Abstract : Reverse engineering is a common method in manufacturing industry. The use of reverse methods for mold design and manufacture is an important means to shorten product development cycle, improve product quality, and then establish its own design and manufacturing system. This article describes the reverse manufacturing method and process of the top cover mold of the LZ6460 passenger car, and gives an example of generating a curved surface from a point cloud. Keywords: reverse engineering; CAD/CAM; mold; top cover introduction In the product development and manufacturing process, the application of geometric modeling technology is quite extensive. However, due to various reasons, there are still many products that are not described by the CAD model. The designers and manufacturers face physical samples. In order to adapt to the development of advanced manufacturing technologies, it is necessary to transform these physical objects into CAD models through certain means so that they can be processed or managed using advanced technologies such as CAD, CAM, RPM, and PDM. At present, this technique for obtaining mathematical models of products from physical prototypes has developed into a relatively independent category in CAD/CAM and is called "Reverse Engineering". By reverse engineering the CAD model of real objects, it is possible to make products based on physical products possible to fully utilize advanced manufacturing and management technologies such as CAD, CAM, RPM, and PDM during design and manufacturing. At the same time, due to implementation in reverse engineering, physical samples can be copied in a very short time. Therefore, it is an important foundation and supporting technology for the implementation of concurrent engineering. 1 Design and manufacturing method of traditional automotive panel molds For self-designed models, the covers are hand-made by sculptors. Such samples are inevitably flawed. Occasionally, copying parts are also used to perform profiling directly. Profiling copies all the defects on the sample to the mold. The final product also inherits all the defects of the sample, causing the cover to have poor smoothness, low accuracy, and poor coordination. In addition, the traditional mold manufacturing methods require large manual repairs, uneven gaps, repeated mold repairs, and unstable quality and long cycle times. If numerical control equipment is used to process the mold, wax test must be performed to verify the correctness of the CNC tool path. The fatal weakness of the traditional method is that the resulting mold cavity does not have the ability to modify and redesign. 2 CAD/CAM system-based design and manufacturing methods The use of CAD/CAM integration technology is a requirement for modern mold design and manufacturing, which can effectively improve the deficiencies of the traditional methods. The product model created by CAD can directly generate NC commands and realize data communication with the machine tool through the DNC interface, so that the The original use of analog transfer of analog to data transfer, so that design and manufacturing links directly. Moreover, in the CAD system, appearance analysis, product assembly, inspection and interference with the components can be performed, the NC machining process can be simulated, the machining process and interference can be checked, and the product design and modification can be realized. Therefore, manual labor can be greatly reduced, new product development cycles can be shortened, and product quality can be significantly improved. The application of CAD/CAM technology to the manufacture of the LZ6460 passenger car top cover mold is based on a mathematical model. Therefore, the resulting product has a smooth, beautiful, symmetric appearance with high accuracy of the contour line and good coordination. Convenient (reform, innovation), manufacturing based on a long shelf life. 3 Reverse Design Method and Process 3.1 CAD/CAM software selection At present, there are more commercial CAD/CAM integrated system software with more complete functions. The author chose Delcam/Copycad for reverse design and processing. The specific contents include: (1) Input and processing of digitized points, including data input and digital point data transformation and processing; (2) Triangulation can be used to triangulate the digital model according to the user-defined tolerance, and then used directly for programming; Select Imageware/Surfacer to generate the surface. In the reconstruction of the surface, the main method of the system has the following two points: the surface can be directly generated from the scan point without the need to go through the process of building the curve, or the peripheral curve can be built first, and then the boundary and its internal scan point group are used to generate the surface . First, construct a NURBS curve in the swept point group and then generate a curved surface based on the curve. Specific content includes: (1) Scanning point analysis and processing - can receive data from different sources, such as scanning with a machine, or data obtained using photographic techniques (ASC file); (2) surface model construction - fast and accurate scan point transformation into NURBS surface model; (3) surface model accuracy, quality