EDM

Materials and Manufacturing Processes, 22: 833–841, 2007Copyright © Taylor & Francis Group, LLCISSN: 1042-6914 print/1532-2475 onlineDOI: 10.1080/10426910701446812

Investigation into the Effect of Voltage Excitationof Pre-Ignition Spark Pulse on the Electro-Discharge

Machining (EDM) Process

M. Ghoreishi and C. Tabari

Department of Mechanical Engineering, K. N. Toosi University of Technology, Tehran, Iran

Studying the variation of the electro-discharge machining (EDM) process outputs due to the change in shape of the generated pulse is oneresearch aspect in the EDM process. In this study, the effects of voltage excitation of the pre-ignition spark pulse on the process outputs materialremoval rate (MRR), electrode wear ratio (EWR), and surface roughness (Ra) have been investigated. Experiments were designed using design ofexperiments (DOE), and the results were analyzed using analysis of variance (ANOVA). Based on the results, it has become clear that applyingvoltage excitation of the pulse produces effective pulses, which in turn lessens EWR, increases MRR, and improves surface quality. Hence, thesuitability of this method has been verified for EDM.

Keywords Analysis of variance (ANOVA); Design of experiments (DOE); Electrode wear; Electro-discharge machining (EDM); Materialremoval rate; Surface roughness; Voltage excitation of pulse.

1. Introduction

Electro-discharge machining (EDM) is a widely usednontraditional machining process in which material isremoved by electrical discharges between the tool andworkpiece electrode immersed in a dielectric medium.Electrical discharge is established due to excitation of a

pulse generator which produces square-shape pulses withadjustable on and off times, voltage level, and dischargecurrent. In this process, there is no mechanical contactbetween the tool and the workpiece [1]. Recent researchhas shown that material removal is based on thermal effectsof electrical discharge and formation of a plasma channelbetween tool and workpiece electrode, with a temperatureof about 20,000�C which would melt the surface of bothelectrodes. Also, it is the mechanical effect of pressureoscillation in the plasma channel which ejects moltenmaterial from the workpiece and tool electrode surfaceinto the dielectric as “flares”. Therefore, tool wear is aninevitable phenomenon in this process [2].According to reported results [1], increasing discharge

current and pulse on time increases material removal rateto a certain degree, and electrode wear is also decreasedwith an increase in current and decrease in pulse on-time.Source voltage does not have considerable effect on theprocess outputs, and is only set according to the machiningconditions.The control of the EDM process is based on the

recognition of different generated pulses during machining.The pulses can be classified into four specified groups ofopen circuit, spark, arc, and short circuit, which with theexception of the spark are all undesirable. This classification

Received September 6, 2006; Accepted December 20, 2006Address correspondence to M. Ghoreishi, Department of Mechanical

Engineering, K. N. Toosi University of Technology, P.O. Box 16765-3381, Tehran, Iran; E-mail: [email protected]

depends on the ignition delay time which affects materialremoval rate (MRR), electrode wear ration (EWR), andsurface roughness (Ra) directly [3]. Hence, recognitionand classification of these pulses can be beneficial incontrolling the process. In recent studies, different modelshave been presented by employing neural network methods[4], or the process has been controlled by applying differentcontrol systems. Therefore, pulses were classified anddesired outputs could be achieved by changing machiningparameters. Many of these systems are established based onmeasuring machining the gap voltage [5, 6]. It should benoted that, due to high frequency (HF) noise, gap voltage isnot an appropriate measure for dynamic changes that occurduring the discharge phase. Thus, measuring the ratio ofdischarge time over the pulse on-time has been introduced asan appropriate criterion for recognizing undesirable pulses.In this method, for example, pulses without ignition delaytime, are considered to be arc pulses. In this respect, acontrol system has also been designed that adjusts tool feedrate with respect to workpiece directly according to theratio of discharge duration in the gap [7]. Models have alsobeen developed using fuzzy logic and genetic algorithmsfor controlling the EDM process [8–10].Cao et al. [11] introduced a new method to fabricate

tungsten microelectrode by single electrical discharge.They have announced that this method can shortenthe microelectrode fabrication time. The fabricatedmicroelectrode can be used as a drilling tool in micro-EDM.In another study, Amorim and Weingaertner [12] carried outan experimental study on the variation of discharge current,discharge duration, tool electrode polarity, and generatoractuation mode (isoenergetic and relaxation circuit) andtheir effects on EDM performance.Egashira et al. [13] have investigated the possibility of

EDM with ultralow discharge energy. They employed anRC discharge circuit at low open-circuit voltages. They haveconcluded that the volumetric electrode wear ratio can be

833

834 M. GHOREISHI AND C. TABARI

Figure 1.—The pulse generator circuit used for applying voltage excitation.

