CAM

3D Micro-EDM Using CAD/CAM K. P. Rajurkar (2), Z. Y. Yu Center for Nontraditional Manufacturing Research, Department of Industrial and Management Systems Engineering, University of Nebraska-Lincoln, Lincoln, USA Received on January 3,2000

Abstract It is necessary to integrate CAD/CAM systems with micro-EDM to generate tool paths when simple shaped tools are used to machine three-dimensional (3D) micro parts. Currently available CAD/CAM systems cannot be directly used because of the continuous tool electrode wear during machining. This paper proposes an approach to integrate CAD/CAM systems with micro-EDM while accounting for tool wear using a recently developed uniform wear method. This approach is verified by successfully generating very complex 3D micro cavities. Additionally, the feasibility of the approach is illustrated by generating complex macro cavities using conventional EDM with single simple shaped electrodes.

Keywords: EDM, Micro-machining, Wear

1 INTRODUCTION Most microfabrication techniques such as etching, deposition, lithography and laser etc. are suitable to fabricate surface structures in mass production. However, these methods, in general, lack the ability of machining three-dimensional shapes because of poor machining control in the Z axis. It is possible to machine a metal micro part having a three-dimensional shape by traditional manufacturing methods such as milling, however, the size of the machined part will be limited by the mechanical strength of the part and the tools used. Micro-EDM, where work material is removed by electro-erosion and not by mechanical contact, offers an effective solution to the problem of tool and work deformation. Additionally, microEDM can be used to machine any electrically conductive material regardless of its hardness and strength. However, proper compensation of tool wear should be considered for producing micro parts by micro-EDM. Micro-EDM has been successfully used in many applications such as the fabrication of micro-pins with various cross sections by Wire Electrodischarge Grinding (WEDG) [ I ] [2] [3]. It has been used to prepare micro tools for micro-drilling, micro-ultrasonic, micro-punching and micro-milling machining [4] [5] [6]. It has also been used to fabricate micro parts such as micro nozzles [7] [8]. The micro-EDM equipment has improved over the last few years. Now, micro-EDM is capable of machining micro holes and slots as well as 3D micro cavities [9]. Three types of electrodes (wire, simple shaped electrodes and complex shaped electrodes) are used in micro-EDM. Although the wire EDM (wire diameter is 20pm-30pm) can cut the micro parts from a thin sheet, it is not suitable to fabricate micro cavities or molds. The complex shaped electrodes made by the LlGA technique have been used in die sinking operations [lo]. However, making complex shaped electrodes by the LlGA method is expensive due to the high cost of the LlGA equipment. Additionally, several electrodes are necessary to achieve the final design requirements because of electrode wear during machining. It is difficult to align electrodes at the same

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position. Moreover, LlGA electrodes cannot be used to machine inclined surfaces or spherical surfaces because the LlGA technique can produce electrodes only with straight sides. 3D micro-shaped machining is realizable by controlling the movement of simple shaped electrodes along designed tool paths. However, the application of micro-EDM in generating 3D complex cavities and therefore 3D microparts is limited due to lack of the necessary Computer Aided Design and Manufacturing (CADICAM) systems to generate the tool paths [ I l l . Although some attempts on the application of CAD/CAM in conventional die-sinking EDM were reported [12][13], a CAD/CAM system applicable to micro-EDM that takes tool wear into account is currently not available. In this paper, an approach is proposed to solve the above mentioned problems. A recently developed technique called Uniform Wear Method was applied to account for the electrode wear and it was integrated with a CAD/CAM system. Using this proposed approach, some complex 3D micro shapes were machined successfully. The feasibility of the proposed approach in conventional EDM was also verified in this research. INTEGRATION OF UNIFORM WEAR METHOD WITH CAD/CAM SYSTEM The extensive functions of commercially available CAD/CAM systems can be utilized for micro-EDM by appropriately integrating it with the uniform wear method to compensate for tool wear. The uniform wear method includes the rules of generating tool paths and the compensation equation. Generating tool paths based on the uniform wear method assures that the shape of the electrode tip remains unchanged even if there is electrode wear during machining. The rules of uniform wear method are [14]: 2

Machining by the bottom of the electrode, or layer-bylayer machining with a small electrode feed for one layer. To-and-fro scanning. Tool paths overlapping.

