An overview of experimental investigation of near dry electrical discharge machining process
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An overview of experimental investigation of near dry electrical discharge machining process Document Transcript

  • 1. INTERNATIONAL JOURNAL OF ADVANCED RESEARCH INInternational Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME ENGINEERING AND TECHNOLOGY (IJARET)ISSN 0976 - 6480 (Print)ISSN 0976 - 6499 (Online)Volume 3, Issue 2, July-December (2012), pp. 22-36 IJARET© IAEME: www.iaeme.com/ijaret.htmlJournal Impact Factor (2012): 2.7078 (Calculated by GISI)www.jifactor.com ©IAEME AN OVERVIEW OF EXPERIMENTAL INVESTIGATION OF NEAR DRY ELECTRICAL DISCHARGE MACHINING PROCESS Mane S.G.1, Hargude N.V.2 1,2 Department of Mechanical Engineering, PVPIT Budhgaon, Sangli 416416,Maharashtra, India. E-mail: shrikant_mane3665@rediffmail.com; nvhargude@gmail.com .ABSTRACT EDM has achieved a status of being nearly indispensable in the industry because of itsability to machine any electrically conductive material which is difficult-to-machineirrespective of its mechanical strength. Out of the three EDM processes viz. wet, dry &near-dry; near-dry EDM is proved to be most environment-friendly. Further some otherproblems like higher discharge energy requirement in wet EDM and the reattachment ofdebris to the machined surface in dry EDM can be overcome in near-dry EDM. Also, it isfound that near-dry EDM has the advantage in finish operation with low discharge energyconsidering its higher MRR than wet EDM and better surface finish quality than dry EDM.In view of these factors, near-dry EDM may prove to be the most prominent processamongst the three EDM processes in near future to finish machine the difficult to machinematerials. Significant work has been done in the parametric optimization of wet EDMprocesses. Efforts are also on in the parametric optimization of dry EDM processes.However, irrespective of its inherent advantages over wet and dry EDM processes, notmuch attention has been given towards the parametric optimization of the near-dry EDMprocess. It is essential to have information on the optimum operating conditions to makethe near dry EDM process cost effective and economically viable one. If applied as the postprocess of direct metal deposition (DMD), the near-dry EDM milling processes can betargeted to finish the near-net-shape parts produced by DMD. Hence the authors feel that,there is a wide scope to work in this area to optimize the vital parameters of near-dry EDMprocess. The experimental investigations of near dry electrical discharge machining processcarried out by a handful of researchers have been overviewed in present work.Keywords: Electrical discharge machining, near dry EDM, material removal rate, Surface roughness 22
  • 2. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME1. INTRODUCTION Electrical discharge machining (EDM) is often used to machine difficult-to-machinematerials. EDM has achieved a status of being nearly indispensable in the industry because ofits ability to machine any electrically conductive material irrespective of its mechanicalstrength. EDM removes work material by melting and vaporizing it through a series ofdischarging electric sparks. The spark removes the work material. Conventional EDM processes use liquid dielectric fluid. However, the dielectric fluid,particularly hydrocarbon oil itself is one of the main sources of pollution in die sinkingelectrical discharge machining. Wastes of dielectric oil are very toxic, cannot be recycled andneed to be disposed of appropriately; otherwise, there is a possibility of both the land andwater being polluted[8].The process generates gases and fumes due to the thermaldecomposition of the dielectric. Another main problem of die sink EDM is the high amountof energy consumed. The energy consumed in the spark gap, which is the effective energy forthe erosion of the material, is usually less than 20% of the total input of electrical energy. Onthe other hand, the energy consumed by the dielectric system may represent 50% of the totalinput of electrical energy, especially when low values of peak current are used [8]. Another emerging technology, viz. powder mixed EDM, increases the cost ofmachining and also environment-unfriendly like conventional EDM. Dry EDM is another technique, which employs gas as a dielectric medium instead ofliquid. Due to the reattachment of debris to the machined surface, dry EDM may havelimitations of meeting the combined material removal rate (MRR) and surface roughnessrequirements. The accuracy of surface profile deteriorates with the debris deposition. Themajor challenges in dry EDM process are low stability of arc column, low material