INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 12, No. 2, pp. 195-201 APRIL 2011 / 195 DOI: 10.1007/s12541-011-0027-6 1. Introduction Demands of micro parts or micro molds with high precision are greatly increasing in various industries. Therefore, micro machining technologies have advanced in order to satisfy industrial requirements. Among them, micro electrochemical machining (micro ECM) and micro electrical discharge machining (micro EDM) are attractive methods for machining difficult-to-cut metals. Micro ECM can be applied to produce micro holes, 3D structures and arrays of micro grooves. 1-3 However, MRR is relatively low, and specific electrolytes are needed according to the materials being machined. Therefore, great amount of research has been conducted to improve machining performance. On the other hand, micro EDM has been widely adopted for a range of purposes. It can produce various shaped micro structures including simple holes and 3D structures as well as complex features on various metals. 4-6 Micro EDM usually utilizes an RC circuit as the pulse generator; the RC circuit produces a significantly short pulse with nanosecond duration. 7 Hydrocarbon oil (kerosene) is widely used as the dielectric fluid. Deionized water is also, resulting in high MRR and low tool wear. 4 However, machined structures suffer from electrolytic corrosion when deionized water is used. Fig. 1 shows a schematic of electrolytic corrosion during micro EDM using deionized water. In micro EDM, positive polarity is usually connected to a workpiece which is called straight polarity. Therefore, anodic dissolution around the machined structure results in poor accuracy and severe damage. Fig. 2 shows voltage and current waveforms of a conventional RC circuit. High positive voltage at the workpiece during time without discharges causes the anodic dissolution of the workpiece. Therefore, deionized water is seldom used in micro EDM using conventional pulse generators except for machining materials with high resistance to corrosion, such as stainless steel. Micro structures of stainless steel were successfully fabricated in deionized water with a machining gap of a few μm. 4 In addition, electrochemical dissolution was used to finish inner surfaces of micro EDM holes on stainless steel. 8 On the other hand, tungsten carbide with a cobalt binder (WC-Co) severely suffers from electrolytic corrosion because cobalt easily dissolves in water. Fig. 3 shows a machined hole using an RC circuit in deionized water. The workpiece was WC-Co with a thickness of 100 μm, and machining conditions were 80 V and 1000 pF. The machining accuracy deteriorated and the surface around the hole was severely damaged. In order to prevent electrolytic corrosion, immense research has been conducted on anti-corrosion power sources. An AC power source has been used for wire EDM. 9,10 A positive and a negative Machining Characteristics of Micro EDM in Water using High Frequency Bipolar Pulse Do Kwan Chung 1 , Hong Shik Shin 1 , Min Soo Park 2,# and Chong Nam Chu 1 1 School of Mechanical and Aerospace Engineering, Seoul National University, Gwanak 599 Gwanak-ro, Gwanak-gu, Seoul, Korea, 151-744 2 Department of Product Design and Manufacturing Eng., Seoul National University of Science and Technology, 172 Gongreung 2-dong, Nowon-gu, Seoul, Korea, 139-743 # Corresponding Author / E-mail: pminsoo@seoultech.ac.kr , TEL: +82-2-970-6356, FAX: +82-2-976-5173 KEYWORDS: Micro EDM, High frequency bipolar pulse, Electrolytic corrosion, Dielectric fluid Micro EDM using conventional pulse generators such as the RC type or transistor type with water as the dielectric fluid suffers from poor accuracy of machined structures due to electrolytic corrosion. In this study, a new high frequency bipolar pulse generator for micro EDM in water was developed in order to prevent electrolytic corrosion. The new pulse generator produced a high frequency bipolar pulse possessing a positive pulse duration of several hundred nanoseconds with a high repetition rate provided to the machining gap. Discharge characteristics of micro EDM using the new pulse generator were investigated. Machining characteristics were also investigated according to machining conditions such as the repetition rate, positive voltage, capacitance and resistivity. Using the new pulse generator, micro holes without electrolytic corrosion were successfully fabricated in deionized water and tap water. Machining time, tool wear and clearance using the new pulse generator in deionized water decreased compared with those using the RC circuit in kerosene. Manuscript received: August 1, 2010 / Accepted: December 12, 2010 © KSPE and Springer 2011