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CAD Module

CAM Module

for machined features calculating area of each sliced surface and compensating electrode wear length based on the uniform

Post processor

Transferring NC codes to micro-EDM

-b

-

NC codes

\

Figure 1: Integrating the uniform wear method with CAD/CAM system. Machining the central part and the boundary of the machined surface alternately. To compensate for the electrode wear in length, the electrode feed for each sliced layer is calculated based on equation (1) derived from the definition of electrode volume wear ratio and the assumption that the electrode feed of each layer consists of two parts, the wear length and the remaining length which equals the average machined depth of the layer [9][14]. AZ = L, ( v SJS,+l).

(1)

where v -electrode volume wear ratio; Se-cross section area of the electrode (X-Y plane); S-area of the machined layer (X-Y plane); AZ-depth of cut (electrode feed): L-machined depth. Figure 1 shows the structure of the integration of a CAD/CAM system with the uniform wear method. The tool paths are generated based on the data in the CAD module, and then the tool paths are regenerated using the uniform wear method followed by the compensation of electrode wear in the NC codes. The NC codes are finally transferred to the micro-EDM. In this way, an existing CAD/CAM system can be integrated with the uniform wear method to fabricate 3D micro shapes by EDM using simple shaped electrodes. 3 EXPERIMENTS The proposed approach has been used to produce micro as well as macro cavities. Some sample examples are described below. 3.1 Application in Micro-EDM

Figure 2 shows a computer generated complex micro cavity consisting of a square cavity (480x480x40pm3)with inclined surfaces, a cylindrical cavity (radius 200pm, depth 100pm) with a curved side surface (radius 100pm) and a pyramid (60x60~1 20pm3) in the center. The experiment was carried out on a commercial micro-EDM with a

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Figure 2: CAD design of a complex cavity. Pulse generator Open circuit voltage Capacitor Workpiece material Electrode material

I

Relaxation type

I 80V

1OOpF Stainless Steel AlSl 304 Tungsten

Table 1: Machining Conditions in micro-EDM. WEDG unit. Table 1 lists the machining conditions. A cylindrical electrode (diameter 52pm) was prepared using the WEDG unit on the micro-EDM. When the electrode is used to machine a complex 3D micro shape, the generated tool paths must satisfy the rules of the uniform wear method to keep the tip shape of the electrode unchanged. Figure 3 (a and b) shows the pattern of tool paths (similar to milling operation) in the XY plane for each sliced surface generated by the CAD system. The cut angle (angle between the direction of tool paths and the X-axis) is 0" in Figure 3 (a) and is 90" in Figure 3 (b). In each case, however, the same thickness (machined depth) of 0.5pm has been used for slicing. In order to assure uniform wear and complete removal of material, the overlap between the two adjoining paths has been set at 35pm, less than the diameter of electrode but larger than the radius of it. In order to improve the precision of the machined surface and to obtain a more uniform electrode wear, the cut angle for each subsequent layer is changed. The electrode moves along the tool path (cut angle 0") in Figure 3 (a) to machine one layer, and

Figure 4: SEM photograph of machined cavity.

