Development of Micro Electrical Discharge Machine and Micro-hole Machining Using Multiple Micro Electrodes
Article information
Abstract
Recent advances in microtechnology have highlighted the need for fabricating microscopic parts with complex three-dimensional structures. This, in turn, has increased the demand for innovative tools. Micro electrical discharge machining (MEDM) is one such technique that has proven to be effective in producing parts that are difficult to fabricate using traditional manufacturing methods, such as micro shafts and micro holes. Therefore, this study focuses on the development of a specialized MEDM system that incorporates an optimized wire electrical discharge grinding (WEDG) module. Additionally, the study successfully fabricates microelectrodes for use as cutting tools using the MEDM/WEDG system. These micro-electrodes are then utilized in the precise fabrication of various types of multiple micro electrodes through the use of reverse electrical discharge machining (REDM). These multiple micro electrodes are subsequently employed in precise hole machining operations using MEDM. The application of REDM signifies a significant technological advancement in the precision fabrication of microstructures. Furthermore, it is expected to greatly enhance the versatility and efficiency of microtechnology manufacturing processes. Overall, REDM represents a new approach to manufacturing that is capable of producing more complex and high-precision microstructures.
1 Introduction
With the growing demand for high-precision, small parts, micro-hole machining technology has been increasingly important in a variety of high-tech industries. This technology is employed in the manufacturing of diverse products such as MEMS [1], injector nozzles [2], inkjet printers [3,4], and turbine blades, combustion chambers, turbine discs [5]. To meet the stringent precision and size demands of modern industry, techniques such as photolithography, micro-drilling, micro-punching, micro-turning, and laser beam machining [6–8] are being utilized.
Despite advances in sophisticated machining techniques, micro-hole machining using mechanical methods still faces significant challenges. One of the main issues is related to the fabrication and life of the tools used in this process.
Tools used in micro-hole machining tend to wear out quickly, especially when machining hard materials or parts with complex geometries. This rapid wear makes it difficult to maintain dimensional accuracy, which ultimately shortens tool life, increases production costs, and reduces the quality of the machined part.
Krzysztof Szwajka et al. demonstrated that combining laser and conventional micro-drilling can double the drill life compared to conventional mechanical micro-drilling, particularly when machining the Inconel 625 nickel-based alloy. However, the equipment is expensive to set up, and the laser drilling process can cause the center of the hole to be asymmetrical to the wall, potentially causing the drill to bend or break under high lateral forces during drilling. These issues are particularly problematic when drilling angled holes [9].
Shan Li et al. performed micro-drilling on silver IC substrates and found that drilling force and tool wear increased with the number of holes processed [10]. In addition, Jingyuan et al. found that when drilling SiC/SiCf composite micro-holes in semiconductor manufacturing, the wear behavior and machining quality of ultra-precision brazed diamond abrasive core drills are critical to ensuring the durability and performance of the product [11].
To address these issues, various tool fabrication methods have been investigated. According to Adams et al., FIB has been utilized for the fabrication of fine tools in the tens of micron range and has been applied to the machining of PMMA and aluminum Al6061 alloys. However, despite high machining precision, material removal at the atomic scale is slow and expensive [12,13]. Omori et al. developed the ELID-G system to achieve micro-tool diameters of less than 1 μm in cemented carbide. However, this process continuously supplies hydroxide ions and dissolved oxygen to the grinding zone, affecting the workpiece surface. Therefore, continuous advances are needed to address surface roughness issues associated with oxide layer formation during micro-tooling [14].
Micro-electrical discharge machining (MEDM) is a non-contact machining method that utilizes electrical discharges to minimize mechanical stress on the material, thereby reducing damage to the microstructure of machined parts and enabling the machining of parts with complex shapes and very small dimensions. This technology also offers high machining efficiency and precision for a variety of very hard or less ductile materials [15].
Wire electrical discharge grinding (WEDG) and Reverse electrical discharge machining (REDM) are two primary types of MEDM. These methods use high-hardness materials such as tungsten carbide (WC), polycrystalline cubic boron nitride (PCBN), and conductive polycrystalline diamond (PCD) to manufacture various tool types cost-effectively. For instance, Zheng et al. fabricated a PCD end milling tool with an average surface roughness (Ra) of 2.0 μm to process quartz glass [16], and Lee et al. utilized REDM to fabricate a carbon graphite pin array and a tungsten carbide eccentric tool [17]. Despite being traditional methods, WEDG and REDM have enabled the production of a wider variety of precise microstructures, significantly contributing to the field of micro-tool manufacturing. Consequently, MEDM technology is playing a crucial role in modern manufacturing.