removalrate, arcing, poor surface quality as compared to conventional EDM and odor of burning.However efforts have been made in the experimental investigation and parametricoptimization of dry EDM processes [11-14]. These problems faced in dry EDM can be reduced & overcome in near- dry EDM byreplacing the gas with the mixture of gas and dielectric liquid. The liquid content in the mistmedia helps to solidify and flush away the molten debris and the debris reattachment isalleviated in near-dry EDM. Compared to the conventional EDM process, near-dry EDM hasanother advantage. It does not require a bath of dielectric fluid. Only a small amount of liquiddielectric fluid is used making the process environment-friendly. Further it has the benefit totailor the concentration of liquid and properties of dielectric medium to meet desiredperformance targets. Also, it is found that near-dry EDM has the advantage in finishoperation with low discharge energy considering its higher MRR than wet EDM and bettersurface finish quality than dry EDM.2. PRESENT STATUS AND SCOPE The metal working fluids (MWFs) are extensively used in conventional machiningprocesses. The economical, ecological and health impacts of metal working fluids (MWFs)can be reduced by using minimum quantity lubrication referred to as near dry machining. Innear dry machining (NDM), an air-oil mixture called an aerosol is fed onto the machiningzone [9]. This concept of near dry machining can be well applied in EDM process, theprocess being referred to as near-dry EDM process. The feasibility of near-dry EDM was explored by Tanimura et. al. in 1989, whoinvestigated EDM in water mists in air, nitrogen & argon gases. Further investigation of near-dry EDM was conducted by Kao et. al.(2007) [1], in wire EDM experiments. After the first 23
  • 3. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEMEexploitation by Tanimura (1989), not much study has been conducted on this process untilrecently by Kao (2007) in near-dry wire EDM. Near dry EDM milling as a super-finishing process to achieve a mirror like surfacefinish has been investigated. Near dry EDM exhibits the advantage of good machiningstability and smooth surface finish at low discharge energy input [6]. Advantages of near-dryEDM were identified as a stable machining process at low discharge energy input because thepresence of liquid phase in the gas environment changes the electric field, making dischargeeasier to initiate and thus creating a larger gap distance. In addition, good machined surfaceintegrity without debris reattachment that occurred in dry EDM was attained since the liquidin the dielectric fluid enhances debris flushing. Other potential advantages of near-dry EDMare a broad selection of gases and liquids and flexibility to adjust the concentration of theliquid in gas. The dielectric properties can thus be tailored in near-dry EDM to meet variousmachining needs, such as high MRR or fine surface finish. Also Near dry EDM showsadvantages over the dry EDM in higher material removal rate (MRR), sharp cutting edge andless debris deposition. Compared to wet EDM, near dry EDM has higher material removalrate at low discharge energy and generates a smaller gap distance [10]. Also compared withconventional wet wire EDM, near dry wire EDM consistently produces better Ra values onPCD coated WC work-pieces, but near dry wire EDM produces lower MRR than wet wireEDM under some conditions [3]. However, the technical barrier in near-dry EDM lies in theselection of proper dielectric medium and process parameters. From the review of literature it is seen that experimental investigations have beencarried out in order to study the effect of various input parameters like discharge current, gapvoltage, pulse on time, gas pressure, fluid flow rate, electrode orientation and spindle speedon material removal rate (MRR), surface roughness and tool wear rate and to improve theperformance of near dry EDM process [1-6]. However, it is necessary to optimize the inputparameters for maximum material removal rate (MRR) and minimize the surface roughnessto make the near dry EDM process cost effective and economically viable one.3. PRESENT WORK The experimental investigations of near dry electrical discharge machining processcarried out by a handful of researchers have been overviewed in present work in view of thefollowing points.1) Comparative study of wet, dry and near dry EDM in view of the response variables viz.material removal rate (MRR), surface roughness, gap distance and debris deposition.2) Study of effect of various electrical input parameters viz. discharge current (ie), gapvoltage (ue), pulse on time (te), pulse interval (to), open circuit voltage (ui) on materialremoval rate (MRR), surface finish & tool wear rate (TWR).3) Study of effect of various machining input parameters viz. gas input pressure, fluid flowrate and spindle speed on material removal rate (MRR), surface finish & tool wear rate(TWR).4) Study of effect of the electrode material and dielectric medium (various liquid-gasmixtures) on material removal rate (MRR) and surface finish at high and low dischargecurrents.5) Study of effect of fluid flow rate (concentration of the liquid in gas) and discharge currenton gap distance and debris deposition. 24
