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Journal of Materials Processing Technology 212 (2012) 1463– 1471

Contents lists available at SciVerse ScienceDirect

Journal of Materials Processing Technology

jou rna l h om epa g e: www.elsev ier .com/ locate / jmatprotec

study on pulse control for small-hole electrical discharge machining

i Jianga, Wansheng Zhaob,∗, Xuecheng Xib

School of Mechanical Engineering, Jiangnan University, PR ChinaState Key Laboratory of Mechanical System and Vibration, School of Mechanical Engineering, Shanghai Jiao Tong University, PR China

r t i c l e i n f o

rticle history:eceived 22 September 2011eceived in revised form 29 January 2012

a b s t r a c t

Small-hole EDM has a problem of debris evacuation from the narrow gap between the electrode andworkpiece. The presence and difficulty in evacuating the debris formed during an erosion process limitthe achievable aspect ratio. To address the problem of debris accumulation, a pulse generator, which is

ccepted 30 January 2012vailable online 8 February 2012

eywords:DM

able to shut off harmful pulses and to apply high discharge energy pulses, is developed. A FPGA chip isused as the master controller for the determination of pulse discharge status and MOSFET switching. Aseries of experiments are carried out to examine the machining performance by shutting off harmfulpulses and applying high discharge energy pulses. The experimental results show that the efficiency ofsmall-hole drilling is improved and the aspect ratio is increased.

ulse generator

mall-hole

. Introduction

Small-hole EDM processes are characterized by a high aspectatio and a slow drilling speed, with a higher tool wear rate (TWR)han normal EDM. Small-hole EDM, with a tool electrode diameterf 100 �m, is within the scope of micro-EDM, because (Masuzawand Tönshoff, 1997) described that micro-EDM is specially devotedo the manufacture of micro-components whose sizes range from

to 999 �m. Yu and Rajurkar (2005) presented the fact that theifficulty of removing the debris formed during an erosion pro-ess is a major problem in drilling small-holes, thereby limiting thechievable aspect ratio.

The accumulation of debris also makes the dielectric fluid eas-er to break down, thus inducing arc pulses. Arc pulses prefigure

deterioration of the gap status and the instability of the EDMrocess, which lead to an inefficient EDM process and damage theurface quality in finish machining. The extent to which arc pulseseteriorate the stability of a drilling process is even more drastic

n small-hole EDM processes, even though the percentage of arculses is less than 5%, as Jiang et al. (2011) presented.

Yeo et al. (2009) applied adaptive servo feed control systemo promote the stability of a small-hole EDM process. Kao et al.2008) utilized fuzzy logic to maintain a stable drilling process.lthough servo feed control is helpful in improving the stabilityf a drilling process, harmful arc pulses still cannot be eliminated,

nd the debris cannot be removed directly.

To eliminate harmful pulses, pulse control has been reported toe used for shutting off harmful pulses. Hara and Nishioki (2002)

∗ Corresponding author. Tel.: +86 21 62934959; fax: +86 21 62934959E-mail address: zws@sjtu.edu.cn (W. Zhao).

924-0136/$ – see front matter © 2012 Elsevier B.V. All rights reserved.oi:10.1016/j.jmatprotec.2012.01.022

© 2012 Elsevier B.V. All rights reserved.

presented a current shutdown circuit to shut off short pulses in EDMprocesses. Han et al. (2004) developed a transistor type isopulsegenerator to shut off the discharge current immediately when adischarge occurs within the gap.

To alleviate the problem of debris accumulation, severalapproaches have been applied to drive the debris out of thedischarge gap, such as ultrasonic vibration assistant EDM anddielectric fluid flushing. Jia et al. (2009) utilized an approach ofmachining deep micro-holes by combining EDM with USM in inver-sion installing. Li et al. (2009) applied dielectric fluid flushing toimprove the efficiency of drilling deeply small holes in TC4 alloy.However, the ultrasonic vibration assistant EDM needs auxiliaryfacilities such that its implementation is relatively complex. Also,although flushing techniques are widely used in macro-EDM, theyare difficult to be applied to small-hole EDM with a very smallelectrode, as Yu and Rajurkar (2005) presented.

Non-circular electrodes are also used for debris evacuation. Zhaoet al. (2002) made a single-notch electrode to expand the spacefor debris to be driven away and to increase the machining effi-ciency. Assisted by ultrasonic vibration, Hung et al. (2006) produceda helical electrode for micro-hole machining. The manufacture ofcomplex electrodes is, however, difficult yet expensive.

Bamberg and Heamawatanachai (2009) reported that orbitalelectrode actuation is able to create a larger gap between the workpiece and the electrode, which results in better flushing as wellas increases the efficiency of the small-hole drilling process. Yuet al. (2009) also reported that high aspect ratio micro-holes canbe obtained by the orbital electrode actuation and ultrasonic vibra-

tion assistant EDM drilling, whereas the deviation from the circularshapes of the through-hole orifices is hard to be avoided.

