COST-EFFECTIVE WIRE-CUT EDM SYSTEM FOR PRECISION MOLD MANUFACTURING

Hyun-Goo Kim1, In-Ho Song2, Hyun-Ho Kim1, Kwan-Woo Yoon1

and Sung-Chong Chung1* 1Hybrid System Design & Control LABoratory

Department of Mechanical Engineering Hanyang University, Sungdong-Gu, Seoul 133-791, KOREA

2Division of Mechanical Engineering Ajou University, Yeongtong-Gu, Suwon 443-749, KOREA

ABSTRACT Automatic toolpath generation system of the wire electrical discharge machining (EDM) process is proposed through the CADviewer. The visualization data of it are optimally transformed lightweight CAD files designed for collaborative works. NC toolpath of the wire-cut EDM process is generated from dimensional and sectional informations extracted from the visualization data. Using the developed system, short lead-time is accomplished without an extra design job for the wire-cut EDM NC code generation. This system is applied for 2-axis cutting for precision manufacture. In addition, verification of the generated NC code is performed on the viewer. INTRODUTION Wire-cut EDM(Electrical Discharge Machining) is widely used for precision machining of complex parts. Most mold manufacturers use AutoCAD to generate wire-cut EDM NC codes instead of pre-designed native 3D CAD files. Though some wire-cut EDM CAM softwares use native 3D CAD files, they only support a few CAD files. In addition, they are expensive to be introduced to small mold manufacturers. In previous studies, during wire-cut EDM machining, Ivano et al. [1] proposed a method to reduce cutting error. The method remedies for workpiece of cutting error caused from wire bending by the real-time optical wire position sensor. Yang et al. [2] studied a minimization method of cutting error due to the circular interpolation by using R-map. In study of toolpath generation, Daniel et al. [3] studied to generate an appropriate toolpath of CNC machining, they studied creation of it from a CAD file. Wang et al.[4] proposed a method of toolpath generation using the centerline of the wire without considering thickness of wire. In this

like, during EDM machining, it continuously studied for upgrading product quality through CAM system and for optimizing CNC machining. But, NC toopath generation system, using lightweight CAD file that is developed for sharing design information among collaborators, is not studied. The developed viewer system offer only dimensional verification function. In this paper, an integrated wire-cut EDM CAM system is proposed as an extra module of the previously developed CAD viewer. This is an exclusive viewing system, which is developed for mold manufacturing, enabling dimensional inspection, draft check, and interference examination. By combining the two systems, short lead-time is accomplished without an extra design job for the wire-cut EDM NC code generation. This system is applied for 2-axis cutting for precision manufacture. In addition, verification of the generated NC code is performed on the viewer. Generation procedure of the NC code is as follows: First, visualize a CAD file including same or different shape of top and bottom planes on the viewer. Secondly, gather information of both top and bottom plane shapes of the translated data on the basis of Z-axis. Thirdly, sort out the entity information of the gathered plane shapes. Fourthly, generate cutting toolpath of X and Y 2-axis based on the sorted entity information. Fifthly, equalize each number of X, Y points, and then match each point for the smooth cutting. Sixthly, generate optimal NC toolpath code dependent on several types of wire-cut EDM machines by constructing a database of machining conditions. Seventhly, display a feature of wire in the developed CAD viewer, and combine the simulation module for the verification of the generated NC code. By using the proposed system, NC codes for 2-axis wire cutting are generated from the CAD viewer without an expensive CAM system, and short

lead-time is achieved. The efficiency of the system is confirmed on the CADviewer. CAD DATA INTERFACE Fig. 1 shows the structure of the translation server. Fig. 1 shows lightweight files produced from several CAD systems. Fig. 1(b) represents the InterOp Reader of Spatial Inc., which converts the ACIS kernel data into lightweight files. Fig. 1(c) shows the 3D modeling ACIS kernel. Finally, Fig. 1(c) represents the developed data extractor module, which generates lightweight files through the ACIS kernel API. All the processes described so far are automatically managed through the translation server. Various commercial CAD files (Fig. 1(a)) are converted into lightweight files through the proposed system. The CAD entities are mapped to the ACIS kernel entities and then to the lightweight entities [5, 6].

