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International Journal of Machine Tools & Manufacture 48 (2008) 446–458

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A new method for monitoring micro-electric dischargemachining processes

Muslim Mahardikaa,�, Kimiyuki Mitsuib

aSchool of Integrated Design Engineering, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama Kanagawa 223-8522, JapanbDepartment of Mechanical Engineering, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama Kanagawa 223-8522, Japan

Received 24 May 2007; received in revised form 24 August 2007; accepted 28 August 2007

Available online 7 September 2007

Abstract

Micro-electric discharge machining (m-EDM) is a very complex phenomenon in terms of its material removal characteristics since it is

affected by many complications such as adhesion, short-circuiting and cavitations. This paper presents a new method for monitoring

m-EDM processes by counting discharge pulses and it presents a fundamental study of a prognosis approach for calculating the total

energy of discharge pulses. For different machining types (shape-up and flat-head) and machining conditions (mandrel rotation and tool

electrode vibration), the results obtained using this new monitoring method with the prognosis approach show good agreement between

the discharge pulses number and the total energy of discharge pulses to the material removal and tool electrode wear characteristic in

m-EDM processes. On applying tool electrode vibration, the machining time becomes shorter, because it removes adhesion. The effect of

tool electrode vibration in order to remove adhesion can be monitored with good results. In order to achieve high accuracy, the tool wear

compensation factor has been successfully calculated, since the amount of tool electrode wear is different in each machining type and

condition. Consequently, a deeper understanding of the m-EDM process has been achieved.

r 2007 Elsevier Ltd. All rights reserved.

Keywords: Micro-electric discharge machining; Online monitoring; Discharge pulses counting methods; Discharge pulses energy; Tool electrode vibration

1. Introduction

Recently, there has been an increasing demand formicromachining due to the reduction in size and weight oftechnological devices. When producing microcomponents,it is critical to achieve high accuracy and precise dimen-sions. It is difficult to produce complex microcomponentssuch as microdies made of high-hardness materials usingconventional machining methods such as micromilling ormicroturning. This is because for such methods, theremoval process depends on the hardness of the workpiece,and high-hardness materials are difficult to machine. Oneway to overcome this problem is to employ novelmachining methods, such as micro-electric dischargemachining (m-EDM). This method has many advantages,including the fact that the machining process is indepen-

e front matter r 2007 Elsevier Ltd. All rights reserved.

achtools.2007.08.023

ing author. Tel.: +8145 563 1141x42087;

1495.

ess: dr064588@hc.cc.keio.ac.jp (M. Mahardika).

dent of the hardness of the workpiece, but depends insteadon its electrical and thermal conductivities and its meltingpoint. Consequently, it has become one of the mostimportant methods for machining micron and submicroncomponents [1–3].Conventional cutting or grinding processes have limita-

tions for fabricating microdies; thus, the use of m-EDM isgaining popularity. 3-D surface machining EDMs thatutilize simple-shaped electrodes (e.g., cylindrical, rectan-gular cylinder and pipe-shaped electrodes) are becomingimportant. Unlike microturning or micromilling machin-ing, m-EDM can be used to machine highly complexgeometrical shapes using a simple-shaped tool electrode [4];however, the tool electrode wear ratio is high, makingshape compensation become difficult. Thus, an accuratemonitoring method for tool electrode wear is required toachieve high precision machining.Adhesion, short-circuiting and cavitations occur fre-

quently during machining processes in m-EDM, making thedischarge pulses become unstable and machining time

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becomes excessively long. When the discharge pulsesbecome unstable, there will be many feedback movementsfor the table in order to remove the melted debris from thetool electrode. That is why the machining time cannot beaccurately predicted. The fact that the machining timecannot be estimated is a great problem for achievingunrestricted machining.

Another drawback in the m-EDM processes is that shapecompensation for tool electrode wear is difficult to achieve,thus making it difficult to produce very high accuracies. Itis also difficult to fully determine the effect of processparameters such as the charge voltage and capacitance,because of the stochastic thermal nature of the m-EDMprocesses. This is the main reason why knowledge-basedsystems for assisting the planning of m-EDM operationsstill await development [5].

Monitoring m-EDM processes is crucial for overcomingthese drawbacks. In this study, we propose a new methodthat uses discharge pulses counting for monitoring m-EDMprocesses. We also present a fundamental study based onthe prognosis approach for gaining a deeper understandingof the material removal rate and tool electrode wearcharacteristic to achieve precise and accurate machining.