(b) Cut angle is 90". Figure 3: Tool paths. then moves along the tool path (cut angle 90") shown in Figure 3 (b) to machine the next layer. The electrode volume wear ratio (0.012) obtained in the preliminary experiments by slot machining and the cross sectional area of the electrode are known before machining. The area of the machined layer, Sw, is obtained in the CAD module. The electrode feed for each layer in the Z axis changes with the variation in the crosssectional area under the same machining conditions. The calculated maximum electrode feed is 2.29pm and minimum electrode feed is 1.16pm for machining this cavity. The calculated total wear length of the electrode is 69.68pm. After regenerating the tool paths, the NC codes are transferred to the NC controller in micro-EDM. Figure 4 and Figure 5 show the machined cavity and the electrode (after machining the cavity), respectively. The electrode shape after machining has been found to be the same as the original one. Figure 6 illustrates the second complex cavity machined by micro-EDM using a cylindrical electrode. The radius of the spherical cavity is 150pm. The conical cavity (top diameter 240pm, bottom diameter 120pm, depth 60pm) is connected with the spherical cavity by a slot with inclined surfaces. 3.2 APPLICATION IN CONVENTIONAL EDM The proposed approach is also applied to generate 3D macro cavities using a conventional EDM. A rectangular cavity (35x25~1 lmm3) with a raised % ball (radius 10mm) in the center was machined using a cylindrical electrode (diameter 6.46mm). The related machining parameters are listed under machining conditions 1 of Table 2. The electrode volume wear ratio is 0.013. The total electrode wear length is 6.63mm. The machining of the cavity (Figure 7) took 20 hours. The machining efficiency can be improved by dividing the machining into roughing and finishing. The major portion of work material is eroded in rough machining. The final design requirements are achieved in the finishing stage. Figure 8 shows a complex cavity machined by three electrodes for rough to finish machining operations. The cavity consists of three features. The left portion consists of a cylindrical cavity (diameter 38mm, depth 13mm) with a square column (10x10x12mm3)in the center. The right portion is a rectangular cavity (20x25x17mm3). The connecting channel has a rectangular cross section (14.5x7mm2)with 15.5 mm in length. The selection of electrode size and shape is very

Figure 5: SEM photograph of electrode after machining.

Figure 6: SEM photograph of conical and spherical cavity.

1

Machining condition Open circuit voltage Peak current of discharge Discharge duration Pulse interval time Workpiece material Electrode material

I

1

1

2 1 3 1 80V 9A 55A 7A 48ps 256ps 16ps 32ps 64ps 16ps Steel 1020 Graphite

I

Figure 7: A raised A ' ball inside a rectangular cavity. important in 3 0 shape machining by EDM using simple shaped electrodes. It depends not only on the size of the

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Further investigations into the optimization of micro-EDM parameters, process monitoring, and controlling for improving the process performance are continuing. 5 ACKNOWLEDGMENTS Authors are thankful for the support from the Nebraska Research Initiative Fund (NRI) and NSF MTAMRI Grant #DMl-9320944. Dr. K. P. Rajurkar thankfully acknowledges the support from the U. S. National Science Foundation under the Intergovernmental Personnel Act program.

Figure 8: A complex cavity by rough and finishing workpiece, shapes of features and the space between features, but also on the desired material removal rate. Although a square electrode is suitable to machine the left square cavity, a cylindrical electrode (diameter 6.29mm) was used to remove most of the material in rough machining because its material removal rate (0.243mm3/min) was found to be larger than that of a square electrode with the same size (0.12 mm3/min) under the same machining conditions 2 listed in Table 2. The remaining thickness (2mm) of each rough machined surface including the round corners in the left cavity can be removed in finish machining. In finishing, half of the size of electrode (one side of square electrode or diameter of cylindrical electrode) should be less than the remaining thickness of the machined surface. A smaller electrode will result in longer machining time. A cylindrical electrode with 3.25mm in diameter was used to finish the left cylindrical cavity using the machining conditions 3 listed in Table 2. A square electrode (3.24x3.24mm2)was used to machine the right cavity under the same machining conditions because of its right angle corners. The connecting part can be machined using either the cylindrical electrode or the square electrode. Experimental results indicated the same material removal rate for both electrodes for this case. However, the electrode wear ratio of the cylindrical electrode was smaller than that of the square electrode. Therefore, a cylindrical electrode was used to machine the connecting part. The rough and finish operations took 16.5 hours and 54.6 hours, respectively. Although the machining time may be longer, this approach substantially reduces the cost and time of preparing the complex shaped electrodes and therefore, the overall productivity may be higher. The roughness values (Ra) of the bottom and side surfaces in Figure 7 and Figure 8 are 8pm and 3pm, respectively. The large roughness value of the spherical surface in Figure 7 was caused by the tool path marks. It seems that the use of gentle machining parameters and adjusting the electrode feed for each layer will reduce these marks. The dimensional errors for both cavities are within +O.lmm. These errors may have been caused by the eccentricities of cylindrical electrodes and the thermal deformation of the mechanical structures of the EDM.

