BEST SELECTION OFPROCESS PARAMETER DURING ELECTRICAL DISCHARGE MACHINING OF H-13 TOOL STEELUSINGCOPPER ELECTRODE WITH INTERNAL FLUSHINGBYTAGUCHI METHOD
Rajesh Kumar Mohanty1*,Prof.Madan Kishore Goel2 ,Dr Vijay Kumar3
1Deparment of Mechanical Engineering, HIET,Ghaziabad, U.P., India
2Department of Mechanical Engineering, HIET, Ghaziabad, U.P., India
3Department of Mechanical Engineering, IIMT,Greater Noida, U.P., India
*Corresponding author:,Tel.No.-9818227231
ABSTRACT
Correct selection of process parameters is considered very important aspect in the majority of manufacturing processesparticularly, in processes related to Electrical Discharge Machining (EDM). EDM is a material removal process capable of machining geometrically complex or hard material components, which are precise and difficult-to-machine such as heat treated tool steels, composites, super alloys, ceramics, carbides, heat resistant steels etc. being widely used in die and mould making industries, aerospace, aeronautics and nuclear industriesfor long time now.In EDM thermalenergy is used to machine all electrically conductivematerials of any hardness & toughness.However, very little research dataon electric discharge machining has been reported onthe applications of electrode with internal flushing route. From this consideration dimensional accuracy is measured in term of overcutusing internal flushing typeU-shaped copper tool electrode duringelectric discharge machining of AISI H-13 tool steel byvarying different machining parameter onEDM in the present research work. Theprocess input parameters(such aselectrode thickness,discharge current,pulse on time) are varied and the overcut obtained is analyzed with a view to improve processperformance.In this work an attempt has been made to study the effect of different machining parameters.Hence number of experiments were conducted and results obtained were analyzed by Taguchi Methodology and analysis of variance. The effect of controlfactors were examined for obtaining the best machining parameter setting. This analysis reveals thatdischarge current, pulse on time and thickness of tool have significant effect on the overcut.
KEYWORDS: Electric discharge machining, Taguchi method, overcut, pulse on time
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1.INTRODUCTION
Considering the challenges brought on byadvanced technology, the Electrical DischargeMachining (EDM) process is one of the bestalternatives for machining an ever increasingnumber of high-strength, non-corrosion, and wearresistant materials. AISI H13 tool steelis considered as asignificant material that has awidespreadapplication in mould industries.Electrical discharge machiningis a non-traditional machining method commonly used to produce die cavities with the erosive effect of electrical discharges. It uses thermoelectric energy sources for machining low machinability materials and complicated intrinsic-extrinsic shaped jobs regardless of hardness. EDM finds its wide applicability in manufacturing of plastic moulds, forging dies, press tools, die castings, automotive, aerospace andsurgicalcomponents.No direct contact is made by EDM between the electrode and the work piece.It annihilates mechanical stresses, chatter and vibration problems during machining. Various types of EDM process are available, but here it is Die-Sinking type EDM machine which requires the electrode to be machined in the exact contradictory shape as the one in the work piece.Electrical discharge machining utilizesrapid, repetitive spark discharges from a pulsatingdirect-current power supply between thework piece and the tool submerged into a dielectricliquid.The thermal energy of the sparksleads to intense heat conditions on the work piececausing melting and vaporizing of the work piecematerial.
EDM PROCESS
EDM, as shown in figure 1.1,is basically a thermo electric process, and has ability to machine any conducting materials regardless of their mechanical and chemical properties. As no contact is required between the tool and the work piece, it is very efficient and effective in machining very hard and high strength materials.
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In EDM, a power supply hands over high-frequency electric pulses to the electrode tool and the work piece. The gap between the tool and work piece is flushed with a flow of dielectric liquid. When an electric pulse is delivered from the electric supply, the insulating property of the die electric fluid is temporarily made ineffective. This permits a small spark to fly the shortest distance between the tool and work piece. A small pool of molten metal is formed between the work piece and the tool at the point of discharge. A gas boil forms around the discharge and the molten pool. As the electric pulse ends andthe discharge disappears, the gas boil collapses. The wave of cool dielectric causes the molten metal to be ejected from the work piece and the tool, leaving small craters. This action is repeated no. of times each second during EDM processing. This removes material from the work piece in a shape corresponding to that of the tool. Depending on the kind of material used positive or negative polarity can be applied. When gap width between the tool and the electrode achieves the maximum sparking gap width, a micro-conductive ionized path appears and the electric spark occurs achieving temperatures up to 15,000 or 20,000°C [2].
