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A study on influence of polarity on the machining characteristics of sinker edm


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A study on influence of polarity on the machining characteristics of sinker edm

  1. 1. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 3, April (2013), © IAEME158A STUDY ON INFLUENCE OF POLARITY ON THE MACHININGCHARACTERISTICS OF SINKER EDMA. Parshuramulu1, K. Buschaiah2*and P. Laxminarayana31Asst. Professor, University College of Technology,OsmaniaUniversity, Hyderabad, A.P. – 5000072*Scientist, Department of Mechanical Engineering, UniversityCollege of Engineering, Osmania University,Hyderabad, A.P. –500007.3Professor, Department of Mechanical Engineering,University College of Engineering, Osmania University,Hyderabad, A.P. –500007.ABSTRACTElectrical discharge machining (EDM) has been recognized as an efficient productionmethod for precision machining of electrically conducting hardened materials. Electrical DischargeMachining is a machining method primarily used for hard metals or those that would be impossibleto machine with traditional techniques. One critical limitation, however, is that EDM only works withmaterials that are electrically conductive. Sometimes referred to as spark machining or spark eroding,EDM is a nontraditional method of removing material by a series of rapidly recurring electric arcingdischarges between an electrode (the cutting tool) and the work piece, in the presence of an energeticelectric field. The most important study of this paper is the effect of the polarity on the machiningtool / work piece using electrical discharge machining to the material removal rate, electrode wearand surface roughness, to determine the optimum condition, and to determine the most significantfactor.In this paper an elaborative methodology is suggested to choose option between electrode andwork piece as terminal positive or terminal negative for different categories of tools and work pieces.The wrong polarity can have significant implications on wear, and stability. A set of experiments areconducted and the results are represented numerically and graphically.The most important output parameters are material removal rate, electrode wear and surfaceroughness. From the obtained data one can easily determine the optimum parameters and mostsignificant factors related to material removal rate (MRR), electrode wear and surface finish veryeasily.Index Terms: Material removal rate (MRR), Straight and reverse polarity, Surface roughness.INTERNATIONAL JOURNAL OF ADVANCED RESEARCH INENGINEERING AND TECHNOLOGY (IJARET)ISSN 0976 - 6480 (Print)ISSN 0976 - 6499 (Online)Volume 4, Issue 3, April 2013, pp. 158-162© IAEME: Impact Factor (2013): 5.8376 (Calculated by GISI)www.jifactor.comIJARET© I A E M E
  2. 2. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 3, April (2013), © IAEME159I. INTRODUCTIONElectrical Discharge Machining (or EDM) is a machining method primarily used for hardmetals or those that would be impossible to machine with traditional techniques. One criticallimitation, however, is that EDM only works with materials that are electrically conductive. EDM cancut small or odd-shaped angles, intricate contours or cavities in pre-hardened steel without the needfor heat treatment to soften and re-harden them as well as exotic metals such as titanium, hastelloy,and inconel.The control parameters optimization for individual machining characteristic is concerned withseparately maximize the material removal rate, separately minimize the tool wear ratio and separatelyobtained a good surface finish. There are many input parameters which can be varied in the EDMprocess which have different effects on the EDM machining characteristics. Sometimes referred to asspark machining or spark eroding, EDM is a nontraditional method of removing material by a seriesof rapidly recurring electric arcing discharges between an electrode (the cutting tool) and the workpiece, in the presence of an energetic electric field. The EDM cutting tool is guided along the desired.path very close to the work but it does not touch the piece [1]. Consecutive sparks produce a series ofmicro-craters on the work piece and remove material along the cutting path by melting andvaporization. The particles are washed away by the continuously flushing dielectric fluid. It is alsoimportant to note that a similar micro-crater is formed on the surface of the electrode, the debris fromwhich must also be flushed away [2].II. THEORYExperimental research generally targets regression analysis of process parameters andmodeling to optimize the process characteristics. This involves maximization of machining rate andminimization of tool wear and surface roughness. This also helps in the development of adaptivecontrol systems. The advances in computer applications in manufacturing processes and their controlhas led to the development of artificial intelligence approaches in the form of expert systems, neuralnetworks and fuzzy logic towards optimization and other control systems like prevention of wirerupture. However most of the experimental research has a simplistic approach and tries the variationof dielectric (hydrocarbons and water based) and electrode materials (Copper, tungsten, graphite etc.),method of gap flushing (tool rotation, vibration or oscillation, magnetic and ultrasonic field’sapplication) and studies of surface integrity (hardness, residual stress, defects like micro cracksalloying with electrode material). The other type of popular research area is hybridization of EDMwith another assisting process for combining the beneficial features of both processes. In the case ofEDM the assistance of Electro Chemical Machining, Ultrasonic Machining and magnetic field havebeen reported. In spite of being so extensively researched there are considerable grey areas in theliterature on the EDM process and the associated theories and mechanism [3].III. STRAIGHT AND REVERSE POLARITYFor a better understanding of spark erosion mechanism, and the final surface characteristicswhich includes morphological, metallurgical and textural features it is always necessary to study theeffect of polarity which is defined as reverse polarity (electrode positive) and straight polarity(electrode negative) [4].IV. DESCRIPTION OF EXPERIMENTAL SETUPAn experimental study was carried out on CREATOR CR-6C (SY CNC PC-60) Electricdischarge machine with hydrocarbon oil (ED-30 oil) as the dielectric fluid. The selected work piecematerial is stainless steel in the form of rectangular shapes of 20mm×20mm×5mm. A cylindrical copper
