Water jet machining of MEDM tools

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1 Water jet machining of MEDM tools O. Blatnik a, H. Orbanic a, C. Masclet b, H. Paris b, M. Museau b, J. Valentincic a, B. Jurisevic a and M. Junkar a a University of Ljubljana, Slovenia b University of Grenoble, France Abstract This contribution presents an investigation about the possibilities of using Water Jet (WJ) technology in combination with Micro Electrical Discharge Machining (MEDM) for tooling production in micro manufacturing. In the first phase the tool copper used in MEDM is produced by WJ machining. Afterwards, the final tool in steel is produced by MEDM. Such kinds of tools intend to be used in processes like hot embossing, molding, and other replication technologies in the field of micro manufacturing. The first results are very promising and the proposed tooling strategy, which involves besides MEDM also WJ technology, shows a lot of potential especially in the design and developing phase of micro-fluidic devices. Keywords: tooling, non-conventional technologies, MEDM, WJ, micro-fluidics 1. Introduction Machining of tools used to produce micro components is usually expensive and time consuming. Thus, the tool used for replication technologies such as hot embossing or molding has to be well defined in advance. Once the tool is produced the corrections or modifications are difficult to implement. The main objective of this contribution is to present a new tooling strategy based on the application of WJ machining, which would allow relatively quick and cost effective production of prototype micro components. In the first step the tool for MEDM is produced in copper by WJ technology. Then the copper tool is used by MEDM technology to produce the final tool in tool steel, which may be further used for replication processes such as hot embossing, pressure molding and others. The complete process chain is shown in Figure 1. Figure 1. The production sequence of micro-fluidic channel. The proposed tooling strategy offers high flexibility and cost effectiveness. Additionally, it provides more freedom and testing opportunities during the development of new micro devices. The most common used tooling strategy is direct manufacturing of the tool by micro milling. When the features of the tool are rather ribs then grooves, the tooling strategy proposed in Fig. 1 has a great advantage over micro milling tool manufacturing, which is the most common used tooling strategy. In the latter case, an end-mill with a relatively small diameter has to remove relatively big volume of the tool which is time consuming and not cost effective. The main application field of the proposed tooling strategy is the design and development of microfluidic devices. Typically, these devices require a well-controlled geometry and surface roughness [1]. With this technology these devices can be manufactured relatively fast and in a cost effective way. Therefore many new concepts and designs can be experimentally validated during the development phase in order to improve the performance of the final product. In the actual context of R&D, flexibility in the manufacturing process enables variety and innovation in the design. The proposed tooling strategy consumes most of its machining time in MEDM machining while WJ machining accounts just for a small portion of the total machining time. However, facing this sequence of different processes, WJ machining of the MEDM tool has an important influence on the final result. The paper is organized as follows: after an introduction to the proposed tooling strategy the second section describes the WJ machining process applied to the machining of the electrode. In the third section, MEDM is described, as well as the manufacturing of the tool by the MEDM process using the electrode made by WJ. The results are presented in the fourth section. They constitute the basis for the analysis and discussion that is held in the fifth section. Finally, conclusions are given on the proposed tooling strategy in the last, sixth section. 2. WJ machining of EDM tools WJ machining is a non-conventional machining process in which a high-speed jet of water is used as the tool to remove the workpiece material by erosion. In order to generate such a tool, the water is first

pressurized. The water pressure is usually set up to 400 MPa and is generated with a hydraulic intensifiers specially designed for this technology.