2. Experimental set-up and method

A 300-mm diameter of brass workpiece was used in thisstudy. It has a thermal conductivity of 159W/mK and amelting point of 1173K. Tungsten–silver (Ag–W) squareplates with dimensions of 10mm� 10mm� 1.2mm wereused as tool electrode. In order to obtain higher materialremoval rate in m-EDM, the polarity of the workpiece wasset as anode and tool electrode was set as cathode.Generally, in m-EDM when making a microhole, micro-contour and microdies, the tool electrode is of micron sizeand workpiece is bigger than tool electrode. But, when

Fig. 1. Experimental set-up and measurement method. (a) Micro-elec

making microprobe, microshaft and microcutting tool formilling machine, the workpiece will be smaller than toolelectrode. In this experiment, we used workpiece which issmaller than the tool electrode.The machining process was performed using a Panasonic

MG-ED72 micro-electro-discharge machine with an RCcircuit and a positioning accuracy of 0.1 mm. It detects the0.0 mm point as the starting point for the machiningprocesses, and commences calculating the feed of themachining immediately after the first discharge has beendetected by the m-EDM. The feed of machining (the depthof cut) is displayed on the display monitor. Experimentswere conducted using kerosene as the dielectric fluid. Thetool electrode was vibrated using a piezoelectric transducer(PZT, AE1010D16, NEC Tokin) having a maximumdisplacement of 12.373.5 mm. Fig. 1 shows the experi-mental set-up of the machine.Fig. 1 also shows the experimental set-up for discharge

pulses counting and the prognosis methods used tocalculate the total energy of discharge pulses during themachining processes. The signals from the discharge pulseswere captured using a current probe sensor (TCP202,Tektronix) and the discharge voltage signals were mea-sured using a voltage probe (P6109B, Tektronix). Thedischarge pulse signals were subsequently divided into twochannels. The first channel was used for discharge pulsescounting; its signals were amplified using an amplifier(5307, NF) then cut off using a zener diode to ensure that itcomplied with the transistor–transistor logic (TTL) condi-tions. The TTL logic is defined as ‘‘low’’ when the voltageis between 0 and 0.8V and ‘‘high’’ between 2.4 and 5V.A discharge pulse signal after compiling with TTL condi-tion will be counted as one pulse. It was then fed to thehigh speed counter (DAQ 6025E, National Instrument),which counted the number of discharge pulses that cancount up to 20 million pulses per second. The LabView

tric discharge machine and (b) Enlarged view of machining part.

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software package was used to obtain plots of the pulsesnumber versus machining time. The second channel wasdirectly connected to an oscilloscope (TDS 640A Tektro-nix), to determine the energy per discharge pulse togetherwith the signals from the voltage probe. The energy perdischarge pulse was calculated using

Edp ¼ VQ, (1)

where Edp is the energy per discharge pulse, V the dischargevoltage measured using the voltage probe and Q theintegrated current of the discharge pulse measured usingthe current probe.

Prognosis methods were used to calculate the totalenergy of the discharge pulses since the discharge pulseswere too fast to measure using an A/D converter; theintegration function of the oscilloscope was used tocalculate the integrated discharge current pulses. Theoscilloscope can integrate the discharge current pulsestogether with the discharge voltage approximately every450ms at a sampling rate of 2GS/s. The amount of datafor the integrated discharge pulses and discharge voltagecaptured by the oscilloscope depends on the machiningtime. For example, for a machining time of 26 s, theoscilloscope calculates about 61 integrations of thedischarge pulses current Q and discharge voltage V.The following equation was used to calculate the averagedischarge pulses energy during the machining period:

Eav ¼

Pndpi¼1Edp

ndp, (2)

where Eav is the average discharge pulses energy, Edp theenergy per discharge pulse and ndp the number of data of

