j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 3374–3383

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Optimization of machining parameters in magnetic forceassisted EDM based on Taguchi method

Yan-Cherng Lina,∗, Yuan-Feng Chena, Der-An Wanga, Ho-Shiun Leeb

a Department of Mechanical Engineering, Nankai University of Technology, Caotun, Nantou 54243, Taiwanb Graduate Institute of Engineering Technology, Chungchou Institute of Technology, Yuanlin, Changhau 51003, Taiwan

a r t i c l e i n f o

Article history:

Received 22 February 2008

Received in revised form

19 July 2008

Accepted 29 July 2008

Keywords:

EDM

Magnetic force

Taguchi method

Material removal rate

a b s t r a c t

A versatile process of electrical discharge machining (EDM) using magnetic force assisted

standard EDM machine has been developed. The effects of magnetic force on EDM machin-

ing characteristics were explored. Moreover, this work adopted an L18 orthogonal array

based on Taguchi method to conduct a series of experiments, and statistically evaluated

the experimental data by analysis of variance (ANOVA). The main machining parameters

such as machining polarity (P), peak current (Ip), pulse duration (�p), high-voltage auxiliary

current (IH), no-load voltage (V) and servo reference voltage (Sv) were chosen to determine

the EDM machining characteristics such as material removal rate (MRR) and surface rough-

ness (SR). The benefits of magnetic force assisted EDM were confirmed from the analysis of

discharge waveforms and from the micrograph observation of surface integrity. The experi-

mental results show that the magnetic force assisted EDM has a higher MRR, a lower relative

electrode wear ratio (REWR), and a smaller SR as compared with standard EDM. In addition,

Surface roughness

Debris

the significant machining parameters, and the optimal combination levels of machining

parameters associated with MRR as well as SR were also drawn. Moreover, the contribution

for expelling machining debris using the magnetic force assisted EDM would be proven to

attain a high efficiency and high quality of surface integrity to meet the demand of modern

industrial applications.

die making industries. During the EDM process, the tool elec-

1. Introduction

The development of manufacturing technique is very rapid tofit the demands of recent industrial applications. In addition,the multi-variety and small batch product has become a majortrend. In order to accommodate the development in mod-ern manufacture and market lead for commercial purpose,industrial producers must shorten the time-to-market in theexploitation of a new product. Consequently, to develop a new

process with high efficiency, high quality of surface finishingand reliability are crucial points to support the developmentof modern industrial applications. Indeed, electrical discharge

∗ Corresponding author.E-mail address: ycline@nkut.edu.tw (Y.-C. Lin).

0924-0136/$ – see front matter © 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.jmatprotec.2008.07.052

© 2008 Elsevier B.V. All rights reserved.

machining (EDM) is widely used in mold and die manufac-turing. Fallbohmer et al. (1996) surveyed applications of theEDM process and pointed out that almost 90% of mold anddie makers employed the EDM process to finish the productsin USA, Germany, and Japan. Therefore, the EDM technique isan essential approach for mold and die making industries tofabricate their products with superior performance and accu-racy. The EDM technique is extensively applied in mold and

trode and the workpiece are separated by a small gap (about5–100 �m). The gap width between workpiece and electrodeis extremely small, so the machining debris resulted from

t e c h

ETpdtdsmawtafmbi

frwaiait(mmipiisaniidmmn(emwMfwsawtenbmpfmw

j o u r n a l o f m a t e r i a l s p r o c e s s i n g

DM process is difficult to remove out of the machining gap.he state of dielectric fluid isolation does not recover com-letely during a short interval, if an enormous amount ofebris is clogged in the machining gap. Hence, abnormal elec-rical discharges will inevitably form in the machining zoneuring EDM process. In general, the stability of EDM progressignificantly affects the machining characteristics. When theachining debris is expelled from the machining gap fast

nd easily, the machining characteristics of EDM processould be improved. Therefore, when magnets were attached

o the standard EDM machine, the machining zone gener-tes magnetic forces to drive the suspending debris expelledrom the discharge gap. Thus, the debris stacked on the

achining zone can be reduced, so the machining conditionecomes more stable and the machining performance will be

mproved.Several researchers focused their efforts on magnetic

orce applications to promote the manufacturing techniqueecently, and the beneficial effects of magnetic force processere verified from the experimental findings. The feasibilitynd reliability of the magnetic abrasive media for finish-ng machined surface have been investigated. The magneticbrasive media could be applied in various fields of surface fin-shing; Khairy (2001) used magnetic abrasive finishing methodo refine the machined surface of silver steel. Chang et al.2002) investigated the finishing characteristics of unbounded

