A Reverse Engineering Approach to Generating Interference-Free Tool Paths in Three-Axis Machining from Scanned Data of Physical Models

An NC tool path is usually generated by sweeping parametric surfaces of a CAD model. In modern design, freeform or sculptured surfaces are increasingly popular for representing complex geometry for aesthetic or functional purposes. Traditionally, a prototype is realised by machining the workpiece using the NC codes generated from a CAD model. The machined part can then be compared with the CAD model by measurement using a coordinate measuring machine. In this paper, a reverse engineering approach to generating interference-free tool paths in three-axis machining from the scanned data from physical models is presented. There are two steps in this procedure. First, a physical model is scanned by a 3D digitiser, and multiple data sets of the complex model are obtained. A surface registration algorithm is proposed to align and integrate those data to construct a complete 3D data set. A shortest-distance method is used to determine the connecting sequence of the neighbouring points between two adjacent scan lines, such that the scanned data are converted into triangular polygons. Tool paths are then generated from the tessellated surfaces. Using the Z-map method, interference-free cutter-location data are calculated, relative to the vertex, edges, and planes of the triangles. The algorithms for tool-path generation are usually different for cutters of different geometries. Some algorithms found in the literature require complex numerical calculations and are time consuming. In this paper, an efficient algorithm is developed to calculate interference-free cutter-location data by easy geometric reasoning without complex computation. This robust method is suitable for generally used cutters such as ball, flat, and filleted end mills, and the time taken to obtain full tool paths of compound surfaces is short. Some real applications are presented to validate the proposed approach.     

从测量的散乱数据点云直接生成数控刀具路径的研究

提出了一种直接从测量的散乱数据点云用球头刀对自由曲面进行三轴数控加工时生成刀具路径的方法。不同于现有散乱点云基于逆向工程的刀具路径生成方法,本法考虑并估计了曲面加工误差和粗糙度。将散乱数据点云向XY平面投影,以获得的投影边界为刀具路径的主方向,然后根据曲面所需的加工误差和残留高度要求划分该投影数据点云,得到一系列刀位网格单元。通过最小化每个刀位网格单元的加工误差以确定每个刀位网格的节点位置,加权平均相关联刀位网格节点来对齐相邻刀位网格单元的边缘。为了缩短加工时间,裁去刀位路径上多余的线段,最终生成高效合理的数控加工刀具路径。已用实测的数据点云验证了本法直接生成刀具路径的有效性。     

Mesh-based tool path generation for constant scallop-height machining

This paper presents a new approach to mesh-based tool path generation for obtaining constant scallop heights. The mesh surface has recently become the focus of considerable interest, because its geometric computation is simpler and more robust than that of the parametric surface. These advantages make it easy to check and remove interference in the process of tool path generation. The previous tool path generation method based on the mesh surface, however, can generate only one topology of iso-planar type where tool paths have evenly spaced tool path intervals. As constant scallop heights cannot be obtained from evenly spaced tool path intervals, unevenly spaced tool paths based on offset meshes are necessary for reducing the machining time and for easy interference removal. This paper proposes and compares four methods to estimate curvatures from the mesh surface; the curvature is essential for calculating unevenly spaced tool path intervals. This paper also proposes an improved drive surface method to propagate CL-paths unevenly and to generate tool paths with various topologies.     

Experimental performance analysis of an inverse dynamics CNC compensation scheme for high-speed execution of curved toolpaths

Experimental results from the implementation of an inverse dynamics compensation scheme on a three-axis CNC mill with an open-architecture software controller are presented. The scheme amounts to imposing a continuously variable displacement on the commanded toolpath, which compensates for the physical limitations (inertia and damping) of each machine axis, producing accurate execution of the prescribed toolpath. Numerical values for the dimensionless parameters that characterize the second-order inverse dynamics model are determined by feeding input and output data from machine runs into a system identification software tool. For a basic P-type controller, exact a priori solutions for the modified path geometry are possible, allowing implementation by simple alterations to the real-time interpolator. The experimental results based on a P-type controller indicate that the inverse dynamics scheme is highly effective in suppressing both feed (lag/lead) error and contour error (deviation from the desired path), even at high feedrates along strongly curved toolpaths. The scheme thus provides a practical means of achieving smooth and accurate execution of free-form paths, without appealing to more complicated “active” control strategies.     