  • 4. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME3.1 Comparative study of wet, dry and near dry EDM in view of the response variablesviz. material removal rate (MRR), surface roughness, gap distance and debrisdeposition.3.1.1 Wire EDM cutting MRR envelopes, which illustrate feasible EDM process regions, have been studiedby Miller . MRR envelopes of wet and dry EDM cutting of 1.27- mm- thick Al6061 arepresented as the baseline data for the comparison with two new envelopes of the near dryEDM. In each envelope, to was varied to find the maximum achievable wire feed rate,which was then converted to MRR. Four levels of te were selected: 4, 10, 14, and 18 µs.The upper and lower boundaries of the MRR envelope correspond to the minimum andmaximum values of te (4 and 18 µs). The specific machine limits, maximum andminimum to (1000 and 6 µs), as well as wire breakage and short- circuit limitations, formthe left and right envelope boundaries of the MRR envelope. The average pulsecurrent ie is about 25 A. To investigate the relationship between the gap distance anddielectric fluid properties, the grooves machined at various water flow rates (0, 5, 8,15, 21, 35, 50, 75 ml/min), as summarized in Table 1 , were studied. The groove qualityand groove width were examined and measured using an optical microscope at 100xmagnification. Three repeated tests were conducted in each experimental setup [1]. Table 1. Average gap distance in EDM cutting under wet, dry & Near dry conditions3.1.2 EDM drilling Two sets of EDM drilling experiments were conducted. The first set was toevaluate the drilling speed and hole quality, including the shape variation and debrisdeposition, of wet, dry, and near dry EDM. The average pulse current was set at 10 A. Thework-piece used was 1.27- mm-thick Al6061. For wet EDM, the flow rate of de- ionizedwater was 107 ml/min. For dry EDM, the air jet pressure was set at 0.62MPa. For near dryEDM, the water flow rate and the pressure of the carrying air jet were set at 21 ml/min and0.62MPa, respectively. The hole quality was inspected using an optical microscope at100x magnification. The second set investigates the effects of water flow rates on EDMdrilling speeds with ie values at 10, 12, and 15 A. Diameters of drilled holes at different waterflow rates were also measured for the investigation of the relationship between the gapdistance and dielectric fluid properties. The water flow rate was varied as 5, 8, 15, 21, and 35ml/ min as shown in Table 2 [1]. 25
  • 5. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEMETable 2. Average gap distance in EDM drilling under wet, dry and near dry conditions Fig.1 shows three MRR envelopes which outline the feasible regions for the wet, dry,and near dry wire EDM cutting of 1.27- mm- thick Al6061. The average of threerepeated test results is presented. The range of variation of three tests is within 10% of thenominal value and is consistent for all experimental conditions. For wet and dry wire EDM,the region of feasible MRR is bounded by the wire breakage, short circuit, and machine limitsof maximum and minimum te (18 and 4µs) and maximum to (1000 µs).Fig 1.Comparision of boundaries of feasible MRR envelopes for wet, dry and near drywire EDM The wet EDM has a significantly higher MRR than that of the dry EDM(21.9mm3/min vs. 0.98mm3/min). At low pulse intervals of to , frequent EDM pulsesgenerate concentrated heat and lead to wire breakage. The minimum value of to that canbe reached at high level of te without wire breakage, is greatly dependent on thedielectric fluid used. For wet EDM, due to the higher thermal conductivity of the bulkwater than that of the water–air mixture, to can be as low as 100 µs at te = 18 µs. For thenear dry EDM using water–air mixture at a water flow rate of 5.3 ml/min, the envelopeboundary falls between the wet and dry EDM. The maximum MRR is improved, from0.98mm3/min in dry EDM, to 2.53mm3/ min. The near dry EDM has a consistentlyhigher MRR than that of dry EDM for all to and te . However, the wire breakage, due to the 26
