五轴数控加工复杂曲面时局部干涉的处理与避免.doc
【机械类毕业论文中英文对照文献翻译】五轴数控加工复杂曲面时局部干涉的处理与避免
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机械类毕业论文中英文对照文献翻译
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【机械类毕业论文中英文对照文献翻译】五轴数控加工复杂曲面时局部干涉的处理与避免,机械类毕业论文中英文对照文献翻译,机械类,毕业论文,中英文,对照,文献,翻译,数控,加工,复杂,曲面,时局,干涉,处理,避免
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五轴数控加工复杂曲面时局部干涉的处理与避免摘要：刀具干涉问题是表面雕刻加工面临的最关键问题，本文提出采用五轴数控加工复杂曲面时对干涉进行处理和退避的一种方法 。采用这种方法加工出的表面被划分成凸起区域和非凸起的地区。在凸起的区域没有局部干涉存在，而对于非凸起区域，以对不同的局部干涉进行分析为基础，局部过切干涉可以通过选择最佳的刀具取向而首先被得到解决和避免。后方过切干涉处理和避免的运算法则分别是为了获得简单的光滑表面和复杂形状而被提出的。本文介绍的这种技术能够用来产生无干涉刀具路径。现实结果表明这种方法不仅切实可行而且很可靠。关键词: 环形刀具 五轴数控加工 局部刨削 后方刨削 被雕刻表面1绪论采用五轴数控加工技术进行复杂曲面加工已经广泛应用于航空航天，造船， 汽车工业，玻璃器皿，陶瓷，模子和模子产业。由于比三轴加工机器多了两个旋转自由度，五轴数控加工较之三轴加工更具优势。然而五轴的加工机制也面临了一些问题，比如投资大，刀具干涉处理与避免的运算法则过于复杂等等。刀具干涉问题是复杂曲面加工面临的最关键问题。采用五轴数控加工进行复杂曲面的加工的刀具干涉主要可以分为两类：（1）全局干涉刀具侧表面与被加工表面、加工环境中的机构表面和工作夹具的碰撞干涉。（2）局部干涉本文主要谈的就是局部干涉。局部干涉包括局部过切干涉和后方过切干涉【1、2】如图1所示。有时刀具主切削刃部分延伸到设计表面下面，这将比设计表面轮廓公差去除更多的材料。这就造成了局部过切干涉（图1.a）。当局部表面曲率比刀具半径还小时就会出现局部过切干涉。后方过切干涉是刀具底部切口或者后缘造成的与局部干涉类似的效果（如图1.b、c）。后方过切干涉可能是由采用了大号刀具或者选择了不正确的切削刀定位造成的。刀具干涉的处理与避免是一个很坚韧的问题。很多研究员都研究了干涉问题，但是到多数都集中在三坐标数控加工的干涉处理与避免上。由于刀具运动复杂和复杂曲面曲率分布不规则，使得采用五轴数控加工进行复杂曲面加工时，刀具干涉问题尤其尖锐。Jensen 等倾向于采用平头立铣刀作为正常的复杂曲面刀具接触点，它是基于瞬时掏槽刀侧面与复杂曲面刀具接触点相匹配，来消除局部过切干涉的。Choi等提出了一种通过刀具接触点产生最佳刀具位置的方法，这种方法藉由明确阐述一个基于刀具瞬间切割剖面而被迫减少到最小限度的难题。Li 等人为复杂曲面产生无干涉刀具路径提供了一个有效的运算法则。Lee等已经开发了一种可以消除碰撞和后方过切干涉的运算法则。他们还就预防局部过切干涉这一议题发表演说。Sarema提出了一种在采用平头立铣刀进行五轴数控加工复杂曲面中处理和消除后方过切干涉的新方法。这种方法是通过准确地寻找刀具瞬间切削剖面来使得后方过切干涉能够在两维中实现侦测与消除运算。Rao等人在刀具接触点处接触平面的各个方向对正常的设计表面曲率和刀具剪切平面曲率进行比较。通过对曲率进行广泛匹配，局部过切干涉在采用平头立铣刀进行五轴数控复杂曲面加工中得到处理和消除。这样环形刀具刀具干涉的处理与避免就只需做很少一部分的工作，Lee等人所做的工作就是一个很好的例子。图【1】本文我们为采用平头立铣刀进行五轴数控复杂曲面加工提供了一种系统的研究方法。加工出来的表面被分成了凸起区域和非凸起区域，在凸起区域没有局部干涉存在，对于非凸起区域，刀具干涉根据三个特定情节分成三个阶段被解决。在第一阶段，最佳的剪切方位藉由刀具瞬间切削剖面与加工表面想匹配来决定，尽可能地使它们相互靠近以避免局部过切干涉。在第二个阶段，对于简单光滑表面来说，后方过切干涉的处理与避免可以通过计算加工表面的偏移量与刀具柱面的偏移量的交集来实现。在第三阶段，以复杂形状表面为例，先找出可能出现后方过切干涉的地方，然后将加工表面分成一批三角形的小平面，后方过切干涉的处理可以通过将刀具平面底部和三角形小平面的至高点的相关位置分类来实现。如果刀具下方三角形的某个至高点在刀具底部平面之上，将会产生后方过切干涉。如果后方过切干涉被侦测到，刀具的方位将调整以消除过切干涉。这一过程将持续到所有的联咯数据被核查。刀具描述与刀具定位磨床中运用到寻多类型的刀具。成型铣刀常常被用来复杂曲面加工，用于五轴数控加工复杂曲面的成型铣刀主要有三种类型：平头立铣刀、环型刀具 和 球头铣刀。一种典型的平头立铣刀可以很容易的替代平头立铣刀和球头立铣刀。因此，本文将考虑用环型铣刀、平头立铣刀 和球头立铣刀进行五轴数控表面雕刻作为专门的案例。一把环型铣刀由一个服务于下表面的平面和一个连接圆柱面的圆形环面组成，如图【2】所示。环型铣刀的切削面T (, )能通过两个表面参数联立得到等式1：R是圆形底部的半径，r是刀具的环形半径，是部分环形的角度，且 【0, /2】, 角是X轴的夹角，且 0, 2五轴加工除了可以沿着数控机床的三个平移轴平移外，刀具还可以绕着三个轴中的两个轴旋转。 图【3】如图【3】所示，在刀具接触点建立了局部调整系统（XL-YL-ZL）。XL轴的方向沿着刀具瞬间切削的路径方向，ZL轴的方向则沿着刀具接触点所在表面的法线，而YL轴则是根据XL轴和ZL轴通过右手法则确定。首先刀具以接触点为支点绕着YL轴旋转，第一个旋转角度定义为倾斜角度，然后刀具以接触点为支点绕着ZL轴旋转，第二个旋转角定义为倾斜角，这样刀具的定位就被这两个角度完全限制了。Marciniak提出了一个结论：当刀具接触点沿着曲线的最小曲率法则移动，直到停留在倾斜角 = 0的位置，将会得到最大的加工带宽度（或者最大的材料去除量）。因此我们选择了XL轴作为刀具运动和通过与YL轴的倾斜角旋转的方向，这都是通过以下的旋转矩阵Ry的不断相乘来完成的。这就给出了一个关于刀具绕着YL轴旋转的方程式，如等式（2）Local interference detection and avoidance in five-axis NC machiningof sculptured surfaces Abstract ：The tool interference problem is the most critical problemfaced in sculptured surface machining. This paper presentsa methodology for interference detection and avoidance in five axis NC machining of sculptured surfaces with a filleted-end cutter. The surfaces to be machined are divided into convex and non-convex regions. There is no local interference inside the convex regions. For the non-convex regions, based on the analysis of the different local interference, local gouging is first detected and avoided by determining