agnetic abrasive. Kim (2003) discussed an application ofagnetic abrasive composed of WC-Co powder using in an

nternal polishing system. Yamaguchi and Shinmura (2004)resented a magnetic field assisted finishing process for the

nner surfaces of alumina ceramic components, and thisnvestigation also revealed a mechanism to smooth the innerurface of alumina ceramic tube and to improve the formccuracy. Wang and Hu (2005) proposed a technique of inter-al magnetic abrasive finishing for producing highly finished

nner surfaces of tubes. The process principle and the finish-ng characteristics of unbounded magnetic abrasive were alsoescribed in this work. Moreover, Yan et al. (2004) adoptedagnetic abrasive finishing to improve the quality of EDMachined surface. Singh and Shan (2002) employed a mag-

etic field around workpiece in abrasive flow machiningAFM) process to improve the machining performance. Thexperimental results showed that magnetic abrasive finishingethod is ideally suited to finish surface integrities associatedith both external and internal surfaces of the components.ori et al. (2003) clarified the mechanism of magnetic abrasive

or a non-magnetic material to examine the magnetic field asell as acting force, and in this research a fundamental under-

tanding of the process mechanism was provided. Jayswal etl. (2005) evaluated a distribution of magnetic forces on theorkpiece surface by means of the finite element method in

he magnetic abrasive process. Singh et al. (2006) reported thexperimental findings about the acting forces during the mag-etic abrasive finishing process and provided a correlationetween the surface finish and the acting force. The experi-ental results suggested that applying the magnetic abrasive

rocess was one of the most promising processes for sur-ace finishing. Kim et al. (1997) investigated the effect of a

agnetic field on the electrolytic finishing process associatedith the migration of ions. They found that the migration

n o l o g y 2 0 9 ( 2 0 0 9 ) 3374–3383 3375

path of the electrolytic ion would be changed by the mag-netic field. In addition, the surface finishing effects wouldbe improved to obtain a high quality surface integrity effec-tively and quickly. De Bruijn et al. (1978) investigated theeffect of magnetic field on the gap cleaning and indicatedthat the magnetic field can improve gap cleaning. Never-theless, there are few relative reports associated with themagnetic force used in the EDM process to improve machiningcharacteristics.

The ability of expelling debris out of the machining gap wasa crucial point to maintain the EDM stability, so the machiningefficiency and the machined surface quality of the EDM pro-cess were directly affected by the debris expulsion. The effectsof dielectric flushing methods and ejection mechanisms ofmachining debris for EDM have been investigated. The debrisformation, size distribution, and ejection mechanism wereinvestigated theoretically and experimentally. Rajurkar andPandit (1988) derived an expression for the sizes of debris tounderstand the influences of machining debris on machiningperformance during the process. Cetin et al. (2004) developeda debris exclusion model to calculate and simulate the debrisconcentration in the machining gap. Lou (1997) conducted astudy in discharge transitivity upon the gap debris, in thisstudy the function of the debris was the dominant factor inrealizing the discharge movement. Masuzawa et al. (1992) pro-posed a dynamic jet flushing method to improve the debrisdistribution and evacuation. Soni (1994) thoroughly exploredthe debris shape, composition, and size distribution, and theexperimental result suggested that the size distribution ofdebris revealed a stochastic nature during their formation.In order to improve the debris expulsion and to prevent thedebris from clogging in the machining gap, Kremer et al. (1991)treated an EDM assisted with ultrasonic vibration of the elec-trode, and a benefit effect of the ultrasonic vibration wasdetermined in this investigation. Thoe et al. (1999) dealt withthe problem of drilling small diameter holes using a combi-nation process of ultrasonic machining (USM) with EDM. Theexperimental results specified that using ultrasonic vibrationduring EDM greatly increased the machining efficiency, and amore stable discharge as well as a lower incidence of abnor-mal arcing could be obtained in the combination process ofEDM with USM. Lin et al. (2000) conducted an experimentalinvestigation of the machining characteristics of Ti–6Al–4Valloy using a combination process of EDM with USM. Theexperimental results concluded that the combination processof EDM with USM could increase the machining efficiencyand decrease the thickness of the recast layer. Zhang et al.(2002) proposed a new method of ultrasonic vibration elec-trical discharge machining (UVEDM) in a gas medium, andthe experimental results showed that the ultrasonic vibrationof the workpiece could improve the machining performanceduring the process. Their research findings revealed thatthe prevention of the debris accumulated on the machin-ing zone had an important benefit for promoting machiningefficiency. Furthermore, Lin et al. (2001a) studied the feasi-bility of the added abrasives that could be regarded as the

surface strengthening agent transferred to the machined sur-face through the ionization of discharge column during theprocess, and the added abrasives could be acted as the USMmedia for the combined process of EDM with USM. The com-