Fig. 1.2 Parts of EDM
In particular, EDM machining experiments were conducted on AISI H13 samples using copper electrodes to investigate the correlations between the EDM parameters (pulse on-time and current) and the EDM characteristics of such a work piece. The output factors investigated were the overcut. This experimental study results in the selection of optimum process parameters.
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Table 1 Mechanical and physical properties of AISI H 13
Temperature (o C) / Density (kg/dm3) / Specific heat (J/Kg-K) / Electrical resistivity in (Ω mm2/m) / Modulus of elasticity (N/mm2) / Thermal conductivity (W/m.K)20 / 7.80 / 460 / 0.52 / 215x103 / 24.30
500 / 7.64 / 550 / 0.86 / 176x103 / 27.70
600 / 7.6 / 590 / 0.96 / 165x103 / 27.50
Liquidus temperature 1454oC / Solidus temperature 13150C
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2. LITERATURE REVIEW
Garg R.K. et al. [1], found that hard composite materials can be machined by many non-traditional methods likewater jet and laser cutting but these processes are limited to linear cutting only. Electrical discharge machining(EDM) shows higher capability for cutting difficult tomachine shapes with high precision for these materials. Kumar S., et al. [2], studied the surface modification by electrical discharge machining using conventionalelectrode and powder metallurgy electrode. Ho K.H., et al. [3], found that with continuous improvement in the metal removal efficiency and incorporation of numerical control, the viability of the EDM process in terms of the type of applications can be considerably extended. Ho, S.K., et al. [4], reportedthat powder metallurgy electrodes produced greateralloying than the solid electrodes. This was a function of electrode conductivity and the level of bonding of copper particles, withthe electrodes. It wasobserved that powder metallurgy electrodes usedwithpositive polarity produced thicker recast layers.Whenusing a solid copper electrode under negativepolarity, recast/alloyed workpiece hardness was ingeneral of lower or of comparable hardness to thebulk material. Rajurkar, K., et al. [5], concentrated on achievingfaster and more efficient metal removal rate coupledwith a reduction in tool wear and improved surfacecharacteristics.Gangadhar A., et al.[6], studied surface deposition by EDM in a liquid dielectricusing powder metallurgy compact tool electrode.Deposition of tungsten carbide on flank and rake of aHSS tool using electrodecontaining 40%WC and60% Fe (zinc sterate as lubricant) with reversepolarity and kerosene as dielectric resulted in lowvariation in cutting forces. Wang Z.L., etal. [7],described a new method of surfacemodification called electrical discharge coating(EDC). The process of EDC begins with electrodewear during EDM, and a kind of hard carbide iscreated by the chemical reaction between the wornelectrode material and the carbon particlesdecomposed from EDM oil (kerosene) under veryhigh temperature. The carbide is piled up on a work piecequickly and becomes a hard layer of ceramic ofthickness about 20μm within few minutes. Thisresearch paper studied the principle and process ofEDC systemically by using a Ti powder greencompact electrode. Experiments and analyses showthat a compact Ti-C ceramic layer can be created onthe surface of the metal work piece. The hardness ofthe ceramic layer is more than three times that of thebase body, and the hardness changes gradually fromthe surface to the base body. Mohri N., et al. [8],introduced a new surface modification by usingcomposite structured electrode in electrical discharge
machining. Work piece surfaces were modified in hydrocarbon oil using composite electrode. Workpiece materialswere carbon steel. Copper, aluminium,tungsten carbide and titanium were used as electrodematerial. It was concluded that there is electrodematerial on the work surface layer and these surfacehas few cracks. Kruth, J.P., et al.[9], reported that withconventional metallicelectrodes operating withstandard polarity, the reported level of work piecealloying is very low <1%, but this alloying can beincreased immensely with the use of powdermetallurgy (PM) electrodes, either in the form ofgreen compact, partially sintered or fully sinteredproducts and suitable operating regimes. Uno Y., et al.[10], reportedthat in the conventional powder suspended EDM,which mainly works as a removal process, powdermaterial is dispersed into a work piece by a depth ofseveral micro meters below the surface of the workmaterial.Singh,p., et al[11] found during electric discharge machining of H-13 toolsteel using the Taguchi approach thatbest parametric setting for minimumovercut is with –ve polarity, CuSiC (Cu 85%SiC 15%) tool electrode, 13 amp current,150 μsec pulse on time, 0.80 duty cycle and60 volts gap voltage and 3mm of retractdistance.It is also found that powder metallurgy tool with reverse polarityelectrode gives the better result forminimum overcut.It is possible to obtain minimum overcuteven at higher values of peak current andgap voltage gives with Powdermetallurgytool andaverage value of duty cycle and retractdistance gives the better result for minimumovercut.