Fig. 2. Examples of energy per discharg

Fig. 3. Machining types. (a) Flat-he

integrated discharge pulses captured by the oscilloscope. Inthe above example, ndp is 61. Image processing analysis wasperformed using MATLAB to verify the accuracy of theintegration of the discharge pulses by the oscilloscope.The unit for the integrated discharge pulse used by theoscilloscope was nVs; 1 nVs is equivalent to 10 nCaccording to the current probe sensor’s specifications.Three different examples of discharge pulse energy

calculated by the oscilloscope are shown in Figs. 2(a)–(c).When discharge occurs, the voltage decreases until thedischarge disappears, after which the capacitor will start torecharge. The width and height of the discharge pulses varyduring the machining periods; thus, the discharge pulsesenergy also varies.There were two machining types, namely flat-head and

shape-up, since in the m-EDM processes, only these kindsof parameters can be used. Fig. 3(a) shows the flat-headfeed movement in the Z direction. The shape-up typemachining is illustrated in Fig. 3(b), its feed movement is inthe X direction. There were four different categories ofmachining conditions used in this study (see Table 1). Themandrel rotation was set as 3000 rpm since it is thecharacteristic of the machine. The frequency and halfamplitude of the vibration of tool electrode in the X

direction were 1 kHz and 1.5 mm (see Fig. 1) because thistype of vibration is the maximum capability of the drivingunit amplifier in order to vibrate the PZT, and result in thehighest material removal rate. The waveform of thevibration was sinusoidal. The charge voltage was 110V,with capacitance of 3300 pf and feed rate of 5 mm/s, andthese parameters produced the highest material removalrate.

e pulse calculation by oscilloscope.

ad type and (b) Shape-up type.

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Table 1

Machining conditions

Type Mandrel rotation at

3000 rpm

Vibration in X direction

(1 kHz, 1.5 mm)

A On Off

B On On

C Off Off

D Off On

Table 2

Machining results for different depths of cut (flat-head type, with mandrel

rotation and without vibration)

Xdoc (mm) Machining time (s) Ndp Eav (mJ) ndp Et ¼ NdpEav (mJ)

100 26.0 205,930 16.84 61 3,468,000

200 50.8 403,096 16.86 120 6,796,000

300 77.0 614,386 16.95 179 10,410,000

400 99.4 838,167 16.65 229 13,960,000

500 124.2 1,022,867 16.80 288 17,180,000

600 146.1 1,273,207 16.53 339 21,050,000

700 173.1 1,483,776 16.51 401 24,500,000

800 196.7 1,659,408 16.71 454 27,730,000

900 219.3 1,871,767 16.64 507 31,150,000

1000 246.5 2,075,783 16.62 570 34,500,000

Average 16.71

StDev 0.15

FUa 0.9%

aFU: Fractional uncertainty.

Fig. 4. Discharge energy distribution for depth of cut of 100mm, flat-head,

with mandrel rotation and without vibration.

M. Mahardika, K. Mitsui / International Journal of Machine Tools & Manufacture 48 (2008) 446–458 449

3. Experimental results and discussion

The main aims of this study were to develop a new methodfor monitoring m-EDM processes and to perform afundamental study of the prognosis approach in order togain a deeper understanding of material removal and toolelectrode wear characteristics to achieve precise and accuratemachining. Table 2 gives the results of machining at differentdepths of cut (Xdoc) for flat-head type, with mandrel rotationand without vibration. The amount of material removedincreases with increasing depth of cut, which requires a largernumber of discharge pulses. Hence, the total energy of thedischarge pulses (Et) increases, which is reasonable in view ofthe increasing number of discharge pulses. Although thetotal discharge pulses energy increases, the average dischargepulses energy (Eav) remains constant; this is because themachining type and machining condition remain similar. Thetotal energy Et is given by

Et ¼ NdpEav, (3)

where Ndp is the number of discharge pulses and Eav theaverage discharge pulses energy.

The average discharge pulses energy is determined usingaveraging methods. Figs. 4 and 5(a)–(h) show the histo-gram analysis of the averaging methods used in thisexperiment; the axis scales used in these figures differdepending on the distribution of the discharge pulsesenergy. There is a linear correlation between the amount ofmaterial removed with discharge pulses number and thetotal energy of the discharge pulses (see data given inTable 2). The equation for discharge pulses number is

Ydp ¼ 2092:2Xdoc � 5883 with R2 ¼ 0:9995, (4)

and the equation for total energy of the discharge pulses is

YEt ¼ 34; 654Xdoc þ 13; 651 with R2 ¼ 0:9998, (5)

where Xdoc is the depth of the cut in mm, which wasmeasured with high accuracy since the minimum driveinterval of the m-EDMmachine is 0.1 mm. The relationshipsof Eqs. (4) and (5) are; when the depth of cut (Xdoc) ofmachining increases, the discharge pulses number (Ydp)and total energy of discharge pulses (YEt) will also increasebecause it requires bigger amount of discharge pulses andtotal energy of discharge pulses in order to machine biggeramount of material removed (see Fig. 6). Table 2 showsthat even though the amount of material removed varies,the average discharge pulses energy remains constant. The

fractional uncertainty of this approach that uses theaverage discharge pulses energy is only 0.9% (indicatingthat the values are almost similar). The fractionaluncertainty is standard deviation divided by its averagein percent. Thus, this approach has the potential tomonitor m-EDM processes.The correlation coefficient of the depth of cut versus tool

electrode wear is R2¼ 0.9825. The effect of electrode wear

was not taken into consideration in the above analysis or inSection 3.1, since the tool electrode wear varies linearlywith the depth of cut for the same machining type andmachining condition.