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Table 1 – Chemical compositions of SKD 61

Element wt.%

C 0.32–0.42Si 0.8–1.2Mn <0.5

on SKD 61 steel. The dielectric fluid was cycled by a pumpand delivered to the machining tank. The dielectric fluid deliv-ery system operated without flushing jet in the machiningzone, but the electrode jumped with a setting height (2 mm) as

Table 2 – Essential properties of copper electrode

Essential properties Descriptions

Specific gravity (g/cm3) 8.94Melting range (◦C) 1065–1083

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bined process of EDM with USM had the potential to preventthe accumulation of debris, to improve the machining effi-ciency, and to modify the machined surface. However, todesign USM equipment for machining was intensively con-stricted, and the degree of tool fastened on USM system wasrigorous. Therefore, the construction of the combined pro-cess of EDM with USM for various workpiece dimensions withconvenience, effectiveness, and economy was a complex andreal challenge. Especially, using EDM process to manufac-ture molds and dies with large projection area is requiredurgently in the industrial applications recently. The ability toexpel machining debris in such situation was relatively diffi-cult during EDM. Consequently, a magnetic force device wasattached to EDM machining zone to facilitate and drive themachining debris expelled from the machining zone, and theeffects of this attached device necessitated a comprehensiveunderstanding on improving the stability of EDM progressowing to preventing debris accumulation on the machiningzone.

Taguchi method has been widely used in engineering anal-ysis, and is a powerful tool to design a high quality system.Moreover, Taguchi method employs a special design of orthog-onal array to investigate the effects of the entire machiningparameters through small number of experiments. Recently,the Taguchi method was widely employed in several indus-trial fields and research works. Liao et al. (1997) used thismethod to determine the optimal parameter setting in Wire-EDM. Lin et al. (2001b) adopted the Taguchi method to obtainthe optimal machining parameter of a hybrid process of EDMwith ball burnish machining. Yang and Tarng (1998) employedthis approach to find the optimal cutting parameter for turn-ing operation. Moreover, Bagci and Ozcelik (2006) used theTaguchi method to explore the effects of drilling parameterson the twist drill bit temperature for a design optimizationof cutting parameters. Their works revealed that the Taguchimethod was a powerful approach using in design of experi-ment. The parameter design via Taguchi method can optimizethe machining characteristics through the settings of processparameter and reduce the sensitivity of the system perfor-mance to sources of variation. The high quality of machiningcharacteristics can be achieved without increasing the opera-tion cost.

In this investigation, the benefits of magnetic force assistedEDM were determined. Moreover, the essential EDM parame-ters such as machining polarity (P), peak current (Ip), auxiliarycurrent with high voltage (IH), pulse duration (�p), no-load volt-age (V), and servo reference voltage (Sv) were varied to exploretheir effects on MRR and SR, which were conducted by mag-netic force assisted EDM. An L18 orthogonal array based onthe Taguchi experimental design method was utilized to planthe experiments of this work. In addition, the experimentaldata were transferred to signal-to-noise (S/N) ratios and wereassessed by the analysis of variance (ANOVA) to determine thesignificant machining parameters and to obtain the optimalcombination levels of machining parameters for MRR and SR.Therefore, the optimal machining parameters of the magnetic

force assisted EDM were established to achieve a sophisti-cated process with high efficiency and high quality of surfaceintegrity to evolve the EDM applications for modern industrialrequirements.