2 pressurized. The water pressure is usually set up to 400 MPa and is generated with a hydraulic intensifiers specially designed for this technology. To generate an effective WJ, a specially designed nozzle with an orifice made in a sapphire insert is used. Typically, the diameters of orifices used for this technology are between 0.3 and 0.08 mm, depending on the application. This technology is mostly used to cut softer materials like wood, plastics, rubber, etc. but can be also applied for cleaning and engraving in harder materials. For metals and other hard to machine materials Abrasive Water Jet (AWJ) machining is more appropriate due to a higher Material Removal Rate (MRR) comparing to WJ machining [2]. In spite of all the advantages that WJ technology brings when producing electrodes for MEDM, the main problem is how to control the penetration depth of the WJ into the workpiece material. Investigations in the field of AWJ milling [3, 4] show that it is difficult to predict and control the depth of penetration of the AWJ into the workpiece material. In general, the depth of penetration depends on several process parameters. One of the most important and easiest to control is the traverse velocity of the cutting head, which defines the exposure time of the workpiece to the WJ. This means that only 2.5 D tools for MEDM can be produced with this technology. The difficulty of controlling the WJ penetration consequently causes that all features on the EDM electrode are deeper than required for the EDM process as shown in Figure 2. Thus the bottom of the slots of the EDM tool machined with WJ will not be functional surfaces. Figure 2. Specifics of MEDM tool production with WJ. As a case study, the manufacturing of a tool for a lab-on-chip application was chosen as presented in Figure 3. Figure 3. Lab-on-chip feature. Machining of the MEDM tool took place on the OMAX type 2652A/20HP abrasive water jet cutting system powered by a Böhler Ecotron 403 hydraulic intensifier capable of reaching water pressures up to 410 MPa. In this case a WJ cutting head was used with the orifice diameter of 0.1 mm. According to previous experience the water pressure was set at 300 MPa, the traverse velocity of the cutting head was 10 mm/min and the stand-off distance between the cutting head and the workpiece was kept constant at 2 mm. 3. MEDM production of the embossing tool Electrical Discharge Machining (EDM) is a machining technique through which the surface of a metal workpiece is formed by discharges occurring in the gap between the tool, which serves as an electrode, and the workpiece. The gap is flushed by the third interface element, the dielectric fluid. The process consists of numerous randomly ignited monodischarges. During a discharge, a plasma channel is formed which serves as the current conductor and the heat generator. As a consequence, a crater appears on the spot of the discharge. The size of the crater depends on the discharge energy, which can be set on the machine by adjusting the discharge current and its duration. The discharge voltage, which also determines the discharge energy, cannot be adjusted on the machine explicitly since it depends on the gap width between the workpiece and the electrode [5]. The MRR is determined by the crater size and the frequency of craters generation, i.e. discharge energy and the frequency of discharges. The latter is influenced by the discharge duration and the pulse interval between two discharges. The gap width between the workpiece and the electrode is in the range of 0.01 and 0.1 mm. The MRR is around 100 times higher on the workpiece than on the electrode. The main difference between EDM process used for macro workpieces and MEDM is in the accuracy for the electrode feeding system, also called servo system, and machining parameters. The comparison is given in Table 1. μ μ μ μ Table 1. EDM and MEDM machining parameters machining EDM MEDM parameter discharge current up to 200 A up to 3 A discharge duration up to 1000 m up to 50 m pulse interval up to 1000 m up to 200 m The MEDM machining is usually performed with rod electrode, whose path is controlled by CNC controller. The commercially available rod electrodes have diameters down to 0.05 mm, but usually bigger electrode diameters, for instance 0.15 mm, are often used due to limitations in electrode handling systems. Smaller electrode diameters are obtained by applying wire EDM grinding or etching after clamping of the electrode on the MEDM machine [6]. In this research, the tool for embossing of the microfluidic channels is made by employing a sinking MEDM, where the electrode has a negative shape of

In this case of manufacturing the pin-like shape of the tool (Figure 1), the sinking EDM achieves much higher MRR than MEDM milling since the size of the machining surface of the electrode (Figure 4)

The following machining parameters were used on the machine IT Elektronika 200M-E: discharge current i e=1 A, ignition voltage u i=180 V, discharge duration t e=8 µs and pulse interval t o=36 μ s.