FIGURE 1. Architecture of the translation server.

FIGURE 2. A triangle and a section plane.

FIGURE 3. 2D profile and triangular meshes.

NC CODES FOR 3D GEOMETRIC SHAPE Wire-cut EDM is a process of metal machining, it machine a workpiece using a thin wire has tension and through electrical erosion by generating between workpiece and the traveling wire. Among the wire-cut EDM machine, 2D machining type that vertical type machine most widely is used, is for machining a workpiece which is same shapes of upper and lower. Coordinate of workpiece surface is defined a local position in X and Y axes, it generates NC code through circular interpolation. During linear and circular interpolation, G-code is used mainly as follows: G01: linear interpolation, X, Y G02: circular interpolation CW [clockwise direction], X, Y, I, J G03: circular CCW [counterclockwise direction], X, Y, I, J I, J: The signed distance from start point to center As above, 2D machining is consist of G-code of G01, G02 and G03 etc. Usually, NC pro-gramming method of EDM obtains a profile of 2D shape by inputting DXF file of AutoCAD, and is a type converting to G-code through it. The commercial CAM system generates profile of 2D shape using offered design module from each commercial system without using AutoCAD, and uses a generating method of corrected NC code through machining parameter and post-process. GENERATION OF 2D PROFILE 2D profile will be generated toolpath, is determined by a polyline which is connecting line with each points after calculating the intersection point between the inputted section from user and the triangle mesh of visualization data. Computing the intersection point between plane and triangle for generating a polyline, be able to obtain the vector efV , using two points e , f of Fig. 2 and Eq. 1. The plane equation of

plane D inputted from user is Eq. 2. If it substitutes the vector efV and coordinate of the intersection point 1h 0 0 0( , , )x y z of section D for Eq. 1, Eq. 3 about parameter 0t is able to be obtained. The plane D also satisfies because through the intersection point 1h . The parameter

0t is able to be obtained using Eq. 2, 3. The coordinate of the intersection point is computed substituting the parameter 0t for Eq. 3.

, ,1 2 3V x k t y k t z k tef f f f= + + + (1 )

0 0 0ax by cz d+ + = (2)

0 0 0 0 0 0, ,f f fx x t y y t z z t= + = + = + (3) If the calculated intersection points be connected, the polyline is generated. Fig. 3 illustrates the intersection point between triangle and section with CAD visualization data. Fig. 3 illustrates to generate the section that based on XY plane after converted visualization data of injection molding CAD file is imported the proposed viewer system. In the proposed system, we can generate the section that based on XY, YZ, ZX plane, arbitrary three points, and perpendicular about a straight line. NC CODE GENERATION In this paper, the method of generating a NC code is that generated polyline in previous chapter is defined a toolpath of wire-cut. Toolpath is generated by G01 linear interpolation. If CAD file that is same shape of upper and lower is generated the section which based on XY plane, YZ plane and ZX plane, using an proposed viewer system in Fig. 4, the section automatically is located in centre position of CAD file by the perpendicular direction to the section. 2D profile in the Fig. 5 is consist of the linear interpolation that includes the circular part. After determine a point that based on this, wire-cut EDM toolpath is generated to G code computing local coordinate about a global center point. Local coordinate of each dimension be able to be computed using a coordinate is obtained from the commercial CAD data.

FIGURE 4. 2D section generation on the XY plane.