Accurate spiral tool path generation of ultraprecision three-axis turning for non-zero rake angle using symbolic computation

An accurate spiral tool path generation method of ultraprecision three-axis turning free form surface is proposed based on symbolic computation in this paper. Many analytic optical free form surfaces often need to be machined to submicron in form error, such as optical nonaxisymmetric aspheric surfaces, but current mainstream CAM systems usually use nonuniform rational basis spline (NURBS) to describe the designed surface and generate tool path. If we want to use these systems, the analytical optical surfaces must be approximated using NURBS surfaces, but it will introduce approximation error and may be difficult to achieve the approximation error less than submicron. More importantly, there is no effective tool path generation method for the special three-axis turning machine tool in current mainstream CAM systems. In this context, we propose to calculate the tool path directly from these analytic surfaces by using symbolic math. The proposed method can be used to generate accurate spiral tool paths for zero/negative/positive rake angle in a unified way. Finally, several examples are given to prove its effectiveness.     

三次样条曲线回转面的自动编程与加工

以三次样条曲线回转面的数控车削加工为例,介绍了基于UG车削加工三次样条曲线回转面的自动编程方法,阐述了针对不同数控系统的数控车床,如何创建后置处理器。UG自动编程的步骤包括：首先使用UG NX6.0的建模功能完成三次样条曲线回转面的三维模型;然后进入车削加工模板,根据加工工艺生成所有工序的车削刀路,再根据车削刀路进行仿真模拟;最后进行后置处理得到车削加工程序。结果表明加工精度符合图纸要求,基于UC的自动编程可以提高NC程序的正确性和安全性,同时还能提高工作效率。     

Local interference detection and avoidance in five-axis NC machining of sculptured surfaces

The tool interference problem is the most critical problem faced in sculptured surface machining. This paper presents a 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 .     

Contour parallel milling tool path generation for arbitrary pocket shape using a fast marching method

Contour parallel tool paths are among the most widely used tool paths for planer milling operations. A number of exact as well as approximate methods are available for offsetting a closed boundary in order to generate a contour parallel tool path; however, the applicability of various offsetting methods is restricted because of limitations in dealing with pocket geometry with and without islands, the high computational costs, and numerical errors. Generation of cusps, segmentation of rarefied corners, and self-intersection during the offsetting operations and finding a unique offsetting solution for pocket with islands are among the associated problems in contour tool path generation. Most of methods are inherently incapable of dealing with such problems and use complex computational routines to identify and rectify these problems. Also, these rectifying techniques are heavily dependent on the type of geometry, and hence, the application of these techniques for arbitrary boundary conditions is limited and prone to errors. In this paper, a new mathematical method for generation of contour parallel tool paths is proposed which is inherently capable of dealing with the aforementioned problems. The method is based on a boundary value formulation of the offsetting problem and a fast marching method based solution for tool path generation. This method handles the topological changes during offsetting naturally and deals with the generation of discontinuities in the slopes by including an “entropy condition” in its numerical implementation. The appropriate modifications are carried out to achieve higher accuracy for milling operations. A number of examples are presented, and computational issues are discussed for tool path generation.     

自由曲面数控加工刀具轨迹映射算法

对自由曲面的数控加工,寻求最优的刀具轨迹生成方法至关重要。本文基于开源3D库Open CASCADE(OCC)和编程开发环境Microsoft Visual Studio 2010(VS2010),应用B样条表达的自由曲面,采用“投影法”思想,研究“重用已有相似刀具路径”方法,提出了处理自由曲面的NC刀具轨迹映射算法。为了验证算法的有效性和可行性,实验建立5类自由曲面,启动设计的轨迹映射算法,输出曲面的NC加工轨迹质量可以满足设计要求,实现了刀具轨迹的重用。     

