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Rapid die design based on digital reverse engineering

Abstract: using coordinate measuring machine and reasonable curve detection algorithm to transmit the measured curve results to pro/engin in IBL format, the experiment is becoming complex and long-time eer software. Using the curve and modeling functions of pro/engineer software, the rapid design of machine and die is realized

key words: CMM; Machine mould; Reverse engineering; Mold manufacturing

1 reverse engineering and its implementation process

reverse engineering is the process of accurately and quickly measuring the three-dimensional coordinate points of the physical surface of the existing parts or physical prototypes by using the 3D digital equipment, and reconstructing the CAD model of the physical object by using the three-dimensional geometric modeling method according to these coordinate points. Reverse engineering has a completely different design process from the traditional design and manufacturing process, which can be generally divided into four stages:

(1) digitization of part prototype. Generally, CMM or laser scanning and other measuring devices are used to obtain the three-dimensional coordinate values of the prototype surface points

(2) extract the geometric features of the part prototype from the measured data. The measured data are segmented according to their geometric attributes, and the design and machining features of the part prototype are obtained by the method of geometric feature matching and recognition

(3) reconstruction of part prototype CAD model. The segmented 3D data are fitted to the surface model in the CAD system, and the CAD model of the prototype surface of the part is obtained through the intersection and splicing of each surface piece

(4) inspection and correction of reconstructed CAD model. According to the obtained CAD model, the method of re measuring and machining samples is used to test whether the reconstructed CAD model meets the requirements of accuracy or other test performance indicators. Repeat the above process for those who do not meet the requirements until the design requirements of the parts are met. Figure 1 is a typical reverse engineering flow chart

Figure 1 reverse engineering flow chart

2 free curve detection method

the CAD model is established by the physical object, and the surface of the physical object is required to be digitized first. It can be seen that the key technology of reverse engineering has two aspects: one is the surface data acquisition technology of physical model, that is, digital measurement technology; The other is surface reconstruction technology [1]

data processing is a key link in reverse engineering. The results of the company's recent approval by the state will directly affect the quality of later model reconstruction. The traditional measurement method is the contact measurement represented by CMM. At present, the surface modeling function of CAD system basically adopts the modeling method of forming lines from points and then surfaces from lines. The solid modeling is also improved based on the downstream demand, but there are some special requirements in curve processing. Therefore, from the perspective of CAD modeling, the focus of the three coordinate detection method should be on the free curve detection. The reasonable curve detection will bring great convenience to the subsequent CAD modeling [2,3]. The key point of CMM is the path planning of curve detection. If the probe can measure with the trajectory desired by the designer, the measurement accuracy and work efficiency of CMM will be improved

The complexity of free curve detection is mainly reflected in two aspects: (1) the number of sampling points; (2) Degree of automation of detection. In view of the two problems in free curve detection, literature [2] proposed a compromise detection method, that is, taking into account the density of sampling points, as well as the problems of detection automation and manual intervention. The number of sampling points and detection paths shall meet the requirements to the greatest extent with the minimum intervention of operators

the basic idea is: first, collect a few points on the free curve to be detected, and use these points to roughly describe the detection path; Then on the basis of the detection path described in these points, the automatic planning of the detection path is carried out, and the detection path is subdivided according to the density requirements of the detection. This can not only flexibly control the spatial trajectory of the detection action, but also get a large amount of point information with less intervention, which improves the automation and rationality of the detection

2.1 description of detection path

at present, in all CAD systems, the description of free curve basically adopts cubic spline curve. This description method is flexible to control the curve, and local changes have little effect on the overall properties of the curve. It is powerful in dealing with the continuity and smoothness of the curve

when describing cubic spline curve, the control polygon is usually used, that is, several control points control the shape of the curve. Control points are used to determine the position and shape of curves and surfaces, and the corresponding curves and surfaces do not necessarily pass through points. In order to make the generated curve pass through the given points (here we call them type value points), the control points passing through them should be obtained from these type value points, and then the generated curve can be described by these control points