3.EXPERIMENTAL SETUP AND PROCEDURE
3.1EXPERIMENTAL SETUP
The work piece material used in this studywas AISI H13 tool steel. Prior to EDM processing,the work piece was cut in a rectangular shape(5 mm and 6 mm thickness) and after it was filed to an U-shape,drilled inside for the purpose of internal flushing. Themain mechanical and physical properties of work piece material at different temperatures aregiven in Table 1.
The tool material was forged commercialpure copper with the main properties given inTable 2. Experiments were performed on a diesinking EDM machineELECTRONICA M2S EMS 5030 with servo-head (constantgap) and positive polarity for electrode.Commercial grade EDM oil (specific gravity= 0.763, freezing point= 94°C) was used as dielectric fluidfor internal flushing of U-shaped Cu tool with a pressure of 0.2 kgf/cm2.The pulsed discharge current was applied invarious steps in positive mode.Machining tests were carried out for two electrode thickness at three pulse current settings, as well as three pulse ontimesettings.
Table 2 Physical properties of Copper electrode
Physical properties / CopperThermal conductivity(W/m.K) / 380.7
Melting point,oC / 1083
Boiling temperature,o C / 2595
Specific heat(Cal/g-oC) / 0.092
Specific gravity at 20 o / 8.9
Coefficient of thermal expansion[x10-6(1/Oc)] / 17
As a result, 18 experiments could be designed as per Taguchi method. Each machining test was performed for 60 minutes. Table 3 presents the experimental testconditions.
OC is expressed as half the difference of diameter of the hole produced to the tooldiameter that is shown in equation no 1.
OC=(Tjt–Tt)/2 ______1
Where Tjt =Thickness of cavity produced in workpiece
Tt =Thickness of tool
Table 3 Experimental test conditions
Generator type / ELECTRONICA M2S EMS 5030Dielectric fluid / Commercial grade EDM oil
Voltage / 415 V,3 phase,50 HZ
Open gap voltage / 135 V
Pulse range / 2 to 650 µs
Pulse on time / 100,200,400 µs
Max. Current / 25A
Pulse current / 1A,3A,5A
Load / 3 KVA
Power factor / 0.8 approx.
3.2WORK PIECE
AISI H 13 tool steel material work piece after machining isshown in Fig 3.1.It shows 18 no. of experiments(9 no. of experiments per side)performed in this job.
Fig.3.1 H-13 Work piece after machining with tool
3.2 TOOL MATERIAL
In this experiment,copper tool electrode was used and the designof copper tool is U- shaped with internal flashingas shown in Fig 3.2.
. Shapes of the tool aresame as that of the cavity to be produced inthe work piece. Using the U-shaped tool an U-shaped cavityto be produced on the work piece.
3.3 MECHANISM OF MATERIAL REMOVAL
The mechanism of material removal in EDM process is most widely established principle. It is the conversion of electrical energy into thermal energy. During the process of machining thesparks are produced between work piece and tool .Thus each spark produces a tiny crater, andcrater formation of the material along the cutting path by melting andvaporization, results in eroding the work piece to the shape of the tool.