Cr 4.5–5.5Mo 1.0–1.5V 0.8–1.2

2. Experimental methods

2.1. Experimental materials

The workpiece material was a SKD 61 steel, widely usedin die and mold manufacturing industry, and its dimen-sions were 50 mm × 50 mm × 5 mm in this investigation. Thespecimens were initially milled and ground to ensure the par-allelism before conducting the experiments. Table 1 presentsthe chemical composition of the workpiece materials. Theelectrode material was electrolytic copper, which is the mostcommon material of tool electrode used in EDM industries.The electrode front face was Ø35 mm with 5 mm thicknessto create a consecutive electrical discharge during the EDMprocess, and the stem diameter of electrode was Ø8 mm with40 mm length to fasten on the spindle of EDM machine. Thefront face of the electrode against the workpiece was groundusing 600, 800, and 1200 grit emery paper to guarantee sur-face finishing and the flatness of each electrode at the samelevel. Table 2 shows the essential properties of electrolytic cop-per. Kerosene (commercial grade) was employed as a dielectricfluid in this investigation.

2.2. Equipment and procedure

In this investigation, a transistor controlled EDM machinewith built-in capacitors in the circuit was used, which wasa commercial type die-sinking EDM (Model YAWJET-5 manu-factured by YIHAWJET Corp., Taiwan). A novel self-designedmagnetic force assisted device was attached to the stan-dard EDM machine, so the EDM process and the magneticforce device operated synchronously. In addition, this versa-tile apparatus was used to conduct a series of experiments toexplore the performance of the magnetic force assisted EDM

Thermal conductivity (W/m·K) 388Specific heat (J/kg K) 385Electrical resistivity (�-cm) 1.7 × 10−6

Thermal expansion coefficient (1/◦C) 16.7 × 10−6

j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h

Ff

wemusptqectcwctdl(wsdcoeaa

raes(

S = i, S = �2 − S (3)

ig. 1 – Demonstration of the configuration with magneticorce assisted EDM.

ell as a fixed period (2 s). The magnetic force assisted devicemployed a rotational disk, which was fastened with two sym-etric magnets and was driven by an electrical motor set

nder the EDM machining zone. Fig. 1 schematically demon-trates the configuration of the magnetic force assisted EDMrocess. This magnetic force assisted device would facilitatehe debris expelling from the machining zone more easily anduickly. Moreover, a fast oscilloscope was connected to the twolectrodes (tool and workpiece) to detect the waveforms of dis-harge current and voltage during the EDM process, and thenhe discharge waveforms were utilized to explore the benefi-ial effects of the magnetic force assisted EDM as comparedith the standard EDM process. The stability of the EDM pro-

ess could be evaluated and determined through diagnosinghe discharge waveforms and counting the number of effectiveischarge waveforms during a fixed period. A fast digital oscil-

oscope (Tektronix TD 2014) that coupled with current probeChauvin Arnoux E3N) and a passive voltage probe (P2200)as used in the experiments. The electrical discharge power

upply system used in this work was an iso-energy type; theischarge waveforms would be distinguished into normal dis-harge, arcing, and short. Since the elapsed machining timef EDM process was prolonged, the machining debris wouldasily aggregate in the machining gap. The intensive debrisccumulated in the machining gap would inevitably result inbnormal electrical discharges such as arcing and shorting.

The machining characteristics such as material removalate (MRR, mm3/min), electrode wear rate (EWR, mm3/min)

nd surface roughness (SR, Ra/�m) were adopted to assess theffects of machining parameters. The workpiece and electrodepecimens were weighed by a precision electronic balancePercisa XT 220A) with 0.1 mg resolution before and after each

n o l o g y 2 0 9 ( 2 0 0 9 ) 3374–3383 3377

experiment to calculate the MRR and EWR. In addition, therelative electrode wear ratio (REWR) was defined as the ratioof EWR to MRR. The measurement of SR employed a precisionprofilometer (Mitutoyo Surfest 4) to evaluate the quality of themachined surface. The SR values were obtained by averagingfive measurements made stochastically at different positionson the front machined surfaces.

2.3. Experimental design based on Taguchi method

The experimental design was according to an L18 orthogonalarray based on the Taguchi method, while using the Taguchiorthogonal array would markedly reduce the number of exper-iments. The L18 orthogonal array had eight columns and 18rows, so it had 17 degrees of freedom to manipulate oneparameter with two levels and seven parameters with threelevels. Thus, eight machining parameters can be apportionedto the columns and the rows designate 18 experiments withvarious combination levels of the machining parameters. Inthis investigation only six machining parameters were con-sidered, so two columns were empty. The orthogonality ispreserved, even if two columns of the array remained empty.

Two observed values of MRR and SR were examined. Thelevels of each machining parameter were set in accordancewith the L18 orthogonal array, based on the Taguchi experi-mental design method. The S/N ratios are calculated from theobserved values. In this work, the experimentally observedMRR value is “the higher the better” (HB), and the SR valueis “the lower the better” (LB). Therefore, the optimal observedMRR was its maximum value, and the optimal SR value, incontrast, was the minimum value.

