1. Manish Vishwakarma, V.K.Khare, Vishal Parashar / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 2, Issue 4, July-August 2012, pp.185-189 Response surface approach for optimization of Sinker Electric Discharge Machine process parameters on AISI 4140 alloy steel 1 manish Vishwakarma, 2v.K.Khare, 3vishal Parashar 1,2,3 Department of Mechanical, M.A.N.I.T, Bhopal, Madhya Pradesh,IndiaAbstract : The purpose of this paper is to determine the To evaluate the performance of EDM on AISIoptimal factors of the electro-discharge machining 4140 with respect to various responses such as(EDM) process investigate feasibility of design of surface quality,material removalrate(MRR).experiment techniques. The workpieces used were To establish mathematical model for allrectangular plates of AISI 4140 grade steel alloy. The responses involved which are surface quality,study of optimized settings of key machining factors like material removal rate (MRR).pulse on time, gap voltage,flushing pressure,input Full factorial method from design of experimentcurrent and duty cycle on the material removal,surface (DOE) used in order to analyze and determineroughness is been carried out using central composite feasible solutions for optimal cuttingdesign. The output responses measured were material parameters of EDM operation.removal rate (MRR) and surfaceroughness.Mathematical models are proposed for the 1.1 PROCESS PARAMETERSabove responses using response surface methodology The process parameters can be divided into two(RSM).The results reveals that MRR is more influenced categories, i.e., electrical and non-electrical parameters.by peak current, duty factor.Finally, the parameters wereoptimized for maximum MRR with the desired surface 1.1.1 Electrical parametersroughness Main electrical parameters are peak current ,discharge voltage,pulse duration and pulseKeywords: Response Surface Methodology, Central interval,polarity,electrode gap.Discharge voltage isComposite Design,MRR. related to spark gap and breakdown strength of the dielectric fluid.The open-gap voltage before electric1.INTRODUCTION discharge increases until ionization path is created Electrical Discharge Machining (EDM) is a between workpiece and electrode. Once the current startsunconventional manufacturing process based on removal flowing, the voltage drops and stabilizes at the workingof material from a part by means of a series of repeated gap level. Thus a higher voltage setting increases theelectrical sparks created by electric pulse generators at gap, which improves the flushing conditions and helps toshort intervals between a electrode tool and the part to stabilize the cut. Peak current is the amount of powerbe machined emmersed in dielectric fluid.At used in discharge machining and is considered as mostpresent,EDM is a widespread technique used in industry significant process parameter. The current increases untilfor high precision machining of all types of conductive it reaches a preset level during each pulse on-time, whichmaterials such as metallic alloys, metals, is known as peak current. Peak current is governed bygraphite,composite materials or some ceramic materials. surface area of cut.Higher peak current is applied duringThe selection of optimized manufacturing conditions is roughing operation and details with large surface area.one of the most important aspects to consider in the die- This is the most important parameter because thesinking electrical discharge machining (EDM) of machined cavity is a replica of tool electrode andconductive steel, as these conditions are the ones that are excessive wear will hamper the accuracy of machining.to determine such important characteristics: surface New improved electrode materials like graphite, canroughness,electrode wear (EW) and material removal work on high currents without much damage [1].Pulserate (MRR). In this paper, a study will be perform on the duration is commonly referred to as pulse on-time andinfluence of the factors of peak current, pulse on time, pulse interval is called pulse off-time. These areinterval time and power supply voltage. Isothermal aging expressed in units of microseconds. Since all the work isof material varies the EN-19 steel, which is mainly used done during pulse duration, hence this parameters andin various elements like gears, axels, drive the number of cycles per second (frequency) areshafts,induction hardening pins and high strength important. Material removal rate (MRR) depends uponshafts[1]. Design of experiments (DOE) technique to the amount of energy applied during the pulse durationselect the optimum machining conditions for machining [13]. Increased pulse duration also allow more heat toAISI 4140 using EDM.The objectives of this paper are sink into the workpiece and spread, which means thestated as follows: recast layer will be larger and the heat-affected zone will be deeper. Material removal rate tends to decrease after an optimal value of pulse duration. Pulse interval mainly 185 | P a g e