  • 6. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEMElower capability of water–air mixture to relieve the concentrated heat from the wire electrode,still limits the MRR in near dry EDM at low to. Nevertheless, near dry EDM shows twoadvantages. First, there is no short circuit limit at the lower boundary. Second, in theregion of very low- energy input (te= 4 µs and to >150 µs), the MRR in near dryEDM is higher than that of the wet EDM. The close up view of MRR (below 4 mm3/min) vs. to for the wet and near dry EDM isshown in Fig.2. Three regions, designated as I–III, are identified. In Region I (to >650 µs), the near dry EDM has higher MRR than that of wetEDM because the lower thermal conductivity and heat capacity of the water–airmixture contribute to less heat dissipation during discharge and a larger portion ofdischarge energy for material removal. At the very low discharge energy setup, te= 4 µs,wet EDM fails to cut due to the short circuit, but near dry EDM still works with fairly lowMRR. The higher dielectric strength of the water medium generates a narrow gapdistance and causes a frequent short circuit in wet EDM. In Region II (250 < to <650 µs), the MRR of near dry and wet EDM is roughly thesame. At the highest te ( = 18 µs), the MRR of wet EDM starts to exceed that of near dryEDM. Under higher energy input, the higher viscosity of the water dielectric fluid in wetEDM generates larger explosion force, which contributes to the high MRR. In Region III (to<250 µs), a significant MRR difference exists between wet and neardry EDM. The MRR drops in near dry EDM and, wire breakage occurs as to isfurther reduced. The dielectric fluid viscosity is critical to the MRR in Region III.Fig. 2.Comparison of MRR performances of wet and near dry wire EDM under varied t0 and te andthree regions based on near dry and wet EDM performance (ie= 25 A, ue= 45 V). Optical micrographs of top and cross-sectional side views of EDM drilled holes and thedrilling time under the wet, dry, and near dry conditions are shown in Fig. 3. The dryEDM takes 428 s to drill a hole through the 1.27- mm thick Al6061. This is verylong Compared to the 11 and 13 s drilling time for the wet and near dry EDMrespectively. The dry EDM also has a severe debris deposition problem, whichsubsequently creates a tapered hole. The taper in wet EDM also exists but is not assignificant as in dry EDM. The smallest taper exists in holes drilled by near dry EDM,which generates a straight hole with sharp edges. 27
  • 7. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME The electrode wear in near dry EDM is 3.7 mg per hole, which is larger than the 2.7 mgper hole in wet EDM. The higher thermal load on the electrode in near dry EDM likelycauses the higher electrode wear. The same phenomenon also exists in near dry wire EDM.As shown in Fig.2, at low to the wire breakage due to electrode wear limits the MRR in neardry wire EDM. The groove width in wire EDM is used to estimate the gap distance. The average gapdistance under the wet, dry, and near dry wire EDM and the associated dielectric strength andviscosity of the dielectric fluid are listed in Table 1.Fig.3. Optical micrographs on holes drilled on 1.27mm Al6061: (a) wet, (b) dry, and (c) neardry EDM conditions (ie= 10 A, te= 10 µs, to= 70 µs, ue= 60 V).The gap distance of wet EDM is wider than that of near dry EDM. This is likely causedby the lower viscosity of the water–air mixture. Similarly, in near dry EDM, higher waterflow rate generates larger gap distance. No debris deposition is observed for near dry EDM. This occurs because water–air mixture has a better flushing capability than the air jet in dry EDM [1].3.1.3 EDM milling Fig. 4 illustrates the configuration of the EDM milling process. Grooves of 8 mm inlength and varied depth for different processes were made. To measure the surface roughnessat the bottom of the slot, a Taylor Hobson Form Talysurf profilometer with a 2 µm stylusradius was used. The cutoff length was set to 0.25 mm for the finished surface and 0.8 mmfor the roughened surface. The measurement length was set to 8 mm. The weight of the partbefore and after machining was measured using an Ohaus GA110 electronic scale with a 0.1mg resolution and converted to the volumetric material removal and MRR.[2]. The experimental investigation of dry and near-dry EDMs was carried out in three setsof experiments, marked as Expts. I, II, and III. 1. Expt. I. Dielectric medium and electrode material selection: Experiments wereconducted to select the dielectric medium and electrode material at high and low dischargeenergy levels for roughing and finishing operations, respectively. The depth of cut and theinput pressure were set at 0.1 mm and 480 kPa, respectively, for the roughing operation andat 0.02 mm and 480 kPa, respectively, for the finishing operation. 2. Expt. II. Exploratory experiments: Based on the selected dielectric medium andelectrode material, several sets of experiments were conducted to investigate the effects of 28
