Automated accessibility analysis and measurement clustering for CMMs

The aim of inspection planning tasks is to ensure the generation of efficient operation for coordinate measurement machines (CMMs). This paper presents a methodology to automatically define the accessibility domain of measurement points and then group them into a set of clusters. The methodology uses the computer-aided design (CAD) model of the workpiece and tolerance information as input to an algorithm for defining points accessibility. For each measurement point, the algorithm determines all feasible inspection probe orientations, as a subset of the total set of available orientations for a given CMM probe. A heuristic algorithm then groups the measurement points into a set of clusters which provide the maximum number of probe orientations. This methodology was applied to three examples which contain solid model and free-form surface representations.     

Tool path re-planning in free-form surface machining for compensation of process-related errors

Free-form surfaces are widely used in many applications in today’s industry. This paper presents a new approach to identify and compensate process-related errors in machining of free-form surfaces. The process-related errors are identified online by a newly developed in-process inspection technique. In this technique, the surface is first machined through an intermediate semi-finishing process that is specifically designed to machine different geometric shapes on the surface with different process parameters. An inspection method is developed to identify the process-related errors in the selected regions on the semi-finished surface. The relationship between the machining/surface parameters and process-related error is then achieved using a neural network. This relationship is used to predict the process-related errors in the finishing process. The process-related errors, together with the machine tool geometric errors identified using a method developed in our previous work, are compensated in the finishing tool paths through tool path re-planning. Experiment has been conducted to machine a part with a free-form surface to show the improvements in the machining accuracy.     

A new machine strategy for sculptured surfaces using offset surface

A new CNC tool path planning method is developed for accurate and efficient finishing cutting of sculptured surfaces. The proposed method generates CNC tool paths based on an offset surface of an object instead of the actual surface. The new algorithm consists of two main computational techniques: offset surface generation technique and offset CNC tool path planning technique. In offset surface generation, approximate parametric offset surfaces are generated accurately from an original parametric surface by employing bi-cubic surface patch, surface conversion, and surface subdivision algorithms. In offset CNC tool path planning, the precise geometric models of chordal deviation and cusp height on an offset surface are established. The effectiveness of this proposed CNC tool path planning method is verified by geometric simulation and verification which detect and identify errors in CNC tool paths.     

Automatic segmentation and approximation of digitized data for reverse engineering

Reverse engineering is the process of developing a computer-aided design (CAD) model and a manufacturing database for an existing object. This process is used in CAD modelling of part prototypes, in designing moulds and in automated inspection of parts with a complex surface. This paper reports on the automatic segmentation and approximation of three-dimensional digitized points for reverse engineering. Based on an innovation that uses the properties of a non-uniform rational B-spline (NURBS) or B-spline and makes ordered digitized points be control points directly to construct a NURBS or B-spline surface, which takes less computation time than traditional algorithms in calculating surface normals and curvatures at digitized points, an algorithm was developed for automatic segmentation and NURBS surfaces fitting for digitized points.     

Flexible inspection system based on a vision guided coordinate measuring machine

The objective of this paper is to develop a flexible three-dimensional inspection system for sculptured surfaces by employing 'Coordinate Measuring Machine (CMM), CAD database and vision system technology. New surface identification and rough localization methods are suggested using a concept called ‘Z-layers’ and a vision guided system. The Z-layers are generated from a CAD database which classifies the three-dimensional models. The three-dimensional models are reconstructed using a vision system that identifies the model and roughly identifies the orientation and location of the object implementing Z-layers. Then inspection planning and fine localization methods are mapped out which are based on the CAD database. The experimental results indicate that the developed inspection system, with the proposed algorithm, can be implemented in modern industry to reduce measurement errors and increase productivity.     