3.1. Validation experiments

3.1.1. Validation experiments for equal depth of cut

In order to validate the reliability of this new method,many trials using similar machining type and machining

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Fig. 5. Discharge energy distribution for depth of cut of 1000mm for the different machining conditions and machining types. (a) Flat-head A; (b) Flat-

head B; (c) Flat-head C; (d) Flat-head D; (e) Shape-up A; (f) Shape-up B; (g) Shape-up C and (h) Shape-up D.

M. Mahardika, K. Mitsui / International Journal of Machine Tools & Manufacture 48 (2008) 446–458450

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Fig. 6. The relationship between discharge pulses number, total energy of discharge pulses to the depth of cut.

Table 3

Repeated trials for flat-head machining type, with mandrel rotation and without vibration

Xdoc

(mm)

Machining

time (s)

Ndp Eav (mJ) ndp Et ¼ NdpEav

(mJ)Errordp ¼

Ndp � Ydp

Ydp

����

����� 100% ErrorEt ¼

Et � YEt

YEt

����

����� 100%

250 62.7 513,814 16.86 145 8,663,000 0.6 0.2

250 63.0 516,091 16.88 147 8,712,000 0.2 0.4

250 61.2 518,644 16.81 142 8,718,000 0.3 0.5

250 62.7 519,373 16.84 145 8,746,000 0.4 0.8

250 64.1 517,191 16.82 148 8,699,000 0.0 0.3

250 65.1 505,146 16.95 144 8,562,000 2.3 1.3

250 66.1 507,974 16.88 142 8,575,000 1.8 1.2

Average 63.6 514,033 16.86 8,668,000

StDev 1.7 5473 0.05 72,330

FUa 2.7% 1.1% 0.3% 0.8%

aFU: Fractional uncertainty.

M. Mahardika, K. Mitsui / International Journal of Machine Tools & Manufacture 48 (2008) 446–458 451

condition were conducted, namely flat-head machiningwith mandrel rotation and without vibration. Somerepresentative results of these trials are given in Table 3.They show that when the machining type, machiningcondition and amount of material removed are similar, themachining time, discharge pulses number, average dis-charge pulses energy and total energy of the dischargepulses are also similar. The fractional uncertainties fordischarge pulses number, average discharge pulses energyand total energy of the discharge pulses are 1.1%, 0.3%and 0.8%, respectively. The fractional uncertainty formachining time is larger, about 2.7%, since the machiningtime is affected by disturbances to the machine. As men-tioned in the introduction, m-EDM is highly complex and itis very difficult to predict the machining characteristics,such as machining time, even when similar conditions areused. This is a result of many complicating factors such asadhesion, short-circuiting and cavitations. Adhesion occurswhen the melted component of the workpiece becomesattached to the tool electrode, causing the discharge pulseto become unstable as a result of short-circuiting between

the workpiece and tool electrode, and inhibiting theinsulation recovery of the EDM machine [6]. Adhesioncauses the machining time to lengthen because of thefeedback of the table required to remove the melted debrisfrom the tool electrode (see Fig. 7). When adhesion, short-circuiting and cavitations occur, it is not easy to accuratelypredict the machining time. Accordingly, the machiningtime depends on the occurrence of these complications.Table 3 shows that for the same machining type and

condition, the fractional uncertainties of the dischargepulses number and total energy of the discharge pulses aresmaller than the fractional uncertainty of machining time.Thus, the discharge pulses number and total energy of thedischarge pulses are independent of the disturbances andmachining processes and only depend on the amount ofmaterial removed.For the same machining type and condition, the average

discharge pulses energy distributions are similar, althoughthe amounts of materials removed are different. Forexample, Figs. 4 and 5(a) show the histograms for theresults of machining with depth of cuts of 100 and 1000 mm,

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Fig. 7. The effect of vibration on the adhesion process. (a) Without vibration; (b) Feeding back action and (c) With vibration.