  • 8. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEMEexternal air jet, depth of cut, gas input pressure, discharge current, and pulse duration in dryEDM roughing and near-dry EDM finishing.Fig. 4 Configuration of EDM milling: Table 3. Roughing and finishing DOE(a) Overview and (b) close-up view of Roughing process Finishing process the electrode and cutting region experiments experiments ________________________________________ Run te ie ui ue to Run te ie ui ue to (µs) (A) (V) (V) (µs) (µs) (A) (V) (V) (µs) _____________________________________________ 1 4 20 160 40 20 1 2 1 160 80 12 2 12 20 160 40 8 2 4 1 160 40 4 3 4 30 160 40 8 3 2 3 160 40 12 4 12 30 160 40 20 4 4 3 160 80 4 5 4 20 260 40 8 5 2 1 260 80 4 6 12 20 260 40 20 6 4 1 260 40 12 7 4 30 260 40 20 7 2 3 260 40 4 8 12 30 260 40 8 8 4 3 260 80 12 9 4 20 160 80 8 9 4 3 260 40 4 10 12 20 160 80 20 10 2 3 260 80 12 11 4 30 160 80 20 11 4 1 260 80 4 12 12 30 160 80 8 12 2 1 260 40 12 13 4 20 260 80 20 13 4 3 160 40 12 14 12 20 260 80 8 14 2 3 160 80 4 15 4 30 260 80 8 15 4 1 160 80 12 16 12 30 260 80 20 16 2 1 160 40 4 17 8 25 210 60 14 17 3 2 210 60 8 18 8 25 210 60 14 18 3 2 210 60 8 19 8 25 210 60 14 19 3 2 210 60 8 20 8 25 210 60 14 20 3 2 210 60 8 _____________________________________________ 3. Expt. III. DOE: Two DOE tests based on the 25-1 fractional factorial design wereperformed to study the effect of five process parameters (ie, te, ue, to, and ui ) and theirinteractions. Four center points were used in the design to test the curvature effect of themodel. The design matrices are listed in Table 3. Analysis of variance (ANOVA) was appliedto analyze the main effects and interactions of input parameters. The DOE results can identifydirections for further process optimization.3.2 Study of effect of various electrical input parameters viz. discharge current, gapvoltage, pulse on time, pulse interval, open circuit voltage on material removal rate(MRR), surface finish & tool wear rate (TWR). The electrical parameters are among the most important factors in EDM. The dischargecurrent (ie ), pulse duration (te) and gap voltage (ue) determine the discharge energy per pulse;the pulse interval (to) decides the time available for gap reconditioning between twoconsecutive discharges; the open circuit voltage (ui ) controls the discharge gap distance; andthe polarity influences the material removal ratio between the electrode and work-piece. Inthis study, different levels of these electrical parameters are selected to study both theroughing and finishing, and dry and near dry EDM processes. Figure 5 shows the effect of discharge current, ie, in the roughing operation. Higherdischarge current increases the discharge energy, removes more work material, and generatesa rougher surface. The increase of MRR and surface roughness with ie is significant.Experiments with higher ie was limited due to the maximum current limit of the rotaryspindle. 29
  • 9. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEMEFig. 5 Effect of the discharge current on high Fig. 6 Effect of the discharge current on theenergy input dry EDM with oxygen finishing EDM (t =4 µs, t0 =8µs, ue =60 V, e( te =4µs, t0 =8µs, ue=60 V, and ui =200 V) and ui =200 V copper electrode) Figure 6 shows the effect of discharge current using water-nitrogen mixture in near-dryEDM finishing. The surface finish was improved from 2.5 µm to 0.8 µm Ra by reducing thedischarge current from 20 A to 1 A. The reduced discharge current lowered the dischargeenergy per pulse and generated finer craters and lower surface roughness. However, the MRRalso dropped quickly, from 0.81 mm3 /min to 0.13 mm3 /min. Statistical analysis using ANOVA for dry EDM drilling reveals that discharge current , ieis the most significant parameter due to the highest F value. With a variation in current from12 to 15 A, and further increase up to 18 A, a linear increase in average MRR has beenobserved . From ANOVA table for MRR, a very higher F value (248.5) indicates thatdischarge current ie is more significant than gap voltage V. The gap voltage (V) is also asignificant parameter at 95 % confidence level. An increase in voltage appears to causea decrease in MRR. An increase in gap voltage from 50 to 65 V causes a decrease inaverage MRR by 1.69 % . As the voltage changes from 65 to 80 V, further reduction in MRRby 18.26 % has been observed.