Besides the approaches mentioned above, high discharge energypulses are also found to be beneficial to debris removing. Pradhan

1464 Y. Jiang et al. / Journal of Materials Processing Technology 212 (2012) 1463– 1471

E1 C1

T1 T2

Usg1 Usg2Usg3 T3 U

R1 R2

emrpcesbagui

dtpppceao

fceph

2

tpd

2

Mct

vocd

ped

dfam

E1C1

T1 T2

Usg1 Usg2 Usg3 T3U

R1 R2

T4 T5

Usg4 Usg5

R3 R4

C2

E2

Sweep pu lse

circuit

Main discharge

circuit

Fig. 1. Structure of the main discharge circuit.

t al. (2009) described that the material removal rate increasesonotonously with the increase in peak current. This increase

esults form the higher discharge energy in the small-hole drillingrocess. Yu and Rajurkar (2005) reported that a high aspect ratioan also be achieved with high discharge energy. A higher dischargenergy delivered within a short time causes a higher energy den-ity, strengthening the explosion effect of discharges. The boomingubbles generated by evaporation are conducive to destroying theccumulated debris and sweeping the debris out of the dischargeap. Interruptive high discharge energy pulses have already beensed in die sinking EDM for debris ejecting to increase the machin-

ng efficiency, as Boccadoro (2004) presented.In this paper, a high discharge energy pulse with a short pulse

uration and a high peak current is referred to as a sweep pulse. Andhe techniques of shutting off harmful pulses and applying sweepulses are referred to as pulse control techniques. One advantage ofulse control for small-hole EDM is that its implementation is sim-le. Compared to the ultrasonic vibration assistant EDM, the pulseontrol approach needs neither auxiliary facilities, nor complexlectrodes, such as single-notch electrodes or helical electrodes. Inddition, the deviation from the circular shapes of the through-holerifices caused by orbital electrode actuation can be avoided.

In this paper, a pulse generator capable of shutting off harm-ul pulses, such as arc pulses, is developed. Moreover, an auxiliaryircuit for applying sweep pulses is integrated in the pulse gen-rator. A series of experiment are carried out to investigate theerformance of the pulse control techniques, including shutting offarmful pulses and applying sweep pulses.

. Development of pulse generator

A rectangle pulse generator was developed for small-hole elec-rical discharge machining. The main discharge circuit, the sweepulse circuit and the pulse detection and control module wereesigned.

.1. Main discharge circuit

The structure of the main discharge circuit is shown in Fig. 1.As shown in Fig. 1, the main discharge circuit consists of three

OSFETs (Metal-Oxide-Semiconductor Field-Effect Transistor), aapacitor and two current-limiting resistors. Capacitor C1 is usedo provide spark discharge energy.

When T1 is turned on, T2 is turned off and T3 is turned on. Theoltage between the electrodes is pull down to zero. The processf spark discharge is shutoff. At the same time, capacitor C1 isharged by an external voltage source E1, getting ready for the nextischarge cycle. This time period is the so-called pulse interval.

When T1 is turned off, T2 is turned on and T3 is turned off, aulse with a voltage of U is loaded between the electrodes, and thelectrical energy stored in capacitor C1 is then released in the sparkischarge. This time period is so-called pulse duration.

One feature of this main discharge circuit is that the charge and

ischarge processes of the capacitor C1 are separated. With thiseature, the electric energy of each spark discharge is controllable,nd the uniformity of the pulse is improved, which is beneficial toachining performance.

Fig. 2. Structure of the sweep pulse circuit.

2.2. Sweep pulse circuit

In order to apply sweep pulses, which have higher dischargeenergy and higher peak current as compared with normal pulses,a sweep pulse circuit is needed besides the main discharge circuit.The structure of the sweep pulse circuit is shown in Fig. 2.

Two MOSFETs, T4 and T5, and a capacitor C2 are added to themain discharge circuit. When the pulse generator runs in nor-mal mode, the MOSFETs in the sweep pulse circuit, T4 and T5,are turned off, and no sweep pulse is applied. When the pulsegenerator is switched to sweep mode, the MOSFETs in the maindischarge circuit, T1 and T2, are turned off. At this moment, T4and T5 start to operate, and sweep pulses are applied between theelectrodes.

The structure and operation mode of the sweep pulse circuit arealmost the same to those of the main discharge circuit. However,in the sweep pulse circuit, the capacitor C2, the external sourceE2, and the current-limiting resistors, R3 and R4, are different fromthose of the main discharge circuit. Therefore, the pulses generatedby the sweep pulse circuit are different from those by the maincircuit.

