Precision Engineering

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1 Precision Engineering 37 (2013) Contents lists available at SciVerse ScienceDirect Precision Engineering jou rnal h om epage: Tool wear compensation in 3D micro EDM based on the scanned area Jian-Zhong Li, Lu Xiao, Hui Wang, Hui-Lan Yu, Zu-Yuan Yu School of Mechanical Engineering, Dalian University of Technology, #2 Linggonglu, Dalian , Liaoning, PR China a r t i c l e i n f o Article history: Received 29 November 2012 Received in revised form 3 February 2013 Accepted 25 February 2013 Available online 4 March 2013 Keywords: Micro-EDM 3D micro cavity wear compensation Machining performance Scanned area a b s t r a c t Complex 3D micro-structures can be fabricated layer by layer in alloys by micro electrical discharge machining (EDM) using simple shaped electrodes. However, electrode wear is a serious problem which significantly affects machining accuracy. The electrode wear compensation method is a key issue in micro EDM milling and effectively solves this problem. This paper proposes a new compensation method based on the scanned area (BSA) in each layer machining. The new method is integrated with a CAD/CAM system to generate 3D micro cavities. Experimental results have been compared with those using the uniform wear method (UWM) and those using a combination of linear compensation with UWM. It was found that using the proposed method machining efficiency was improved and tool wear ratio was reduced Elsevier Inc. All rights reserved. 1. Introduction Nowadays the demand for micro structures, micro components and minimization of products is expanding, which thereby promotes the development of micro machining technology. As a non-traditional micro machining processes, micro EDM s ability of machining any electrically conductive materials regardless of their hardness and strength is gaining more and more attention. Because of these advantages, micro EDM is used extensively in drilling micro holes and fabricating micro parts and micro molds. In micro EDM milling, simple shaped electrodes with various cross-sections are applied to fabricate arbitrary three dimensional cavities. During the machining, however, the tip shape of electrodes quickly changes due to electrode tool wear, which dramatically deteriorates the machining accuracy. In order to solve this problem, a substantial amount of research effort has been done on micro EDM 3D milling. Generally, the compensation methods of electrode tool wear for micro EDM can be divided into two types: on-line and off-line. The advantage of the on-line method is that the electrode wear is compensated based on the data of real machining conditions. In on-line compensation methods, the electrode wear is measured by a digital image system installed on the machine or by the discharge signals that cross the gap [1 4]. The disadvantage of the on-line method is the low measurement accuracy. After an electrode is used, it has to be cleaned before measurements. It is a challenge to identify the edges of a tiny electrode with amplified image [1]. In micro EDM, Corresponding author. Tel.: ; fax: address: zyu@dlut.edu.cn (Z.-Y. Yu). shortening the discharge time allows for little electrical discharge energy from each pulse. It is a difficult task to catch these signals and analyze them in a short time [2 4]. Therefore, the measurement accuracy of on-line compensation may affect the accuracy of 3D micro shapes by micro EDM. The off-line electrode wear compensation method uses the previously obtained electrode wear data to compensate electrode wear during machining by adjusting the tool path. This method was used to study the simulation of geometric formation of micro hole and 3D micro shape under the conditions of electrode wear [5,6]. So far, all above mentioned electrode wear compensation methods have no consideration of tip shape change of electrodes due to wear. To machine 3D micro shapes using simple shaped electrodes by micro EDM accurately, the electrode shape has to remain unchanged. This is achievable to design the tool paths following the rules of uniform wear method (UWM) [7]. Furthermore, the uniform wear method is integrated with CAD/CAM to generate complex 3D micro shapes layer by layer [8]. Several micro cavities were successfully generated by micro EDM using simple shaped electrodes. The electrode tip shape is same as the original one after machining. The electrode wear length of each layer is compensated at the start point of the tool path from said layer by feeding the electrode with the wear length from the same layer. When the electrode is fed into the workpiece deeply, the sides of the electrode are involved in machining, which leads to frequent abnormal discharges and results in low machining performance of micro EDM. One solution is to divide the total compensated wear length of one layer into several parts [9]. The total length of the tool path in one layer is also divided into the corresponding sections. The divided wear length of the electrode is compensated at the beginning of each section of tool /$ see front matter 2013 Elsevier Inc. All rights reserved.

