Examination of machining parameters on surface roughness in EDM of tool steel

Journal of Materials Processing Technology 191 (2007) 141–144<br /> <br /> Examination of machining parameters on surface<br /> roughness in EDM of tool steel<br /> M. Kiyak a,∗ , O. Cakır b<br /> ¸<br /> a<br /> <br /> Department of Mechanical Engineering, Yildiz Technical University, 34349 Istanbul, Turkey<br /> b Department of Mechanical Engineering, Dicle University, 21280 Diyarbakir, Turkey<br /> <br /> Abstract<br /> Electrical discharge machining (EDM) is one of the important non-traditional machining processes and it is widely accepted as a standard<br /> machining process in the manufacture of forming tools to produce molds and dies. Since its introduction to manufacturing industry in late 1940s,<br /> EDM became a well-known machining method. The method is based on removing material from a workpiece by means of a series of repeated<br /> electrical discharges, produced by electric pulse generators at short intervals, between an electrode (tool) and a part being machined in dielectric<br /> fluid medium. This paper is devoted to a study of the influences of EDM parameters on surface roughness for machining of 40CrMnNiMo864 tool<br /> steel (AISI P20) which is widely used in the production of plastic mold and die. The selected EDM parameters were pulsed current (8, 16 and<br /> 24 A), pulse time (2, 3, 4, 6, 12, 24, 48 and 100 ␮s) and pulse pause time (2 and 3 ␮s). It was observed that surface roughness of workpiece and<br /> electrode were influenced by pulsed current and pulse time, higher values of these parameters increased surface roughness. Lower current, lower<br /> pulse time and relatively higher pulse pause time produced a better surface finish.<br /> © 2007 Elsevier B.V. All rights reserved.<br /> Keywords: EDM; Tool steel; Current; Surface roughness<br /> <br /> 1. Introduction<br /> Electric discharge machining (EDM) is one of the most popular non-traditional material removal processes and has became<br /> a basic machining method for the manufacturing industries of<br /> aerospace, automotive, nuclear, medical and die-mold production. The theory of the process was established by two Soviet<br /> scientists B.R. and N.I. Lazarenko in the middle of 1940s. They<br /> invented the relaxation circuit and a simple servo controller tool<br /> that helped to maintain the gap width between the tool and the<br /> workpiece. This reduced arcing and made EDM machining more<br /> profitable and produced first EDM machine in 1950s. Major<br /> development of EDM was observed when computer numerical<br /> control systems were applied for the machine tool industry. Thus,<br /> the EDM process became automatic and unattended machining<br /> method [1].<br /> The process uses thermal energy to generate heat that melts<br /> and vaporizes the workpiece by ionization within the dielectric<br /> <br /> ∗<br /> <br /> Corresponding author.<br /> E-mail addresses: kiyak@yildiz.edu.tr (M. Kiyak), ocakir@dicle.edu.tr<br /> (O. Cakır).<br /> ¸<br /> 0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved.<br /> doi:10.1016/j.jmatprotec.2007.03.008<br /> <br /> medium. The electrical discharges generate impulsive pressure<br /> by dielectric explosion to remove the melted material. Thus, the<br /> amount of removed material can be effectively controlled to produce complex and precise machine components. However, the<br /> melted material is flushed away incompletely and the remaining material resolidifies to form discharge craters. As a result,<br /> machined surface has microcracks and pores caused by high<br /> temperature gradient which reduces surface finish quality.<br /> There have been many published studies considering surface<br /> finish of machined materials by EDM. It was noticed that various machining parameters influenced surface roughness and<br /> setting possible combination of these parameters was difficult<br /> to produce optimum surface quality. The influences of some<br /> machining parameters such as pulsed current [2–9], pulse time<br /> [2–6,8,9], pulse pause time [2,5,9], voltage [4,6], dielectric liquid pressure [4,6,8,10] and electrode material [11] have been<br /> examined. One study examined P20 tool steel and provided useful information the effects of some machining parameters on<br /> surface roughness, but the selected of pulsed current values was<br /> very low 1–8 A [12].