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The application of reverse engineering in mold manufacturing

Abstract: reverse engineering is a common means in the manufacturing industry. Using reverse methods for mold design and manufacturing is an important means to shorten the product development cycle, improve product quality, and then establish their own design and manufacturing system. This paper introduces the reverse manufacturing method and process of lz6460 passenger car top cover mold, and gives an example of generating surface from point cloud

key words: reverse engineering; CAD/CAM; Mold; Continuous research and development of top cover in terms of intelligent and automatic performance


in the process of product development and manufacturing, geometric modeling technology is widely used. However, for various reasons, there are still many products that are not described by CAD models, and designers and manufacturers face physical samples. In order to adapt to the development of advanced manufacturing technology, these objects need to be transformed into CAD models through certain ways, so that they can be processed or managed by using advanced technologies such as CAD, cam, RPM and PDM. At present, this technology related to obtaining product mathematical models from physical samples has developed into a relatively independent category in cad/cam, which is called "reverseengineering". Through reverse engineering to reproduce the CAD model of the physical object, it is possible for those products based on the physical object to make full use of advanced manufacturing and management technologies such as CAD, cam, RPM and PDM in the process of design and manufacturing. At the same time, due to the implementation of 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 automobile panel mold

for the self-designed and developed vehicle model, it is a latent heart valve tissue engineering scaffold material for future reference. The panel is a product made by sculptors by hand, and such samples inevitably have defects. Sometimes, the panel samples are also used for direct profiling processing. While profiling will copy all the defects on the sample to the mold, and its final product also inherits all the shortcomings of the sample, resulting in poor appearance smoothness, low accuracy and poor coordination of the panel. In addition, the traditional mold manufacturing method of manual repair has large modulus and uneven gap, which requires repeated mold repair and test, with unstable quality and long processing cycle. If NC equipment is used to process molds, in order to test the correctness of NC tool path, wax mold trial cutting is also required. The fatal weakness of traditional methods is that the generated mold cavity does not have the ability of modification and redesign

2 design and manufacturing method based on cad/cam system

adopting cad/cam integration technology is the requirement of modern mold design and manufacturing, which can effectively improve the shortcomings of traditional methods. The product model established by CAD can directly generate NC instructions, and realize data communication with machine tools through DNC interface, so that the original shape simulation transmission in production is changed to data transmission, and the design and manufacturing links are directly communicated. Moreover, it can carry out appearance analysis, product assembly, check the interference of mating parts in the CAD system, simulate the NC machining process, check the machining process and interference, and realize the design and modification of products. Therefore, the amount of manual labor can be greatly reduced, the development cycle of new products can be shortened, and the product quality can be significantly improved

the cad/cam technology is applied to the manufacturing of lz6460 passenger car top cover mold, and its manufacturing basis is the mathematical model. Therefore, the appearance of the finished product is smooth, beautiful, symmetrical, with high accuracy of the matching contour, good coordination, convenient modification (modification, innovation), and the manufacturing basis has a long shelf life

3 reverse design method and process

3.1 cad/cam software selection

at present, there are many commercialized cad/cam integrated system software with relatively perfect functions. The author chose delcam/copycad for reverse design and processing. The specific contents include:

(1) input and processing of digital points, including data input and transformation and processing of digital point data

(2) triangle division can triangulate the digital model according to the user-defined tolerance, and then be directly used for processing programming

select imageware/surfacer to generate surfaces. In the aspect of surface reconstruction, the main methods of the system are as follows: the surface can be generated directly from the scanning points without the process of curve construction, and the peripheral curve can be built first, and then the surface can be generated by using the boundary and its internal scanning point group. Firstly, NURBS curves are constructed in the scanning point group, and then surfaces are generated according to the curves. The specific contents include:

(1) analysis and processing of scanning points - data from different sources can be received, such as machine scanning, or data obtained by photographic technology (ASC files)

(2) surface model construction - quickly and accurately transform scanning points into NURBS surface models

(3) accuracy and quality analysis of surface model

(4) surface modification - curves and surfaces can be modified interactively in real time