Sculptured surface tolerance verification with design datums

Sculptured or free-form surfaces are widely used in many fields with extensive applications. Once such surfaces are manufactured, surface inspection compares the manufactured surfaces with the surface design specifications to verify conformance. Although significant research and development efforts have been devoted to the design and manufacturing of products consisting of partial or sole free-form surfaces, the inspection of these surfaces is still a difficult task. For many engineering applications, a free-form surface is assigned a profile tolerance with reference to design datums for assembly, functionality and other manufacturing requirements. The paper discusses developments of surface inspection techniques for profile tolerance of free-form surfaces. The concept of datum direction frame is proposed to find the transformation information that localizes measurement data to design model. The technique consists of two major steps: localization of measurement data to the design system, based on the datum reference information; and further localization based on the information from free-form surfaces. Testing examples were carried out to validate the developed techniques. The new method does not need corresponding points from the datums of the design model and measured surfaces. Therefore, it is simpler, yet more robust. It can also be used conveniently in manufacturing processes.     

基于分数阶傅里叶变换的NURBS曲面拟合

为了实现对自由曲面模型的高精度非均匀有理B样条(NURBS)曲面拟合,提出了一种基于分数阶傅里叶变换的NURBS曲面拟合方法。首先,借助分数阶傅里叶变换对自由曲面模型点云数据的高程图像展开分析,从中提取出表征自由曲面表面三维结构的特征点;然后,利用外切圆取点法结合提取的特征点选取用于NURBS曲面拟合的数据点;最后,利用分数阶傅里叶变换滤波及反插节点法对拟合曲面形状进行优化,提高拟合精度。实验结果表明:该方法与传统NURBS拟合方法相比,在不同调整次数下拟合结果的均方根误差降低了28%,取得了更好的拟合效果。     

自由曲面光学器件检测技术

自由曲面光学器件尽管有其突出的优点,但还远不能进入到现代光学系统的主流中去,问题之一就是精密检测．自由曲面光学器件的检测对于其精密加工不可或缺,并且两者通常具有不可分割的联系．文中阐述了与不同加工阶段相关的自由曲面光学器件检测中的问题和对策．在抛光前后宜分别采用坐标测量机和光学干涉仪,两种方法都存在一些问题有待解决．实际上坐标测量方法是近十几年来自由曲面测量的主流,这方面的研究主要集中于定位、误差补偿及采样策略等问题上．相比之下,自由曲面光学器件的光学测试是一个新的技术,其中不仅是分析软件上的存在问题,首要的还是缺乏适当的测量手段．尽管也可应用专门的轮廓测量仪,但它还存在诸多限制．结合子孔径拼接技术的干涉仪在某些自由曲面光学器件的测量中前景良好,不过对于更复杂的曲面,它同样无法进行测量．     

Automatic metric 3D surface mesh generation using subdivision surface geometrical model. Part 2: Mesh generation algorithm and examples

In this paper, a new metric advancing front surface mesh generation scheme is suggested. This new surface mesh generator is based on a new geometrical model employing the interpolating subdivision surface concept. The target surfaces to be meshed are represented implicitly by interpolating subdivision surfaces which allow the presence of various sharp and discontinuous features in the underlying geometrical model. While the main generation steps of the new generator are based on a robust metric surface triangulation kernel developed previously, a number of specially designed algorithms are developed in order to combine the existing metric advancing front algorithm with the new geometrical model. As a result, the application areas of the new mesh generator are largely extended and can be used to handle problems involving extensive changes in domain geometry. Numerical experience indicates that, by using the proposed mesh generation scheme, high quality surface meshes with rapid varying element size and anisotropic characteristics can be generated in a short time by using a low‐end PC. Finally, by using the pseudo‐curvature element‐size controlling metric to impose the curvature element‐size requirement in an implicit manner, the new mesh generation procedure can also generate finite element meshes with high fidelity to approximate the target surfaces accurately. Copyright © 2003 John Wiley & Sons, Ltd.     