The differences between the main circuit and the sweep pulsecircuit are as follows. The capacitance of C2 in the sweep pulse cir-cuit is higher than that of C1 in the main circuit. Therefore, when thepulse generator runs in sweep mode, the discharge energy, whichis delivered by a single pulse and then released between the elec-trodes, is higher. Meanwhile, the voltage loaded by the externalsource E2 is higher than that of E1. A higher charge voltage E2 meansnot only a higher electrical energy stored in the capacitor C2 andreleased in a discharge process, but also a higher open voltage ofthe sweep pulses. Furthermore, the resistance of R4 is lower thanthat of R2, so that the peak current of the pulse is raised.

The pulse duration of sweep pulse is smaller than that of normalpulse. Therefore, when the pulse generator runs in sweep mode, theswitching rate of T4 and T5 is faster than that of T1 and T2 in normalmode. In sweep mode, the on and off states of T3 are determinedby the parameter settings for sweep pulse.

2.3. Pulse detection and control module

The control of pulses includes shutting-off of harmful pulses andapplication of sweep pulses. The transition between the normalmode and the sweep mode, as well as the MOSFET switching, ismanaged by the control module of the pulse generator. The blockdiagram of the pulse generator is shown in Fig. 3.

As shown in Fig. 3, a field programmable gate array (FPGA) chipis utilized as the master controller. FPGA has the characteristics ofhigh speed, functional agility and high integration, which are suit-able for high-speed pulse control at micro-second scale. To avoid

electrical interference, photo couplers are used to isolate the con-trol module from the discharge module.

Fig. 3 shows that the MOSFET switch signal is generated by theFPGA and then sent out to the discharge module. Accordingly, the

Y. Jiang et al. / Journal of Materials Processing Technology 212 (2012) 1463– 1471 1465

Maddttt

pshdbTg

utsbtdat

ttTFptrsvdtot

Ahtpopn

bp

Fig. 4. EDM voltage waveform of harmful pulse shutting off.

Fig. 3. Block diagram of the pulse generator.

OSFETs in the main discharge circuit and the sweep pulse circuitre switched by the MOSFET drivers. At the same time, the EDMischarge signal between the tool electrode and the work piece isetected by a pulse discharge status detecting unit and fed backo the master FPGA controller. The discharge status of the pulse ishen identified, and the corresponding control decision is made byhe FPGA.

EDM pulses can be distinguished into open pulses, ideal sparkulses and harmful pulses. Harmful pulses include arc pulses, tran-ient arc pulses and short pulses. Arc pulses and transient arc pulsesave very short ignition delay, because the dielectric fluid breaksown very quickly. Also, when short pulse occurs, the gap voltageetween the tool electrode and the work piece is essentially zero.herefore, it is feasible to identify harmful pulses by detecting theap voltage at the front edge of a single pulse.

A high-speed comparator with a voltage threshold setting issed for detecting the gap voltage. If the gap voltage is higher thanhe threshold at a sampling instant, the pulse can be regarded as apark pulse or an open pulse, because the dielectric fluid does notreak down at this instant. By contrast, if the gap voltage is lowerhan the threshold, meaning that the dielectric fluid has brokenown, the pulse can be regarded as a harmful pulse. Thus, as longs the gap voltage is acquired by FPGA at the front edge of a pulse,he discharge status of the pulse is determined.

Once a harmful pulse is identified, the FPGA gives an instructiono shut off the pulse so that the damage of the harmful pulse tohe machining stability and machining quality can be prevented.his shutting-off can be carried out as follows. At first, referring toig. 1, MOSFET T1 is turned on and T2 is turned off. After that, theulse generator is set to pulse interval state, which is also referredo as shutoff pulse interval. Since a harmful pulse implies dete-ioration of the gap status and instability of the EDM process, ahutoff pulse interval should be longer than a normal pulse inter-al such that enough time is provided for gap status recovery andischarge debris removal. A normal pulse interval is in the order ofens of micro-seconds, while a shutoff pulse interval is in the orderf over a hundred of micro-seconds. The EDM voltage waveform ofhe harmful pulse shutting off is shown in Fig. 4.

As shown in Fig. 4, the open pulses have a peak voltage of 42 V. spark pulses has an ignition delay time as long as 6 �s, whereasarmful pulses have very short ignition delay. In a harmful pulse,he discharge occurs before the peak voltage is attained, and theulse can be therefore regarded as an arc. Fig. 4 also shows thatnce detected, a harmful pulse is shut off. At the same time, theulse generator is set to pulse interval state for 240 �s. After that,ormal pulses are applied again.

In small-hole EDM processes, the generated debris is difficult toe evacuated from the discharge gap. Although shutting off harmfululses is helpful in avoiding deterioration of the gap status, it is

Fig. 5. EDM voltage waveform of sweep pulse.

not helpful in removing the debris. For the purpose of ejecting thedebris effectively, a sweep pulse is thus used.