3.2 reverse design method

3.2.1 data measurement

in recent years, with the development of laser technology, the characteristics of good laser directivity are fully used in scanning, and non-contact measurement methods are adopted, such as grating method, holographic method, depth image three-dimensional measurement method, laser triangulation method, etc., which effectively overcome many systematic errors in mechanical contact measurement, such as the compensation and rigidity of the detection rod in the coordinate measuring instrument, So that reverse engineering has developed to a new level

use the scanning function of the high-speed CNC milling machine to digitize the top cover surface, and the compensation of the measuring probe must be considered in the measurement. Due to the existence of measurement error, the obtained performance data should be processed, including the removal of bad points, the supplement of measurement blind area data, data homogenization and smoothing, etc. The cloud diagram of the top cover is shown in Figure 1

3.2.2 geometric modeling

geometric modeling is the key of reverse engineering. The process of modeling is also the process of data processing. The purpose of data processing is to obtain the basic characteristic information of input data. Its general means mainly include data adjustment, replication, area pruning, data density modification, data smoothing, noise removal, sharp corner retention, etc., so as to obtain the required graphics. The quality of building a model depends not only on the fitting degree of the model to the shape of the real object, but also on whether the surface is smooth and whether the connection of the surface is smooth. The fairing of curves and surfaces should be paid special attention to. Generally speaking, the conditions to meet the fairing of curves are: 1. Second order smoothness, that is, continuous curvature; 2. There is no redundant inflection point in the curve; 3. The curvature changes were compared by in-situ synthesis process. The conditions for surface fairing are: 1. Fairing the key curves that make up the surface; 2. The grid line of the surface has no redundant inflection points; 3 the Gaussian curvature of the surface changes uniformly

there are two methods of modeling: one is to use CopyCAD software to directly generate triangular surface (STL model) from point cloud, and use triangular surface model for NC machining programming; The second is to use surfacer software to generate NURBS surface model from point cloud for mold design and machining

(1) general steps for CopyCAD to process the scanning file of lz6460 passenger car top cover:

1) read the file (ASC file), transfer it to the scanning file of the top cover, as shown in Figure 1:

2), delete the redundant points

3) generate two-way scanning lines

4) offset model, which offsets the probe radius based on the current model to generate the model of the top cover itself

5) generate triangular surfaces, as shown in Figure 2

in this way, the triangular surface in Figure 2 can be directly used for programming processing

(2) general steps of generating surfaces with surfacer:

1) read the ASC file, as shown in Figure 3 (the point cloud is a corner of the door inner panel)

2) draw the boundary of the built surface, as shown in Figure 4

3) use the surface/fitw/cloudandcurves command to generate a surface and extend the trim. The results are shown in Figure 5

4) analyze and compare the generated surface with the original point cloud, and get the error cloud of the surface, as shown in Figure 6 and 7

5) output the above two surfaces into IGS or VDA format is carried out in professional CAD software (such as UG, pro/e, Solidworks, etc.). Figure 8 is a cloud diagram of rounding in pro/e and Gaussian curvature analysis of the surface

After the surface is generated, the mold design and programming processing can be carried out

4 application of reverse engineering in other aspects

4.1 restoration of damaged or worn parts

when parts are damaged or worn, CAD models can be reconstructed directly by reverse engineering method to restore and repair the damaged part surface. The measurement error is caused by the wear and damage of the measured part surface, which requires the reverse engineering system to have the ability of reasoning and judgment. For example: symmetry, standard size, parallel and vertical between planes, and finally machined parts

4.2 digital model detection

scan and measure the processed parts, and then use the reverse engineering method to construct a CAD model. By comparing the model with the data of the original CAD model on the computer, the manufacturing error can be detected and the detection accuracy can be improved

5 conclusion

the reverse engineering has successfully completed the design and manufacture of six sets of molds for the two parts of lz6460 passenger car open sunroof top cover and front sunroof reinforcing plate. The surface matching of this part meets the requirements, and the design and manufacturing cycle is shortened from the original 6 months to 4 months

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