analysis; (4) Surface Modification - Curves and surfaces can be interactively modified in real time. 3.2 Reverse Design Method 3.2.1 Data Measurement In recent years, with the development of laser technology, full use has been made of good laser directivity characteristics in scanning, and non-contact measurement methods such as a raster method, a holography method, a depth image three-dimensional measurement method, a laser triangulation method, etc. are effectively used. It overcomes many systematic errors such as mechanical contact-type measurements such as the compensation and rigidity of the probe bar in the coordinate measuring machine, which makes the reverse engineering develop to a new height. Using the scanning function of the high-speed CNC milling machine, the surface of the top cover is digitized, and the compensation of the measuring probe must be considered in the measurement. Due to the presence of measurement errors, the resulting performance data should be subjected to data processing, including the removal of dead pixels, the measurement of blind zone data, data homogenization, and smoothing. The top cover cloud is shown in Figure 1. 3.2.2 Geometric Modeling Geometry modeling is the key to reverse engineering. The modeling process is the process of data processing. The purpose of data processing is to obtain the basic characteristic information of the input data. Its common methods include data adjustment, copying, area trimming, data density modification, data smoothing, noise removal, sharp corner retention, etc., to obtain the desired graphics. The quality of the established model not only depends on how well the model fits the physical shape, but also on whether the surface is smooth and whether the surface connection is smooth. The smoothness of curves and surfaces is a problem that should be paid special attention. In general, the conditions satisfying the smoothness of the curve are: 1 second-order smoothness, ie, continuous curvature; 2 curves without extra inflection points; 3 curvature variation is relatively uniform. The conditions for satisfying the smoothness of the surface are: 1 the key curve smoothing that constitutes the surface; 2 the grid lines of the curved surface have no extra inflection points; 3 the curvature of the surface Gaussian changes uniformly. There are two methods for modeling: First, use the Copycad software to generate triangular surfaces directly from the point cloud (STL model), and use the triangular surface model to program NC machining; second, use the Surfacer software to generate the NURBS surface model from the point cloud to perform the mold design. And processing. (1) General steps of COPYCAD processing LZ6460 passenger car top cover scanning file: 1) Read the file (ASC file) and load it into the top cover scan file, as shown in Figure 1: In this way, you can directly use the triangular surface of Figure 2 for programming and processing. (2) General steps to generate surfaces using Surfacer: 1) Read the ASC file, as shown in Figure 3 (the point cloud is the corner of the door inner panel); 2) draw the boundary of the built surface, as shown in Figure 4; After the surface is generated, mold design and programming can be performed. 4 Reverse Engineering in Other Applications 4.1 Restoration of damaged or worn parts When parts are damaged or worn out, CAD models can be reconstructed directly by reverse engineering to restore and repair damaged parts. Due to the wear and damage of the surface of the tested part, measurement errors are caused, which requires the reverse engineering system to have reasoning and judgment capabilities. For example: symmetry, standard size, parallel, perpendicular to the plane, etc. Finally, the part is machined. 4.2 Digital Model Detection Scanning and measuring the processed parts, and then using the reverse engineering method to construct a CAD model, by comparing the model with the original design of the CAD model on the computer data, you can detect manufacturing errors and improve the detection accuracy. 5 Conclusion The application of reverse engineering has successfully completed the design and manufacture of 6 sets of molds for the two parts of the LZ6460 passenger car sunroof top cover and front sunroof reinforcement plate. The part's profile matching meets the requirements, and the design and manufacturing cycle is shortened from 6 months to 4 Months. Glass Tube,High Borosilicate Glass Tube,Transparent Glass Tube,Quartz Glass Tube HEBEI DINGSHUO GLASSWARE CO.,LTD , https://www.dsglasspipe.com
2) Edit and delete redundant points 3) Generate two-way scan lines;
4) Offset model, based on the current model bias probe radius, produce the model of the top cover itself;
5) Generate a triangular surface, as shown in Figure 2. 
3) Use the Surfacer/Fitw/CloudandCurves command to generate the surface and extend the trim. The result is shown in Figure 5;
4) Analyze and compare the generated surfaces with the original point cloud, and obtain the error condition cloud of the surface as shown in Figures 6 and 7;
5) Export the above two surfaces to .igs or .vda format and edit them in professional CAD software (such as UG, PRO/E, SOLIDWORKS, etc.). FIG. 8 is a graph of a Gaussian curvature analysis of the case of a round in Pro/E and a curved surface; FIG. 