Sweep pulses are applied when a number of harmful pulsesoccur successively. Under this circumstance, the shutoff pulseinterval, which is longer than the normal pulse interval, is not ableto make the gap status return to a normal state. In order to stop theaccumulation of debris as well as to recover the gap status, sweeppulses can be applied such that their explosion effect can be madeuse of. Once a number of harmful pulses occur successively, theFPGA gives an instruction to exit the normal mode and enter intothe sweep mode. Referring to Fig. 2, MOSFETs T1 and T2 in the maindischarge circuit are turned off, and MOSFETs T4 and T5 in the sweepcircuit starts to operate. The EDM voltage waveform of the sweeppulse is shown in Fig. 5.

Fig. 5 shows that, at the beginning of the waveform, harmfulpulses occur successively and they are shut off. The threshold num-ber of successive harmful pulses is set to be eight, and the numberof the sweep pulses applied is set to be five. Therefore, when eightsuccessive harmful pulses appear, the pulse generator is turned intosweep mode and five sweep pulses are then applied. As shown inFig. 5, the sweep pulses have very short pulse duration. The openvoltage of a sweep pulse is 55 V, which is higher than 42 V of anormal pulse. Moreover, both the peak current and the dischargeenergy of a sweep pulse are higher than those of a normal pulse.Once five sweep pulses have been applied, the sweep mode ter-minates and the normal mode returns. It can be noticed from thewaveforms that after applying the sweep pulses, no more harmfulpulse occurs, and open pulses appear, which implies the improve-ment in the gap status.

3. Experimental results and discussions

A series of experiments were carried out to examine the machin-ing performance by shutting off harmful pulses and applying highdischarge energy pulses. The effects of the parameters for both

1466 Y. Jiang et al. / Journal of Materials Processing Technology 212 (2012) 1463– 1471

X-axis Stage

Y-axis Stage

Z-axis

Stag eCirculatory

Fluid

Supplying

System

Vision System

Indus trial PC

Motor

Drivers

Motion

Control

Card

Average

gap voltage

detecting

Rectangle Pulse

Generator (with sweep

pulse circuit)

Tool

Elect rod e

Dielectri c

Fluid

Workpiece

Rotati ng

Spindle

of th

hw

3

amsiws

1oTnTcnswFta

ohs

TP

Besides the machining efficiency, tool wear rate (TWR) is alsoan important machining performance index. A comparison of thetool wear rate during the experiments is shown in Fig. 8.

100

Fig. 6. Schematic diagram

armful pulse and sweep pulse were studied. Deep small-hole testsere also carried out.

.1. Experimental setup

Small-hole EDM experiments of shutting off harmful pulsend applying sweep pulse were carried out on a CNC micro-EDMachine with the rectangular pulse generator developed in this

tudy. The schematic diagram of the experimental system is shownn Fig. 6. The average voltage between the tool electrode and the

ork piece is measured as an input signal to the servo controlystem.

The tool electrode is a tungsten micro-shaft with a diameter of00 �m. The work piece is a stainless steel plate with a thicknessf 1 mm. Through holes were drilled in the stainless steel plate.he retract cycle time of the tool electrode is 5 s. The parameters oformal pulse, shutoff harmful pulse and sweep pulse are listed inable 1. It can be seen from the table that the open voltage, peakurrent and nominal capacity of sweep pulse is higher than that oformal pulse. Besides, the pulse duration of sweep pulse is muchhorter. The detecting time in the table is defined as the time period,hich starts from the moment a pulse begins to the moment the

PGA acquires the gap voltage. The shutoff pulse interval in theable is defined as the time period in which a pulse interval lastsfter a harmful pulse is shut off.

The polarity of the tool electrode is positive. The rotational speed

f the spindle is 6000 rpm. The threshold number of successivearmful pulses is set to be 8, and the number of the sequence ofweep pulses is set to be 5, as shown in Fig. 5.

able 1arameters of the three different EDM modes.

Normalpulse

Shutoffharmful pulse

Sweeppulse

Pulse duration 30 �s 30 �s 5 �sPulse interval 40 �s 40 �s 20 �sOpen voltage 40 V 40 V 55 VPeak current 1.3 A 1.3 A 5.5 ANominal capacity 6800 pF 6800 pF 0.1 �FDetecting time – 4 �s –Shutoff pulse interval – 180 �s –Successive harmful pulses – – 8Number of sweep pulses – – 5

e experimental system.

3.2. Experimental results and discussion

A comparison of machining time is made among three cases ofsmall-hole drilling with each case repeated for five times. Morespecially, these three cases are (a) applying only normal pulses;(b) shutting off harmful pulses without applying sweep pulses and(c) applying sweep pulses. The comparison results are shown inFig. 7.