<br /> The present study examines the effects of pulsed current,<br /> pulse time and pulse pause time on surface roughness in the<br /> 40CrMnNiMo864 tool steel (AISI P20) tool steel.<br /> <br /> 142<br /> <br /> M. Kiyak, O. Cakır / Journal of Materials Processing Technology 191 (2007) 141–144<br /> ¸<br /> <br /> Table 1<br /> Properties of workpiece material<br /> Chemical composition (%)<br /> C<br /> <br /> Cr<br /> <br /> Mn<br /> <br /> Mo<br /> <br /> Ni<br /> <br /> S<br /> <br /> Si<br /> <br /> 0.37<br /> <br /> 2<br /> <br /> 1.4<br /> <br /> 0.2<br /> <br /> 1<br /> <br /> Max. 0.01<br /> <br /> 0.3<br /> <br /> Hardness<br /> Elasticity module (E)<br /> Thermal conductivity<br /> <br /> 290–341 HB<br /> 205 GPa<br /> 29 W/m K<br /> <br /> Fig. 1. Effects of pulse time and pulsed current on surface roughness of workpiece (2 ␮s pulse pause time).<br /> <br /> 2. Experimental procedures<br /> The experimental study was carried out on AJAN EDM 982 machine. The<br /> dielectric liquid was a brand of Cuttex Fel Ultra. The viscosity of dielectric fluid was 2.2–2.7 at 40 ◦ C and the lightning point was 105 ◦ C/min. The<br /> selected workpiece material was 40CrMnNiMo864 (AISI P20) that was widely<br /> used in die and mold manufacturing industry. The properties of workpiece<br /> material were presented in Table 1. The workpiece specimen was prepared<br /> 20 mm × 70 mm × 315 mm of dimension and the surfaces of workpiece were<br /> milled and finish ground before EDM method application. The possible more<br /> influential machining parameters were selected according to literature review.<br /> The list of selected machining parameter values was given in Table 2.<br /> A cylindrical pure copper with a diameter of 22 mm was used as an electrode<br /> which was finish ground before experimental study. It was mounted axially in<br /> line with workpiece.<br /> The surface roughness of the machined surface was measured by using Taylor<br /> Hobson Surtonic 3+ surface test equipment. The surface roughness, which is<br /> measured by central line average (Ra ) was employed to asses the quality of the<br /> machine surface quantitatively. Each surface roughness value was obtained by<br /> averaging five measurements at various positions of workpiece and electrode<br /> surfaces for each machining condition. The surface measurement filter of this<br /> equipment was 2CR type. The cut-off length was set as 2.5 mm. The evaluation<br /> length was selected as the maximum value which was 2.5 mm. Stylus type was<br /> diamond with 5 ␮m radius.<br /> <br /> 3. Experimental results and discussion<br /> First part of the experimental study carried out for machined<br /> workpiece surface finish quality.<br /> Fig. 1 gives the experimental result of surface roughness when<br /> 2 ␮s pulse pause time was used with different pulsed current and<br /> pulse times. It was observed that surface roughness increased<br /> when higher pulse time was used. Similar result was noticed<br /> for pulse current values, higher currents produced poor surface<br /> finish.<br /> When pulse pause time increased from 2 to 3 ␮s, similar<br /> results were obtained (Fig. 2). Higher pulse time and pulsed<br /> current produced poor surface finish. However, up to certain<br /> pulse time (6 ␮s), surface roughness was different comparing to<br /> 2 ␮s pulse pause time results. For the same machining conditions, surface roughness was 2–3 ␮m for 2 ␮s pulse pause time<br /> and 6 ␮m for 3 ␮s pulse pause time. Pulsed current showed a<br /> similar trend, increasing current gave high surface roughness.