【作者】 刘威；

【导师】 周来水；

【作者基本信息】 南京航空航天大学 ， 航空宇航制造工程， 2013， 博士

【摘要】 逆向工程中，对点云模型生成数控加工刀轨，通常要先构造曲面或者网格模型，再对曲面或网格生成加工刀轨。但构造曲面或网格模型是一个复杂、费时的过程，直接对点云生成数控加工刀轨，可以省去重构的过程，大幅缩短数控加工刀轨的计算时间，因此该方法已经成为国际上的研究热点之一。论文回顾了国内外点云刀轨生成技术的研究成果，并对点云模型直接生成加工刀轨的关键技术进行了深入研究，提出了点云模型的层切法粗加工刀轨生成算法、等残留高度法精加工刀轨生成算法、截面线等误差法精加工刀轨生成算法和点云模型局部修改后的刀轨重新生成算法。论文的主要研究成果与创新点概括如下：提出了一种点云模型的层切法粗加工刀轨生成算法。算法采用分层切削法规划刀轨，为了高效地求出点云刀位面，将点云划分到立方体栅格中，利用反置刀具法计算出由栅格组成的刀位面，以此规划平底铣刀在切削层上的等间距行切刀轨。给出了一种根据刀轨之间的投影和距离连接刀轨的方法，该方法无需获取加工区域边界和刀轨之间关系，只需计算刀轨端点切削区域是否相连，在相连处直接连接刀轨端点，形成连续的刀轨，使平底铣刀在抬刀前走过所有可走的刀轨。为了减少平底铣刀切削后的台阶状余料，规划了球头刀行切刀轨并给出了刀轨优化的方法。提出了一种点云模型的等残留高度法精加工刀轨生成算法。算法对球头刀生成三坐标加工刀轨，首先对当前行的刀位点建立局部坐标系，计算刀具投影范围内的点云与刀位点切矢平面的交点点集，以点集中每个点为圆心、残留高度值为半径创建圆，等价地表示残留高度面，与切矢平面上的刀具圆求交，具有坐标极值的交点即为要求的残留高度点；再对残留高度点建立局部坐标系，计算出位于残留高度点的刀具圆上无干涉的刀位点，即相邻行刀位点。依此类推，连接刀位点形成刀轨。该算法不需要将点云转化为曲面，也不需要计算点云的等距面，解决了一直以来因离散点云难以生成等距面而无法对其直接生成等残留高度刀轨的问题。提出了一种点云模型的截面线等误差法精加工刀轨生成算法。算法对截面线法获得的刀具路径上密集的刀位点逐点建立以弓高误差为半径的圆，以首个刀位点作为首个等误差刀位点，计算到后续刀位点弓高误差圆切线之间区域的交集，直至交集为空，获取最小非空交集区域，在此交集区域内规划的刀轨与遍历过的刀位点弓高误差圆相交或相切，满足了误差要求。以此区域边界与遍历过的刀位点连成的折线求交，距离该等误差刀位点最远的交点即为下一个等误差刀位点。依此类推，可求出所有等误差刀位点。算法解决了现有算法无法对刀位点点集计算等误差刀位点的问题。为了提高刀轨光顺性，给出了圆弧插值算法，可对等误差刀位点生成了G1连续的圆弧刀轨。提出了一种点云模型局部修改后的刀轨重新生成算法。算法将点云划分到立方体栅格中，对比修改前后的点云，运用栅格表示修改区域，只识别出修改区域内外受修改影响的刀轨并重新生成，继续沿用未修改区域的刀轨，从而最大限度地重用已有成熟刀轨。算法先识别出受修改影响的粗加工刀轨，运用提出的粗加工刀轨生成算法重新计算，再识别出精加工刀轨，逐行重新计算刀位点的坐标，通过新增刀位点使其满足步长方向上的误差，然后计算新生成的刀位点与邻行刀轨的残留高度，通过新增若干行刀轨使其满足残留高度要求。论文研究提出的方法和算法均已在OpenCASCADE几何平台和Visual C++6.0平台下实现，并通过例子进行了验证。更多还原