Based on the Taguchi method, the S/N ratio calculation wasdecided as “the higher the better, HB” and “the lower the better,LB” as are given in the following equations:

HB : � = −10 log

[1n

n∑i=1

yi−2

](1)

LB : � = −10 log

[1n

n∑i=1

yi2

](2)

where � denotes the S/N ratio calculated from the observedvalues (unit: dB), yi represents the experimentally observedvalue of the ith experiment, and n is the repeated number ofeach experiment. Notably, each experiment in the L18 array isconducted three times.

The S/N ratios determined from the experimental observedvalues were statistically studied by ANOVA to explore theeffects of each machining parameter on the observed valuesand to elucidate which machining parameters significantlyaffected the observed values. The related equations are asfollows:

(∑�)2 ∑

m 18 T i m

SA =∑

�2Ai

N− Sm, SE = ST −

∑SA (4)

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Table 3 – Experimental conditions

Working conditions Descriptions

Workpiece SKD 61Electrode Electrolytic copperMagnetic flux density 0.3 TPolarity (−), (+)Duty factor 0.5No-load voltage 120, 160, 200 VPeak current 5, 10, 20 AAuxiliary current with high voltage 0.4, 0.8, 1.2 APulse duration 20, 50, 150, 350, 460 �s

Servo reference voltage 10, 20, 40 VDielectric fluid Kerosene (commercial)Working time 45 min

VA = SA

fA, FA0 = VA

VE(5)

where Sm is the sum of squares based on the mean, ST is thesum of squares based on the total variation, SA is the sum ofsquares based on parameter A (for example, A = P, Ip, IH, �p,V and Sv), SE is the sum of squares based on the error, �i isthe value of � in the ith experiment (i = 1–18), �Ai is the sum ofthe ith level of parameter A (i = 1, 2 or i = 1–3), N is the repeat-ing number of each level of parameter A, fA is the numberof degrees of freedom of parameter A, VA is the variance ofparameter A, and FA0 is the F-test value of parameter A.

Herein, the contribution of the input parameters is definedas significant if the calculated FA0 values exceed F0.05,n1,n2.

2.4. Experimental conditions

The effects of machining parameters associated with themagnetic force assisted EDM on machining characteristicswere extensively investigated in this study. Moreover, thesignificant parameters and the optimal combination levelsof machining parameters were determined. The machiningparameters, such as machining polarity (P), peak current (Ip),auxiliary current with high voltage (IH), pulse duration (�p),no-load voltage (V), and servo reference voltage (Sv) were var-ied to determine their effects on the machining characteristicsMRR and SR. The detailed machining conditions used in thisinvestigation were given in Table 3. Moreover, According tothe L18 orthogonal array based on the Taguchi experimental

design method, the machining polarity was set to two levels,and other machining parameters were set to three levels, indi-vidually. Table 4 presents the experimentally observed values,machining parameters (control parameters) and the levels of

Table 4 – Experimental observed values and levels of machinin

Observed values Control parameters

Material removalrate, MRR(mm3/min)

A. Machining polarity (P)B. Peak current (Ip)C. Auxiliary current with high voltage

Surfaceroughness, SR(�m)

D. Pulse duration (�p)E. No-load voltage (V)F. Servo reference voltage (Sv)

c h n o l o g y 2 0 9 ( 2 0 0 9 ) 3374–3383

the machining parameters based on Taguchi method. The S/Nratios were calculated from the experimental observed values,according to Eqs. (1) and (2). The optimal combination levels ofthe machining parameters correlated with the magnetic forceassisted EDM that yielded high MRR and low SR were deter-mined by analyzing the S/N ratios. Moreover, the significantmachining parameters associated with MRR and SR were alsodetermined by ANOVA.

3. Results and discussion

3.1. Discharge waveforms of analysis

Fig. 2 shows the comparison of discharge waveforms betweenthe magnetic force assisted EDM and standard EDM obtainedat the elapsed machining time 35 min. As shown in this figure,the number of effective discharge waveforms obtained by themagnetic force assisted EDM was larger than that by standardEDM. As mentioned above, the stability of EDM progress canbe monitored and determined from inspecting the dischargewaveforms. If the number of effective discharge waveformsmoves up, the stability of the EDM progress is ameliorated,and then the machining efficiency will be increased. Whenthe elapsed machining time is extended, more machiningdebris would be produced and suspended in the machininggap. The debris accumulated on the machining zone wouldalso affect the EDM progress due to the abnormal discharge.Consequently, from the results of assessing the dischargewaveforms, the magnetic force assisted EDM had a bettermachining stability, since the debris driven by the magneticforce would be expelled more completely to reduce the abnor-mal discharge.