2. Manish Vishwakarma, V.K.Khare, Vishal Parashar / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 2, Issue 4, July-August 2012, pp.185-189affects machining speed and stability of cut. Shorter evaporation[7]. The working principle of EDM is showninterval, results in faster machining operation. in Fig. 1. This technique has been developed in the late 1940s [8]. The electrode moves toward the workpiece2. LITERATURE REVIEW reducing the spark gap so that the applied voltage is high Electrical Discharge Machining (EDM) – enough to ionize the dielectric fluid [9]. Short durationSinker EDm is one of the most widely used discharges are generated in a liquid dielectric gap, whichunconventional material removal process.Its typical separates electrode and workpiece.feature of using thermal energy to machine electricallyconductive job piece regardless of hardness has been itsdistinctive advantage in the manufacture of mould, die,automotive, defence, aerospace and surgicalcomponents. In addition, EDM does not make directcontact between the electrode and the workpieceeliminating mechanical stresses, chatter and vibrationproblems during machining. [2].Based on Yussni (2008),the variables parameters are have great effects to themachining performances results especially to thematerial removal rate (MRR), electrode wear rate andsurface quality. There are two major groups of Figure 1: Working principle of EDM[10]parameters that have been discovered and categorized[2]: The material is removed from tool and workpiece with1) Non-electrical Parameters the erosive effect of the electrical discharges [10]. Thea. Injection flushing pressure dielectric fluid serves the purpose to concentrate theb. Rotational of speed electrode discharge energy into a channel of very small cross sectional areas. It also cools the two electrodes, and2) Electrical Parameters flushes away the products of machining from the gap.a. Peak current The electrical resistance of the dielectric influences theb. Polarity discharge energy and the time of spark initiation [11].c. Pulse duration Early discharge occurs due to low resistance.If resistanced. Power supply voltage is more, the capacitor will obtain higher charge value before initial discharge. A servo system is present toIn the other hand, Van Tri (2002) categorized the compares the gap voltage with a reference value and toparameters into five groups[3]: ensure that the electrode moves at a proper rate to maintain the correct spark gap, and also to retract the1) Dielectric fluid; type of dielectric, temperature, electrode if short circuiting occurs. When the measuredpressure,flushing system . average gap voltage is higher than that of the servo2) Machine characteristics; servo system and stability reference voltage, preset by the operator, the feed speedstiffness, thermal stability and accuracy. increases.On the contrary,the feed rate decreases or the3) Tool; material, shape, accuracy electrode is retracted when the average gap voltage is4) Workpiece lower than the reference voltage, which is the case for5) Adjustable parameters; discharge current, gap smaller gap widths resulting in a smaller ignition delay.voltage,pulse duration, polarity, charge frequency, Thus, short circuits occured by eroded particles andcapacitance humps of discharge a crater are avoided. and tool materials. It has been observed that there is very little or no workErden [4] proposed that material removal mechanism represented on AISI 4140 & no model is available torelating to three phases of sparking,namely breakdown, represent MRR for AISI 4140 with combination of workdischarge and erosion. Also, it was found that reversing piece and electrodes And hence the study is focused onthe polarity of sparking alters the material removal AISI 4140 machining cavilty with various inputphenomenon with an appreciable amount of electrode parameters like current, pulse on time, gap voltage, dutymaterial depositing on the workpiece surface [5]. cycle to obtain better MRR.These have been done usingGadalla and Tsai [6] investigated the material removal of Central composite design technique of design ofWC–Co composite. They attributed the material removal experiments.Central composite design technique isto the melting and evaporation of disintegrated Co design of experiments (DoE) utilized because thefollowed by the dislodging of WC gains, which have a experimental design and analyze of the results can belower electrical conductivity. However, thermal spalling done with less effort and expenses. Since the methodcontributes to the material removal mechanism during enormously reduces the number of experiments, qualitythe sparking of composite ceramics. This is because the loss of results must be taken into account [12].physical and mechanical properties promote abrupttemperature gradients from normal melting and 186 | P a g e
