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Effect of gap region medium on acoustic emission wave bysingle pulse discharge
Yoshiaki Akematsu, Kazuro Kageyama, Naotake Mohri and Hideaki Murayama
Dept. of Environmental & Ocean Engineering, School of Eng., The Univ. of Tokyo
Address: 7-3-1, Hongo, Bunkyo-ku, Tokyo, 113-8656, JAPANE-Mail: akematsu@giso.t.u-tokyo.ac.jp

Abstract: In this study, the mechanism of material removal in electrical discharge machining (EDM)
was investigated by acoustic emission (AE) technique. Twice burst AE wave was detected by opticalfiber sensor during single pulse discharge.
Copyright 2006 Optical Society of America
OCIS code: (320.3980) Microsecond phenomena; (320.7120) Ultrafast phenomena

1.

Introduction

Electrical discharge machining is high-precision processing technology. Recently, electrical dischargemachining (EDM) is widely used for manufacturing various kinds of die and molds. Especially, it is a machining
method used for hard metals or those that would be impossible to machine with traditional techniques. EDM is a
nontraditional method of removing material by a series of rapidly electric discharges between an electrode andthe work piece. Electrical discharge was the sudden and momentary electric current flows. The EDM cutting
tool is guided along the desired path very close to the work piece but it does not touch the work piece.
Consecutive sparks produce a series of micro-craters on the work piece and remove material along the cutting
path by melting and vaporization [1]. The particles were washed away by the continuously flushing dielectric
fluid. However, the mechanism of material removal in EDM has not been sufficiently clarified, because
discharge is short distance and time phenomenon [2]. In order to clarify the mechanism, acoustic emission (AE)method has been applied [3]. But it is difficult to detect the AE wave during a discharge because of piezo
electric sensor is usually affected on electrical noise. In 2001, optical fiber sensor which was not affected onelectrical noise was developed in our laboratory [4, 5]. So, this sensor was applied to detect the AE wave during
single pulse discharge. In this study, it was investigated that effect of gap region medium on AE wave by singlepulse discharge.
2.

Experimental setup

Figure 1 shows schematic view of opticalfiber

sensorsystem.

Laser

Coupler

Doppler

Velocimeter (LDV) is used to detect thenm),

andheterodyne

interference technique. An acousto-opticalmodulator (AOM) changes the frequencyof the reference light source from f0 toLight Source

AOM
Couplerlaser (output power; 1mW, wavelength.
632.8

f0

Sensorfrequency shift. Light source is He-Ne
λ0:

Half Mirror

Optical Fiberf0+ fMfM+ fDf0+ fD
Half Mirror

Detector

Fig. 1 Schematic view of optical fiber sensor systemOutput

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f0+fM (fM = 80 MHz) in order to produce
beating signals with frequency of fD+fM.
Optical fibers consist of core, cladding andcoating. The core has a higher index ofrefraction than the cladding, so that totalreflection

occursatthecore–cladding

interface. We used single-mode optical fibers
with a coating diameter of 150µm and acladding diameter of 125µm. The core
diameter of the single-mode fiber is small(4.2µm), so we employed single-mode fibers
in optical fiber sensor to improve sensitivity.

Fig. 2 Schematic view of optical fiber sensor

The split light was fed into each sensor andthe transmitted light signals were detected

200mm
200mm

using photodiodes. The fiber-optic signals

m0m
04
Aluminum plate
Thickness 1mm

received by the photodiode were transmitted
to a PC. Manufactured optical fiber sensors
were attached to the surface of the specimens.
In order to make several µs discharge

500mm

duration, condenser circuit was applied.

Optical fiber sensor
Diameter 16mm,
Number of loop 2Condenser capacity was changed from
18.8nF to 300nF in order to changeFig. 3 Experimental setup

discharge duration from 0.5µs to 2µs.The current flowing through the anode is monitored with a current probe(Pearson Electronics, Model-110). The applied voltage was 100V, which is defined by the charging voltage ofthe capacitor. Discharge interval time was controlled several 10ms by 1MΩ resistance. Figure 2 show the opticalfiber sensor. Optical fiber sensor (Radius 8mm, Number of loop 2) was bonded on the aluminum plate. Figure 3shows the experimental setup. The electrical discharges were made to fall on aluminum (1000 series) plate
400mm * 400mm * 1mm. The optical fiber
3024analysis were carried out with 12 bit resolution
and sampling rate of 10MHz.
3.

Results and discussion

3.1 Condenser characteristics

Current [A]aluminum plate so as avoid the effect ofreflection of elastic wave. Data acquisition and80
Current
Voltage

60

1840

1220

600-20-6Figure 4-400current signal goes up 25A and down -4Aduring 2µs. The voltage goes down from 75V
to -20V during 2µs. It was typical behavior ofcondenser discharge. It is reported thatdischarge duration is proportional to multiply
inductance by condenser capacity of root [6].
Discharge duration is expressed Eq. (1).