In this study, we developed a WEDG module that uses two motors and a magnetic clutch to provide a constant feed rate and tension. Our module allows precise control of wire tension and feed rate, enabling the fabrication of cemented carbide microelectrode tools with improved accuracy. Furthermore, the WEDG module and EDM bath are installed on a single stage to minimize electrode attachment and detachment, enabling runout-free microelectrode fabrication and machining.
We then applied REDM technology to facilitate tool fabrication to more efficiently fabricate microelectrodes with different pin configurations. MEDM was applied to a brass plate to produce micro-holes with different numbers of electrode pins in one process.
2 Materials and Methods
2.1 Micro Electrical Discharge Machining (MEDM)
The configuration of an electrode and workpiece for connection to a discharge circuit for the MEDM process used herein is shown in Fig. 1.
Schematic diagram of electrode and workpiece configuration for electrical discharge machining (MEDM) process
MEDM is set up with the tool electrode as the cathode and the workpiece as the anode in a bath filled with insulating fluid. When the tool electrode and workpiece approach a critical gap, a discharge spark is initiated. This spark causes an explosion due to the high thermal energy and pressure difference, which melts the material and releases debris. Thus, MEDM is an extension of traditional electrical discharge machining technology, where the discharge energy (E) is influenced by the supply voltage (V) and storage capacitance (C), which is expressed by the discharge equation, given here as Eq. (1):
This indicates that increasing the voltage will increase the discharge energy and, hence, the material removal rate. However, the increased voltage can also increase the thermal impact on the workpiece. Conversely, reducing the voltage reduces the discharge energy, which can allow for finer machining, but may reduce the efficiency.
Meanwhile, increasing the capacitance allows more energy to be stored, thereby increasing the discharge power, which affects both the removal rate and machining precision, while reducing the capacitance reduces the energy released, which prioritizes precision over speed.
MEDM is based on a non-contact mechanism that requires a small distance to be maintained between the electrode and the workpiece. In this respect, a machining system capable of precise feeding is essential to avoid problems such as unintentional material removal or electrical short circuits, which can reduce the overall efficiency of the process.
In addition, the gap between the electrode and the tool directly affects the machining speed and precision, which indicates the need for precise gap management in order to achieve good quality results.
In the present work, a precision machining instrument (DT110, Microtools Co.) with X-Y-Z-U axis was used, as shown in Fig. 4. The specifications of the machine are detailed in Table 1. In addition, a PMAC motion controller (Turbo PMAC, Delta Tau Data Systems Co.) was used to control precisely the position of the X-Y-Z-U axis.
Photographic image (a) and schematic diagram (b) of MEDM system
The specifications of the MEDM system
2.2 Wire Electrical Discharge Grinding (WEDG)
In typical MEDM machining, not only is material removed from the workpiece, but the electrodes are also worn away in the opposite direction. This wear has a significant impact on the dimensional accuracy of small workpieces. To address these issues, Masuzawa introduced wire electrical discharge grinding (WEDG) as a new approach that specifically targets micro-electrode machining, as shown in Fig. 2 [18,19].
Schematic diagram of the WEDG
WEDG uses electrical discharge energy generated between the workpiece and a continuously fed wire electrode to move along a wire guide with an applied voltage. This configuration significantly reduces the effects of electrode wear due to the constant feed rate of the wire electrode. In addition, the precision provided by the concentrated discharge area is particularly advantageous for micromachining operations, such as the fabrication of very small parts such as micro-pins or axles.
In a WEDG system, wire feed and tension management are critical to both the machining time efficiency and the quality of the final product. Hence, the WEDG module in this study is equipped with a DC motor (KD3429S1, GGM Co.), magnetic clutch, wire guide, wire bobbin, and wire roller on a 3D-printed base frame, as shown in Fig. 3. Precise movement of the wire electrodes along the wire guide is essential to generate a regular discharge.
Schematic diagram of the WEDG setup
The magnetic clutch, combined with the motor, plays an important role in maintaining precise wire tension in the WEDG process. Controlling the tension reduces wire friction by preventing slippage if the tension becomes too high, while ensuring accurate wire delivery and constant control of tension to improve the machining precision and quality.
Here, a DC motor is used to set the wire at 30 mm/min through the feed rollers, roller guides, and wire guides. Made of cemented carbide, these components are highly resistant to high temperatures and wear, thereby ensuring stable wire feed and high-precision machining.
2.3 Reverse Electrical Discharge Machining (REDM)
REDM is a special kind of machining that deviates from traditional EDM technology. Instead of using an electrode to create a small hole or shape in a workpiece through a high-temperature electrical discharge, as in a typical EDM process, REDM prepares the tool electrode to the desired shape in advance [20].
The method involves applying a reverse polarity voltage to the electrode in order to mimic the shape of the electrode directly on the workpiece. Multiple micro electrodes that were fabricated using REDM and a plate electrode that was consumed after machining are shown in Fig. 5.