0.2% at voltages lower than 40V, while it is more than 1%for RC discharge circuit.Also, in another study, it was revealed that deleting pulses

of very short ignition delay times (which can be consideredas arc pulses), has no effect on MRR, but increases EWR.Thus, avoiding short ignition delayed time pulses will alsonot produce desirable output [14]. Taking these results intoaccount, it seems that applying an excitation for startinga spark will result in more effective spark pulses, betterprocess controllability, and more desirable outputs.Having effective spark pulses and improving process

controllability reduce machining time and produce bettersurface quality which makes the EDM process moreeconomical for tool and die makers.In this research, the use of voltage excitation of pulse

as a pulse stimulator to make electrical discharges startfaster (i.e., shortening ignition delay time) is studied. Inconventional EDM, pulses of constant voltage (e.g., 90V)start and at the beginning of electrical discharge, the gapvoltage drops to about 20V or 30V.To use the technique of voltage excitation of a pulse, the

initial voltage level is set at a high value (e.g., 180V), andthen comes back to the previous level (90V) immediatelyafter the beginning of electrical discharge, so that dischargewould be done at the same level of 20V to 30V. Usingthis voltage excitation does not change the nature of thespark. The output of source voltage passes into a diodicswitch (dotted line in Fig. 1), and is commanded by themachining gap. When there is no electrical discharge in thegap (before the beginning of a spark), the output voltage ofthe diodic switch is −90V, i.e., a voltage of −90 to 90Vor 180V is applied to the oscillator circuit. At the momentof breakdown and start of electrical discharge, the outputvoltage from the diodic switch will become zero, and hence,a voltage of 0 to 90V is applied to the oscillator. Figure 1shows the circuit of this pulse generator. The amount ofresistance, R, is so large that high current cannot pass it.When the passing current is zero (before the beginningof electrical discharge), the voltage difference between thetwo ends of resistance is zero, i.e., the −90V of voltagedifference at the negative outlet of diode D comes to themachine generating pulse circuit that produces a 180V pulsevoltage. It is recommended that a diode permits currentto flow only from the positive to the negative pole. Onceelectrical discharge begins, the resistance R will no longerallow current to pass, so the voltage difference of the twoends will increase (potential becomes zero at the diodenegative outlet), therefore, current is established from the

Figure 2.—The comparison between pulses shapes in the two cases of appliedvoltage excitation.

diode positive outlet to the negative, and the incomingvoltage to the pulse generator would be 90V. Howeverin normal generators, the gap voltage would be constantduring the pulse on-time. If a pulse is converted into aspark, the applied voltage difference to the gap and theelectrical discharge voltage will be dissipated as heat. Thecomparison between pulse shapes in the case of appliedvoltage excitation (as is the present generator) and thenormal case without excitation is illustrated in Fig. 2. As isclear, introducing voltage excitation has shortened ignitiondelay time which in turn results in increasing spark time ina pulse.In this study, the effect of voltage excitation on the

spark time, numbers of effective pulses, and finally processoutputs have been investigated. Design of experiments(DOE) technique and analysis of variances (ANOVA) havebeen used for carrying out the experiments and analyzingresults, respectively [15].