Table 4

Confirmation experiments to predict the depth of cut

Ndp Depth of cut as shown

from m-EDM machine

(mm)

Depth of cut

from calculation

by Eq. (4) (mm)

Error

(mm)

% Error Depth of cut

from calculation

by Eq. (5) (mm)a

Error

(mm)

% Error

132,176 64.9 66.0 1.1 1.7 63.3 �1.6 2.5

432,610 210.1 209.6 �0.5 0.2 208.2 �1.9 0.9

1,051,810 504.6 505.5 0.9 0.2 506.8 2.2 0.4

319,497 157.6 155.5 �2.1 1.3 153.7 �3.9 2.5

390,911 190.0 189.7 �0.3 0.2 188.1 �1.9 1.0

623,630 296.0 300.9 4.9 1.7 300.3 4.3 1.4

616,383 298.2 297.4 �0.8 0.3 296.8 �1.4 0.5

989,559 464.6 475.8 11.2 2.4 476.8 12.2 2.6

1,262,546 606.0 606.3 0.3 0.05 608.4 2.4 0.4

1,688,751 812.4 810.0 �2.4 0.3 813.9 1.5 0.2

Note: Eq. (4) is for pulse number and Eq. (5) is for total energy of discharge pulse.aTo calculate the total energy of discharge pulse, the average discharge pulse energy of 16.71mJ is used (as mentioned in Table 2).

M. Mahardika, K. Mitsui / International Journal of Machine Tools & Manufacture 48 (2008) 446–458452

respectively. The average discharge pulses energies mostlylie in the same range of 16.50–17.00 mJ.

Table 3 gives the estimated discharge pulses number andtotal energy of discharge pulses used for machining theworkpiece with a depth of cut of 250 mm. By substitutingXdoc ¼ 250 mm in Eqs. (4) and (5), we obtain Ydp=517,167pulses and YEt=8,677,228.27 mJ, respectively. Comparingthese values with the experimental results, for a depth ofcut of 250 mm, flat-head machining with mandrel rotationand without vibration, demonstrates that this new methodproduces no significant errors. The error ranges are smallfor both discharge pulses number (0.0–2.3%) and totalenergy of discharge pulses (0.2–1.3%).

3.1.2. Validation experiments for different depth of cuts

Another experiment was conducted to forecast the depthof cut. While machining with the flat-head type, withmandrel rotation and without vibration, we stopped themachine and obtained the value of the depth of cut directlyfrom the display of the m-EDM machine and the dischargepulses number from the pulses counter. This validation wasused to demonstrate the ability of the new monitoringmethod to predict the amount of material removed. Bysubstituting the discharge pulses number in Eqs. (4) and(5), the depth of cuts was estimated and the resultscompared with the results obtained from the m-EDMmachine. In order to predict the depth of cut using Eq. (5),

the total energy of the discharge pulses was first calculatedusing Eq. (3). The average discharge pulses energy used tocalculate the total energy of the discharge pulses was16.71 mJ (see Table 2). Representative examples of thesevalidation results are given in Table 4. It shows the errorrange in forecasting the depth of cut from the dischargepulses number by using Eq. (4) is 0.05–2.4%, and the errorrange in forecasting the depth of cut from the totaldischarge pulses energy by using Eq. (5) is 0.2–2.6%. Theseresults show that there is no large difference in the depth ofcuts predicted from both calculations. Hence, the method isverified to be accurate.

3.2. Flat-head and shape-up type machining

To compare the two m-EDM processes in terms ofmachining time, discharge pulses number, average dis-charge pulses energy and total energy of the dischargepulses, several tests were performed for different machiningtypes and machining conditions for a depth of cut of1000 mm (see Table 5). From the viewpoint of machiningtime, the fractional uncertainty is relatively high at 42.1%.However, the values for discharge pulses number,

average discharge pulses energy and total energy of thedischarge pulses are almost the same since the amountsof material removed are similar. The fractional uncertain-ties for these parameters are 5.7%, 1.7% and 4.2%,

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Table 5

Results for the different machining types and conditions of machining for the same depth of cut (1000mm)

Machining

condition

Machining

typeaMachining

time (s)

Ndp Eav (mJ) ndp Et ¼ NdpEav

(mJ)Electrode

wear (mm3)