As shown in Fig. 7, the average machining times by using normalpulse (case a) and shutting off harmful pulse (case b) are 57.6 minand 35.8 min, respectively. By shutting off harmful pulses, the dete-rioration of the gap status is avoided and the stability of the EDMprocess is improved. As a result, the efficiency of the small-holeEDM process is increased. The average machining time by apply-ing sweep pulses (case c) is 19.4 min, which is only about one thirdthat by using normal pulse. When sweep pulses are applied, due tothe explosion effect, the debris generated in the erosion process isejected out of the discharge gap, and the accumulation of the debrisis destroyed. Therefore, the gap status is improved considerably,and the machining time is reduced remarkably.

1 2 3 4 50

20

40

60

80

Drilled hole number

Mac

hini

ng ti

me

(min

)

(c) Sweep pulse loaded(b) Harmful pulse shutoff(a) Normal pulse

Fig. 7. Machining time of the small-hole EDM.

Y. Jiang et al. / Journal of Materials Processing Technology 212 (2012) 1463– 1471 1467

1 2 3 4 50

20

40

60

80

100

Drilled hole number

TW

R (

%)

(c) Sweep pulse loaded(b) Harmful pulse shutoff(a) Normal pulse

patpsbr

iabhtcbapa

ma

e1o(pootth

btesIap

aaot

Fig. 8. Tool wear rate of the small-hole EDM.

Fig. 8 shows that the average tool wear rate by using normalulses (case a) is 71.8%. By shutting off harmful pulses (case b), theverage tool wear rate dropped to 8.5%. The significant reduction ofhe tool wear rate implies that the damage resulting from harmfululses onto the tool electrode is a major cause of tool wear. Byhutting off harmful pulses, the damage to the tool electrode causedy harmful discharge is weakened. Therefore, the tool wear rate iseduced significantly.

By applying sweep pulses (case c), the average tool wear rates 33.6%. Although the machining efficiency is very high, the aver-ge tool wear rate by applying sweep pulses is higher than thaty shutting off harmful pulse. The increased TWR results from theigher discharge energy and higher peak current. This experimen-al result shows that the sweep pulses with high discharge energyannot be used as a major means of material removal. This is mainlyecause the high tool wear rate brought over by sweep pulses is notcceptable. Sweep pulses, therefore, can only be adopted as a sup-lementary means for the improvement in gap status, and only bepplied when necessary.

The entrances and exits of the small holes drilled by using nor-al pulse, shutting off harmful pulses and applying sweep pulses

re shown in Figs. 9–11, respectively.By using normal pulses, the diameters of the small hole at the

ntrance and exit, as shown in Fig. 9(a) and (b), are 147 �m and21 �m, respectively. By shutting off harmful pulses, the diametersf the small hole at the entrance and exit, as shown in Fig. 10(a) andb), are 136 �m and 127 �m, respectively. Hence, by using normalulses, the difference between the diameters of the two orificesf the through hole is 26 �m, whereas the difference by shuttingff harmful pulses is 9 �m. It is thus evident that the taper of thehrough small-hole drilled by shutting off harmful pulses is lowerhan that of normal pulses, and the precision of the through small-ole is improved.

It can be noticed from Fig. 9 that a number of small size dischargey-products, such as metal debris or carbon deposition, stick tohe edges of the through hole, both on the entrance and on thexit sides. By comparison, the edges of the through hole drilled byhutting off harmful pulse, as shown in Fig. 10, are much cleaner.t is thus concluded that the quality of the through small-hole islso improved. This is because the damage resulting from harmfululses to the work piece is alleviated.

The diameters of the small hole at the entrance and exit by

pplying sweep pulse, as shown in Fig. 11(a) and (b), are 140 �mnd 123 �m, respectively. The difference between the diametersf the two orifices of the through hole is 17 �m. The taper of thehrough small-hole drilled by applying sweep pulses is lower than

Fig. 9. Small hole drilled by using normal pulses: (a) hole entrance and (b) hole exit.

that by normal pulses, but higher than that by shutting off harmfulpulses.

It can be noticed from Fig. 11 that small size discharge by-products stick to the edges of the through hole. Sweep pulses arehelpful in ejecting metal debris or carbon deposition out of thedischarge gap. Therefore, the edges of the through hole drilled byapplying sweep pulse are very clean.

In conclusion, shutting off harmful pulses is helpful in improv-ing the machining efficiency and decreasing the tool wear rate. Thequality of the through small-hole drilled by shutting off harmfulpulses is better than that by using normal pulses. Applying sweeppulses, on one hand, contributes to the increase in the machin-ing efficiency, on the other hand, brings over a higher tool wearrate than that by shutting off harmful pulses. Also, the taper of thethrough small-hole drilled by applying sweep pulses is higher thanthat by shutting off harmful pulses, even though the edges of thehole are cleaner.