3. Manish Vishwakarma, V.K.Khare, Vishal Parashar / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 2, Issue 4, July-August 2012, pp.185-1892 EXPERIMENTAL WORK Parameters Range Discharge current (I) 0-19/35/49 Amp2.1 Test Piece and Electrode MaterialThe test piece material was AISI 4140 with chemical Pulse-on time (ton) 1 to 99 μscomposition given in Table 1.AISI 4140 is preferred in Flushing pressure 0-10 kg/cm2defence and health care due to its specific properties Duty cycle 1-9essential in manufacturing.Components of mediums & Gap Voltage 1-20 voltslarge cross section, requiring high tensile strength &toughness for automatic engineering and gear & engine Table 2. Machining conditionsconstruction such as crane shafts steering knockingconnecting rods are also made from this material. 2.3 Measurement ProcedureChemical Composition(%) The test piece was weighed before and after drilling operation using a digital precision scale. MaterialChe Car Sili Mang Chro Molybd Sulp Removal Rate (MRR) for each experiment wasmical bon con anese mium enum hur calculated by the following formula:% 0.36 0.1 0.70- 0.90- 0.25- 0.03 - 0- 1.00 1.20 0.35 5 MRR (mg/min) = initial weight - final weight / .044 0.3 max machining time 5 3 Mathematical ModellingTable 1. 3.1 Design of ExperimentsCharacteristics: Design of Experiments (DOE) is a method to obtain• A 6% allowance should always be made for removal of useful information about a process by conducting onlysurface defects during machining minimum number of experiments [13].Each controllable• Machinability good variable (Ip, Ton,Vgap,, Tduty cycle,Flushing pressure) can be• Ideal for 60 ton tensile applications up to 100mm max. set on EDM machine at five consecutive levels from 1 to• Sizes above limited ruling section will be heat treated 5, and hence the design consisting of 30 experimentsto a hardness only, with no based on Central Composite Design (CCD) wasguarantee on mechanical properties generated at these levels using Minitab® statistical• Weldable software.Other factors given in Table 2 were kept constant. Table 3 shows the design matrix with2.2 Typical Applications: experimental plan. MRR predicted within error range of Axles,Connecting Rods,Drive ± 16% (except experiment no. 29) and ± 19%, Shafts,Crankshafts,High Tensile Bolts,Studs respectively.The selected three parameters have different And propeller Shaft Joints influences on the machining performance. The MRR is calculated 3by dividing the volume of material removed2.2 Experimental Setup by the actual machining time and expressed in mm3/min. The coefficients of the response surface equation wereThe experiments were performed on AISI 4140 test determined by using Minitab15 software.pieces (100 mm x 100 mm x 21mm) using ElectronicaENC series EDM machine shown in fig below in figure2.Figure 2(ENC series EDM machine)Table 2.presents the machining conditions for removal of3mm depth cavity of size by the tool electrode size of40mm x 20mm x 3mm. 187 | P a g e
4. Manish Vishwakarma, V.K.Khare, Vishal Parashar / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 2, Issue 4, July-August 2012, pp.185-189 CCD Coded levels of Actual levels of variables variables MRR Tduty Tduty (mm3/min) Ton Vgap Ip Ton Vgap Ip cycle cycle - 0.6465 1 -1 -1 -1 74.5 01 01 10 1 - 0.7627 2 1 -1 -1 25.5 01 01 10 1 - 0.9122 3 -1 1 -1 74.5 20 09 10 1 - 0.7219 4 -1 -1 1 74.5 01 09 10 1 5 -1 -1 -1 1 74.5 01 01 49 0.8627 - 0.9755 6 1 1 -1 25.5 20 01 10 1 - 0.7998 7 1 -1 1 25.5 01 09 10 1 8 1 -1 -1 1 25.5 01 01 49 0.9713 - 1.118 9 -1 1 1 74.5 20 09 10 1 10 -1 -1 -1 1 74.5 01 01 49 0.5554 11 -1 -1 1 1 74.5 01 09 49 0.6761 - 0.8156 12 1 1 1 25.5 20 09 10 1 13 1 1 -1 1 25.5 20 01 49 0.6491 14 1 -1 1 1 25.5 01 09 49 0.7686 15 -1 1 1 1 74.5 20 09 49 0.9998 16 1 -1 1 1 25.5 01 09 49 0.6783 17 -2 0 0 0 01 10.5 05 29.5 0.8799 18 2 0 0 0 99 10.5 05 29.5 1.0291 19 0 -2 0 0 50 01 05 29.5 0.4771 20 0 2 0 0 50 20 05 29.5 0.5998 21 0 0 -2 0 50 10.5 01 29.5 0.7154 22 0 0 2 0 50 10.5 09 29.5 0.5553 - 0.6877 23 0 0 0 50 10.5 05 10 2 24 0 0 0 2 50 10.5 05 49 0.8584 25 0 0 0 0 50 10.5 05 29.5 0.6408 26 0 0 0 0 50 10.5 05 29.5 0.8113 27 0 0 0 0 50 10.5 05 29.5 0.9484 28 0 0 0 0 50 10.5 05 29.5 0.8856 29 0 0 0 0 50 10.5 05 29.5 0.6365 30 0 0 0 0 50 10.5 05 29.5 0.7527 Table 3. Experimental plan in accordance to CCD. for five factorial experiment.The experiments were run3.2 Statistical analysis in random order according to a CCD configuration forA five factors, five level Central Composite design was five factors. The coded values above in table 3 wereuse to determine the optimal levels of machining obtained as per the following relationships in table 4 :parameters of sinker EDM machine for AISI 4140 alloy Code Actual value of variablesteel. The central composite design (CCD) with a -α xminquadratic model was employed.[14] Five independent -1 {xmax+xmin/2} - { xmax-xmin/2 α }variables namely current, voltage gap,duty cycle,flushing 0 {xmax+xmin/2}pressure are chosen. Each independent variable had 5 +1 {xmax+xmin/2} + { xmax-xmin/2 α }levels which were – 1, 0 and +1. A total 30 different +α xmaxcombinations including 8 axial runs and 6 center point Table 4.Relationship between coded values and actualruns.The value of alpha here is (-2,2) as per CCD chart values of variables 188 | P a g e