  3. 3. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 3, April (2013), © IAEME160tool with a diameter of 12mm was used as an electrode which was finish ground before experimentalstudy and was mounted axially in line with work pieces. 12 specimens of stainless steel have been takenfor experiment. Those have been made as rectangular shape of 20mm×20mm×5mm pieces. By varyingthe current (6, 8,10,12,14 & 16 Amps) i.e., 6 specimens each for both Straight and Reverse polaritymachining is carried out keeping time of observation as 20 minutes. The specimens are weighed beforeand after the machining, the difference of both is considered to arrive at the Material Removal Rate(MRR). Surface roughness for 12 specimens of stainless steel has been measured by using stylus typeinstrument. The sampling length taken was 5mm. Surface morphology was also studied in depth usingSEM photographs on all the 12 specimens (six specimens in straight polarity and six specimens inreverse polarity)V. RESULTS AND DISCUSSIONSA. Material Removal Rate:The effect of polarity was interesting whereby negative polarity produced shallow and smallcraters whereas positive polarity led to larger craters.B. Morphology and integrity of EDM surfaces:The physical and metallurgical studies of EDM surfaces show some interesting aspects. The sparkeroded surfaces are matty in appearances owing to the overlapping spark craters. Ideally these cratersare expected to be spherical as a result of melting from the spark energies with the assumptions of apoint source of heat with dissipation radially there from. In practice the evacuation of molten metalfrom spark craters is not complete and owing to variables energies in the spark trains the crater sizesalso differ, leading to the formation of a randomly varying surface morphology (Fig.1). Erosion inmolten form is also evident from spherical debris trapped in the resolidified residual layer. This apartthe eroded surfaces exhibit typical EDM characteristics of micro cracks and gas pockets. Theabsorption of carbon from the pyrolysis of hydrocarbon dielectric leads to the formation of hardcarbides, which form a hard surface layer, which is not etchable by the conventional etchant andappears and so names as “White Layer” [5], [10], [11].(a)(b)Fig. 1. SEM photographs after EDM of steel surfaces (a) Straightpolarity and (b) Reverse polarity
  4. 4. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 3, April (2013), © IAEME161Electrode positive setup is superior in the form of high erosion of work material. This inmachining terminology is equivalent to high machining rates. This is due to higher liberation of sparkenergy at cathode which absorbs ions higher mass through the plasma channel dimensions comparedto electrons absorbed at anode which are of negligible mass and result in large and expanded channeldimensions due to mutual repulsion leading to low energy concentration. Next in importance is theeffect of pulse current on erosion rate. But this is a well established fact. The pulse energy is being aproduct of pulse voltage, current and on time, naturally any increase in these variables results inhigher erosion rates both at work and electrodes surfaces. Of all these the effect of pulse current ishigher [6].(A) (B)Fig. 2. EDAX photographs after EDM of steel surfaces(a) Straight polarity and (b) Reverse polarity.There is evidence of inter-electrode mass transfer whereby the anodic electrode material getsdiffused on to the cathodic work surface. EDXA analysis shows such a mass transfer where electrodeis anode (Fig.2). One can anticipate higher roughness to be associated with higher erosion rates and iscertainly evident in most of the results. The only exception being the pulse times. The factor, whichpromotes spark erosion, also promotes surface roughness owing to larger size of spark craters.Consequently the effects of polarity, pulse current and pulse voltage are similar on erosion rates androughness. But in the case of pulse on time the roughness is reduced though erosion rate increasesignificantly. This phenomenon is attributed once again to expansion of spark channel, which not onlyreduces the energy concentration but also results in higher diameter of spark craters. Though theeffect of both pulse current and on time are similar in increasing the size of spark craters , leading tohigher erosion rates, their effect on the geometry of the spark craters is different. Increase in currentfor the same pulse on time results in deeper craters owing to higher energy concentration. On theother hand the increase in pulse on time promotes plasma channel expansion thus leading to largerdiameter of spark craters.VI. CONCLUSIONSSummarizing the main features of the present experimental work, the following conclusionswere drawn:1. It is evident from the results obtained that, the polarity and current setting have dominant effecton erosion rates of steel.