In order to find out the reason why the machining efficiency isimproved by applying sweep pulses, further investigation on theEDM drilling process was carried out. The EDM discharge signalsduring the drilling process were recorded and analyzed. The cate-gory of each pulse was identified offline. The percentages of each ofthe four types of discharge pulse (open, short, arc and spark) duringthe drilling process, whose sum equals to 100%, are shown in thearea graphs in Fig. 12. The data was filtered by a low-pass filter togive the major trends of each discharge status.

It is noticed that the machining times by applying sweep pulsesand by shutting off harmful pulses (but no sweep pulse is applied)

are 1124 s and 2291 s, respectively. The corresponding materialremoval rates (MRR) of these two drilling processes can, therefore,

1468 Y. Jiang et al. / Journal of Materials Processing Technology 212 (2012) 1463– 1471

Fh

bp

oasa

botha3d

ntTer

TA

identification of harmful pulses. On one hand, a too small detectingtime leads to a failure in identifying some arc pulses, whose ignitiondelays are longer than the detecting time. On the other hand, a toolarge detecting time leads to mistaking some spark pulses for arc

0 400 800 11240

50

100

Pul

se r

atio

(%

)

Spark pulseArc pulseShort pulseOpen pulse

Sweep pulse applied

ig. 10. Small hole drilled by shutting off harmful pulses: (a) hole entrance and (b)ole exit.

e worked out by the machining times. The MRR by applying sweepulses is 103% higher than that by shutting off harmful pulses.

The average pulse percentages of the four types of pulses in theverall process are shown in Table 2. The percentages of spark pulsend open pulse by applying sweep pulse are higher than those byhutting off harmful pulse, while the percentages of short pulse andrc pulse are lower.

It can be noticed from the table that the percentage of arc pulsey applying sweep pulse is remarkably lower than 44% by shuttingff harmful pulse, which suggests that sweep pulses are conduciveo the improvement in gap status. Arc pulses cause local over-eating, which is not only harmful to the gap environment, butlso detrimental to material removal. Although the percentage of.4%, as shown in Table 2, is not very high, arc pulses significantlyeteriorate the stability of the drilling process.

By applying sweep pulses, the percentage of arc pulses is sig-ificantly reduced and the gap status is improved, which implieshat the debris is easier to be evacuated from the discharge gap.

he machining efficiency is thus increased. In addition, the cleanerdge of the hole by applying sweep pulses also results from theeduction of arc pulses.

able 2verage pulse percentage in drilling process.

% Sweep pulse applied Harmful pulse shutoff

Spark pulse 16.0 14.7Arc pulse 3.4 7.6Short pulse 16.5 18.9Open pulse 64.1 58.8

Fig. 11. Small hole drilled by applying sweep pulses: (a) hole entrance and (b) holeexit.

3.3. Effects of harmful pulse shutoff parameters

The parameters for shutting off harmful pulses include thedetecting time and the shutoff pulse interval. Referring to Table 1,the detecting time was set to be 4 �s, and the shutoff pulse intervalwas set to be 180 �s.

Detecting time at the beginning of a pulse is a crucial factor in the

t (s)

0 400 800 1200 1600 2000 22910

50

100

t (s)

Pul

se r

atio

(%

)

Harmful pulse shutoff

Fig. 12. Pulse percentage during drilling process.

Y. Jiang et al. / Journal of Materials Processing Technology 212 (2012) 1463– 1471 1469

90 120 150 180 2100

20

40

60

80

Shutoff pulse interval (µs)

Ave

rage

mac

hini

ng ti

me

(min

)

2 µs3 µs4 µs5 µs

Detecting time

po

tsvpn

etin23tp

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imrdtsoavoWmia

h

3 5 8 120

5

10

15

20

25

30

Number of sweep pulses

Mac

hini

ng ti

me

(min

)

4 harmful pulses8 harmful pulses12 harmful pulses

Also, it can be seen from the figure that the tool wear rate rises as

Fig. 13. Effects of harmful pulse shutoff parameters on machining time.

ulses, because the detecting time is longer than the ignition delayf normal spark pulses.

Shutoff pulse interval is important for gap status recovery. Aoo small shutoff pulse interval is insufficient to recover the gaptatus. On the contrary, although a too large shutoff pulse inter-al is sufficient for gap status recovery, the duty ratio of theulse is also reduced and the machining efficiency is affectedegatively.

Therefore, it is necessary to investigate the effects of the param-ters for harmful pulse shutoff, including the detecting time andhe shutoff pulse interval. Through small-hole EDM drilling exper-ments were carried out, in which harmful pulses are shut off ando sweep pulse is applied. The detecting times are set to be from

�s to 5 �s. The shutoff pulse intervals are set to be from 60 �s to00 �s. Other experimental parameters are as same as the parame-ers settings in Section 3.1. The effects of the harmful pulse shutoffarameters on machining time are shown in Fig. 13.