Overcut is the difference by which the machined hole in the work piece exceeds the electrode size .During the process of machining cavity produced are always larger than the electrode. This difference (size of electrode and cavity) is called Over Cut (OC). It becomes important when close tolerance components are required to be produced for space application and also in tools, dies and moulds for press work.
3.4 TAGUCHI DESIGN EXPERIMENTS
The goal of robustexperimentation is to find an optimal setting of controlling factors that providesrobustness against (insensitivity to) noise factors.
A Taguchi design or an orthogonal array is the method of designing the experimentalprocedure using different types of design, like two, three, four, five, and mixed levels. In thestudy, a three factor mixed level setup is chosen to conduct a total of eighteen numbers of experimentsand for that OA L18 set up was chosen. As a few more factors were to be added for further study with thesame type of material, it was decided to utilize the L18 setup, which in turn reduced thenumber of experiments at the later stage and the comparison of the results becamesimpler.
The levels of experimental parameters i,e. electrode thickness(t), spark on time (Ton), anddischarge current (Ip) are tabulated in Table 4 and the design matrix is depicted in Table 5.
Table 4 Machining parameters and their levels
Machining Parameter / Symbol / Unit / Level1 / 2 / 3
Electrode Thickness / t / mm / 5.2 / 6.2 / -
Spark on time / Ton / µs / 200 / 300 / 400
Discharge Current / Ip / A / 1 / 3 / 5
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Fig.3.2 U-Shaped Cu Tool Design
Table 5Design matrix and Observation table
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Run / Thickness
(mm) / Ip (A) / Ton
(μs) / Cavity dia.
(mm)
Djt
1 / 5.2 / 1 / 200 / 5.365
2 / 5.2 / 1 / 300 / 5.320
3 / 5.2 / 1 / 400 / 5.350
4 / 5.2 / 3 / 200 / 5.480
5 / 5.2 / 3 / 300 / 5.485
6 / 5.2 / 3 / 400 / 5.500
7 / 5.2 / 5 / 200 / 5.685
8 / 5.2 / 5 / 300 / 5.540
9 / 5.2 / 5 / 400 / 5.240
10 / 6.2 / 1 / 200 / 6.230
11 / 6.2 / 1 / 300 / 6.210
12 / 6.2 / 1 / 400 / 6.210
13 / 6.2 / 3 / 200 / 6.650
14 / 6.2 / 3 / 300 / 6.775
15 / 6.2 / 3 / 400 / 6.650
16 / 6.2 / 5 / 200 / 6.900
17 / 6.2 / 5 / 300 / 6.780
18 / 6.2 / 5 / 400 / 6.220
4. RESULT AND DISCUSSION
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The response table for OC is shown in Table 6
Table 6 Response table
Run / Thickness(mm) / Ip (A) / Ton
(μs) / OC
(mm)
1 / 5.2 / 1 / 200 / 0.0825
2 / 5.2 / 1 / 300 / 0.0600
3 / 5.2 / 1 / 400 / 0.0750
4 / 5.2 / 3 / 200 / 0.1400
5 / 5.2 / 3 / 300 / 0.1425
6 / 5.2 / 3 / 400 / 0.1500
7 / 5.2 / 5 / 200 / 0.2425
8 / 5.2 / 5 / 300 / 0.1700
9 / 5.2 / 5 / 400 / 0.0200
10 / 6.2 / 1 / 200 / 0.0150
11 / 6.2 / 1 / 300 / 0.0050
12 / 6.2 / 1 / 400 / 0.0050
13 / 6.2 / 3 / 200 / 0.2250
14 / 6.2 / 3 / 300 / 0.2875
15 / 6.2 / 3 / 400 / 0.2250
16 / 6.2 / 5 / 200 / 0.3500
17 / 6.2 / 5 / 300 / 0.2900
18 / 6.2 / 5 / 400 / 0.0100
4.1 INFLUENCES ON OC
The S/N ratios for OC are calculated as given in Equation 4.1. Taguchi method is used toanalyse the result of response of machining parameter for smaller is better (SB)criteria.