2. Experimental details

The EDM machine used for carrying out experiments isof a special type capable of creating voltage excitation, andis equipped with a pulse generator which can provide 20 Acurrent (Fig. 3).To avoid dielectric circulation difficulties encountered

commonly in the EDM drilling process, planning wasselected as the machining mode. Cylindrical shape tools of10 mm diameter were used to erode a workpiece of the sameshape and diameter. Surfaces of the tool, and workpiece

EFFECT OF VOLTAGE EXCITATION OF PRE-IGNITION SPARK PULSE 835

Figure 3.—The EDM machine used and the oscilloscope connected.

samples were ground to the same roughness to provide auniform gap width.Commercial copper and AISI 01 (DIN: 100Mn CrW4)

cold work tool steel were used as tool and workpieceelectrode, respectively. The workpieces were heat-treatedup to 800�C, quenched in oil and then tempered at 100�C toreach a hardness of about 60 to 64RC. Also kerosene wasused as the dielectric liquid.The measured outputs, MRR and electrode wear ratio

EWR were calculated using Eqs. (1) and (2), respectively.The volume of removed material from the workpieceand tool electrode surfaces was estimated by dividingthe weight difference of each electrode before and aftermachining by its density, using a digital single panbalance (precision = 0�0001 gr). Also, the time in whichthe tool electrode goes up to permit effective flushingduring machining was not considered as the real machiningtime for each test. Ra was measured by means of aMahr Perthometer-PGK 120 profilometer with Ra value inmicrons at a cut-off length of 5.60mm according to the DINEN ISO 4287 standard.

MRR = volume removed from workpiecetime of machining

(mm3/min

)(1)

EWR = volume removed from toolvolume removed from workpiece

(%)

(2)

The experiments were done in the roughing regime (highdischarge current and long pulse on-time) and semi-

Table 1.—Design of experiments.

Factors

Voltage Generator Pulse Pulse Currentexcitation (V) mode on-time (�s) off-time (�s) (A)

Levels 90 (without) Iso-frequency 100 50 9.5 (Semifinishing)180 (with) Iso-pulse 200 100 15.5 (Roughing)

300 20500700

No. of experiments = 2 ∗ 2 ∗ 5 ∗ 2 ∗ 3 = 120

finishing (low discharge current) using the DOE techniqueas full factorial in mixed levels mode.The EDM machine generator can be selected manually

as either iso-frequency or iso-pulse type. Pulse on and offtimes were selected from 100 to 700�s and 50 to 100�s,respectively. All experiments were carried out with andwithout applying pulse voltage excitation. Table 1 showsthe scheme of experimental design.In order to reduce the possible errors caused by the change

in conditions of machining on different days, experimentswere planned in ten blocks according to the experiment’srunning day. The number of experiments was not the samein these blocks, and their running orders were also random.To monitor the pulse’s real shape during the experiments,

the EDM machine was directly connected to an EZelectronic digital oscilloscope, model: DS-1150 (Fig. 3).The oscilloscope was also linked to a personal computer forsimultaneous recording of the pulse’s shape in soft. View1.3 software environments.

3. Discussion

3.1. MRRThe ANOVA results for MRR are given in Table 2.

According to the obtained P-value, the smaller the P-value,the more effective the parameter is. It becomes clear that

Table 2.—ANOVA table for MRR.

Source DF F P-value

Blocks 9 5�63 0.000Voltage excitation 1 16�65 0.000Generator mode 1 4�46 0.038Pulse on-time 4 10�3 0.000Pulse off-time 1 2�32 0.132Setting current 2 270�03 0.000Vol Exc* Gen Mod 1 0�79 0.377Vol Exc* P On 4 7�09 0.000Vol Exc * P Off 1 0�72 0.400Vol Exc * Set Cur 2 1�57 0.216Gen Mod * P On 4 1�36 0.255Gen Mod * P Off 1 0�04 0.850Gen Mod * Set 2 0�32 0.724P On * P Off 4 1�03 0.396P On * Set Cur 8 1�54 0.160P Off * Set Cur 2 2�3 0.108Error 72Total 119

DF: Degrees of freedom; F : Number; P-value: Probability value.


Figure 4.—Normal probability plot for MRR.

pulse voltage excitation, pulse on-time, adjustable current,and generator mode (iso-pulse or iso-frequency) are themain influential parameters, while pulse off-time has lesseffect on process outputs compared to the mentioned ones.Also, the interaction between pulse voltage excitation andpulse on-time as well as pulse on-time and discharge currentare effective. As the experiments were done in differentdays, the negative effect of nuisance parameter has beendeleted as a result of blocking. In other words, the days ofexperiments have also shown to be effective.The normality of residual distribution for MRR, EWR,

and Ra (Figs. 4–6) verify the appropriate application of themethod in modeling and ANOVA application.Figure 7 shows a special trend for the MRR changes with

respect to pulse-on times. Increasing pulse on-time increasesMRR up to a maximum value, and after that MRR falls.This is mainly attributed to the enhanced depth and diameterof the molten crater at the beginning of electrical dischargein the plasma channel. However, when the plasma channelreaches its maximum compression, MRR increase stops,and afterwards no more effective material removal takesplace, as a result of pressure decrease in the plasma channel[3]. In Figs. 7 and 8, which show the variation of average

Figure 5.—Normal probability plot for EWR.