NURBS machining and feed rate adjustment for high-speed cutting of complex sculptured surfaces

Owing to the enormous engineering advancement in modern industry, the competition in the development of advanced manufacturing technologies has been increasingly intense as can be seen in automobile, aerospace and various industries. Not only has product quality improved tremendously, but also the demand over production speed has become higher. Therefore, high-speed and high-accuracy production has become a very important goal of modern manufacturing companies. Thanks to ever increasing computer speed and the open architecture of new controllers in computer-controlled machines, the capability and flexibility of today's CNC machines allow us to machine contoured geometry using non-uniform rational B-spline (NURBS) curves directly from the free-form surfaces of a complex CAD/CAM model. As a result, machining accuracy and product quality can be greatly improved while machining time is also significantly reduced. This paper proposes a post-processing approach to convert G1 NC codes from most CAD/CAM systems to NURBS NC paths for highspeed contour machining. The NURBS interpolation strategy that takes into account the optimized cutting feed rate based on machine dynamic response and curvature of the NURBS curve is also developed. Experiments show that the implemented NURBS cutting can significantly improve machining accuracy and reduce cutting time and therefore satisfies the requirements of today's highspeed and-accuracy machining needs.     

Combining physical shell mapping and reverse-compensation optimisation for spiral machining of free-form surfaces

Machining of free-form surfaces has an important role in industrial manufacturing, but conventional tool-path generation strategies for free-form surfaces machining have the drawbacks of serious flattening distortion and poor tool-path continuity. Therefore, a novel method is developed to generate a spiral tool path for the machining of free-form surfaces by improving surface-flattening distortion and tool-path continuity. First, physical shell mapping is presented to flatten a free-form surface into a plane, which takes stretching energy, bending energy, and global energy into account. Then, the spatial spiral polyline is rounded to generate a spiral path by proposing reverse-compensation optimisation. Therefore, the free-form surfaces can be quickly flattened with less distortion, remaining free of overlap, and can in addition be machined at high speed along a C² continuous spiral tool path. Further, the flattening error, tool-path length, mean curvature, mean scallop-height error of the spiral path, machining time and surface roughness are obviously reduced. Finally, simulation results are given to show the effectiveness and feasibility of the presented strategy.     

Haptic modeling for a virtual coordinate measuring machine

Introducing a haptic device into coordinate measuring machine (CMM) inspection path planning leads to the proposal of a novel CMM off-line inspection path planning environment, a haptic virtual coordinate measuring machine (HVCMM), which makes use of the haptic modeling technique for CMM off-line programming. The HVCMM is an accurate model of a real CMM, which simulates a CMM's operation and its measurement process in a virtual environment with haptic perception. In this paper, a simple and effective mechanics model is implemented for the proposed HVCMM. The HVCMM enables CMM off-line programming to take place exactly as if an operator were in front of a real CMM and moving a real CMM probe. Even more, operators can feel the collision between the CMM and a part. Since there is a force feedback when the probe reaches the surface of the part, besides showing the contact in the HVCMM environment, it is much easier to generate a collision-free probe path than using other off-line inspection planning methods. The HVCMM not only facilitates inspection path planning, but also speeds it up because the operator does not need to slow the probe down when it is approaching an object. Combined visual and force feedback is the best indicator for selecting measurement points.     

Verification of Articulated Arm Coordinate Measuring Machines Accuracy Using LaserTracer System as Standard of Length

This article describes a novel approach to verification of articulated arm coordinate measuring machines (AA CMMs) based on ISO 10360-2 standard. The approach utilizes the LaserTracer (LT) system as standard of length and automation of calibration procedure using an industrial robot. In this method, the robot is programmed to repeatedly move the AA CMM. The retroreflector is integrated with the stylus of AA CMM. Location of the retroreflector is constantly tracked by LaserTracer and, in select positions, the measurements of distance are performed by both the AA CMM and LT system. The verification of AA CMM accuracy is carried out through the comparison of the two measurement values. The developed procedure is recommended mainly for checking AA CMMs with large measuring volume, where the application of common artefacts is usually insufficient, or time-consuming, due to the fact that measurements have to be performed for several subspaces in the AA CMM measuring volume.     

