Electrical Discharge Machining in Dielectric-Powder Media

The paper reports investigations of EDM using powder suspended working fluid instead pure dielectric. The EDM characteristics obtained using hydrocarbon dielectric (kerosene) and mixture deionized water with abrasive powder have been compared. The relationship between surface roughness parameters, material removal rate and operating parameters of EDM have been determined for different kind of powder and its concentration in kerosene/water.The investigation results show that there are chances for replacing the conventional dielectric with water and that would imply considerable economic and ecology advantages. 1. INDRODUCTION During the last decades, EDM has nearly obtained a first position in some important areas due to its capability of machining any material with electrical conductivity more than 0.01 S/cm with high cutting speed, high precision and satisfactory surface finish. Conventional EDM surface is generally matt, since it is generated by network of individual craters as result of discharges. Therefore, hand smoothing has been usually used for finishing plastic molds, dies and casting molds. Mohri, et al. [1, 2, 3] found that an EDM finishing process using dielectric mixed with silicon powder provides a mirror surface of up to 500 cm, area. Recently this machining method has been introduced in commercial machine tools and practically applied in industry. The effect of silicon and others kind of powders mixed with the dielectric on the surface generation mechanism was investigated [4, 5]. According to investigations carried out in Japan, positive influence of electrical conductive or nonconductive particles on AEDM performance is the effect of better dispersion of sparks over a workpiece [1 4]. This dispersion is much related with gap distance between the electrode and workpiece which becomes bigger by mixing powder into dielectric. In order to estimate the effect of powder suspension on the gap distance, when sparks are occurs, the maximal electrical field intensity in the gap with grains was determined [6]. Estimation of distribution of electric field in dielectric with spherical particles shows that electrical breakdown in the gap with conductive or nonconductive particle should be at 3 or 1.5 times larger distance than in the case EDM in pure dielectric, respectively. The experiments described in the paper [4], confirm the conclusion that the conductive particle enlarges the working gap significantly more compared to that of nonconductive particles. Bigger gap distance makes more uniform flushing conditions, sparks stable, and consequently they disperse over wider area of a workpiece. Moreover, when the gap distance is bigger, the stray capacitance inserted in the gap is smaller, which affects the surface quality [1, 4]. Simultaneously, environmental and work protection legislation, which is constantly becoming more and more stringent, leads to considerable obligations in the operation of erosion systems and the handling of operating materials, particularly working media and filtration aids. A possible solution lies in replacing the oil-based dielectrics, which are used as standard for EDM-sinking by water-based fluids. Extended investigations carried out in Germany show high efficiency application of mixture water and glycerin for reducing disadvantages in EDM-sinking using pure water [7, 8]. This paper reports investigation on EDM sinking using mixture deionized water and abrasive powders. For comparison, results using powder suspended oil-based dielectric is also presented. 2.EXPERIMENTAL PROCEDURE Experimental investigations have been performed on modified EDM machine tool FORM 2LC CNC by incorporating developed dielectric system for deionized water and mixture powder/dielectric (Figure 1). As working fluids were used: • kerosene FLUXELF 2 with resistivity 7.4x10 Ωcm; • deionized water with conductivity 15 μS/m. • Figure 1. Schematic diagram of dielectric/powder supply system: 1tool electrode holder,2 container, 3 tool-electrode TE, WP-workpiece, 4 pipe for dielectric delivering to TE, 5 workpiece holder, 6 pomp These fluids were mixed with powder: • silicon carbide SiC with 180 grade; • Al2O3 with 600, 800 and 1200 grade; • Al with 180 grade. A copper cylinder of 20 mm in diameter has been used as a tool electrode; the hole has been made in the cylinder in order to pump dielectric into to area of machining. The tool steel NC6, in compliance with Polish Standard, was used as a workpiece. To determine of basic EDM relationships between input parameters such as pulse current, on-time, duty factor, and output parameters namely material removal rate (MRR) and surface roughness (Ra, Rz), factorial design and multiple regression analysis have been used. First series of experiments were carried out using kerosene and kerosene/powder mixture as dielectric and second series with using deionized water and deionized water/powder mixture. During machining negative and positive polarity of tool electrode was used for investigation of polarity effect, it is an especially important when water-based dielectric is used. 3. EXPERIMENTAL RESULTS FOR EDM USING POWDER SUSPENDED KEROSENE The experiments confirmed of previous investigations presented in papers [1 6], that application of powder suspended fluids reduces roughness of EDM-ed surfaces. The relations between the surface roughness obtained with EDM process carried out in pure kerosene (Ra and Rz) and in SiC powder suspension Ra(0.2) and Rz(0.2) of two different concentration (0.2 % and 2 %) and various current values is illustrated in Fig. 2 and 3. The largest differences have been obtained for low pulse current, which is advantageous as the EDM finishing is carried out for low current values. Higher grain concentration also contributes to the improvement in surface smoothness (Fig. 3). The EDM using powder suspended kerosene can make surface roughness values 2.5 less than those without powder. 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 0,0 2,0 4,0 6,0 8,0 10,0 12,0 14,0 current [A] R a/ R a Ra/Ra(0.2) Ra/Ra(2) tp = 100 μs t0 = 25 μs dielectric: kerosene Figure 2. Effect of current on relative value Ra / Ra: Ra / Ra (0.2) at powder concentration of 0.2 %, Ra / Ra(2) at powder concentration of 2 % (Ra roughness of surface machined in kerosene) 0,0 0,5 1,0 1,5 2,0 2,5 3,0 0,0 2,0 4,0 6,0 8,0 10,0 12,0 14,0 current [A] R z/ R z