Figure 6.—Normal probability plot for Ra.

MRR with respect to the order of running experiments, theincrease in MRR using pulse voltage excitation is clearlyobvious. These enhancements are due to the increasednumber of effective pulses, resulting in normal discharge,and also the decrease in spark delay times.

3.2. EWRANOVA results for EWR output are given in Table 3.

Based on the calculated P-value, it is clear that themain parameters of pulse voltage excitation, pulse on-time,and their interactions as well as the interactions betweengenerator mode and pulse on-time, pulse-on and off times,and pulse on-time and discharge current have great effecton the EWR process output, while generator mode, pulseoff-time, and current are not effective.Figure 9 shows the decrease of EWR with respect to

the pulse on-time, which is caused by the longer electricaldischarge times. Results of past experiments have revealedthat at the beginning of electrical discharge (up to 2�s),more negative particles than positive are in motion. Inother words, the electron current predominates in theearly stage of discharge, since the positive ions oftenbeing more massive than the electrons, are less mobilethan the electrons. If more particles of one kind movetoward the target electrode, then more heat is generated

Figure 7.—Variation of the mean MRR vs. pulse on-time.


Figure 8.—Variation of the mean MRR vs. experiments run numbers.The table shows the settings of each run number.

on it. The positively-charged particles generate more heatwith the same impact velocity due to their greater size.Therefore, with long pulse on-times, positive ions haveenough time to bombard the negatively-charged workpieceelectrode surface. Hence, more material is removed from theworkpiece than tool, i.e., EWR decreases with an increasein pulse on-time. From Figs. 9 and 10, which present the

Table 3.—ANOVA table for EWR.

Source DF F P-value

Blocks 9 2 0.052Voltage excitation 1 299�88 0.000Generator mode 1 0�44 0.508Pulse on-time 4 132�77 0.000Pulse off-time 1 0�21 0.646Setting current 2 1�6 0.210Vol Exc* Gen Mod 1 1�87 0.175Vol Exc* P On 4 102�78 0.000Vol Exc * P Off 1 0�24 0.629Vol Exc * Set Cur 2 1�51 0.227Gen Mod * P On 4 2�88 0.028Gen Mod * P Off 1 0�49 0.487Gen Mod * Set Cur 2 0�74 0.480P On * P Off 4 2�87 0.029P On * Set Cur 8 7�09 0.000P Off * Set Cur 2 1�17 0.318Error 72Total 119

Figure 9.—Variation of the mean EWR vs. pulse on-time.

variations of average EWR with respect to the experimentsrunning order, it is evident that applying pulse voltageexcitation results in drastic reduction of EWR, reachingfrom about 17% down to almost less than 2%. This decreaseis caused by the longer discharge time produced withinpulse on-time as a result of this technique that lessens delaytime before the initiation of a discharge.In normal EDM, when there is no pulse voltage excitation,

there are a lot of pulses with long delay times which produceshort discharge times, and hence, increase tool electrodemetal removal or EWR (Figs. 17–20).

3.3. RaTable 4 gives the ANOVA results of Ra. Again, from

the P-value, it is inferred that pulse voltage excitation,pulse on-time, and discharge current of the main parametershave high influence on this output, while the effect ofpulse off-time is lower than the aforementioned variables.Interactions of voltage excitation and current, generatormode and current, and pulse on-time and current areeffective parameters influencing Ra.The effects of pulse on-time on Ra for current settings of

9.5A, 15.5, and 20 are depicted in Figs. 11–13, respectively,in two cases of voltage excitation and without voltageexcitation.It is also observed that increasing pulse on-time decreases

surface roughness. This can be explained by the fact that in

Figure 10.—Variation of the mean EWR vs. experiments run number.


Table 4.—ANOVA table for Ra.