大型复杂曲面的荒加工研究

提出了大型曲面的荒加工概念 ,它是对加工余量较大的区域进行的局部加工 ,是粗加工的预处理。其关键之处在于如何求取“荒岛”的边界 ,由此确定荒加工区域 ,安排走刀路径。利用基于 Herm ite离散点曲面造型的新算法构造了余量分布曲面 ,避开了毛坯面与零件曲面等距面的求交问题 ,求取余量分布曲面的等高线即可获得“荒岛”的边界。荒加工能减少大量空行程 ,大幅度提高加工效率。     

微光成型法固化成形路径的生成和检验

为了对平面实体进行曝光固化 ,需要同时得到平面图形的轮廓和内部填充的扫描轨迹。为此 ,利用CNC自动编程系统中的“C刀补”功能 ,实现了平面轮廓环扫描路径的自动生成 ;并在此基础上 ,用“辅助直线法”生成了平面实体填充固化所需的“行栅式”扫描路径 ;最后 ,利用VC的编程功能 ,分别实现了轮廓扫描和行栅扫描路径的实时检验     

Numerically Controlled Machining of Freeform Curves Using Biarc Approximation

To machine a shape incorporating freeform curves, the free-form curves must be approximated and represented in a form suitable for numerically controlled (NC) tool-path generation. Instead of the commonly used line segment approximation, a new method using biarc approximation is presented here. The purpose is to reduce sudden changes in tool movement and increase continuity and smoothness. A biarc can be described as two consecutive circular arcs with an identical tangent at the connecting point. Therefore, tangent continuity can be maintained. Furthermore, curvature continuity can be improved if the difference between the two curvatures is minimised. In this research, biarc segments are used to approximate the freeform curves by controlling the largest deviation distance between the curves and the biarcs. The test results show that the biarc approximation method generates fewer tool-path segments than the traditional line approximation. The definitions of the biarcs are used to generate NC tool paths. A new tool-path generation method is developed for processing rough cutting and finish cutting paths. Example parts modelled with B-spline curves are tested and machined.     

Tool path generation for machining compound surfaces with extended cut region method

Presented in the paper is a novel tool path generation method for 3-axis NC machining of compound surfaces. Tool path generation procedure consists of two core steps: firstly, the extended cut region (ECR) of every surface is constructed; secondly, CL-curves are computed on every ECR and the final tool paths are achieved by sorting, trimming and connecting the CL-curves. The developed ECR method can solve the gouging problems caused by tangent discontinuity, surface overlap or surface gap among surface elements. The proposed method has been implemented and applied to tool path generation in a general CAM system that has been developed by authors. Our case study has demonstrated the validity of the proposed algorithms.     

Real-time simulation and visualization of robotic belt grinding processes

Real time simulation and visualization are important for robot programmers to verify and optimize the path planning for the robotic belt grinding process. A new free-form surface representation based on discrete surfel element is developed to facilitate the system implementation, which exploits the advantage of the new development of point-based rendering technology in computer graphics. A local process model is integrated to calculate the material removal rate by considering the local geometry information and non-uniform force distribution. The final surface grinding error is easy to be assessed and visualized for quality evaluation. The experiments show that the simulation error is below 15%, even for a non-uniform contact under stable cutting conditions.     

STEP-NC的可扩展标记语言实现方法研究

STEP-NC是新发展的数控编程国际标准（ISO14649），依据该标准生成的数控程序通常采用ISO10303 Part21物理文件格式。但该格式的数控程序不但不易解释，而且不适合在网上传输。为此，提出了采用可扩展标记语言作为STEP-NC数控程序的文件格式。首先，分析了STEP-NC数据模型核心内容及ISO10303 Part21物理文件格式数控程序的缺点，研究了可扩展标记语言格式STEP-NC数控程序的生成，包括文档结构的选择、EXPRESS向可扩展标记语言的映射等，并将其作为机床控制器的输入。将机床控制器软件划分为3个模块，即可扩展标记语言形式数控程序的解释、微观工艺规划并自动生成刀具路径和控制机床运动的数控指令的生成。重点研究了可扩展标记语言数控程序的解释。     

数控铣削仿真系统中刀具几何体生成算法

针对球头铣刀给出了用于三轴数控铣削仿真系统的一种快速、精确的刀具扫描体生成算法；基于数控仿真技术的曲线建模方法，给出了数控切削物理仿真所需的关键几何因素－－刀具切削刃的几何表达式。     