2.2 subdivision of detection path

the detection path can be described more completely through the spline curve established in 2.1.1. Through this detection path, the probe can move in space in a reasonable way, and the point information with good performance can be obtained. If the number and distribution of the given points are more reasonable, the detection path is more similar to the actual curve, and the difference between the two is smaller. The purpose of path subdivision is to detect these differences and make the collected point information truly reflect the actual curve

in order to facilitate the operator to control the subdivision degree of the detection path, the method of path subdivision based on distance is adopted to make the distribution of the generated measurement points on the curve as equidistant as possible

after subdividing the detection path, a specific detection program representing the latest image of the company can be generated. The contact mode of the probe shall be vertical downward. Therefore, two key factors of point detection can be determined: one is the touch direction, which is vertically downward; The second is the spatial position of the point to be measured, which is obtained by subdividing the detection path

according to the mathematical model of the detection path, the detection program with different number of acquisition points can be generated by taking different detection steps. Fig. 2 is a curve detection example, (a) Fig. is a detection path in the form of cubic spline curve generated by six points, wherein the triangular points represent six detection path control points, and the curve represents the detection path generated by these six points

; (b) The figure shows the subdivision of the detection path with a step length of 10 mm, · is the encryption point, and point O represents the detection starting point corresponding to each point

Figure 2 curve detection path

3 example

3.1 data measurement

the shell of the machine is composed of many complex surfaces. Since the contact force between the probe and the surface is very small during measurement, the machine shell is fixed on the workbench with four support planes of the base and double-sided adhesive tape. Take the worktable as the xoy coordinate plane and the normal direction of the worktable as the Z axis. Take any point on the transverse symmetry center line of the machine shell as the origin. The machine is horizontally Y axis and longitudinally X axis [4]. Three coordinate detection

3.2 CAD modeling

use the above steps to complete the data measurement, and output the 3D point data measured by the CMM in Qi tech format. In order to meet the data format of pro/engineer, open arclength and begin section must be added at the beginning of the file, and begin curve must be added at the beginning of each section to generate the data format that pro/engineer can recognize IBL data format

the scantool module of pro/engineer software is used to remove the bad points of the characteristic curve and smooth the characteristic curve (as shown in Figure 3), so as to ensure the smoothness of the surface

Figure 3 characteristic curve of the machine shell

the measurement and treatment of some regular holes on the machine shell will not be repeated here. Through surface modeling and solid modeling, the shell model as shown in Figure 4 is obtained [5,6]

Figure 4 machine shell modeling figure

using the mold design module provided by pro/engineer software, considering the shrinkage coefficient, the female and male mold of the machine mold are obtained, as shown in Figure 5

Figure 5 mold drawing

4 conclusion

based on digital reverse engineering technology, with the help of contact coordinate measuring machine, adopting efficient curve detection algorithm, combined with the powerful modeling function of pro/engineer, the seamless connection between digital measurement and cad/cam can be realized. This method provides a feasible path for the rapid tooling manufacturing of parts, which can speed up the R & D cycle of products and improve the market competitiveness of enterprises


[1] He Jie Application of cad/cam reverse engineering in mold development [j] Computer aided engineering, 2000, (3): 55- 56

[2] wangguangyan Research on Key Technologies of machining center inspection based on CAD [d] Master Thesis of Hebei University of technology 2002:21- 26.

[3] wuyongqing Research on some key technologies in the inspection planning of intelligent CMM [d] Doctoral dissertation, Tianjin University, 2001:46- 56

[4] Liu Bin Numerical analysis of plastic extrusion flow and optimization of die structure design [d] Doctoral dissertation, Dalian University of technology, 2003

[5] xiaoyaoxian, Ke Yinglin Application of reverse engineering modeling based on physical model [j] Mechanical design and manufacturing, 2002, (3): 77- 78

[6] caiyujun Overview of NC machining of complex surfaces [j] EDM and die, 2003, (3): 46- 50 (end)

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