【Abstract】 In Reverse Engineering, the method of point clouds NC tool path generation usually fits pointclouds to surface or triangular mesh surface, and generates tool path based surface or triangular meshsurface. However, surface fitting is a complex, time-consuming procedure. Directly generating toolpath based on point clouds avoids surface fitting and greatly shortens the NC tool path computingtime. So it has been one of the popular focuses of international research.After reviewing the domestic and foreign research achievement on point clouds tool pathgeneration, this paper conducts in-depth research into the technologies of point clouds tool pathgeneration, and proposes new algorithms about roughing tool path generation based layer cuttingmethod, constant scallop-height finishing tool path generation, equal-error finishing tool pathgeneration with curves of cross sections and tool path regeneration for point clouds designmodification. The main research results and innovation are summarized as follows.A new algorithm of directly generating roughing tool path based on stratifying method for pointclouds is proposed. Layered milling is employed to plan roughing tool path. Firstly, the point cloud isdivided into3D cell grids. Inverse tool offset method (ITO) is used to compute CL(Cutter-Location)point enveloping surface formed by cell grids, and flat-end cutter direction-parallel tool path in eachcutting layer is obtained. Based on the projection and the distance between tool paths, a new tool pathlinking algorithm is proposed to reduce the number of tool retraction motions. In the algorithm, themachining region boundary and the relationship between tool paths are not needed. When the cut areaof a tool path’s CL point connects with current tool path, their CL points will be linked directly. Toreduce the remaining step material of flat-end cutter machining, roughing tool path generation usingball-end cutter is proposed and an algorithm of optimize tool path is given.A new algorithm of directly generating constant scallop-height finishing tool path for pointclouds is proposed. The algorithm generates three-coordinate NC tool path for ball-end cutter. A localcoordinate system centered at each CL point of the current path is built to calculate the correspondingscallop point. Intersection points of CL point tangent plane and data points located in tool projectionarea are obtained. Circles centered at intersection points are created with tool radius to substitutescallop-height surface. The circles intersect the tool circle centered at CL point, and the intersectionpoint with the maximum coordinate is the wanted scallop point. A similar local coordinate system is built at each scallop point to calculate the wanted CL point of the next tool path. The interference-freeCL point located on the tool circle centered at scallop point is calculated, which is the wanted CLpoint. Point clouds (especially discrete point clouds) are hard to obtain offset point clouds so that it ishard to generate constant scallop-height tool path for point clouds. However, in this algorithm, offsetpoint clouds are not needed and the problem is solved.A new algorithm of generating equal-error finishing tool path generation with curves of crosssections is proposed. For dense CL points on tool path, circles centered at CL points are created withmaximum allowed step error value. The first CL point is obtained as the first equal-error CL point.Tangent lines from the equal-error CL point to the circles are obtained to compute the intersection ofthe areas between tangent lines. When the intersection area is empty, the minimum non-emptyintersection area can be calculated. Tool path computed in the non-empty intersection area is tangentto or intersected with the traversed CL points’ step error circles, which meets step error requirement.Of all intersection points of the intersection area’s boundaries and lines of CL points, the point farthestaway from the equal-error CL point is the wanted next equal-error CL point. Similarly, all equal-errorCL points can be calculated. To improve tool path fairness, an arc spline tool path interpolationalgorithm is proposed to generate G1arc tool path.A new algorithm of tool path regeneration for point cloud design modification is proposed. Thepoint cloud is divided into3D cell grids. The modified region is expressed by cell grids. Tool path outof the modified region is retained. Only CL points in the modified region are identified andrecalculated, which reuses existing mature NC tool paths as many as possible. Firstly, roughing toolpath is identified and recalculated by proposed roughing tool path generation algorithm. Finishing toolpath is identified and line-by-line recalculated, and new CL points are added to meet tolerancerequirement. The scallop height values for the modified region are calculated to judge if it is greaterthan the allowable value. New lines of CL points are added to maintain the required surface finish.The methods and algorithms proposed in this paper have been implemented in theOpenCASCADE and Visual C++6.0platforms. And practical examples are given to show thevalidity and effectiveness.更多还原