Contour-parallel offset machining for trimmed surfaces based on conformal mapping with free boundary

The parametric surfaces of some manufactured parts are often subjected to the Boolean operation of other objects; generating suitable NC tool paths from such trimmed surface remains a challenge. This paper presents a new planar development-based method to generate contour-parallel offset paths of trimmed surfaces. To avoid direct frequent identifications and removals of interferences of offset curves on 3D-trimmed surface possibly with multiple holes or restricted regions, the original surface is flattened in the plane domain using a two-stage approach which consists of conformal mapping with free boundary and further nonlinear accuracy improvement. Then, sequent offsets of the boundary curves of the planar region are generated, and the global interferences are detected and removed using an efficient and robust divide-and-conquer strategy. Based on a tree data structure, a tool-path linking algorithm is also given with less or no tool retractions, and subsequently, the resulting planar paths are inversely mapped to the physical space. Illustrated examples have been conducted to testify the affectivity and the applicability of the proposed contour-parallel offset machining method.     

叶片等残留高度数控螺旋铣加工刀具轨迹的规划算法

根据叶片的曲面特征,提出了基于等残留高度的叶片五坐标数控螺旋铣加工方法,设计了刀具轨迹生成算法,在UG二次开发环境下编程实现.该方法满足相邻刀轨之间的等残留高度要求,无冗余刀轨.避免了刀轨间的抬刀,可实现连续的切削加工,在满足误差要求的前提下可显著提高加工的效率.     

Tool Positioning Algorithm Based on Smooth Tool Paths for 5-axis Machining of Sculptured Surfaces

The current research of the 5-axis tool positioning algorithm mainly focuses on searching the local optimal tool position without gouging and interference at a cutter contact(CC) point,while not considering the smoothness and continuity of a whole tool path.When the surface curvature varies significantly,a local abrupt change of tool paths will happen.The abrupt change has a great influence on surface machining quality.In order to keep generated tool paths smooth and continuous,a five-axis tool positioning algorithm based on smooth tool paths is presented.Firstly,the inclination angle,the tilt angle and offset distance of the tool at a CC point are used as design variables,and the machining strip width is used as an objective function,an optimization model of a local tool positioning algorithm is thus established.Then,a vector equation of tool path is derived by using the above optimization model.By analyzing the equation,the main factors affecting the tool path quality are obtained.Finally,a new tool position optimization model is established,and the detailed process of tool position optimization is also given.An experiment is conducted to machine an aircraft turbine blade by using the proposed algorithm on a 5-axis blade grinding machine,and the machined blade surface is measured with a coordinate measuring machine(CMM).Experimental and measured results show that the proposed algorithm can ensure tool paths are smooth and continuous,improve the tool path quality,avoid the local abrupt change of tool paths,and enhance machining quality and machining efficiency of sculptured surfaces.     

Virtual workpiece: workpiece representation for material removal process

In this paper, an efficient methodology to generate a virtual workpiece (VWP) is presented. VWP is a workpiece in a virtual environment in which the geometric, kinematic, and thermo-mechanical effects of the process and resources can be reflected. VWP encompasses not only the macro-information corresponding to the shape of the “virtually” machined intermediate workpiece, but also the micro-information, such as the surface roughness, scallop heights, chatter mark, etc. To represent VWP, swept volume (SV) of geometrically defined cutters is generated first by envelope profiles which are calculated by the intersection of the Tool map with the Contact map of the tool moving direction. Then SV is tessellated to conduct elementary 1D Boolean subtraction of SVs from the IPW. The Boolean subtraction is realized by means of an efficient ray-triangle intersection test using Barycentric coordinates. Finally, VWP is reconstructed as a triangular mesh (STL, stereolithography data format) from the orthogonal triple-dexel model (TDM) which predicts machined surface quality, such as surface roughness, gouging and sharp edges and is reused for further operations, e.g., tool path generation, simulation and geometric metrology, etc. To demonstrate the validity of VWP modeling, several material removal processes, e.g., milling and micro-EDM operations, have been tested and the proposed approach has been proven to be applicable to enhance the quality of NC simulation and verification.