5. Manish Vishwakarma, V.K.Khare, Vishal Parashar / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 2, Issue 4, July-August 2012, pp.185-189 [3] Van Tri etal(2002). Electrical dischargeA second-order polynomial equation was proposed to machining of aluminum alloy using classicalexpress the MRR as a function of independent design of experiment approach. Universitivariables.Response surface model is hereby employed as Teknologi Malaysia: Master Thesis.a hybrid mathematical and statistical method in which aresponse(MRR) of interest is affected by several [4] Erden, A., 1983, “Effect of materials on thevariables and the objective is to optimize this response mechanism of electric discharge machining[13]. In this study, a second-order polynomial was (EDM)”, J. Eng. Mater. Technol., 105,132–138.selected to develop empirical equations to representresponses MRR in terms of controllable variables (I p, [5] Gangadhar, A., Sunmugam, M.S., Philip, P.K.,Ton,Tduty cycle,Vgap) : 1992,Pulse train studies in EDM with controlled pulse relaxation”, Int.J. Mach. Tools Manuf., 32Y(MRR) = b0 + b1x1 + b2x2 + b3x3 + b4x4 + b11x12 + (5), 651–657.b22x22 + b33x32 + b44x42+ b12x1x2 + b13x1x3 + b14x1x4 +b23x2x3 + b24x2x4 + b34x3x4 [6] Gadalla, A.M., Tsai, W., 1989, “Machining of WC–Co composites”, Mater. Manufacturing4. Experimental Results & Scope Processes, 4 (3), 411– 423.4.1. Effect of Voltage on MRR [7] Lee, T.C., Lau, W.S., 1991, “Some characteristicsIn EDM operations, the voltage is one of the important of electrical discharge machining of conductiveparameters. To study the effect of voltage,the spark gap ceramics”, Material Manufacturing Processes, 6and capacitance were kept constant. With analysis, it is (4), 635–648.observed that, with increase in voltage, the MRR alsoincreases. This occurs because of the energy discharge [8] Singh, S., Maheshwari, S., Pandey, P.C., 2004from the electrode increases with increase in voltage. “Some investigations into the electric dischargeWith increase in discharge energy, higher temperatures machining of hardened tool steel using differentare generated between the electrodes. This results higher electrode materials”, Journal of Materialsin MRR. Processing Technology, 149, 272– 277.The capacitance is also an influencing parameter in [9] Bojorquez, B., Marloth, R.T., Es-Said, O.S., 2002EDM.To study the effect in capacitance, other two “Formation of a crater in the workpiece on anparameters such as voltage and spark gap are kept electrical discharge machine”,Engineeringconstant.This is another scope for further study and Failure Analysis, 9, 93– 97.analysis in EDM research. [10] Marafona, J.; Chousal, A.G., 2006 “A finite5. Conclusion element model of EDM based on the JouleA Central composite design was used to conduct the effect”, Int. J. Mach.Tools Manuf., 46 (6),595-experiments. The effect of important process 602.parameters in EDM: voltage,current,duty cycle andflushing pressure on MRR has been studied. [11] Kuneida, M., Lauwers, B., Rajurkar, K.P.,The following are some of the salient conclusions that Schumacher, B.M., 2005 “Advancing EDMcould be drawn based on the studies: through fundamental insight into the process”,Annals of CIRP, 54 (2), 599–622.1.Increase in voltage results in higher MRR andsimilarly higher capacitance results in higher MRR. [12] Dr. S. S. Khandare & Mitesh a. Popat (2009),2. In case of spark gap, with increase in spark gap the Experimental Investigations of EDM toMRR decreases. optimize Material Removal Rate & Surface3. The results obtained would be a good technical Roughness through Taguchi’s Technique ofdatabase for the defence/automotive industries. Design of Experiments. IEEE explore,ICETET-09, pg 476 – 482 .6. References[1] A.Gopichand etal (2012). Analysis and Estimation [13] Montgomery, D.C. (1997) Design and analysis of Attenuation Coefficient of Aging EN-19 Steel, of experiments. Wiley (New York), ISBN: 0- Int. Journal of Engineering Research and 471-15746-5. applications Vol. 2, Issue 1,pp.590-595. [14] Box, G.E.P., Hunter, W.G., Hunter, J.S. (1978),[2] Mohd Yussni etal 2008).Electrical Discharge Statistics for experimenters, John Wiley, New machine Die Sinking.Universiti Tun Hussein On York. Malaysia (UTHM). 189 | P a g e
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