，再使用二叉树法精确搜索笔式加工区域边界点，最终求出笔式加工区域。整个搜索过程避免了大量的微分计算，提高了搜索效率。由于笔式刀位轨迹为一系列离散数据点的插值曲线，缺乏光顺性，提出了一种三次B样条曲线逼近笔式加工轨迹的优化算法。在满足逼近精度的条件下，提高了笔式加工轨迹的光顺性，减少了笔式加工轨迹的NC代码量。
针对复杂多面体模型的笔式加工，根据相邻曲面片之间的夹角与残留高度之前的关系，提出了一种利用曲面片法矢量之间的夹角关系来判断曲面局部干涉的方法。该方法通过刀具与加工面接触的有效长度逆推求取笔式加工区域，能够快速简洁地确定出多面体模型的笔式加工区域范围。通过偏置法生成刀具加工轨迹，然后运用改进的遗传算法对生成的轨迹进行优化排序，最终生成辅助轨迹长度短的刀具轨迹。
针对刀具轨迹的干涉处理做了系统的研究，将生成的刀具轨迹与笔式加工区域进行求交运算，删除干涉区域内的刀具轨迹线，生成无局部干涉的刀具轨迹。针对全局干涉问题，提出了一种通用性较强的全局干涉检测算法，利用坐标系变换原理进行刀具轨迹干涉检测，通过最小二乘法确立一个最小包络面来调整刀具姿态避免全局干涉，此算法快速简洁。最后基于Vericut软件实现了刀具轨迹的加工仿真实验，验证了本文算法的可行性及正确性。
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.
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