INSIDE-OUTSIDE CHECKING Using a method introduced in the previous section, inner profiles of a workpiece such as a pocket or hole is generated together. However, shape of EDM in this paper needs to determine the inside-outside of a 2D profile through the odd-even test as shown in Fig. 5. The odd-even test is the most widely used test for making inside-outside decisions. When p is a point inside a polygon, any ray emanating from p and going off to infinity must cross an odd number of edges. Any ray emanating a point outside the polygon and entering the polygon crosses an even number of edges before reaching infinity. Hence, a point can be defined as being inside if we draw a line through it and if following this line, starting on the outside, we cross an odd number of edges before reaching it. Usually, we replace rays through points with scanlines, and we count the crossing of polygon edges to determine inside and outside. In this system, the odd-even number is determined through the checking of the polyline. Fig. 6 illustrates a result to keep only a required outside profile through test of the inside-outside [7].

FIGURE 5. Inside and outside of 2D profile.

FIGURE 6. 2D profile which only has an outer.

To generate toolpath of wire-cut EDM has not only to consider a starting point of machining but also radius of wire, cutting speed, oversize and material of workpiece etc [7]. In this paper, the toolpath is generated by using lightweight CAD data, it is applied computing local location of the toolpath to the G code. Fig. 8 shows a part of the G code generated from the toolpath of wire-cut EDM using 2D profile in Fig. 7.

FIGURE 7. A dialog of direction selecting and the 2D profile.

… N120 X-82.0575 N122 Y0. N124 Y-49.9 N126 X-82.0455 Y-49.912 N128 G40 X-81.9955 N130 M50 N132 G0 X0. Y0. N134 M60 N136 S0 D2 N138 G41 G1 X-82.0695 Y-49.988 N140 X-82.0575 Y-50. N142 X67.9425 N144 Y50. N146 X-82.0575 N148 Y0. N150 Y-49.9 N152 X-82.0455 Y-49.912 N154 G40 X-81.9955 N156 M50 N158 G0 X0. Y0. N160 M60 N162 S3151 D1 N164 G42 G1 X-82.0575 Y-49.988 N166 X-81.9825 N168 Y-50. N170 X-82.0575 N172 M01 N174 Y-49.9 N176 Y-49.825 N178 X-82.0455 N180 Y-49.9 …

FIGURE 8. Generated G code of Wire EDM by proposed system.

CONCLUSIONS By designing a cost-effective wire-cut EDM system for precision mold manufacturing, following conclusions are obtained: (1) Using triangular meshes and section curves of a CAD file, NC toolpath generation of the wire-cut EDM process is performed. (2) Using the proposed integrated system, it is possible to generate a toolpath of wire-cut EDM without using extra CAD/CAM modules. (3) An integrated CAD viewer system for injection mold design and manufacture is developed through the NC wire-cut EDM module. (4) The validity and effectiveness of the developed system have been verified through case studies on the CADviewer. For the further study, 4-axis toolpath generation, material DB, machining parameter DB, etc. should be studied. REFERENCES [1] Ivano B, Axel B, Dirk D. A Simplified Post

Process for Wire Cut EDM. Journal of Materials Processing Technology. 1996; 4: 385-389.

[2] Yang M, Lee E. NC Verification for Wire-EDM using an R-map. Computer-Aided Design. 1998; 28: 733-740.

[3] Daniel C H, Yang J J, Chuang Z, Han Z, Ding S. Boundary-conformed Toolpath Generation for Trimmed Free-form Surfaces via Coons Reparametrization. Journal of Materials Processing Technology. 2003; 138: 138-144.

[4] Wang J, Ravani B. Computer Aided Contouring Operation for Traveling Wire Electric Discharge Machining (EDM). Computer-Aided Design. 2003; 35: 925-934.

[5] Song I H, Chung S, Chung S C. Precision Design Verification System of Injection Molds using Web-based 3D Viewing Technology. Proceeding of the ASPE. 2006; 21: 451-454.

[6] Song I H, Chung S C. Geometric Kernel Design of the Web-Viewer for the PDM Based Assembly DMU. Transactions of the KSME(A). 2007; 31: 260-268.

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