As shown in Fig. 13, the curve of the detecting time of 3 �s is sim-lar to that of 4 �s, which shows that the average machining timeecreases gradually as the shutoff pulse interval rises from 60 �s to80 �s, and then increases gradually as the shutoff pulse intervalises from 180 �s to 300 �s. It is thus evident that a shutoff pulsenterval of 180 �s is sufficient for gap status recovery. Meanwhile,he machining efficiency is not found to be affected negatively. Its also noted from the figure that the average machining time withhe detecting time of 4 �s is shorter than that of 3 �s. This resulthows that the detecting time of 4 �s is able to detect the harmfululses more accurately.

Fig. 13 also shows that the curve of the detecting time of 2 �ss similar to that of 5 �s, which demonstrates that the average

achining time increases gradually as the shutoff pulse intervalises from 60 �s to 300 �s. Also, the average machining times withetecting times of 2 �s and 5 �s are higher than that of 4 �s with allhe shutoff pulse intervals investigated in this experiment, whichuggests that 2 �s is a too small detecting time and 5 �s is a too bigne. When the detecting time is 2 �s, many arc pulses are omittednd are not shut off. Along with the increase in shutoff pulse inter-al, the decrease in pulse duty ratio begins to have a large impactn the drilling speed, resulting in an increase in machining time.hen the detecting time is 5 �s, many normal spark pulses areistakenly identified as arc pulses and are thus shut off. With the

ncrease in shutoff pulse interval, the pulse duty ratio decreases,

nd the average machining time increases.

In summary, 4 �s is a suitable detecting time in the sense thatarmful pulses can be detected accurately. 180 �s is an appropriate

Fig. 14. Effects of sweep pulse parameters on machining time.

shutoff pulse interval in the sense that the gap status can recoverbecomingly.

3.4. Effects of sweep pulse parameters

Sweep pulse parameters include the threshold number of suc-cessive harmful pulses and the number of the sequence of sweeppulses applied each time.

The threshold number of successive harmful pulses determinesthe timing of the application of sweep pulses. For instance, if thethreshold number is set to be 8, the sweep pulses should be appliedwhen 8 successive harmful pulses occur. A too small thresholdnumber of the successive harmful pulses cause a too frequent appli-cation of sweep pulse, which is likely to lead to a high tool wear ratebecause of the high discharge energy of the sweep pulse. By con-trast, a too large threshold number is not able to apply sweep pulsesin a timely manner such that the debris accumulation can be sweptaway.

The number of sweep pulses applied in a sequence each timedetermines the intensity of sweep pulses. If the number is too small,the debris accumulation cannot be swept away adequately. On thecontrary, if the number is too large, the tool wear rate is likely toincrease.

In order to investigate the effects of the sweep pulse parameters,a series of experiments were carried out. The threshold numbers ofsuccessive harmful pulses are set to be 4, 8 and 12. The numbers ofsweep pulses applied each time are set to be 3, 5, 8 and 12. Otherexperimental parameters are as same as the parameters settings inSection 3.1. The effects of the sweep pulse parameters on machin-ing time are shown in Fig. 14 while the effects of the sweep pulseparameters on tool wear rate are shown in Fig. 15.

As shown in Fig. 14, the machining time decreases as the num-ber of sweep pulses applied each time increases from 3 to 12, whichindicates that sweep pulses are helpful to the debris evacuation andthe increase in machining efficiency. It can be seen from the figurethat when the number of sweep pulses applied each time is 12 andthe threshold number of successive harmful pulses is 4, the machin-ing efficiency is the highest among the range of investigation.

Fig. 15 shows that the tool wear rate rises as the numberof sweep pulses applied each time increases from 3 to 12. Thisincreased TWR is due to the high discharge energy of sweep pulses.

the threshold number of successive harmful pulses decreases from12 to 4, because a lower threshold number leads to a more frequentapplication of sweep pulses.

1470 Y. Jiang et al. / Journal of Materials Processing Technology 212 (2012) 1463– 1471

3 5 8 120

10

20

30

40

50

60

Number of sweep pulses loaded each time

TW

R (

%)

4 harmful pulses8 harmful pulses12 harmful pulses

ii

3

aco

bGfswbabri

gas

sEYdttneidmos

stid

p

Fig. 16. Longitudinal profile of the deep blind small-holes side (a) and side (b).

Table 3Details of the blind small-holes.

Normal pulse Harmful pulseshutoff

Sweep pulseapplied

Entrance diameter 223 �m 210 �m 219 �mBottom diameter 122 �m 129 �m 129 �mDepth 1957 �m 2223 �m 3300 �m

Fig. 15. Effects of sweep pulse parameters on tool wear rate.