(a) Fabricated multiple micro electrodes and (b) plate electrode after REDM
2.4 RC Circuit System
The RC circuit used for the MEDM system consists of cement resistors and capacitors connected in series, with an additional resistor for measuring the discharge gap in the event of a short circuit between the tool and the workpiece. An oscilloscope (Analog Discovery 3, Digilent Co.) was connected to the measurement resistor (R2) of the RC circuit under the conditions given in Table 2 in order to monitor the voltage applied to the RC circuit during MEDM and the voltage changes in real time. The installed resistance in a series circuit is equal to the sum of the resistances of all the resistors. When an open circuit voltage is applied to the RC circuit, and the electrodes are close to the workpiece, the voltage through each resistor is divided proportionally according to the voltage distribution rule. This division is determined by the resistance ratio (
The specifications of the RC circuit
This systematically distributed voltage is input to the A/D board (ACC-28BP, Delta Tau Data Systems Co.) to ensure a stable discharge gap between the electrode and the workpiece throughout the machining process. When a short circuit occurs between the electrode and the workpiece, the voltage is split due to the resistance ratio, and a signal is input to the A/D board connected to the resistor for measurement.
The system is designed to take this value and move the axis in the opposite direction of travel in order to clear the short circuit. In addition, as the gap between the electrode and workpiece decreases with the speed of axis movement, the system can predict the voltage distribution across the resistor in advance and adjust the machining axis accordingly. The predicted voltage is passed to an A/D board, which converts it from analogue to digital, thereby allowing the electrical discharge machining gap to be monitored and precisely adjusted in real time.
Another advantage of RC circuit is that, despite their low impact factor, they can achieve short pulse widths of less than 0.1 μs and relatively high peak currents relative to the input power supply voltage. These properties make them particularly suitable for micro-electro-discharge circuits that require low-energy and high-frequency discharges, which are essential for precision machining and micro structuring. As a result, RC electrical discharge circuits offer a wider range of applications in MEDM processes and enable the creation of more precise and complex parts [21].
3 Results and Discussion
3.1 Control of the MEDM Micro Gap
As demonstrated in Fig. 6, the measured voltage value obtained at a frequency of 299.401 kHz using the setup described in Section 2.4 enables the direct observation of the voltage distribution within the circuit as a function of the magnitude of the applied voltage. Moreover, by connecting the A/D board that can convert analog voltages in the ±10 V range to 16-bit digital signals, the voltage change can be measured in real time as the 0–10 V value divided by the ratio of the resistors. This allows the axis to be moved according to the existing voltage in order to control precisely the fine spacing of the electrical discharge machining. The precise monitoring and adjustment of the voltage changes in order to control the gap is essential because it affects the efficiency and precision of the MEDM process [22], as demonstrated in the following sections.
Voltage value measured at RC circuit resistance (R2) during electrical discharge machining
3.2 Fabrication of Micro Electrode Based Tools
WEDG module developed herein was used to machine microelectrodes for use as cutting tools or for electrical discharge machining EDM. For this procedure, a tungsten carbide (WC) cylinder with a diameter of 1 mm was mounted on a spindle, a potential of 100 V was applied, and the spindle was rotated at 500 RPM while approaching the wire guide with a continuously fed brass wire. Tungsten carbide was selected for its high hardness, good wear resistance, and high temperature resistance, which make it a widely used tool material [23–25]. First, a rough process cut with a cutting depth of 250 μm and a capacitance of 470,000 pF was performed, followed by a finish cut with a cutting depth of 150 μm and capacitance of 3,000 pF. The detailed machining conditions are displayed in Table 3, while the results from the tools fabricated using the Hitachi FE-SEM (S- 4800) are illustrated in Fig. 7.
The conditions used to fabricate the micro electrode WEDG
FE-SEM images of the micro electrode tool manufactured using WEDG: (a) micro electrode tool and (b) tool surface
3.3 Fabrication of Multiple Micro Electrodes
Fig. 8 shows the fabrication of multiple micro electrodes by the REDM process. In this process, a WC microelectrode fabricated using the developed WEDG technology (Sections 3.1 and 3.2) was used as a tool to drill a hole with a precise diameter of 250 μm in a 300 μm thick brass plate MEDM. Micro-hole machining was performed on the brass plate to achieve the desired shape. A direct current open circuit voltage of 100 V and a capacitance of 10,000 pF were applied to the WC, anode plate, and cathode plate, and the WC was slowly lowered vertically without rotation for accurate replication. The machined plate electrode was then used as a tool to engrave the machined shape on the WC with a diameter of 1 mm. Therefore, the FE-SEM image in Fig. 9 shows that various numbers of multiple microelectrodes were fabricated by machining the desired number of holes into the plate electrode.