3.2. Effects of magnetic force assisted EDM

Fig. 3 depicts a comparison of MRR, REWR, and SR betweenthe magnetic force assisted EDM and standard EDM. Basedon the experimental results, the MRR of the magnetic assistedEDM was almost three times as large as standard EDM, and theREWR was improved from 1.03% to 0.33% when employed themagnetic force assisted EDM. Moreover, the SR of the magneticforce assisted EDM was less than that of standard EDM. Theaverage value of SR reduced from Ra 3.15 to 3.04 �m. The sur-

plus workpiece materials were removed from the machiningarea caused by the EDM removal mechanisms such as melting,vaporization, and dielectric explosion to form the machin-ing debris. The more the discharge energy was delivered to

g parameters

Levels

(−) (+)5 10 20

(IH) 0.4 0.8 1.2

50 150 460120 160 20010 20 40

j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 3374–3383 3379

ssist

mttwitddf

Fm

Fig. 2 – Discharge waveforms of magnetic force a

achining zone, the more the debris was produced withinhe dielectric fluid. Moreover, when the elapsed machiningime is prolonged, the amount of debris produced form theorkpiece was increased. The extensive debris accumulated

n the machining gap would interrupt the progress of EDM, andhe stability of EDM would be deteriorated. When an assisted

evice of magnetic force was attached to the EDM machine, theebris would be driven by the assisted magnetic force to expelrom the machining gap more easily and quickly, and the prob-

ig. 3 – Comparison of MRR, REWR and SR betweenagnetic force assisted EDM and standard EDM.

ed EDM and standard EDM (obtained at 35 min).

ability of abnormal discharge would be diminished. Therefore,the MRR of the EDM was facilitated by the assisting magneticforce, and then the REWR would also be improved when alarger MRR was received in magnetic force assisted EDM. More-over, the un-expelled debris accumulated on the machiningzone would result in abnormal discharge and would re-meltto the machined surface, so the machined surface would bedamaged and the recast layer would be thickened. As a result,the surface roughness obtained by standard EDM was higherthan that by the magnetic force assisted EDM. The machin-ing debris would be driven by the magnetic force to preventtheir clogging in the machining gap, and the debris wouldalso be expelled from the machining gap more completely andquickly. Therefore, the probability of abnormal discharge wasreduced, and the thickness of the recast layer on which there-melted debris to be deposited was also diminished. There-fore, the surface roughness was improved due to the presenceof an additional magnetic force.

3.3. Surface integrities after magnetic force assistedEDM

Fig. 4 shows the comparison of SEM micrographs at 20 Apeak current and 350 �s pulse duration. As shown in the

micrographs, the discharge craters obtained by standard EDMwere bigger and deeper than those by the magnetic forceassisted EDM. In addition, the machined surface of standardEDM depicted more obvious micro-cracks on magnified micro-

3380 j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 3374–3383

by

Fig. 4 – (a–d) SEM micrographs of machined surface obtained�p = 350 �s).

graphs (see Fig. 4(c) and (d)). Since the magnetic force assistedEDM would facilitate the expulsion of the machining debrisin machining gap to reduce the probability of abnormal dis-charge and to prevent the deposition of re-melted debris onthe machined surface to form a thicker recast layer, the surfacecracks were also diminished proportionally on the machinedsurface. Therefore, the machined surface of the magnetic forceassisted EDM process revealed a finer finishing integrity thanthat of standard EDM.