4 SUMMARY This paper presents a method of integrating an existing CAD/CAM system with the uniform wear method to machine complex 3D micro shapes using simple shaped electrodes. The experimental results show that the proposed method successfully addresses the problem of electrode wear in generating tool paths. The feasibility of applying this method in conventional EDM was also verified. The proposed approach has an application potential in micro and macro mold/die-making industries.

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REFERENCES Masuzawa, T., Tlinshoff, H. K., 1997, ThreeDimensional Micromachining by Machine Tools, Annals of the CIRP, 46/2:621-628. Masuzawa, T., Fujino, M., 1990, A Process of Manufacturing Very Fine Pin Tools, SME Technical Paper, MS90-307:l-12. Masuzawa, T., Fijino, M., Kobayashi, K., 1985, Wire Electrodischarge Grinding for Micromachining, Annals of the CIRP, 3411:431-434. Fujino, M., Okamoto, N., Masuzawa, T., 1995, Development of Multi-Purpose Microprocessing Machine, Proceedings of International Symposium for ElectroMachining (CIRP sponsored) Xl:613-620. Masuzawa, T., Yamamoto, M., Fujino, M., 1989, A Micropunching System using Wire-EDG and EDM, Proceedings of International Symposium for ElectroMachining (CIRP sponored) IX:86-90. Egashira, K., Masuzawa, T., 1999, Microultrasonic Machining by the Application of Workpiece Vibration, Annals of the CIRP, 48/1:131-134. Kuo, C. L., Masuzawa,T., Fujino, M., 1992, High Precision Micronozzle Fabrication, IEEE MEMS:I 16121. Langen, H. H., Masuzawa, T., Fujino, M., 1995, Modular Method for Microparts Machining and Assembly with Self-Alignment, Annals of the CIRP, 44/1:173-176. Yu, Z. Y., Masuzawa, T., Fujino, M., 1998, MicroEDM for Three-Dimensional Cavities, --Development of Uniform Wear Method--, Annals of the CIRP, Vol. 4711:169-172. 101 Ehrfeld, W., Lehr, H. Michel, F. Wolf A., 1996, Micro Electro Discharge Machining as a Technology in Micromachining, SPlE 2879:332-337. 111 DeVries, M. F., Duffie, N. A., Kruth, J. P., Dauw, D. F. and Schumacher, B., 1990, Integration of EDM within a CIM Environment, Annals of the CIRP, 39121665-672. 121 Kaneko, T., Tsuchiya, M., 1984, ThreeDimensionally Controlled EDM Using a Cylindrical Electrode (5th Report), --Research on Contour Machining of Inclined Plane and Cylindrical Surface-, Journal of Japan Society of Electrical Machining Engineers, 18136:l-16 (in Japanese). [I31 Lauwers, B., Kruth, J. P., 1992, Computer Aided Manufacturing for EDM Operations based on a Workpiece Description, Proceedings of International Symposium for ElectroMachining (CIRP sponored) X:l90-203. [I41 Yu, Z. Y., 1998, Three Dimensional Micro-EDM Using Simple Shape Electrodes, Ph. D. Thesis, University of Tokyo, Japan.