optimal cutter orientations. Rear gouging detection and avoidance algorithms are then proposed for simple smooth surfaces and complex shaped surfaces, respectively. The techniques presented in this paper can be used to generate interference-free tool paths. The realistic results indicate that the proposed method is feasible and reliable.Keywords ：Filleted-end cutter Five-axis NC machining Local gouging Rear gouging Sculptured surfaces1 IntroductionFive-axis numerically controlled (NC) machining of sculptured surfaces has been widely applied in the aerospace, shipbuilding, automotive, glassware, ceramics, and dies and moulds industries. Because of the two additional degrees of freedom, five-axis NC machining offers some advantages over 3-axis machining. However, five-axis machining suffers from some problems such as a large investment, and complex algorithms for tool interference detection and avoidance, etc. The tool interference problem is the most critical problem faced in sculptured surface machining.Tool interference in five-axis NC machining of sculptured surfaces can be classified into two types: (1) global interference the tool flank surface collides with the machined surfaces and fixtures in the machining environment and (2) local interference. In this paper, the focus is on local interference. Local interference includes local gouging and rear gouging 1, 2 as shown in Fig. 1. Portions of the cutters leading edge sometimes extend below the designed surface, removing more material than is allowed by the designed surface profile tolerance. This leads to local gouging (Fig. 1a). Local gouging occurs when the radius of the local surface curvature is smaller than that of the cutter. Rear gouging is a similar effect caused by the trailing edge or the cutting bottom of the cutter (Fig. 1b,c). Rear gouging may be caused by using a large size cutter or by choosing an improper cutter orientation.The detection and avoidance of tool interference is a tough problem. Many researchers have studied the interference problem, but most concentrate on interference detection and avoidance in three-axis NC machining. In five-axis NC machining of sculptured surfaces, the tool interference problem is much more acute because of the complex tool movements and the irregular curvature distributions of sculptured surfaces. Jensen et al. 3 inclined a flat-end cutter to the normal at the cutter contact (CC) point on a sculptured surface based on matching the curvature of the instantaneous cutting profile of the cutter to that of the sculptured surface at the CC point, to eliminate local gouging. Choi et al. 4 proposed a method for generating optimal cutter location (CL) points from CC points by formulating a constrained minimization problem based on the instantaneous cutting profile of the cutter. Li et al. 5 presented an efficient algorithm for generatinginterference-free tool paths for sculptured surfaces. Lee et al. 68 have developed algorithms to eliminate collisions and rear gouging. They also address issues of local gouging prevention 9. Sarma 1 presented a new method to detect and eliminate rear gouging in the five-axis NC machining of sculptured surfaces with flat-end cutters. The method is based on finding accurately the instantaneous cutting profile of the cutter that enables rear gouging detection and