Electrical discharge machining (EDM) is a non-traditional machining process based on an electro-thermal material removal mechanism. From micro to macro scale, EDM can process any conductive material including metal, alloy, cemented carbide, ceramic and PCD.^1-4 EDM has been widely used to produce dies, molds and mechanical parts in various industries.^5,6

Electrical discharge milling using a simple cylindrical rod electrode or pipe electrode is useful for precisely processing 3D complex shapes. Compared with die sinking EDM, there is no need to manufacture electrodes of different shapes according to the processing shape. Using the simple electrode, various and complex shapes can be processed by controlling the tool path. In EDM, the electrode is also worn along with the workpiece, so for precision machining, machining errors due to electrode wear must be removed. In the electrical discharge milling, additional complicated tool paths are created for the electrode wear compensation.^7-11 The electrode wear compensation method increases processing time and cost, and it is very difficult to completely remove machining errors caused by the electrode wear.

In wire EDM, since new wires are continuously supplied to the machining area, there is no need to consider the compensation of an electrode wear. If a simple rod electrode is used as a wire guide and the wire moves along the rod electrode, electrical discharge milling is possible without the need to consider electrode wear compensation. This type of electrode was developed in this study, and this is called wire electrical discharge milling electrode. A cylindrical rod with a hemispherical end serves as a guide, and the wire moves along it. Fig. 1 shows a conceptual diagram of electric discharge machining using a wire electrical discharge milling electrode developed in this study. The wire electrical discharge milling electrode also rotates in one direction around the guide shaft axis, similar to a ball end mill in which the wire acts as a cutting edge. In the previous studies of EDM using electrodes continuously supplied through guides having a shape, Song et al.^12,13 used continuously supplied metal strip electrodes in EDM turning and milling. In case of using a wire, Gotoh et al.^14,15 studied wire electrical discharge milling using an electrode in the form of a wire conveyed by using a rod electrode as a guide. In the study of Gotoh et al., a 360-degree reciprocating motion was adopted to rotate the electrode around the axis. This is because the wire spool was fixed on the outside separately from the rotating electrode, so the wire was twisted when rotating in one direction. The wire electrical discharge milling electrode developed in this study is designed to rotate the wire supply spool and the wire collection spool together with the electrode, so that it can rotate in one direction instead of a reciprocating motion. In the case of a reciprocating motion, acceleration and deceleration are repeated, and the rotational speed cannot be kept constant. Also, it is difficult to find out the effect of the change in rotational speed. The wire electrical discharge milling electrode developed in this study has the advantage of being able to rotate at a constant speed in one direction, and the rotation speed is also variously controlled. In this paper, the electric discharge machining characteristics according to the change of machining conditions are examined using the developed wire electrical discharge milling electrode, and an electrical discharge milling test is performed.

Fig. 1 
EDM using wire electrical discharge milling electrode

Fig. 2 shows the wire electrical discharge milling electrode developed in this study. The cylindrical guide shaft of the wire electrical discharge milling electrode is 3 mm in diameter, and the lower end of the shaft is hemispherical. The side of the shaft and the center of the hemisphere have grooves for the wire to pass through. The wire travels from the wire supply spool along the grooves to the wire collection spool. A torque controller (Model Name: PHT-1.2D, Manufacturer: Ogura Clutch) for wire tension adjustment was used for the wire supply spool. If the wire tension is low, the wire cannot adhere to the guide groove. If the wire tension is high, the frictional force increases when the wire moves along the groove, making the transfer unstable. In addition, fine chips may be generated due to the grinding of the wire. These chips cause unnecessary electrical shorts during electrical discharge machining. In this experiment, the torque controller was set to 0.04 Nm. A worm geared DC motor (Model Name: JGY-370) was used for wire feed in the wire collection spool, and the wire feeding speed can be adjusted through a speed controller. The size of the worm geared motor is small, and the output shaft of the reducer has a 90 degree angle to the motor shaft, enabling a compact arrangement. Lithium polymer battery was used to power the motor. The plate on which the motor, the battery, the wire supply spool and the wire collection spool are fixed is coupled to the guide shaft, so that they all rotate around the guide shaft axis. A speed control motor was used to rotate the shaft, and the rotational speed was adjusted using a speed controller. The wire electrical discharge milling electrode was fixed to the Z stage of the EDM machine, and the workpiece was fixed to the XY stage.

Fig. 2 
Wire electrical discharge milling electrode

An RC circuit was used as an EDM power source, and the voltage was fixed at 120 V. As for the polarity, the (+) pole was connected to the workpiece, and the (-) pole was connected to the electrode. The wire feeding speed is controlled by the worm geared motor of the wire collection spool, and it was fixed at 50 mm/min in this experiment. Deionized water with a resistivity of 1MΩcm or more was used as a dielectric fluid, and sprayed to the machining area using a nozzle. Through a filtering system, impurities in deionized water were removed and the resistivity was maintained at an appropriate level. For the wire electrical discharge milling electrode, a brass wire with a 0.25 mm diameter was used, and a cylindrical stainless steel with a 3 mm diameter as a guide shaft was used. The workpiece was 1 mm thick stainless steel (AISI304). In order to examine the machining characteristics, the capacitance of the RC circuit and the rotational speed of the wire electrical discharge milling electrode were changed. The experimental conditions are summarized in Table 1.

In this paper, a wire electrical discharge milling electrode was developed, and electric discharge machining characteristics were studied using it. The wire electrical discharge milling electrode is a form in which the wire is conveyed by using a cylindrical rod with a hemispherical end as a guide, and also rotates in one direction around the guide axis. If the wire electrical discharge milling electrode is used in EDM, there is no need to consider electrode wear compensation. The detailed conclusion of the study of electric discharge machining characteristics using the wire electrical discharge milling electrode is as follows.

(1) In the experimental conditions used in this paper, if the capacitance of the RC circuit was small such as 1 and 10 μF, the machining became unstable as the machining depth increased, and the machining did not proceed to the end. If the capacitance was as large as 200 μF, the wire broke during machining. When the capacitances of 40 and 100 μF were used, the machining was successfully proceed to the end. In the case of 100 μF than 40 μF, the machining time was shortened.

(2) The faster the rotational speed of the wire electrical discharge milling electrode, the shorter the machining time. In the case of 60 rpm compared to 4 rpm, the machining time was reduced in half. When the wire electrical discharge milling electrode rotates around guide shaft axis, the wire acts like a cutting edge. Therefore, the faster the rotational speed, the faster the wire removed the material and the machining time was shortened.

(3) A hemispherical shape was machined in two steps of roughing and finishing. In the roughing step, conditions of 100 μF and 60 rpm were used, and in the finishing step, conditions of 1 μF and 60 rpm were used. As a result, a hemispherical shape with good shape accuracy and fine surface finish was fabricated.

(4) Straight and curved shapes were successfully processed using the wire electrical discharge milling electrodes. It is expected that wire electrical discharge milling electrode will be effectively used for 3D machining.