3 the required shape on the workpiece. The accuracy of the electrode shape is directly transferred into the workpiece if the orbital or planetary motion of the electrode (employing CNC controller) is not used. In this case of manufacturing the pin-like shape of the tool (Figure 1), the sinking EDM achieves much higher MRR than MEDM milling since the size of the machining surface of the electrode (Figure 4) is much bigger than the machining surface size of the rod electrode with a diameter less than 0.2 mm [7]. The following machining parameters were used on the machine IT Elektronika 200M-E: discharge current i e=1 A, ignition voltage u i=180 V, discharge duration t e=8 µs and pulse interval t o=36 μ s. the tool (Figure 6), (2) the slot profile of the electrode and the tool (Figure 7), and (3) the surface roughness of the tool The top profile The various measurement points of the top profile of the electrode before and after MEDM machining and of the finished embossing tool were measured by a digital camera (Imaging Source DMK 21F04, 640x480) at positions shown in Figure 6. The results of measurements are collected in Table 2. In the first row the desired dimensions of the microchannels are given. The specific dimensions were chosen in order to evaluate the discrepancy between expected and obtained values. a) Figure 4: Electrode (a) before and (b) after the machining of the tool. The wear phenomenon of MEDM process can be observed in Figure 4(b), while the tool produced by the given electrode is presented in Figure 5. Comparing the machining times of the WJ, MEDM and embossing processes, the following conclusions arisen. In order to produce the electrode given in Figure 3, the machining time of WJ process was less than 5 minutes. Compared to the MEDM time, which took about 12 hours for manufacturing of the tool (Figure 5), the machining time required for making the electrode is really negligible. It is worth noticing that the MEDM machining time is related to the relatively low MRR (0.051 mm 3 /min) and to the relatively large removed volume (36 mm 3 ). Figure 5. Steel tool for hot embossing Lab-on-chip application. b) Figure 6. Drawing of a Lab-on-chip feature measurement points. Table 2. Measurement of the top profile (in mm). Dimension A B C D E Drawing Tool New electrode Worn electrode The side gap between the electrode and the tool during MEDM was approximately 0.1 mm, which was considered in the phase of electrode machining. The tip of the rib of the tool has the width of only 0.04 mm, which can be considered as a consequence of the poor flushing conditions in the gap The channel profile In order to achieve the desired dimensions of the finished part, the channel profile has to be checked more thoroughly, since the shape of the embossed channel is exactly the negative shape of the tool. The new and worn electrodes, as well as the tool were cut in half and the profile was measured by digital camera. The measurements are given in Figure 7, where the measurement points are also marked. The line at 0.7 mm indicates the depth of machining in order to obtain the 0.7 mm high feature of the tool. 4. Property analysis of machined parts As the difficulty of small parts is the measurement of characteristic dimension, the following properties were measured in this study: (1) the dimensions measured from the top profile of the electrode and

4 Figure 7. Measurement of slot profile: (a) new electrode, (b) worn electrode and (c) tool rib. The measurements are in millimeters. The channel on the electrode was found to have very little taper at 0.7 mm, which is the depth of MEDM machining. The chamfer is typical for WJ machining and could be removed by consequent machining before the MEDM phase. In contrast to the electrode, quite a big taper is noticed on the tool, which is related to the poor flushing conditions during MEDM. Thus, the profiles of the new and worn electrode as well as the tool were measured. Results are given in Figure The surface roughness The surface roughness was measured only on the tool, as its surface profile will be transferred on the final product. The measurements were performed by a Hommel Tester T1000. The roughness was first measured on the base, where the stylus measurements is applicable. The measuring distance L t was 1.5 mm. The measured R a was 0.67 µm and R z was 3.99 µm. Although this surface has no influence on the tool, it gives us a good estimation of the EDM process performance. Since it is impossible to measure the surface roughness of the sidewalls, the additional vertical surface was produced by MEDM using the same machining parameters. It should be taken into account that the flushing conditions were not the same as for the machined part, but the measured R a was 0.65 µm and R z was 3.99 µm, the same as the surface roughness of the base surface. 5. Discussion It can be observed that WJ performed very well as the deviation from the desired dimension is not so large. There is a slight problem with the width of the main channel, which should be 0.1 mm wide (the diameter of the WJ) but was wider by 80 %. The reason is the back flow of the water jet that is widening the main channel. But this is balanced by the side gap between the electrode and the workpiece (tool). By applying the orbital motion of the electrode during the machining, the size of the channels could be reduced, and the shape of the channels could be improved. Additionally, the flushing of the gap is also improved. Higher concentration of the debris in the gap results in a higher electrode wear. In the given case, the flushing was performed only by nozzles supplying fresh dielectric in the gap. The flushing could be improved by using additional holes in the electrode for supplying fresh dielectric. The conical shapes of the tool ribs are due to the poor flushing of the gap and due to the slight taper of the sidewalls made by WJ machining of the electrode. The slope of the rib has to be efficiently controlled through orbital motion of the electrode to better integrate the constraints related to further use of the tool. The achieved surface roughness is quite good, however, we still need to better analyze the surface roughness to evaluate the impact on the product made by embossing. Only functional surfaces should be considered, e.g. sidewalls and bottom of the channels. Two prospects have to be developed. First, CNC controlled MEDM that can perform orbital (also called planetary) machining. Second, performing roughing and finishing MEDM. The major job of removing the large part of the material is done by roughing conditions, while the smooth surface and accurate tool rib is done by finishing MEDM parameters. This approach requires fine-tuning of the size of the channels on the electrode, MEDM rough and finishing parameters and orbital motion of the electrode. 6. Conclusions The results of the case study applying the proposed tooling strategy are promising for the future research. The work will be focused on enhancing the MEDM process by improving the flushing of the gap by orbital motion of the electrode. Considering WJ machining the orifice diameter has to be further reduced in order to allow the production of smaller features and improvement of their accuracy. Also other WJ machining strategies such as multi-pass cutting should be considered, since early investigations showed an improvement in surface roughness [8]. Further on, different electrode materials, such as graphite, will be used in experiments in order to determine the best conditions for rough and finish MEDM. Research will also concentrate on the relationship between the expected EDM tool dimensions and the final tool dimensions by taking into account the wear of the electrode, which strongly depends on the machining parameters. Using multiple electrodes in various conditions will surely help to master the key parameters. In general, efforts will be made to characterize the processes and to model them. Existing strong interactions should be highlighted to better master the process. Finally, a more detailed study of the surface roughness should be held in order to refine the process parameter. Micro-fluid flowing behavior is