246810Time [µs]shows the typical waveforms of current andvoltage signal, when electrode was anode. The

Voltage [V]sensor was located at the center of anFig. 4 Typical waveform of the discharge current and voltage

Table 1 Discharge condition
C: Condenser
capacity [µF]

τp: Pulse duration [µs]L:Inductance [µH]18.8×10-3

0.5

1.3

64.8×10-3

1.01.6300×10-3

2.0

1.4

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τ p = π (LC )1 2(1)Where τ p is discharge duration [µs], L is inductance [µH] and C is condenser capacity [µF]. Relation between
discharge duration, inductance and condenser capacity shows Table 1. Condenser capacity increased from
18.8nF to 300nF as inductance was almost constant 1.6µH. It was considered that there were no affect oncondenser circuit characteristics. Moreover, we can easily find the time of electrical discharge.
3.2 Effect of gap region medium

In order to research the effect of gap region medium on AE wave bysingle pulse discharge, gap region medium were selected air and oil. Electrical discharge duration was changed
from 0.5µs to 2µs. Figure 5 shows the typical AE waveforms detected by optical fiber sensor and current signals.
The burst AE wave was detected at 1.7µs after single pulse discharge both in air and in oil. It was thepropagation time which longitudinal wave propagates in the aluminum plate. It was found that the burst AEwave was occurred by single pulse discharge. In the case of electrical discharge in air, the burst AE wave wasdetected only once. Displacement velocity converged on zero after occurrence of the burst AE wave. On theother hand, in the case of electrical discharge in oil, the burst AE waves were detected several times.
Displacement velocity did not converge on zero after occurrence of the burst AE wave. Second burst AE wave
was occurred between several 10µs and several 100µs. Moreover, it was found that first burst AE wave andsecond burst AE wave behavior was almost same. It was reported that several burst AE wave was occurred bycavitations [7, 8]. Single pulse discharge was occurred near aluminum plate without direct electrical discharge
on the plate.
660.1

434020260

Velocity [mm/s]-0.1

Pulse Duration 0.5µs, In Oil

Current [A]-30

Optical fiber
Current

Optical fiber
Current-0.2050100

150-2200-6-2050

Time [µs]10245018-0.2

12Optical fiber
Current-0.6-0.8050

100Time [µs]1506

Velocity [mm/s]

Velocity [mm/s]

30Current [A]Pulse Duration 2µs, In Air-0.4150200

Time [µs]0.40.2100

Pulse Duration 2µs, In Oil

24018-512Optical fiber
Current-100-15-6-20

200

30

60-6050100

Time [µs]Fig. 5 Detected AE waveforms by optical fiber sensor and current transducer

150200Current [A]

Velocity [mm/s]

Pulse Duration 0.5µs, In Air

Current [A]0.2

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3.3 Cavitations

Figure 6 shows model of experimental setup to detect AE wave occurred by cavitations.

Using two cylindrical electrode rods, one was anode and another was cathode, single pulse discharge wasoccurred near aluminum plate at the center of optical fiber sensor. Figure 7 shows burst AE wave in the case ofdischarge duration was 2µs. There were several burst AE wave as same as cathode was aluminum plate.
Displacement velocity converged on zero after occurrence of the burst AE wave. Second burst AE wave wasoccurred after 80µs at first burst AE wave. It was considered that the reason of occurrence several times burst
AE wave was cavitations behavior.

Cu (+)

OilCu (-)
Insulating material

Aluminum plate

Velocity [mm/s]

10Optical fiber

50-5-10-15-20050

100150Fig. 6 Model of experimental setup to detect AE wave occurred byTime [µs]cavitations

Fig. 7 Detected AE waveforms by optical fiber sensor

4.

200Conclusion

In this study, it was investigated that effect of gap region medium on elastic wave by single pulse discharge. As
the results, it was found that burst AE wave was detected in air and in oil without gap region medium. In thecase of electrical discharge in air, the burst AE wave was detected only once. On the other hand, in the case ofelectrical discharge in oil, the burst AE waves were detected several times. Moreover, single pulse discharge wasoccurred near aluminum plate without direct electrical discharge on the plate. There were several burst AE wave
as same as cathode was aluminum plate. It was considered that the reason of occurrence several times burst AE
wave was cavitations behavior.
5.

Acknowledgment

The author was supported through the 21st Century COE Program, “Mechanical Systems Innovation,” by theMinistry of Education, Science and Technology.
6.

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Materials and Structures, 14, 2005, S52-59[5] K. Kageyama et al., : Doppler Effect in Flexible and Expandable Light-Waveguide and Development of New Fiber-Optic
Vibration/Acoustic Sensor, IEEE Journal of Light Wave Technology, Vol. 24 (4), 2006, pp.1768-1775[6] Nagao Saito et al., : Technique of electrical discharge machining, NIKKAN KOUGYOU, 1997, p.35
[7] M. Tubota et al., : Dynamic of Laser – Induced Bubble in Pressurized Liquid Nitrogen, J. Soc. of Mechanical Engineers B, Vol. 61(582),
1995, pp. 365-371[8] X. Chen et al., : Optical investigation of cavitation erosion by laser-induced bubble collapse, Optics & Laser Technology, 36, 2004, pp.
197-203