Tool electrode wear

compensation factor

Flat-head A 247.0 2,090,987 16.58 572 34,670,000 1,300,000 0.018

Flat-head B 261.1 2,097,279 16.53 603 34,670,000 1,300,000 0.018

Flat-head C 540.5 2,116,619 16.16 1247 34,210,000 1,200,000 0.017

Flat-head D 258.5 2,123,489 16.25 594 34,510,000 1,200,000 0.017

Shape-up A 264.9 1,974,056 16.61 546 32,790,000 985,000 0.014

Shape-up B 180.4 1,952,473 16.64 415 32,490,000 937,000 0.013

Shape-up C 375.9 2,279,184 15.96 830 36,380,000 1,336,000 0.019

Shape-up D 165.6 2,283,589 16.00 382 36,540,000 1,417,000 0.020

Average 286.7 2,114,710 16.34 34,530,000

StDev 120.6 121,081 0.28 1,454,000

FU 42.1% 5.7% 1.7% 4.2%

aRefer to Table 1.

Table 6

Calculation of tool electrode wear

Machining

condition

Machining

typeaDiameter of

electrode wear (mm)

Electrode wear

depth (mm)

Cross-section area of

electrode wear (mm2)

Length of electrode

wear (mm)

Volume of electrode

wear (mm3)

Flat-head A 340 14 – – 1,300,000

Flat-head B 340 14 – – 1,300,000

Flat-head C 328 14 – – 1,200,000

Flat-head D 327 14 – – 1,200,000

Shape-up A – – 972 1013 985,000

Shape-up B – – 920 1019 937,000

Shape-up C – – 1325 1008 1,336,000

Shape-up D – – 1396 1015 1,417,000

aRefer to Table 1.

Fig. 8. A conceptual figure of tool electrode wear.

M. Mahardika, K. Mitsui / International Journal of Machine Tools & Manufacture 48 (2008) 446–458 453

respectively, and much smaller than the fractional un-certainty of the machining time. This demonstrates that form-EDM, the total energy of the discharge pulses required toremove the same amount of material are similar even fordifferent machining types and machining conditions.However, the machining time may vary greatly, since it isaffected by many complications, besides machining typesand machining conditions.

As shown in Table 5, for different machining types andmachining conditions, the tool electrode wear and totalenergy of discharge pulses vary considerably and it isnot a linear function of the depth of cut. Furthermore,

tool electrode wear should be taken into considerationin the analysis of the effect of different machining typesand machining conditions because the discharge pulsesenergy is not only used to machine the workpiece butalso used to machine the tool electrode. The tool wearcompensation factor is calculated by dividing the volumeof tool electrode wear with the total volume of materialremoved.The results of tool wear compensation factor are given in

Table 5. The tool electrode wear calculation results aregiven in Table 6 and a conceptual figure of tool electrodewear is shown in Fig. 8.

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3.2.1. Flat-head type machining

The number of pulses required for flat-heads A and B arelesser than those for flat-heads C and D, but the averagedischarge pulses energies are higher (see Table 5). Rotatingthe flat-heads A and B effectively removes debris; thisstabilizes the discharge pulses, since the discharge travelsdirectly from the tool electrode to the workpiece, without

Fig. 9. Profiles of flat-head type tool electrode wear. (a) Flat-head A (diameter

(d) Flat-head D (diameter profile) (e) Flat-head A (depth profile) (f) Flat-head B

profile).

passing through debris. This phenomenon increases theaverage discharge pulses energy relative to those for flat-heads C and D.The situation is different for flat-heads C and D since no

rotation is applied. Consequently, the number of pulsesincreases because there are many low energy dischargepulses, since some discharge pulses do not travel directly

profile) (b) Flat-head B (diameter profile) (c) Flat-head C (diameter profile)

(depth profile) (g) Flat-head C (depth profile) and (h) Flat-head D (depth

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Fig. 10. The mechanism of low energy discharge current.

M. Mahardika, K. Mitsui / International Journal of Machine Tools & Manufacture 48 (2008) 446–458 455

from the tool electrode to the workpiece, but pass throughdebris. Figs. 5(a)–(d) show that there are more dischargepulses energies p15.50 mJ for flat-heads C and D than forflat-heads A and B.