5 very sensible to the surface quality and the ability of the process to obtain the desired roughness will make, with no doubt, a decisive advantage. Acknowledgement This work is supported by the "Multi-Material Micro Manufacture: Technology and Applications (4M)" Network of Excellence, Contract Number NMP2-CT and by the "Virtual Research Lab for a Knowledge Community in Production (VRL-KCiP)" Network of Excellence, Contract Number NMP2-CT , both within the EU 6th Framework Program and by the bilateral PROTEUS Project between the Republic of Slovenia and the Republic of France, Contract Number BI-FR// References [1] S. Colin: Microfluidique, Lavoisier Paris, [2] A.W. Momber, R. Kovacevic: 1998 Principles of abrasive water jet machining, Springer, Berlin. [3] C. Ojmertz: A Study on Abrasive Waterjet Milling, Ph.D. Thesis, Chalmers University of Technology, Goteborg, Sweden, [4] B. Jurisevic, D. Kramar, K.C. Heiniger and M. Junkar: New Perspectives in 3D Abrasive Water Jet Precision Manufacturing. In Bley H., editor, Proceedings of the 36 th CIRP-ISMS: Progress in Virtual Manufacturing Systems, pp , Saarbruecken, Germany, June [5] J. Valentincic, M. Junkar: On-line selection of rough machining parameters. Journal of Material Processing Technology. Vol. 149, Issue 1/3, 2004, Pages [6] S. Bigot, A. Ivanov, K. Popov: A study of the micro EDM electrode wear, First Multi-Material Micro Manufacture (4M) Conference, Karlsruhe, Germany 2004, Pages [7] J. Valentincic, M. Junkar: Detection of the eroding surface in the EDM process based on the current signal in the gap. International Journal of Advanced Manufacturing Technology, Vol. 26, Issue 3/4, 2006, Pages [8] B. Jurisevic, K.C. Heiniger, A. Schuetz, and M. Junkar: Feasibilities of Abrasive Water Jet Multipass Cutting Technique. In Summers D., editor, Proceedings of the 2003 WJTA American WaterJet Conference, paper 2-A, Houston, Texas, USA, August 2003.

WATER JET BASED TOOLING STRATEGIES FOR MICROPRODUCTION

WATER JET BASED TOOLING STRATEGIES FOR MICROPRODUCTION B. Jurisevic 1, H. Orbanic 1, J. Valentinčič 1, O. Blatnik 1, C. Masclet 2, H. Paris 2, M. Museau 2 and M. Junkar 1 1 University of Ljubljana, Slovenia