2 754 J.-Z. Li et al. / Precision Engineering 37 (2013) Nomenclature Z total electrode feed for each layer machining L w machined depth L e electrode tool wear length of each layer N compensation times in each layer S w area of machined layer S e cross-section area of electrode electrode tool relative volumetric wear ratio L electrode moving distance in one layer L i L/N, electrode compensation moving distance in X Y plane, i = 1 N S i electrode compensation scanned area in X Y plane, i = 1 N (4) (3) (7) (8) (6) (10) (13) (9) (12) path. The ratio between the divided wear length and the divided tool path length is linear. The electrode tool wear length is calculated based on the uniform wear method. This method is called combination of linear compensation with uniform wear method (CLU). Experimental results show that the machining performance is improved through factors such as material removal rate (MRR), electrode tool relative volumetric wear ratio (TWR) and surface roughness. The material removal volume is the product of the electrode scanned area and the layer thickness. In the real machining, the tool paths generated by a CAD/CAM system have to cover the area of each layer. The tool paths may overlap. In other words, in the real time one layer machining of CLU, the ratio of removal volume to electrode wear volume does not have a linear relationship if the compensation points are linearly distributed along tool paths of this layer due to the scanning tool path overlap [10]. To compensate electrode wear at accurate points of tool paths, it is necessary to consider the scanned area or removed area when compensation points are set. In this paper, a new electrode wear compensation method is proposed. The compensation points along the tool paths of each layer are set based on the scanned area (BSA) to achieve the constant electrode wear ratio. After the introduction of a new compensation method, experiments are carried out to compare the machining performance of UWM, CLU and the proposed method. Next, experimental results are analyzed and discussed. The conclusions are given in the final section. 2. Principle of the proposed compensation method (BSA) Fig. 1 shows the tool paths for machining a square cavity. One layer of the cavity is scanned by an electrode along the numbered tool paths. To compensate the electrode tool wear length, the tool is fed into the workpiece with the depth of both the layer itself and the electrode wear length for machining said layer obtained by Eq. [7]. In UWM, as shown in Fig. 2(a), the electrode tool wear is compensated at the start point of this layer s machining. When the electrode is fed below the surface of the workpiece, the electrode side surface is involved in machining, resulting in the increase of abnormal discharge occurrence and formation of an inclined surface. In CLU, as shown in Fig. 2(b), the electrode wear is compensated gradually, and is linear to the electrode moving distance of tool paths. In this way, the machining process is stable. The machining performance is improved [9]. It can be seen that the grey area in Fig. 1 is the scanned area when the electrode moves along tool paths 1 4. The dark part in the grey area is the overlapped area by electrode scanning. When the CLU is used, electrode wear is not compensated linearly to the removed volume. The removed volume is the product of the scanned area and the depth of layer. Unscanned area (5) Once-scanned area (11) Fig. 1. Tool paths for machining square cavity. Overlap area Therefore, it is necessary to find the accurate compensation positions to ensure the electrode wear length compensation is linear to the removed volume as shown in Fig. 2(c). ( Z = L w 1 + S ) w S e In this paper, the electrode wear compensation based on the scanned area (BSA) is proposed. In this method, a cavity is sliced into layers of the same thickness. The area of each layer is calculated and recorded. The electrode feed including the layer thickness and electrode wear length is calculated by Eq.. The electrode wear of this layer is compensated evenly. The compensation time, N, is obtained by dividing the electrode wear length of this layer with the minimum movement of stage. In this study, the resolution of stage 0.1 m is used as the minimum movement of stage. Correspondingly, the area of this layer is divided by N as in Eq.. S 1 = S 2 = S n = S w N Each layer is divided into a finite number of grids in X Y plane as shown in Fig. 3 The position in the X Y plane and the status of each grid are recorded in an array. When the electrode moves along the tool path, the statuses of grids covered by the electrode and its discharge gap are checked. The number of unscanned grids is counted and corresponding statuses of grids are converted into scanned grids. When the product of the grid area and the number of grids is equal to or larger than S w /N, the electrode is fed into workpiece of L e /N at that position. In this way, the electrode wear compensation is evenly distributed based on the scanned area. 3. Experiments To verify the proposed electrode wear compensation method, experiments using different electrode compensation methods are carried out and their results are compared and discussed.