[7].3.3 Study of effect of various machining input parameters viz. gas input pressure, fluidflow rate & depth of cut on material removal rate (MRR), surface finish . The effect of the gas pressure input to the spray generator on surface finish and MRR innear-dry EDM finishing with graphite electrode and kerosene-air mixture is shown in Fig. 7.As seen in the figure, the surface roughness is nearly unaffected. Under the stable dischargeconditions, the surface roughness mostly depends on the discharge energy. The MRRgradually increases until the gas pressure reaches 480 kPa. The enhanced gas flow providedbetter debris flushing as well as more oxygen content. In the following DOE of the finishingEDM, the gas pressure was set at 480 kPa. Figure 8 shows the effect of the depth of cut in oxygen assisted dry EDM roughing. TheMRR reached the maximum, 22 mm3 /min, at a 500 µm depth of cut. When the depth of cutis beyond 500 µm, the increase of MRR is limited due to the debris removal problem. Thedebris can bridge between the electrode sidewall and work-piece, resulting in arcing or shortcircuit. This was confirmed by observing frequent servo retraction of the electrode to regulatethe discharge condition. The surface roughness was generally not affected by the depth of cutbecause it does not influence the discharge condition at the bottom of the electrode. In thefollowing DOE roughing experiments with oxygen, the depth of cut was set at 500 µm. 30
  • 10. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEMEFig. 7: Effect of the input pressure of the spray Fig. 8: Effect of the depth of cut on dry EDM delivery device to the finishing process rough cutting with oxygen (ie=30 A, te=4 µs,(ie=1 A, te=2 µs, to=16 µs, ue=20 V&ui=200 V to=8 µs, ue=60 V, and ui=200 V)graphite electrode with kerosene-air mixture) The MRR in near dry EDM under 5.3 and 75 ml/min water flow rates is shown in Fig.9.In near dry EDM, high water flow rates increases the MRR because of improved cooling,more efficient debris flushing, and higher dielectric fluid viscosity due to the higherconcentration of water. It improves the MRR at low to (below 500 µs) for all values of te, andis particularly beneficial when te is high (= 18 µs). The peak MRR rises to 3.9mm3/min at 75ml/ min flow rate. A much higher flow rate is required to increase the MRR because thenozzle is set near the discharge gap and thus not all water droplets are successfully deliveredinto the gap.Fig. 9. MRR envelopes of near dry wire EDM cutting at two de-ionized water flow rates (5.3 and 5ml/min, ie = 25 A, ue = 45 V).3.4 Study of effect of the electrode material and dielectric medium (various liquid-gasmixtures) on material removal rate (MRR) and surface finish at high and low dischargecurrents. Experiments were conducted at high and low discharge energies to study effects of theelectrode material and dielectric medium for roughing and finishing operations, respectively.Figure 10(a) shows the results on MRR and surface roughness at high discharge energy input.The copper electrode was successful at removing the work-material in nearly all dry and 31
  • 11. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEMEnear-dry EDM cases (except the near-dry EDM with kerosene-nitrogen and kerosene-heliummixtures). However, the graphite electrode failed in a high discharge energy setting due tosevere arcing. The deposited workpiece material, similar to that in arc welding, was observedat the outer circumference of the machined spot, as shown in Fig.11(a). The severe arcingcauses discharge localization and large scale material melting, while ideal sparks shoulduniformly distribute over the machining area and erode the material. The arcing was likelystimulated by the excessive amount of graphite powder chipped off from the electrode tip, asshown in Fig.11(b). The high thermal load, due to lower cooling efficiency in dry and near-dry EDMs, cracked the brittle graphite electrode. The resultant graphite powder bridged thework-piece and electrode, causing discharge localization and, thus, arcing. For the effect ofdielectric medium, oxygen, water-oxygen mixture, and kerosene-air mixture are found toachieve comparable MRRs and better surface finish than liquid kerosene in wet EDM. Thelower viscosity of the liquid-gas mixture resulted in shallower craters on the machinedsurface and, thus, better surface finish. Since oxygen was confirmed to have the highest MRR, its potential is further exploitedin this study. Water-oxygen mixture is another good candidate for roughing since it providedhigh MRR close to that of oxygen and had good surface finish. The flushing of water-oxygenmixture is helpful in high discharge energy to solidify and remove the molten debris.However, the water combined with oxygen induces severe electrolysis corrosion on amachined surface. Hence, copper electrode and oxygen gas are selected for further DOEstudy of high MRR roughing EDM Figure 10(b) shows the results of the MRR and surface roughness at low dischargeenergy input. The graphite electrode exhibited its advantage over copper electrode withhigher MRR and comparable surface roughness. In near-dry EDM using water mixture withnitrogen or helium, the graphite electrode achieved a similar quality of the surface finish(0.87−0.95 µm Ra) and twice the MRR as that of copper electrode. At low discharge energyinput, the graphite powder, which exists in much smaller amounts than that at high dischargeenergy, assisted the machining process to improve the discharge transitivity and,consequently, the MRR. It is hypothesized that the carbon powder plays a role in assisting thedischarge ignition and evenly distribute the sparks, as identified by Yang and Cao. 32