Source DF F P-value

Blocks 9 2 0.052Voltage excitation 1 299�88 0.000Generator mode 1 0�44 0.508Pulse on-time 4 132�77 0.000Pulse off-time 1 0�21 0.646Setting current 2 1�6 0.210Vol Exc* Gen Mod 1 1�87 0.175Vol Exc* P On 4 102�78 0.000Vol Exc * P Off 1 0�24 0.629Vol Exc * Set Cur 2 1�51 0.227Gen Mod * P On 4 2�88 0.028Gen Mod * P Off 1 0�49 0.487Gen Mod * Set Cur 2 0�74 0.480P On * P Off 4 2�87 0.029P On * Set Cur 8 7�09 0.000P Off * Set Cur 2 1�17 0.318Error 72Total 119

short pulse on-times, highly concentrated current is not fullydeveloped in the plasma channel, producing deep moltencraters with small diameters which cause more roughnesson the workpiece surface [2]. While, by making pulse on-time longer, the plasma channel gets much time to develop,energy concentration decreases, and craters with largerdiameters are produced which reduce surface roughness.

Figure 11.—Variation of the mean Ra vs. pulse on-time (in 9.5A currentsetting).

Figure 12.—Variation of the mean Ra vs. pulse on-time (in 15.5.A currentsetting).

Figure 13.—Variation of the mean Ra vs. pulse on-time (in 20A currentsetting).

In Figs. 11 and 14, which represent the variation ofaverage surface roughness with respect to experimentsrunning orders at 9.5A discharge current setting, surfaceroughness has decreased using pulse voltage excitation.More regular electrical discharges is the main reason for thisdecrease (refer to Figs. 17 and 19). As 9.5A is in the rangeof semi-finishing regime currents, applying pulse voltageexcitation has produced a more even surface.Figures 12 and 15 show the variations of average surface

roughness vs. experiment running order at 15.5A discharge

Figure 14.—Variation of the mean Ra vs. experiments run numbers (in 9.5Acurrent setting).

Figure 15.—Variation of the mean Ra vs. experiments run numbers (in 15.5Acurrent setting).


Figure 16.—Variation of the mean Ra vs. experiments run numbers (in 20Acurrent setting).

Figure 17.—Surface micrograph and voltage pulse shapes (voltage excitationapplied) other settings are: Iso-pulse mode, Pulse on-time = 100�s; Pulseoff-time = 50�s, Setting current = 9.5A.

Figure 18.—Voltage pulse shapes (without voltage excitation) other settingsare: Iso-pulse mode, Pulse on-time = 100�s, Pulse off-time = 50�s, Settingcurrent = 9�5A.

current. It is clear that applying pulse voltage excitation hasnot changed the surface roughness. However, in Figs. 13and 16, which are the same as Figs. 12 and 15, but at aconstant current of 20A, voltage excitation has increasedsurface roughness. Since 20A current lies in the roughingregime, this increase is not considered to be a problem inthe process result. Here, rough surface can be neglected bythe effect of increased MRR and reduced EWR.

4. Pulse shape during the process

Figures 17 to 20 illustrate the comparison of pulseshapes in two cases of voltage excited and without voltageexcitation.The four shapes of randomly captured pulses recorded

during one test also indicate that applying voltage excitationresults in more sparks as well as nonproductive pulses ofopen, short circuits, and arcs.Using voltage excitation of the pulse results in the

conversion of all pulses to effective sparks with veryshort delay time. Therefore, the process would becomemore stable and controllable. Figures 17 and 19 showthe surface micrographs and the voltage excited pulses,recorded randomly during experimentation. On the otherhand, for normal EDM (Figs. 18 and 20), there exist someopen circuit as well as arc pulses. Even in Fig. 20, shortcircuit occurred. In general, application of voltage excitationleads to a more regular pulse train, free of undesirable pulseforms, which results in more stability and controllability ofthe process.

5. Conclusions

(1) It was observed that EWR has decreased considerablyusing voltage excitation of pulse. Its amount is less than2% of volumetric ratio in the roughing regime. Thisdecrease is the mainly attributed to the longer dischargetimes produced during pulse on-times.In the absence of voltage excitation, there are pulseswith long delay times which have too short discharge


Figure 19.—Surface micrograph and voltage pulse shapes (voltage excitationapplied) other settings are: Iso-pulse mode, Pulse on-time = 700�s, Pulseoff-time = 100�s, Setting current = 20A.