3 J.-Z. Li et al. / Precision Engineering 37 (2013) Moving direction L (a) wear compensation of UWM. Moving direction ΔL 1 ΔL 2 ΔL 3 (b) wear compensation of CLU. Moving direction ΔS 1 ΔS 2 ΔS 3 ΔS 1 = ΔS 2 = ΔS 3 (c) wear compensation of new method (BSA). Fig. 2. Different electrode wear compensation methods. (8) (13) P 2 P 3 P 1 (7) (9) (3) P 4 P 7 (4) (10) P 0 P 9 (6) (12) P 8 Table 1 Machining conditions. Fig. 4. Experimental equipment. Pulse generator Relaxation type Open circuit voltage (V) 80 Capacitance (pf) 220 Dielectric Mineral oil material Stainless steel AISI 304 Polarity of workpiece Positive material Tungsten Polarity of electrode Negative Discharge gap ( m) 5 tool wear volumetric ratio (TWR) 3% Rotation speed of spindle (rpm) Experiment setup Fig. 4 shows the experimental equipment used in this study. It consists of XYZ stages with the resolution of 0.1 m, a high precision spindle with the roundness of 1 m, a wire electrical discharge grinding (WEDG) unit and a set of digital microscope for observation. The machining conditions are listed in Table 1. To machine a complex 3D shape, a CAD/CAM system is necessary to generate tool paths. There is no commercial system available to compensate the electrode tool wear. To machine 3D micro shapes accurately under the conditions of electrode tool wear, a CAD/CAM system has to integrate with a tool wear compensation method. Fig. 5 shows the structure of integrating an electrode wear compensation method with CAD/CAM. First, a 3D part feature is designed and sliced into layer of equal thickness by CAD software. Then, the CAD Module Part Modeling CAM Module Generating Tool path Micro EDM Machine Transferring NC codes from PC to machine Y P 0 X (5) P 5 (11) Area scanned by electrode Areas of Sliced layers Regenerating tool path using the new method NC codes Fig. 3. wear compensation based on scanned area. Fig. 5. Integration of electrode wear compensation with CAD/CAM system.

4 756 J.-Z. Li et al. / Precision Engineering 37 (2013) Fig. 6. Designed cavity. tool paths are generated using CAM software. To compensate the electrode shows the machined cavities using different compensation methods. Although all electrodes were worn over 100 m in length, the shapes of electrode tips are kept unchanged as shown in Fig. 8 tool wear, the tool paths are re-generated by adjusting the electrode feed in the Z axis for each layer using Eq.. In this study, three electrode wear compensation methods including UWM, CLU and BSA are used. The differences of these methods are found in the position and the distribution of electrode wear compensation in each layer s machining. After tool paths are generated, they are converted into NC codes which are transferred into the experimental equipment to fabricate arbitrary 3D micro shapes Experimental results and discussion Fig. 6 shows a rectangular cavity (top size: 444 m 354 m; bottom size: 300 m 210 m; depth: 72 m) with a cylinder (height: 60 m; diameter: 40 m) on the bottom. A cylindrical electrode of 50 m in diameter was fabricated using the WEDG method. Three sets of tool paths based on UWM, CLU and the proposed method are generated. The overlap distance between two adjoining paths is set at 40 m. The tool relative volumetric wear ratio (v = 3%) and the discharge gap (Gap = 5 m) were obtained in preliminary experiments under the same machining conditions. To compare effects of three methods, the cavity is machined by UWM, CLU and the proposed method, respectively. The top layer of machining is taken as the example to explain the difference of the three compensation methods. The thickness of the layer is 0.5 m and the electrode wear length for machining this layer is 1.2 m. At the start point of the tool path of this layer in UWM, the electrode is fed 1.7 m into the workpiece. In the study of this paper, the minimum electrode feed is 0.1 m which is equal to the resolution of Z stage. In CLU, the total length of tool paths for this layer s machining is calculated and divided by 12. At the start point of tool paths, the electrode is fed 0.6 m into the workpiece. After the electrode travels 1/12 of the tool paths of this layer, the electrode is fed 0.1 m into the workpiece at the Z axis to compensate the electrode wear. In BSA, total area of this layer is calculated and divided by 12. This layer is divided into identical grids as shown Fig. 3 and each grid is marked as unscanned. The process used by CLU is the same in the beginning in that the electrode is fed 0.6 m into the workpiece at the start point of the tool paths. During the movement of the electrode along the tool paths, grids covered by said electrode are counted and the corresponding status is changed into scanned. When the scanned grid area is equal to or larger than the 1/12 of this layer area, the electrode is fed 0.1 m into the workpiece at the Z axis. It indicates the electrode wear compensation is based on the scanned area. Table 2 lists machining parameters of three electrode wear compensation methods. Before and after machining, the electrode was controlled to contact to the reference point on the workpiece surface. The difference in the Z axis is the electrode wear length. The machined depth of the cavity was obtained by detecting the surrounding top surface and bottom surface of the cavity electrically. The dimensions of the cavity in the X Y plane were measured using an optical microscope with a resolution of 0.1 m. The diameter of the electrode was measured in the same way. The surface roughness was measured using an interferometer. The machining time was recorded. Fig. 7 This Fig. 7. Machined cavities by three methods. Fig. 8. after machining.