Therefore, although sweep pulses are beneficial to the machin-ng efficiency, the application of sweep pulses should be restrainedn such a manner that the tool wear rate is kept to be in a low level.

.5. Deep small-hole EDM test

In order to explore the combined capability of the twopproaches, i.e., the shutting-off of harmful pulses and the appli-ation of sweep pulses, deep small-hole EDM tests were carriedut.

Two bonding gauge blocks were used as a work piece. A gaugelock is a precision ground and lapped length measuring standard.auge blocks have very flat and good polishing surfaces, whose sur-

ace roughness Ra is less than 0.15 �m. Because of their ultra-flaturfaces, when two gauge blocks slide with respect to each otherith pressure, the air is squeezed out of the joint, and the gauge

locks are able to adhere to each other tightly due to the air pressurend molecular attraction. The blind small-holes are drilled at theonding edge of the gauge blocks. The gauge blocks are then sepa-ated after drilling. The longitudinal profile of the blind small-holess thus obtained.

The diameter of the tool electrode is 150 �m. The material of theauge block is chromium carbide. Other parameters are as sames those in Section 3.1. The longitudinal profile of the deep blindmall-holes is shown in Fig. 16.

As shown in Fig. 16(a), the blind small-hole drilled by applyingweep pulses is the deepest among the three cases. The small-holeDM processes were carried on until the feed rate is nearly zero.amazaki et al. (2005) reported that the influence of the carbonebris adhering to the tip of the rod electrode is large, which makeshe rod electrode as sharp as a needle. Therefore, it can be seen fromhe figure that the bottom diameters of the blind small-holes areoticeably smaller than the entrance diameters, because the toollectrodes are seriously damaged. As shown in Fig. 16(b), the tapern the holes is even more obvious than that in Fig. 16(a). Also, theepth of the holes shown in Fig. 16(b) is shorter, because of theisalignment between the electrode axis and the mating surface

f the two gauge blocks. The details of the blind small-holes arehown in Table 3.

As shown in Table 3, the aspect ratio is the ratio of the blindmall-hole depth to the diameter of the tool electrode. In this study,he diameter of the tool electrode is equal to 150 �m. The taper

s the ratio of the difference between the entrance and bottomiameters to the depth of the blind small-hole.

By applying sweep pulses, the debris generated in the erosionrocess is forced by the explosion effect to be evacuated from the

Aspect ratio 13.0 14.8 22.0Taper 0.052 0.036 0.027

discharge gap in deep small-hole EDM processes. Therefore, with-out vibration assistance or dielectric fluid flushing, an aspect ratioas high as 22 is achieved. This result suggests that the approaches ofshutting off harmful pulses and applying sweep pulses demonstratea great potential in small-hole EDM drilling processes.

4. Conclusion

In this paper, a pulse generator with a sweep pulse circuit wasdeveloped. The voltage of EDM pulses was detected by a high-speedcomparator. A FPGA chip was used as a master controller of thecontrol module for the determination of pulse discharge status andthe MOSFET switching. A series of experiments were carried outto examine the machining performance when shutting off harmfulpulses and applying sweep pulses. Some of the major conclusionsare given as follows.

1. Shutting off harmful pulses is helpful in improving the machin-ing efficiency and decreasing the tool wear rate. The applicationof sweep pulses further improves the machining efficiency,

essing

2

3

4

5

A

doP

R

B

Y. Jiang et al. / Journal of Materials Proc

whereas the tool wear rate is higher than that by shutting offharmful pulses.

. The quality of the through small-hole drilled by shutting offharmful pulses is better than that by using normal pulses. Thetaper of the through small-hole drilled by applying sweep pulsesis higher than that by shutting off harmful pulses. Also the edgesof the hole by shutting harmful pulses are cleaner than those bynormal pulses.

. It is found that the percentage of arc pulses during an EDMprocess is diminished significantly by applying sweep pulses,which indicates that sweep pulses are conducive to the evac-uation of debris and the improvement in gap status. As a result,the machining efficiency is improved.

. The effects of the parameters for both harmful pulse and sweeppulse are studied. The optimal detecting time and the optimalshutoff pulse interval are chosen. Though being beneficial to themachining efficiency, sweep pulses must be used with limit suchthat the tool wear rate is at an acceptable level.

. Shutting off harmful pulses and applying sweep pulses areconducive to obtaining high aspect ratio small-holes. A deepsmall-hole with an aspect ratio as high as 22 is achieved withoutvibration assistance or dielectric fluid flushing.

cknowledgements

This paper is financially supported by the National Science Foun-ation of China with the grant number: 50635040, and the Ministryf Science and Technology of China (project of the “863 High-Techlan”) with the grant number: 2009-AA044205.

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