Automatic segmentation of digitized data for reverse engineering applications

Reverse engineering is the process of developing a Computer Aided Design (CAD)model and a manufacturing database for an existing part. This process is used in CAD modeling of part prototypes, in designing molds, and in automated inspection of parts with complex surfaces. The work reported in this paper is on the automatic segmentation of 3-Dimensional (3-D) digitized data captured by a laser scanner or a Coordinate Measuring Machine (CMM) for reverse engineering applications. Automatic surface segmentation of digitized data is achieved using a combination of region and edge based approaches. It is assumed that the part surface contains planar as well as curved surfaces that are embedded in a base surface. The part surface should be visible to a single scanning probe (21/2D object). Neural network algorithms are developed for surface segmentation and edge detection. A back propagation network is used to segment part surfaces into surface primitives which are homogenous in their intrinsic differential geometric properties. The method is based on the computation of Gaussian and mean curvatures of the surface. They are obtained by locally approximating the object surface using quadratic polynomials. The Gaussian and mean curvatures are used as input to the neural network which outputs an initial region-based segmentation in the form of a curvature sign map. An edge based segmentation is also performed using the partial derivatives of depth values. Here, the output of the Laplacian operator and the unit surface normal are computed and used as input to a Self-Organized Mapping (SOM) network. This network is used to find the edge points on the digitized data. The combination of the region based and the edge based approaches, segment the data into primitive surface regions. The uniqueness of our approach is in automatic calculation of the threshold level for segmentation, and on the adaptability of the method to various noise levels in the digitized data. The developed algorithms and sample results are described in the paper     

Sculptured surface reconstruction from CMM measurement data by a software iterative approach

The reverse engineering approach is being widely applied in designing and manufacturing of sculptured surfaces. The first step in the reverse engineering process is collection of data lying on the surface. When using a coordinate measuring machine (CMM) with a touch trigger probe for digitizing data points, the probe radius must be compensated according to normal vectors. Previous works were addressed on the iterative measurement procedure to reduce the error caused by the compensation. The major problem of iterative methods is time consumption in digitization. This paper proposes a new architecture based on look-up table that keeps the estimated normal vectors of the measurement data to refine the data points digitized by CMM. The digitized data are first fitted into several NURBS curves by interpolation. The skinning method is then applied to interpolate these curves. Thus a smooth NURBS surface can be constructed and more accurate normal vectors of the measurement points can be obtained to compensate for the probe radius. By using this software iterative technique, we need only one digitization procedure. Therefore, much time can be saved and the efficiency of reverse engineering can be improved.     

Visibility theory and algorithms with application to manufacturing processes

Visibility arising from computer vision, geometrical design and complexity analysis is widely used in manufacturing processes. According to the definition of the visibility cone of a point, two kinds of visibility of a feature, namely a complete visibility cone and a partial visibility cone, are defined, and the relation between visibility map and complete visibility cone is also discussed. To solve a kind of accessibility and setup problem in mold parting, NC-machining and CMMs inspection path planning, a procedure is proposed to perform visibility analysis with respect to the geometry of the part, the shape of the effector, and degrees of freedom between part/effector. A new method for computing visibility cones is formulated by identifying C-obstacles in Configuration Space (C-Space), in which a general and efficient algorithm is presented and implemented using visibility culling. Compared with previous methods, the proposed algorithm is efficient even in very complex scenes. Finally, the contributions and limits of our work are discussed.     

Automated feature accessibility algorithm for inspection on a coordinate measuring machine

In this paper, the issue of inspection planning for a coordinate measuring machine (CMM) is addressed. A method is presented for the automation of the probe selection decision, with the ultimate goal of determining the set of workpiece orientations recommended for use on a CMM equipped with an indexable probe. The methodology uses a computer-aided design (CAD) model of the workpiece and a list of tolerance information as input to a feature accessibility algorithm. For each inspection feature, the algorithm generates a feasible set of probe orientations using information about potential probe/workpiece collisions, and computes an estimate of its percentage accessibility.