  • 12. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEMEFig.10. MRR and Ra results of different dielectric fluids for copper and graphite electrode materials:(a) at high discharge energy input (ie=20 A, te=4µs, to=8µs, ue=60 V, and ui=200 V) and(b) at low discharge energy input (ie=1 A, te=4µs, to=8µs, ue=60 V, and ui=200 V) 33
  • 13. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEMEFig. 11 Graphite electrode in near-dry EDM at high discharge current: (a) Damaged workpiecesurface due to arcing and (b) damaged tool (ie=20 A, te=4 µs, to=8 µs, ue=60 V, and ui =200 V) The copper electrode produced slightly better surface finish, 0.80µm and 0.85µm Ra,using water-helium and water-nitrogen mixtures, respectively, but its MRR was lowcompared with graphite. The frequent servo retraction was observed when using the copperelectrode at low discharge energy, probably because the discharge is difficult to initiate.When kerosene or kerosene based mixtures were used as dielectric fluids, the copperelectrode cannot maintain stable discharges because of the narrow gap distance in the lowdischarge energy EDM. Considering the effect of the dielectric medium, near-dry EDM outperformed both dryand wet EDMs to generate better surface finish and higher MRR. The best surface finish of0.8 µm was achieved using the water-nitrogen mixture. The highest MRR of 1.8 mm3 /minwas obtained using the kerosene-air mixture. In dry EDM at low energy input, the MRR waslow, using an oxygen medium, and the surface was rough. The water based mixture generally provided better surface finish than the kerosene basedmixture with the sacrifice of MRR due to its lower viscosity and correspondingly smootherand shallower crater for each discharge. Water-nitrogen and water-helium mixtures yieldedbetter surface finishes(0.95 µm and 0.87 µm Ra for graphite electrode and 0.85 µm and 0.80µm Ra for copper electrode) than the water-air and water-oxygen mixtures (1.68µm and 1.62µm Ra for graphite electrode and 0.98 µm and 1.25µm Ra for copper electrode). A possiblereason is that nitrogen and helium shielded the process from oxygen and thus reduce thecorrosion caused by water electrolysis. The mixture with helium produced a slightly bettersurface finish over that of nitrogen. Nitrogen has the potential to form a hard nitride surfacelayer by alloying with elements in the work-material. Kerosene-air mixture produced higher MRR than that of kerosene with nitrogen orhelium. The oxygen content in the air generates more heat for materialremoval through an exothermic reaction, but the surface finish was adversely affected. Whenkerosene was used as dielectric media, the deterioration caused by electrolysis corrosion wasnot observed. For further DOE study of finishing EDM, the graphite electrode and water-nitrogen mixture are selected. Nitrogen is selected over helium because of the comparableperformance, lower cost, and potential to form a hard nitride surface layer on the machinedsurface for better wear resistance. 34
  • 14. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME 3.5 Study of effect of fluid flow rate (concentration of the liquid in gas) and dischargecurrent on gap distance and debris deposition.Fig. 12. The effect of de-ionized water flow rate and discharge current on the MRR of EDM drilling(te = 10 ms, to= 70 ms, ue = 60 V). Effects of water flow rate and pulse current ie on the MRR in near dry EDMdrilling are shown in Fig. 12 . The efficiency of near dry EDM drilling improves with ahigher water flow rate under all three levels of ie. The MRR is low at ie = 10A due to thelow-energy input. The highest energy input (ie = 15 A), however, does not generate thehighest MRR as expected. This is caused by the debris flushing problem at high-energy input.The medium level of ie(=12A) has the highest MRR by balancing the debris flushing andpower input. The measured average gap distance is calculated using the difference betweenthe average hole diameter and electrode diameter. Table 2 lists the average gap distance inwet, dry, and