  5. 5. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 3, April (2013), © IAEME1622. Pulse current has direct effect on both MRR and surface roughness, which are increasing withincrease in the former for both the polarities.3. Electrode positive and higher currents produce higher erosion rates compared to electrodenegative at higher currents for steel.4. MRR obtained is in the range of 5.94 – 24.46 mm3/min in case of reverse polarity, whereas incase of straight polarity the range obtained is 0.035 – 0.120 mm3/min for same peak pulse currentand time of machining.5. Surface roughness obtained is in the range of 1.72 – 5.92 µm in case of straight polarity, whereasin case of reverse polarity the range obtained is 7.36 – 12.64µm for same peak pulse current andtime of machining.6. It is inferred that higher current promote deeper craters and higher roughness.Finally the following concluding remarks are put forth about the practical significance of thesestudies. For improving erosion rates reverse polarity and higher pulse currents are advisable toelectrode positive. For improving the surface finish slower erosion is advisable which is possible withstraight polarity only.REFERENCES1. S.H.Tomadi et al, “Analysis of the influence of EDM Parameters on Surface Quality, Material removalrate and electrode wear of tungsten carbide”, IMECS 2009 march 18-20, 2009, Hongkong.2. K.H. Ho, S.T. Newman, “State of the art electrical discharge machining (EDM)”, Int. journal ofmachine tools & manufacture 43 (2003) 1287-1300.3. Norliana Mohd Abbas et al, “A review on current research trends in electrical dischargemachining(EDM)”,Int. journal of machine tools & manufacture 47(2007) 1214 -1228.4. C.H. Che Haron et al, “Investigation on the influence of machining parameters when machining toolsteel using EDM”, Journal of Materials Processing Technology 116(2001) 84-87.5. Seong Min Son et al, “Influences of pulsed power condition on the machining properties in microEDM”, Journal of Materials Processing Technology 190(2007) 73-76.6. Ahmet Hascalik, Ulas Caydas, “Electrical discharge machining of titanium alloy (Ti-6Al-4V)”,Applied surface science 253(2007) 9007-9016.7. J.C. Rebelo et al, “Influence of EDM pulse energy on the surface integrity of martensitic steels”,Journal of Materials Processing Technology 84(1998) 90-96.8. Shankar Singh et al, “Some investigations into the electric discharge machining of hardened tool steelusing different electrode materials”, Journal of Materials Processing Technology 149(2004) 272-277.9. Gokul Vasudevamurthy, Travis W. Knight, “Effect of system parameters on size distribution of 304stainless steel particles produced by electrical discharge mechanism”, Materials letters 61(2007) 4872-4874.10. Bhattacharyya et al, “Modelling and analysis of EDMED job surface integrity”, Journal of MaterialsProcessing Technology 189(2007) 169-177.11. S.H.Lee, X.P.Li, “Study of the effect of machining parameters on the machining characteristics inelectrical discharge machining of tungsten carbide”, Journal of Materials Processing Technology115(2001) 344-358.12. A. Hemantha Kumar , Krishnaiah.G, V.Diwakar Reddy, “Ann Based Prediction Of Surface RoughnessIn Turning” International Journal Of Mechanical Engineering & Technology (IJMET) Volume 3, Issue2 , 2012, pp. 162 - 170, ISSN PRINT : 0976 – 6340, ISSN ONLINE : 0976 - 6359.13. P.B.Wagh, R.R.Deshmukh And S.D.Deshmukh, “Process Parameters Optimization For SurfaceRoughness In Edm For Aisi D2 Steel By Response Surface Methodology” International Journal OfMechanical Engineering & Technology (IJMET) Volume 4, Issue 1, 2013, pp. 203 - 208, ISSN PRINT: 0976 – 6340, ISSN ONLINE : 0976 - 6359