<br /> <br /> Table 2<br /> Parameters of examinations<br /> Number of test<br /> 1<br /> Pulsed current (A)<br /> Pulse time (␮s)<br /> Pulse pause time (␮s)<br /> Surface roughness (Ra ) (workpiece)<br /> Surface roughness (Ra ) (electrode)<br /> <br /> 2<br /> <br /> 3<br /> <br /> 4<br /> <br /> 5<br /> <br /> 6<br /> <br /> 7<br /> <br /> 8<br /> <br /> 9<br /> <br /> 10<br /> <br /> 11<br /> <br /> 12<br /> <br /> 13<br /> <br /> 8<br /> 2<br /> 2<br /> 1.8<br /> 1.8<br /> <br /> 8<br /> 4<br /> 2<br /> 3.8<br /> 2.2<br /> <br /> 8<br /> 6<br /> 2<br /> 5.3<br /> 2.6<br /> <br /> 8<br /> 12<br /> 2<br /> 5.7<br /> 3.0<br /> <br /> 8<br /> 24<br /> 2<br /> 7.2<br /> 3.1<br /> <br /> 8<br /> 48<br /> 2<br /> 8.1<br /> 3.3<br /> <br /> 8<br /> 100<br /> 2<br /> 8.7<br /> 3.4<br /> <br /> 8<br /> 3<br /> 3<br /> 1.8<br /> 2.1<br /> <br /> 8<br /> 4<br /> 3<br /> 2.4<br /> 2.6<br /> <br /> 8<br /> 12<br /> 3<br /> 5.0<br /> 3.0<br /> <br /> 8<br /> 24<br /> 3<br /> 6.3<br /> 3.1<br /> <br /> 8<br /> 48<br /> 3<br /> 8.0<br /> 3.2<br /> <br /> 8<br /> 100<br /> 3<br /> 8.9<br /> 3.3<br /> <br /> Number of test<br /> 1<br /> Pulsed current (A)<br /> Pulse time (␮s)<br /> Pulse pause time (␮s)<br /> Surface roughness (Ra ) (workpiece)<br /> Surface roughness (Ra ) (electrode)<br /> <br /> 2<br /> <br /> 3<br /> <br /> 4<br /> <br /> 5<br /> <br /> 6<br /> <br /> 7<br /> <br /> 8<br /> <br /> 9<br /> <br /> 10<br /> <br /> 11<br /> <br /> 12<br /> <br /> 13<br /> <br /> 16<br /> 2<br /> 2<br /> 2.5<br /> 2.2<br /> <br /> 16<br /> 4<br /> 2<br /> 5.0<br /> 3.4<br /> <br /> 16<br /> 6<br /> 2<br /> 6.4<br /> 3.6<br /> <br /> 16<br /> 12<br /> 2<br /> 6.7<br /> 3.7<br /> <br /> 16<br /> 24<br /> 2<br /> 8.4<br /> 4.3<br /> <br /> 16<br /> 48<br /> 2<br /> 9.3<br /> 4.6<br /> <br /> 16<br /> 10<br /> 2<br /> 10.4<br /> 4.7<br /> <br /> 16<br /> 3<br /> 3<br /> 2.3<br /> 2.5<br /> <br /> 16<br /> 4<br /> 3<br /> 3.0<br /> 3.0<br /> <br /> 16<br /> 12<br /> 3<br /> 5.7<br /> 3.8<br /> <br /> 16<br /> 24<br /> 3<br /> 7.0<br /> 3.9<br /> <br /> 16<br /> 48<br /> 3<br /> 8.6<br /> 4.3<br /> <br /> 16<br /> 100<br /> 3<br /> 10.1<br /> 4.3<br /> <br /> Number of test<br /> 1<br /> Pulsed current (A)<br /> Pulse time (␮s)<br /> Pulse pause time (␮s)<br /> Surface roughness (Ra ) (workpiece)<br /> Surface roughness (Ra ) (electrode)<br /> <br /> 2<br /> <br /> 3<br /> <br /> 4<br /> <br /> 5<br /> <br /> 6<br /> <br /> 7<br /> <br /> 8<br /> <br /> 9<br /> <br /> 10<br /> <br /> 11<br /> <br /> 12<br /> <br /> 13<br /> <br /> 16<br /> 2<br /> 2<br /> 3.1<br /> 3.0<br /> <br /> 16<br /> 4<br /> 2<br /> 5.2<br /> 3.5<br /> <br /> 24<br /> 6<br /> 2<br /> 6.7<br /> 3.6<br /> <br /> 24<br /> 12<br /> 2<br /> 8.5<br /> 3.9<br /> <br /> 24<br /> 24<br /> 2<br /> 9.4<br /> 4.4<br /> <br /> 24<br /> 48<br /> 2<br /> 9.7<br /> 4.7<br /> <br /> 24<br /> 100<br /> 3<br /> 11.3<br /> 4.8<br /> <br /> 24<br /> 3<br /> 3<br /> 3.0<br /> 3.0<br /> <br /> 24<br /> 4<br /> 3<br /> 3.2<br /> 3.1<br /> <br /> 24<br /> 12<br /> 3<br /> 6.6<br /> 3.8<br /> <br /> 24<br /> 24<br /> 3<br /> 7.5<br /> 4.2<br /> <br /> 24<br /> 48<br /> 3<br /> 9.0<br /> 4.4<br /> <br /> 24<br /> 100<br /> 3<br /> 10.9<br /> 4.4<br /> <br /> M. Kiyak, O. Cakır / Journal of Materials Processing Technology 191 (2007) 141–144<br /> ¸<br /> <br /> 143<br /> <br /> Fig. 2. Effects of pulse time and pulsed current on surface roughness of workpiece (3 ␮s pulse pause time).<br /> <br /> Fig. 4. Effects of pulse time and pulsed current on surface roughness of electrode<br /> (3 ␮s pulse pause time).<br /> <br /> Surface finish quality was better when applying smaller<br /> pulsed current and pulse time. This is because of small particle size and crater depths formed by electrical discharge. As a<br /> result, the best surface finish will be produced. The selection of<br /> these machining parameters for EDM of any material should be<br /> used for a higher surface quality is required.<br /> It was observed that when pulsed current and particularly<br /> pulse time increased, machined workpiece surface exhibited a<br /> higher surface roughness due to irregular topography. Pulsed<br /> current had an effect on surface roughness at low pulse time,<br /> but the influence of pulse time was more significant than pulsed<br /> current at higher pulse times.