The tool electrode wear for flat-heads A and B is largerthan that for flat-heads C and D (see Table 6). The totalenergies for flat-heads A and B are higher than those forflat-heads C and D since some of the energy is expendedcutting the tool electrode. The rotation of the workpiecefor flat-heads A and B was the main cause for the highertool electrode wear for these flat-heads, since the rotationalaxes of the workpiece and mandrel were not preciselyaligned. The different rotational axes make the diametersof the tool electrode wear for flat-heads A and B are greaterthan that for flat-heads C and D (without rotation), asshown in Figs. 9(a)–(d). Figs. 9(e)–(h) show that the depthof tool electrode wear does not vary much. Figs. 9(a)–(d)were obtained using a laser microscope.

For flat-head type machining, the depth of tool electrodewear was measured using a stylus-type surface roughnesstester (see Figs. 9(e)–(h)). The width of the tool electrodewear for flat-head A is narrower than those of the others.This is because the surface roughness machine did nottravel along a line through the centre of the tool electrodewear. Since the diameter of the tool electrode wear wasabout 300 mm, it was extremely difficult to measure thewear through the exact centre. Although it did not travelacross the centre of tool electrode wear, the measureddepth was not affected. In order to validate the results fromsurface roughness tester in measuring the depth profile oftool electrode wear, Laser microscope (1LM21D, Laser-tech) was used. The results were validated to be accurate.A laser microscope was also used to accurately measure thediameter of the tool electrode wear.

3.2.2. Shape-up type machining

As shown in Table 5, the trends for shape-up machiningare similar to those for flat-head machining. The number ofpulses for shape-ups A and B are lower than for flat-headsA–D and for shape-ups C and D, while the average dischargepulses energies are higher. This is because the rotation of theworkpiece removes debris more effectively for shape-upsA and B, since the centrifugal force removes debris from thesides of the workpiece, thus increasing the number of high-energy discharge pulses for shape-ups A and B relative to theothers. Figs. 5(e) and (f) shows that high-energy dischargepulses occur more frequently for shape-ups A and B than forflat-heads A–D and shape-ups C and D.

The numbers of pulses are higher for shape-ups C and Dthan for flat-heads A–D and shape-ups A and B, but theaverage discharge pulses energies are lower. Figs. 5(g) and(h) show that low discharge energy pulses in the range of3.00–15.00 mJ occur more frequently for shape-ups C and Dthan for flat-heads A–D and shape-ups A and B. Fig. 10shows that the workpiece profile when it touches the toolelectrode has a ‘‘rectangular’’ profile; consequently, thedebris will accumulate in this region. This then results in a

lot of low-energy pulses since the discharge pulses do nottravel directly from the tool electrode to workpiece, butrather pass through the debris. When there are many low-energy discharge pulses, the number of discharge pulseswill increase. Shape-ups A and B also have ‘‘rectangular’’profiles. However, the debris will be expelled by thecentrifugal force of mandrel rotation. As a consequence,the discharge pulses are stabilized and the number of low-energy pulses will decrease.The total discharge pulses energies for shape-ups C and

D are higher than those of the others since the toolelectrode wears are higher. Thus, some of the dischargepulses energy is expended in cutting the tool electrode. Thetool electrode wears for shape-ups A and B are lower thanthose for the others because mandrel rotation for theshape-up type reduces the workpiece diameter as materialis removed, thus the tool electrode wears also degrades.Although the rotational axis of the workpiece is notprecisely aligned with the mandrel rotational axis, theeffect to the tool electrode wear is not as great as for theflat-head type, since the diameter of the workpiecedecreases as material is removed. That is why the totalenergy of the discharge pulses is lower compared to theothers. Figs. 11(a)–(d) show examples of the cross-sectionsof tool electrode wear and show the differences between thecases with and without rotation.For shape-up type machining, the tool electrode wear cross-

section was measured using a stylus-type surface roughness

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Fig. 11. Profiles of shape-up type tool electrode wear. (a) Shape-up A (cross section profile); (b) Shape-up B (cross section profile); (c) Shape-up C (cross

section profile); (d) Shape-up D (cross section profile); (e) Shape-up A (length profile); (f) Shape-up B (length profile); (g) Shape-up C (length profile) and

(h) Shape-up D (length profile).

M. Mahardika, K. Mitsui / International Journal of Machine Tools & Manufacture 48 (2008) 446–458456

tester, and subsequently the area of the cross-sections werecalculated by image processing using MATLAB. A lasermicroscope was used to obtain Figs. 11(e)–(h) which was usedto measure the tool electrode wear lengths.