Figure 20.—Voltage pulse shapes (without voltage excitation) other settingsare: Iso-pulse mode, Pulse on-time = 700�s, Pulse off-time = 100�s, Settingcurrent = 20A.

times. In these pulses, the time spent on tool EWR islonger than that of excited pulses.

(2) Using pulse voltage excitation is effective in increasingMRR. This enhancement is caused by the increasednumber of effective (spark) pulses and shortened delaytimes due to stronger electrical field in the machininggap and faster formation of a plasma channel. In fact, thetime consumed for material removal from the workpieceover the total machining time has been increased, andthe problem of excessive power loss and gap control incase of high voltage (between the two electrodes) hasbeen overcome.

(3) Applying pulse voltage excitation in roughingconditions has no effect to make the surface smooth.Because high MRR and low EWR have been achievedin rough machining, increasing surface roughness is oflittle concern.

(4) Sample recorded pulses during experiments show thatthe use of voltage excitation on the pulse gives riseto the conversion of more pulses into sparks, i.e., thenumber of arcs is decreased which makes the processmore stable and controllable.

(5) Generally, the use of voltage excitation results ineffective pulses that yield noticeably low EWR,relatively high MRR, and improved surface quality,especially in the finishing regime. Therefore, theeffectiveness of this technique in EDM was confirmedexperimentally.

References

1. Ho, K.H.; Newman, S.T. State of the art electrical dischargemachining (EDM). International Journal of Machine Tools andManufacture 2003, 43, 1287–1300.

2. Schumacher, B.M. After 60 years of EDM the dischargeprocess remains still disputed. Journal of Materials ProcessingTechnology 2004, 149, 376–381.

3. Cogun, C. A technique and its applications for evaluation ofmaterial removal contributions of pulses in electrical dischargemachining. Int. J. Mach. Tools Manuf. 1990, 30 (1), 19–31.

4. Kao, J.Y.; Tarng, Y.S. A neutral-network approach for the onlinemonitoring of the electrical discharge machining process. J. Mat.Pro. Tech. 1997, 69, 112–119.

5. Liu, H.S.; Tarng, Y.S. Monitoring of the electrical dischargemachining process by abductive networks. Int. J. Adv. Manuf.Tech. 1997, 13, 264–270.

6. Weck, M.; Dehmer, J.M. Analysis and adaptive control of EDMsinking process using the ignition delay time and fall time asparameter. Ann. CIRP 1992, 41 (1), 243–246.

7. Rajurkar, K.P.; Wang, M.W.; Lindsay, R.P. Real-time stochasticmodel and control of EDM. Ann. CIRP 1990, 39 (1), 187–190.

8. Boccadoro, M.; Dauw, D.F. About the application of fuzzycontrollers in high-performance die-sinking EDM machines,Ann. CIRP 1995, 44 (1), 147–150.

9. Tarng, Y.S.; Tseng, C.M.; Chung, L.K. A fuzzy pulsediscriminating system for electrical discharge machining. Int. J.Mach. Tools Manuf. 1997, 37 (4), 511–522.

10. Tarng, Y.S.; Jang, J.L. Genetic synthesis of a fuzzy pulsediscriminator in electrical discharge machining. J. Intell. Manuf.1996, 11 (7), 311–318.


11. Cao, G.; Zhao, W.; Wang, Z.; Guo, Y. Instantaneous fabricationof tungsten microelectrode based on single electrical discharge.J. Mat. Proc. Tech. 2005, 168, 83–88.

12. Amorim, F.L.; Weingaertnet, W.L. The influence of generatoractuation mode and process parameters on the performance offinish EDM of a tool steel. J. Mat. Proc. Tech. 2005, 166,411–416.

13. Egashira, K.; Matsugasako, A.; Tsuchiya, H.; Miyazaki, M.Electrical discharge machining with ultralow discharge energy.Precision Engineering 2006, 30, 414–420.

14. de Bruyn, H.E.; Pekelharing, A.J. Has the delay time influenceon the EDM Process? Ann. CIRP 982, 31 (1), 103–106.

15. Montgomery, D.C. Design and Analysis of Experiments, 6th ed.;John Wiley and Sons, 2005.

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