near dry EDM at five water flow rates. Following the same trend observed inTable 1 for the wire EDM, higher water flow rate corresponds to larger gap distance. Amodel is developed to investigate the effect of dielectric strength and dynamic viscosity onthe gap distance.4. CONCLUSION Advantages of near-dry EDM can be identified as a stable machining process at lowdischarge energy input because the presence of liquid phase in the gas environment changesthe electric field, making discharge easier to initiate and thus creating a larger gap distance.In addition, good machined surface integrity without debris reattachment that occurred in dryEDM can be attained since the liquid in the dielectric fluid enhances debris flushing. Otherpotential advantages of near-dry EDM are a broad selection of gases and liquids andflexibility to adjust the concentration of the liquid in gas. The dielectric properties can thus betailored in near-dry EDM to meet various machining needs, such as high MRR or fine surfacefinish. However, the technical barrier in near-dry EDM lies in the selection of properdielectric medium and process parameters. From the review of literature it is seen that experimental investigations have beencarried out in order to study the effect of various input parameters like discharge current, gapvoltage, pulse on time, gas pressure, fluid flow rate and spindle speed on material removalrate (MRR), surface roughness and tool wear rate and to improve the performance of near dryEDM process. However, irrespective of its inherent advantages over wet and dry EDM processes, notmuch attention has been given towards the parametric optimization of the near-dry EDM 35
  • 15. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEMEprocess. It is essential to have information on the optimum operating conditions to make thenear dry EDM process cost effective and economically viable one. Authors conclude that, there is a wide scope to work in this area to optimize the vitalparameters of near-dry EDM process.5. REFERENCES[1] C.C. Kao,Jia Tao, Albert J. Shih,”Near Dry Electrical Discharge Machining”,International Journal of Machine Tools & Manufacture 47 (2007), 2273-2281.[2] Jia Tao, Albert J. Shih,Jun Ni,”Experimental study of the Dry & Near-Dry ElectricalDischarge Milling Processes”, Journal of Manufacturing Science & Engineering (Feb2008),Vol.130 / 011002-1- 011002-8.[3]Y Jia, B.S. Kim, D.J. Hu & J Ni, “ Parametric study on near-dry wire electro-dischargemachining of polycrystalline diamond-coated tungsten carbide material”, Proceedings of theInstitution of Mechanical Engineers, Part B : Journal of Engineering Manufacture (2010) Vol.224, 185-193.[4] M. Fujiki, Gap-Yong Kim, Jun Ni, Albert J. Shih,”Gap control for near-dry EDM millingwith lead angle”, International Journal of Machine Tools & Manufacture 51 (2011), 77-83.[5] M. Fujiki, Jun Ni, Albert J. Shih,” Investigation of the effect of electrode orientation &fluid flow rate in near-dry EDM milling”, International Journal of Machine Tools &Manufacture 49 (2009), 749-758.[6] Jia Tao, Albert J. Shih, Jun Ni,” Near-Dry EDM Milling of Mirror-Like Surface Finish”,International Journal of Electrical Machining 13 ( January 2008). 29-33.[7] P. Govindan, Suhas S. Joshi, “ Experimental characterization of material removal in dryelectrical discharge drilling” , International Journal of Machine Tools & Manufacture 50(2010), 431-443.[8] Fabio N. Leao , Ian R. Pashby , “ A review on the use of environmentally-friendlydielectric fluids in electrical discharge machining”, Journal of Materials ProcessingTechnology 149 (2004), 341-346.[9]Viktor P. Astakhov, General Motors Business Unit of PSMI, USA, “ EcologicalMachining : Near Dry Machining”.[10] B.C. Routara, B.K. Nanda, D.R. Patra,” Parametric optimization of CNC wire cut EDMusing Grey Relational Analysis”, Proceedings of the International Conference on MechanicalEngineering ( Dec.2009),RT-24,1-6. [11] S. Abdulkareem, A.A. Khan & Z.M. Zain,”Effect of Machining Parameters on SurfaceRoughness during Wet & Dry Wire EDM of Stainless Steel”, Journal of Applied Sciences 11(10), 1867-1871, (2011). [12] Jia Tao, “Investigation of Dry & Near Dry Electrical Discharge Milling Process”, Adissertation submitted in partial fulfillment of the requirements for the degree of Doctor ofPhilosophy (Mechanical Engineering) in The University of Michigan, (2008).[13] Sourabh K. Saha, S.K.Choudhury, Department of Mechanical Engineering, IndianInstitute of Technology Kanpur, ”Multi-objective optimization of the dry electric dischargemachining process”, (Jan. 2009).[14] Grzegorz Skrabalak, Jerzy Kozak, “ Study on Dry Electrical Discharge machining”,Proceedings of the World Congress on Engineering, London UK, (2010) Vol. III. 36