3.4. Analysis of Taguchi method

Table 5 lists the S/N ratios of MRR and SR correlated with eachexperimental measurement of the magnetic force assisted

Table 5 – L18 orthogonal array, control parameters and S/N ratio

No. Control parameters

P Ip IH �p V

1 (−) 5 0.4 50 1202 (−) 5 0.8 150 1603 (−) 5 1.2 460 2004 (−) 10 0.4 50 1605 (−) 10 0.8 150 2006 (−) 10 1.2 460 1207 (−) 20 0.4 150 1208 (−) 20 0.8 460 1609 (−) 20 1.2 50 200

10 (+) 5 0.4 460 20011 (+) 5 0.8 50 12012 (+) 5 1.2 150 16013 (+) 10 0.4 150 20014 (+) 10 0.8 460 12015 (+) 10 1.2 50 16016 (+) 20 0.4 460 16017 (+) 20 0.8 50 20018 (+) 20 1.2 150 120

magnetic force assisted EDM and standard EDM (Ip = 20 A,

EDM from the L18 orthogonal array based on the Taguchimethod. Moreover, the S/N ratios were considered to evaluatethe effects of machining parameters (control parameters) onMRR and SR, by performing the statistical analysis of ANOVA.The optimal combination levels of the machining parametersto optimize MRR and SR were also determined from the S/Nratios response graphs.

3.4.1. Analysis of MRRTable 6 shows the results of ANOVA associated with MRR

obtained from the L18 array based on Taguchi method. Asshown in Table 6, the machining polarity (P) and peak current(Ip) were the significant parameters affecting MRR associatedwith the essential machining parameters of the magnetic

s

S/N ratios (�)

Sv e1 e2 MRR SR

10 1 1 −12.9776 −3.507320 2 2 −12.3339 −2.891540 3 3 −9.1522 2.390620 3 3 8.7987 −10.077740 1 1 19.8172 −10.742510 2 2 21.1437 −3.144340 2 3 31.0490 −12.053610 3 1 32.4472 −8.662620 1 2 24.1391 −11.414420 2 1 −17.5704 −2.385140 3 2 −25.9989 −3.498610 1 3 −21.5329 −1.979810 3 2 −3.4676 −2.012220 1 3 6.3516 −6.359240 2 1 −7.0600 −4.523240 1 2 11.8307 −4.766910 2 3 5.8927 −5.508520 3 1 1.9406 −4.5433

j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 3374–3383 3381

Table 6 – ANOVA of MRR

Parameter (A) Degree (fA) Square sum (SA) Variance (VA) FA0 F0.05,n1,n2

P 1 1292.7839 1292.7839 45.8745* 5.99Ip 2 3759.4728 1879.7364 66.7026* 5.14IH 2 23.2370 11.6185 0.4123 5.14�p 2 228.8847 114.4424 4.0610 5.14V 2 8.1885 4.0943 0.1453 5.14Sv 2 10.4764 5.2382 0.1859 5.14Ee1+e2 6 169.0851 28.1809

Total 17 5492.1285

∗ Significant parameter.

fdtweppuStT6(

Fig. 5 – S/N ratios response graph of MRR.

orce assisted EDM. The cathode received more electricalischarge energy at a longer pulse duration, and using a nega-ive machining polarity (workpiece connected to the cathode)ould generally increase the MRR. In addition, more electrical

nergy was conducted into the machining zone within a singleulse upon increasing the peak current. Therefore, more sur-lus workpiece materials were removed within a single pulsesing large peak current and negative machining polarity. The/N ratios response graph of MRR plotted in Fig. 5 reveals that

he optimal combination levels of the machining parameters.he optimum MRR of the magnetic force assisted EDM on SKD1 steel are: negative machining polarity (P); 20 A peak currentIp); 0.8 A auxiliary current with high voltage (IH); 460 �s pulse

Table 7 – ANOVA of SR

Parameter (A) Degree (fA) Square sum (SA)

P 1 33.4189Ip 2 108.7009IH 2 19.5129�p 2 21.6425V 2 1.2370Sv 2 14.1970Ee1+e2 6 57.0440

Total 17 255.7532

∗ Significant parameter.

Fig. 6 – S/N ratios response graph of SR.

duration (�p); 120 V no-load voltage (V); 10 V servo referencevoltage (Sv).

3.4.2. Analysis of SRTable 7 depicts the results of ANOVA associated with the mag-netic force assisted EDM of SR. As shown in this table, the peakcurrent (Ip) was a significant parameter affecting SR. As men-tioned above, when the peak current was set at a high level, ahuge discharge energy would be delivered into the machining

zone within a single pulse, so the machined surface presenteda larger crater size. The S/N ratios response graph of SR inFig. 6 shows that the optimal combination levels of machiningparameters for minimum SR include positive machining polar-