Schematic diagram of the fabrication of micro tool by REDM (a) Micro hole machining using MEDM and (b) Electrode processing using REDM
FE-SEM images of the multiple micro electrodes with various numbers of electrodes manufactured using REDM
3.4 MEDM Characteristics When Using Multiple Micro Electrodes
MEDM with a machining depth of 100 μm was performed diameter of 250 μm and the four pins that were made by REDM technology (Section 3.3). The specific conditions of the MEDM process are given in Table 4, and the results of the discharge machining process are revealed by the SEM images in Fig. 10. Here, it can be seen that the sizes and shapes of holes machined on a brass plate are precise and consistent. This is because, as shown in Fig. 10, a consistent discharge was applied to the brass plate where the same pin parts were in contact, thereby supplying all pins with the same constant discharge energy throughout the machining process.
The hole machining parameters when using the multiple micro electrodes
FE-SEM images of the micro holes obtained MEDM using the multiple micro electrodes at (a) 10,000 pF and (b) 20,000 pF
Moreover, a comparison of the results in Figs. 10(a) and 10(b) clearly demonstrates that the roughness of the workpiece surface increases with the increase in capacitance. Taken together, these results demonstrate that the roughness of the brass surface in contact with all four pins of the multiple micro electrode remains constant when a constant voltage and capacitance are applied. In other words, the same constant discharge energy is distributed equally among all four pins. These results provide important information for understanding and controlling the discharge characteristics in precision machining when using multiple micro electrodes, and suggest possible applications in micro hole machining, especially for large numbers of micro holes requiring high precision.
As a demonstration of this application, a multi-microelectrode with four pins was used to machine micro-holes in a brass plate using a discharge of 100 V and capacitance set at 10,000 pF and 20,000 pF. As shown in Fig. 11, four holes were successfully machined, but burrs were observed at the hole entrance. This is because the multi-microelectrode machines the workpiece in only one direction without rotation, so the discharge debris is not sufficiently dispersed, resulting in remelting and frequent short circuits, which rapidly degrade the surface.
FE-SEM image of the micro-machined holes obtained using the multiple micro electrodes at (a) 10,000 pF and (b) 20,000 pF
In addition, as the capacitance increased, the size of the workpiece diameter increased due to the high discharge energy, and the surface finish tended to deteriorate. To address this, we are planning future research to identify optimal machining conditions and reduce burr generation at the inlet. We are planning research to identify optimal machining conditions and reduce burr generation on the inlet side through various methods such as coolant circulation pump and vibration [26–28].
4 Conclusions
Wire electrical discharge grinding (WEDG) module and RC circuit were developed to enable precise micro electrical discharge machining (MEDM). These technological advances expand the scope of MEDM and provide a technique for producing microproducts more precisely and efficiently. The design and fabrication of RC circuits play an important role in increasing the precision and reliability of the electrical discharge machining (EDM) process, and contribute to improving the quality and performance of the entire manufacturing process.
Moreover, the integrated utilization of MEDM, WEDG, and REDM presented herein provides an important technological advancement in the efficient and versatile fabrication of complex three-dimensional micro components. The superiority of this integrated approach was demonstrated by the successful fabrication of micro electrodes using the MEDM system with the developed WEDG module, along with the fabrication of multiple micro electrodes with different numbers of pins using REDM. This demonstrates that REDM can be effectively used to successfully fabricate micro electrodes with a variety of pin configurations.
The results of this study demonstrate that multiple micro electrodes can be utilized as tools to process holes with high precision and microstructure according to the characteristics of the processing conditions, thereby providing broad application potential in high-tech industries. In addition, this study is expected to further stimulate research and development in this field as a fundamental study for innovative improvements in micro component manufacturing processes.
In conclusion, the manufacturing and integrated utilization of MEDM technology demonstrated herein represent an important technological advancement in the field of precise micro-part manufacturing. It paves the way for tooling and applied machining methods that will greatly enhance the versatility and efficiency of micro-part manufacturing, and is expected to contribute to the exploration and development of new manufacturing methodologies in high-tech industries.
Acknowledgement(s)
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (RS-2023-00208717).
Notes
Conflict of interest
There are no conflicts of interest to declare.
Data availability
The datasets generated and/or analysed during the current study are available upon reasonable request from the corresponding author.
References
Biography
Min Chul Shin is currently a Ph.D. candidate in the Department of Mechanical Engineering at Kyungpook National University in Daegu, Korea. His research interests include precision machining, precision manufacturing, and MEMS
Bo Hyun Kim is currently a professor at Soongsil University. His reasearch topic are micro mechanical and non-conventional machining processes.
Gyu Man Kim is currently a professor at Kyungpook National University and has received his master’s and doctoral degrees in Mechanical Engineering from Seoul National University. His areas of interest are precision machining and MEMS.