S/N = -10 Log 10 (M.S.D)...... 4.1
Where M.S.D. stands forMean square deviation and is equal toYi2where Yiisthe value of response variables for ith experiment.
Mean (S/N) ratios response table for overcut is shown in Table 7.From Table8 and 9 it is clearly evident that
(i)In case of over cut, the most important factor is thickness of the tool, then discharge current and then pulse on time.
(ii)All the three parameters have significant effect on the experiment.
Table 7 Mean (S/N) ratios response Table for OC
Symbol / Process parameter / Level 1 / Level 2 / Level 3A / Electrode Thickness / A1=22.643 / A2=25.014 / -
B / Discharge Current / B1=32.854 / B2=14.537 / B3=20.258
C / Spark on Time / C1=18.268 / C2=20.725 / C3=28.656
Table 8 Analysis of Variance of S/N ratios for OC
Results of ANOVA for OCSymbol / Process parameter / Degrees of freedom / Sum of square / Mean square / F0=(Mean square/ Mean square error) / Contribution %
A / Electrode thickness / 1 / 1092.96 / 1092.96 / 100.642 / 41.542
B / Discharge current / 2 / 1053.886 / 526.94 / 48.522 / 40.057
C / Spark on time / 2 / 353.806 / 176.90 / 16.290 / 13.448
Error / 12 / 130.318 / 10.86 / 4.953
Total / 17 / 2630.97 / 100
Fig.4.1 Main effect plot for OC
Fig.4.2 Interaction plot for OC
4.2 SIGNIFICANT EFFECT
Case (a)
IfF0 > FαDOF,Errorthen this process parameter significantly affects the experiment.
Case (b)
IfF0 Fα,DOF,Errorthen this process parameter does not significantly affect the experiment.
The value of process parameters and their corresponding F0 value can be obtained from table 9.
The value ofFα,DOF,Errorcanbe obtained from percentage points of F distribution[17] .
Table 9 Factors which affect the experiment
Sl No. / Process parameter / DOF / F0 / Fα,DOF,Error / Remarks1 / Electrode thickness / 1 / 100.642 / 4.75 / Significantly affects the experiment
2 / Discharge current / 2 / 48.522 / 3.89 / Significantly affects the experiment
3 / Spark on time / 2 / 16.290 / 3.89 / Significantly affects the experiment
4.3 MODEL ANALYSIS OF OC
Fig.4.3 Residual plot for OC
During the process of Electrical discharge machining, the influence of various machiningparameters like Ip, Ton and thickness of tool have significant effect on OC, as shown in maineffect plot for S/N ratio of OC in Fig 4.1. The over cut is the difference between the dimension of the electrode and the size of the cavity. It isinherent to the EDM process which is unavoidable, though adequate compensations are provided in the tool design. To achieve the accuracy, minimization of over cut is essential. Thereforefactors affecting over cut are essential to recognize. The over cut effects each parametersuch as thickness of tool, discharge current and pulse on time. The main effect plot for S/N ratios is shown in Fig 4.1 for over cut.This graph shows that the thickness of tool is directly proportional to the over cut. Increasing in the discharge current from 1 to 3A the OC is increasing, with increase in discharge current from 3A to 5A the OC is decreasing slightly. OC decreases slightly with the increase in pulse on time from 200 to 300 micro second but there is a greater dip in line from 300 to 400 micro second.
5.RESULT
A2B1C3 Optimum level of process parameter
Electrode thickness = 6.2 mm
Discharge current = 1A
Spark on time = 400 µs
Experiments were conducted according to Taguchi method by using the machining set upand the designed U-shaped tubular electrodes with internal flushing. In case ofOCelectrode thickness is most influencing factor and then discharge current and the last is spark on time.
6. CONCLUSION
In the present study of theAISI H 13 tool steel component machining using the U-Shaped cu tool with internal flushingsystem, the effect of machining parametersonOC have been investigated for EDM process. The experiments were conducted undervarious parameters like Discharge Current (Ip), Pulse On-Time (Ton), and thickness of thetool. L-18 OA based on Taguchi design was used foranalysis of the result and these responses were partially validated experimentally.