<br /> It was noticed that excellent machined surface quality could<br /> be obtained by setting machining parameters at a low pulse current and short pulse time. This combination will unfortunately<br /> produce low material removal rate and cause high machining<br /> time, in total high machining cost. When high material removal<br /> rates are needed, high pulsed current and pulse times should<br /> be selected. However, this selection will produce a poor surface finish due to deeper and wider crates on the machined<br /> surface. There is also an influence on dielectric fluid at high<br /> pulsed current; the properties will be lost because of high<br /> temperature.<br /> Second stage of the experimental study considered to examine the surface roughness of electrode. The bottom surface<br /> of the electrode, which is continuously contacted to workpiece, was investigated. For the same machining conditions,<br /> the experimental results were given in Figs. 3 and 4. Low<br /> pulse times and pulsed currents provided a better surface quality for both duration times. It was observed that pulse pause<br /> time was an influential parameter; 2 ␮s of pulse pause time pro-<br /> <br /> vided better surface quality comparing to 3 ␮s of pulse pause<br /> time.<br /> Similar surface roughness results were noticed for electrode<br /> surface roughness. It was observed that the surface roughness<br /> of electrode was better when applying smaller pulsed current<br /> and pulse time. When pulsed current and pulse time increased,<br /> electrode surface presented a higher surface roughness. Pulsed<br /> current had an effect on surface roughness of electrode at low<br /> pulse time, but the influence of pulse time was more significant<br /> than pulsed current at higher pulse times. The effect of pulse<br /> pause time was insignificant.<br /> <br /> Fig. 3. Effects of pulse time and pulsed current on surface roughness of electrode<br /> (2 ␮s pulse pause time).<br /> <br /> 4. Conclusion<br /> The experimental study of the EDM of 40CrMnNiMo864<br /> tool steel (AISI P20) tool steel provided important quantitative<br /> results for obtaining possible high surface finish quality and<br /> machining outputs as follows:<br /> a. Surface roughness increased with increasing pulsed current<br /> and pulse time. Low current and pulse time with high pulse<br /> pause time produced minimum surface roughness that means<br /> good surface finish quality. The selection of these machining<br /> parameters is not useful because machining process generally becomes very slow. Material removal rate will be low<br /> and thus machining cost increases. This combination should<br /> be used in finish machining step of EDM process.<br /> b. High pulsed current and pulse time provide low surface finish<br /> quality. However, this combination would increase material<br /> removal rate and reduce machining cost. As a result, this<br /> combination (high pulsed current and pulse time) should be<br /> used for rough machining step of EDM process.<br /> c. From power point of view, rough and finish machining steps<br /> require different level of machine power. For rough EDM<br /> application, the machine power should be one-fourth of the<br /> produced power with 16 A of current, 6 ␮s of pulse time and<br /> 3 ␮s of pulse pause time. Finish machining should be carried<br /> out at one-half level of power at 8 A of current as well as 6 ␮s<br /> of pulse time and 3 ␮s of pulse pause time.<br /> d. Increasing wear on electrode surface is unavoidable during<br /> EDM process. Therefore, workpiece surface roughness will<br /> be increasing due to wear rate on electrode.<br /> e. Wear on electrode surface is unavoidable during EDM<br /> process. Surface roughness of machined workpiece would<br /> <br /> 144<br /> <br /> M. Kiyak, O. Cakır / Journal of Materials Processing Technology 191 (2007) 141–144<br /> ¸<br /> <br /> increase when surface quality of electrode decreases due to<br /> pulsed current density.<br /> f. For the same pulse pause time, the trends of surface roughness on the workpiece and electrode are similar. Thus, there<br /> will be a relation between wear on electrode and increase of<br /> surface roughness from workpiece surface quality point of<br /> view.<br /> Acknowledgements<br /> Authors would like to thank two companies (OPAS and<br /> ERDEM) for kind encouragement and technical support. They<br /> are also thankful to Mech. Eng. 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