5 J.-Z. Li et al. / Precision Engineering 37 (2013) Table 2 Machining parameters of three methods. UWM CLU BSA Sliced layer number Layer thickness ( m) Compensation times for first layer Compensation times for last layer relation of electrode tool relative volumetric wear ratio (TWR) and the three methods under different layer thickness. It is clear that the electrode tool wear ratio decreases in the order of UWM, CLU and BSA. This means that occurrence of abnormal discharges decreases and machining processes are stabilized under the same machining conditions. Fig. 11 shows the surface roughness, Ra, generated under difference layer thickness using three tool wear compensation methods. In this study, the surface roughness is dominated by the layer thickness. The tool compensation method has no significant influence on the surface roughness. 4. Conclusions Fig. 9. MRRs of three methods. Micro EDM has the ability to machine 3D micro shapes in any electrically conductive material. To generate micro shape correctly, the electrode wear has to be compensated. In this paper, a new method is proposed, which is based on the scanned area (BSA) to achieve accurate electrode wear compensation in 3D micro EDM milling. To verify the proposed method, experiments were carried out. Experimental results indicate that MRR of the proposed method increases and TWR decreases because the electrode wear length is evenly distributed in one layer machining, which reduces the occurrence of abnormal discharges. Acknowledgements Fig. 10. TWRs of three methods. Authors are thankful for the support from the National Science Foundation of China (NSFC) (Nos and ), SRFDP of China (No ) and the National High-Tech R&D Program of China (No. 2009AA044205). References Fig. 11. Surface roughness. indicates that the electrode uniform wear has been achieved in all three methods. Material removal rates (MRR) under different layer thicknesses using different compensation methods are summarized in Fig. 9. It can be seen that the layer thickness influences MRR significantly. When the layer thickness is less than 1 m, MRR is improved in the order of UWM, CLU and BSA. When the layer thickness was set at 1 m, MRR has no significant difference. Fig. 10 shows the [1] Yan MT, Huang KY, Lo CY. A study on electrode wear sensing and compensation in Micro-EDM using machine vision system. International Journal of Advanced Manufacturing Technology 2009;42: [2] Tong H, Wang Y, Li Y. Vibration-assisted servo scanning 3D micro EDM. Micromechanics and Microengineering 2008;18:1 8. [3] Chang YF, Chiu ZH. wear-compensation of electric discharge scanning process using a robust gap-control. Mechatronics 2004;14: [4] Bleys P, Kruth JP, Lauwers B. Sensing and compensation of tool wear in milling EDM. Materials Processing Technology 2003;149: [5] Young HJ, Min BK. Geometry prediction of EDM-drilled holes and tool electrode shapes of micro-edm process using simulation. International Journal of Machine Tools & Manufacture 2007;47: [6] Segon H, Young HJ, Min BK. Virtual EDM simulator: three-dimensional geometric simulation of micro-edm milling processes. International Journal of Machine Tools & Manufacture 2009;49: [7] Yu ZY, Masuzawa T, Fujino M. Micro-EDM for three dimensional cavities development of uniform wear method. CIRP Annals Manufacturing Technology 1998;47: [8] Rajurkar KP, Yu ZY. 3D micro-edm using CAD/CAM. CIRP Annals Manufacturing Technology 2000;49: [9] Yu HL, Luan JJ, Li JZ, Zhang YS, Yu ZY, Guo DM. A new electrode wear compensation method for improving performance in 3D micro EDM milling. Journal of Micromechanics Microengineering 2010;20(5): [10] Yu ZY, Kozak J, Rajurkar PK. Modeling and simulation of micro EDM process. CIRP Annals Manufacturing Technology 2003;52:143 7.