4. Application of pulse counting methods

Pulse counting methods can be used to monitor m-EDMprocesses. Fig. 12 shows the results of the discharge pulsesnumber monitoring for shape-ups C and D for the same

depth of cut (1000 mm). The data shown in Fig. 12 wasobtained using LabView software, since it is able to plot theaccumulated pulse number as a function of machining timedetermined by the pulse counter. The discharge pulsenumbers are almost equal for shape-up C (2,279,184pulses) and shape-up D (2,283,589 pulses). Many smallpeaks and valleys in Fig. 12 for shape-up C representregions that received a low number of pulses. The reasonfor this can be attributed to the feedback of the machinetable when machining disturbances, such as adhesion,

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Fig. 12. The results from the new monitoring system.

Fig. 13. Machining characteristic.

M. Mahardika, K. Mitsui / International Journal of Machine Tools & Manufacture 48 (2008) 446–458 457

occur. As discussed above, the machining time becomeslonger because of adhesion, since adhesion causes short-circuiting between the workpiece and the tool electrodebecause of melted debris attached to the tool electrode. Itresults in feedback of the table in order to remove themelted debris from the electrode (see Fig. 7). As aconsequence, the machining time for shape-up C (375.9 s)is longer than that for shape-up D (165.6 s). For the shape-up D (see Fig. 12) fewer small peaks and valleys are presentbecause the vibration inhibits adhesion. Thus tool electrodevibration improves machining stability and reduces ma-chining time [7]. Subsequently, the discharge pulses becomemore stable and the machining time becomes shorter.

Fig. 13 shows the accumulated discharge pulses numberfrom Fig. 12 for the three machining parts as shown inFig. 10. The discharge pulses numbers for the parts A andC are lower than those for the part B because the volumesof material removed from the workpiece with diameter of300 mm and length of 1000 mm for each of parts A and C:20,600,000 mm3 are lesser than that for the part B: 29,450,000 mm3.

By plotting the discharge pulses number versus machin-ing time (see Fig. 12), the machining condition of them-EDM processes can be effectively monitored.

5. Conclusions

In this study, we have proposed a new method formonitoring m-EDM processes by using discharge pulsescounting, and the results show a good agreement betweenthe discharge pulses number and total energy of thedischarge pulses to the amount of material removed andthe tool electrode wear characteristics for m-EDM.The main outcomes obtained using this new method for

monitoring m-EDM processes and fundamental analysesusing a prognosis approach are summarized as follows:

1.

The experimental results confirm that the new monitor-ing method that uses discharge pulses number cansuccessfully account for the effect of machining types(shape-up and flat-head) and machining conditions(mandrel rotation and vibration) on the amount ofmaterial removed and tool electrode wear in m-EDM.The tool electrode wear compensation factor can becalculated in order to achieve precise dimension, sincethe amount of tool electrode wear is not same in eachmachining type and condition. This compensation isonly valid for tool electrode of Ag–W and workpiece ofbrass. We will continue our research in order to makedatabases from many combinations of tool electrodeand workpiece material.

2.

By using prognosis methods that involve plottinghistograms for averaging, the relationship between thedischarge pulses number, average discharge pulsesenergy and the total energy of the discharge pulses fordifferent machining types and conditions can beeffectively explained. When the material removed isbigger, it will require bigger discharge pulses numberand bigger total energy of discharge pulses, but theaverage discharge pulses energy remains constant at thesame machining type and condition.

3.

In a validation experiment to predict the depth of cut,small errors were obtained, indicating high accuracy. Inorder to reduce the errors, we intend to develop a newultrahigh-speed electronic circuit to directly calculate thetotal energy of the discharge pulses, thus avoiding theneed to use prognosis approach.

4.

The machining times for machining types A, B and Dare lower than for the machining type C since mandrelrotation and vibration effectively remove debris andinhibit adhesion from the tool electrode.

5.

The potential capability of this new monitoring methodfor predicting tool electrode wear has been demon-strated. The effect of tool electrode wear to the totalenergy of discharge pulses has also been known, sincethe discharge pulse energy is not only used to machinethe workpiece, but also used to machine the toolelectrode. This characteristic is very important forprecision micro-3-D surface machining EDMs thatutilize simple-shaped electrodes.

6.

The effect of complications during machining processessuch as adhesion and the effect of tool electrode

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vibration can be monitored with good results by thenew monitoring system using discharge pulses countingmethods.

We also intend to establish a database relating thematerial removal rate and tool electrode wear for variouscombinations of workpiece and tool electrode material.

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