elimination calculations to be done in two dimensions. Rao et al. 10 compared the normal curvatures of the machined surface and the cutter swept surface, in all directions in the tangent plane of the machined surface at the CC point. Based on this comprehensive curvature matching, local gouging in the five-axis machining of sculptured surfaces using flat-and cutters is detected and eliminated. Very little work has been done on tool interference detection and avoidance of the filleted-end cutter. Lee et al.s work 8 is one example. In this paper, we present a systematic methodology for investigating the tool interference in five-axis NC machining of sculptured surfaces using filleted-end cutters. The surfaces to be machined are divided into convex and non-convex regions. There is no local interference inside the convex regions. For the nonconvex regions, the tool interference is solved in three phases according to three scenarios. In phase I, an optimal cutter orientation is first determined by matching the instantaneous cutting profile of the cutter and the machined surface, as close as possible to avoid local gouging. In phase II, rear gouging detection and avoidance is implemented by calculating the intersection between the offset surface of the machined surface and the offset cylinder of the cutter, for simple smooth surfaces. In phase III, for cases of complex shaped surfaces, a search for possible rear gouging is conducted by first dividing the machined surfaces intoa set of triangular facets. Rear gouging is then detected by classifying the relative position of the bottom plane of the cutter and the vertices of the triangular facets. If one of the vertices of the triangles under the cutter shadow is above the bottom plane of thecutter, rear gouging will occur. If any rear gouging is detected, the cutter orientation is adjusted to eliminate gouging. The process continues until all the CC data are checked.2 Cutter description and cutter orientationThere are many types of cutters used in milling applications. An end-mill cutter is often the choice for sculptured surface machining. Basically three types of end-mill cutters are used in the 5-axis NC machining of sculptured surfaces: the flat-end cutter, the fillet-end cutter and the ball-end cutter. A model of a filletend cutter can easily represent both the flat-end cutter and the ball-end cutter. For this reason, this paper will consider 5-axis sculptured surface machining with a fillet-end cutter, and with the flat-end cutter and the ball-end cutter as special cases. A filleted-end cutter consists of a plane that serves as the bottom surface, and a piece of a torus connected to a cylinder, as shown in Fig. 2. The cutting surface T (, ) of a filleted-end cutter can be represented by a two parameter surface with continuous derivatives as in Eq. 1:where R is the radius of the bottom portion, r is the fillet radius of the cutter, describes the corner portion and 【0, /2】, and is the angle from the Xt -axis and 0, 2. In 5-axis machining, besides the translations along the three translation axes of an NC machine, the cutter can be rotated about two of the three translation axes. As shown in Fig. 3, the local coordinate system (XL YL ZL ) is set up at the CCpoint. The XL -axis is along the tangent of th
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