Variance (VA) FA0 F0.05,n1,n2

33.4189 3.5151 5.9954.3505 5.7167* 5.14

9.7565 1.0262 5.1410.8213 1.1382 5.14

0.6185 0.0651 5.147.0985 0.7466 5.149.5073

3382 j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 3374–3383

Table 8 – Results of the confirmation experiments

Initial levels ofmachining parameters

Optimal combination levels ofmachining parameters

Prediction Experiment

MRRLevel A1B2C2D2E2F2 A1B3C2D3E1F1 A1B3C2D3E1F1

Observed values (mm3/min) 13.84 – 42.58S/N ratio (dB) 22.82 26.36 32.58

SR

r

Level A1B2C2D2E2F2

Observed values (Ra/�m) 2.98S/N ratio (dB) −9.21

ity (P); 5 A peak current (Ip); 1.2 A auxiliary current with highvoltage (IH); 460 �s pulse duration (�p); 200 V no-load voltage(V); 10 V servo reference voltage (Sv).

3.4.3. Confirmation experimentThe optimal combination levels of the machining parameterswere determined and confirmed as follows. The estimated S/Nratios are calculated as,

�̂ = �̄m +n0∑i=1

(�̄i − �̄m) (6)

where �̂ is the estimated S/N ratio for optimal combinationlevels of machining parameters, �̄m the total mean S/N ratio,n0 the number of significant parameters, and �̄i is the meanS/N ratio at the optimal level.

Table 8 displays the results of confirmation experiments. Asthis table indicates, the MRR increased from 13.84 (mm3/min)to 42.58 (mm3/min), and SR (Ra) improved from 2.98 (�m) to1.26 (�m). In addition, the S/N ratios correlated with MRR andSR for the optimal combination levels of machining param-eters are 9.76 and 12.48 dB larger than those obtained at theinitial experimental conditions; the initial conditions were setat P (−), Ip (10 A), IH (0.8 A), �p (150 �s), V (160 V) and Sv (20 V).Therefore, the experimental results confirm that the machin-ing parameters of the magnetic force assisted EDM would beoptimized for MRR and SR, so the observed values would thusbe significantly improved.

4. Conclusions

The effects of attached magnetic force to EDM were deter-mined and the optimal machining parameters of the magneticforce assisted EDM were estimated based on Taguchi method.According to the experimental results, and statistical analysisof ANOVA, the following conclusions have been drawn.

(1) The magnetic force assisted EDM had a better machiningstability, since the debris driven by the assisted magneticforce would be expelled more quickly and completely toreduce the abnormal discharge. Moreover, the number of

effective discharge waveforms obtained by the magneticforce assisted EDM was higher than that by standard EDM.

(2) The MRR of magnetic force assisted EDM was almost threetimes as large as the value of standard EDM; the REWR was

A2B1C3D3E3F1 A2B1C3D3E3F1

– 1.26−1.98 3.27

improved from 1.03% to 0.33% when employed the mag-netic force assisted EDM. Moreover, the SR of the magneticforce assisted EDM was less than that of standard EDM.The average value of SR reduced from Ra 3.15 to 3.04 �m.

(3) The discharge craters obtained by standard EDM were big-ger and deeper than those by the magnetic force assistedEDM. Moreover, the machined surface of standard EDMdepicted more obvious micro-cracks than that of magneticforce assisted EDM.

(4) The machining polarity (P) and peak current (Ip) were thesignificant parameters affecting MRR associated with themagnetic force assisted EDM. Moreover, the optimal com-bination levels of machining parameters of maximizedMRR of the magnetic force assisted EDM on SKD 61 steelwere: negative machining polarity (P); 20 A peak current(Ip); 0.8 A auxiliary current with high voltage (IH); 460 �spulse duration (�p); 120 V no-load voltage (V); 10 V servoreference voltage (Sv).

(5) The peak current (Ip) was the significant parameter affect-ing SR associated with the magnetic force assisted EDM.The optimal combination levels of machining parameterswith minimum SR of the magnetic force assisted EDMwere: positive machining polarity (P); 5 A peak current (Ip);1.2 A auxiliary current with high voltage (IH); 460 �s pulseduration (�p); 200 V no-load voltage (V); 10 V servo refer-ence voltage (Sv).

(6) The S/N ratios correlated with MRR and SR for the optimalcombination levels of machining parameters are 9.76 and12.48 dB higher than those obtained at the initial exper-imental conditions. Therefore, the experimental resultsconfirm that the machining parameters of the magneticforce assisted EDM can be optimized for both MRR and SR.

Acknowledgement

The authors would like to thank the National Science Councilof the Republic of China, Taiwan, for financially supportingthis research under Contract No. NSC 92-2212-E-235-005.

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