Advancing EDM through Fundamental Insight into the Process

1 2 3 4
M. Kunieda (2), B. Lauwers (2), K. P. Rajurkar (1), B. M. Schumacher (1)
1
Tokyo University of Agriculture and Technology, Japan
2
Katholieke Universiteit Leuven, Belgium
3
University of Nebraska-Lincoln, USA
4
University of Applied Science St Gallen, Switzerland

Abstract
This paper aims to show the prospects of electrical discharge machining (EDM) technology by interrelating
recent achievements in fundamental studies on EDM with newly developed advanced application technologies.
Although gap phenomena in EDM are very complicated and hence not yet very well understood, recent
improvements in computers and electronic measuring instruments are contributing to new discoveries and
inventions in EDM technology. Such newly acquired insight sometimes raises questions on the validity of the
established theories of EDM phenomena, and EDM processes once believed to be impossible or unrealistic are
now becoming practical.

Keywords:
Electrical discharge machining (EDM), Wire electrical discharge machining (WEDM)

1 INTRODUCTION theories and commonly accepted EDM phenomena.

During the 1930s, attempts were made for the first time to Dramatic improvements in machining characteristics are
machine metals and diamonds with electrical discharge. being achieved, and EDM processes believed to be
V.E. Matulaitis and H.V. Harding of Elox US developed impossible or unrealistic before are now becoming practical.
“Disintegrators” to remove broken taps from valuable Furthermore, applications of EDM phenomena other than
workpiece materials such as cemented carbide and high material removal are also being developed. This paper
speed steel. Erosion was caused by intermittent arc aims to demonstrate the future prospects of EDM
discharges occurring in air between the tool electrode and technology by linking recent achievements in fundamental
workpiece connected to a DC power supply. Arc discharges studies with newly developed advanced application
were initiated through short mechanical contact like welding technologies.
arcs and were interrupted by retraction using vibration of
the tool electrode. AEG developed equipment capable of 2 OVERVIEW OF EDM
eroding diamond using the heat generated by discharges 2.1 Principle
occurring at high frequencies in the gap between a pair of Figure 1 shows the concept of EDM. Pulsed arc discharges
tool electrodes. occur in the “gap” filled with an insulating medium,
These processes are not very precise due to overheating of preferably a dielectric liquid like hydrocarbon oil or
the machining area and are defined as “Arc-Machining”, de-ionized (de-mineralized) water between tool electrode
while “Spark Machining”, as discussed in the following, and workpiece. The insulating effect of the dielectric
works precisely because the workpiece is protected from medium has some importance in avoiding electrolysis
excessive heat attacks. This distinction is apparent from effects on the electrodes during an EDM process. As the
the shop floor application of spark machining. However, it is electrode shape is copied with an offset equal to the
difficult to distinguish between arc machining and spark gap-size, the liquid should be selected to minimize the gap
machining from the physical point of view. (10-100µm) to obtain precise machining. On the other hand
During World War II physicists B.R. and N.I. Lazarenko in a certain gap width is needed to avoid short circuiting,
Moscow conducted studies on the minimisation of wear on especially when electrodes that are sensitive to vibration
electric power contacts. They tested different materials with (like wire-electrodes) or deformation are used. The ignition
discharges of defined energy, generated by a capacitor. of the discharge is initiated by a high voltage, overcoming
B.R. Lazarenko published the paper “To invert the effect of the dielectric breakdown strength of the small gap. A
wear on electric power contacts”, in 1943 [1]. This idea channel of plasma (ionized, electrically conductive gas with
started the development of Electrical Discharge Machining high temperature) is formed between the electrodes and
(EDM), using controlled discharge conditions, for achieving develops further with discharge duration. As the metal
precision machining [2]. removal per discharge is very small, discharges should
occur at high frequencies (103 -106 Hz). For every pulse,
Since then, EDM technology has developed rapidly and discharge occurs at a single location where the electrode
become indispensable in manufacturing applications such
as die and mold making, micro-machining, prototyping, etc. Feed
However, EDM gap phenomena are very complicated and Dielectric liquid Bubbles
hence not yet fully understood. Understanding arc
discharges even in the steady state continues to be a
difficult problem. Electrical discharge phenomena in EDM
occur over a very short time period, in a very narrow space Tool electrode
filled with liquid, and involve evaporation and melting of the
electrodes, thus making both observation and theoretical Workpiece
analysis extremely difficult.
On the other hand, recent developments in computer Gap width: several
technology and measuring and analyzing instruments are Discharge current tens of micrometer
accelerating new findings and knowledge. This trend Discharge column
sometimes leads to results conflicting with established
Figure 1： Concept of EDM.
materials are evaporated and/or ejected in the molten peak value over a short duration are used, current is
phase. As a result, a small crater is generated both on the supplied through both the upper and lower feeding brushes
tool electrode and workpiece surfaces. Removed materials to obtain a quick rise in the discharge current by reducing
are cooled and resolidified in the dielectric liquid forming the inductance and to avoid wire breakage due to Joule
several hundreds of spherical debris particles, which are heating. Tension is applied to the wire to reduce vibration
then flushed away from the gap by the dielectric flow. and deflection, which deteriorates cutting accuracy.
After the end of the discharge duration, the temperature of In WEDM, water is most often used as the dielectric liquid,
the plasma and the electrode surfaces contacting the but its specific electrical conductivity should be decreased
plasma rapidly drops, resulting in a recombination of ions using de-ionizing resins to avoid electrolysis and to keep
and electrons and a recovery of the dielectric breakdown high open voltage. Use of de-ionized water is much
strength. To obtain stable conditions in EDM, it is essential preferable to hydrocarbon dielectrics considering fire
for the next pulse discharge to occur at a spot distanced hazards. In sinking EDM, however, hydrocarbon dielectrics
sufficiently far from the previous discharge location. Such a are normally used because surface roughness is better and
spot may be the place where the gap is small or tool electrode wear is lower compared to de-ionized water.
contaminated with debris particles which may weaken the 2.3 Characteristics and applications
dielectric breakdown strength of the liquid. Accordingly, the
interval time between pulse discharges must be sufficiently Since EDM is a thermal process, hard materials such as
long so that the plasma generated by the previous quenched steel, cemented carbide, and electrically
discharge can be deionized and the dielectric breakdown conductive ceramics can be machined. EDM also allows
strength around the previous discharge location can be machining of complicated shapes. Since the tool electrode
recovered by the time the next pulse voltage is applied. does not need to rotate for material removal like milling or
Otherwise discharges occur at the same location for every grinding, holes with sharp corners and irregular contours
pulse, resulting in thermal overheating and a non uniform can be machined without difficulty. Reaction forces
erosion of the workpiece. generated in the EDM gap are insignificant, which also
facilitates the machining of thin and flexible parts, and deep
2.2 Sinking EDM and Wire EDM grooves and holes which are difficult to machine by milling.
Figure 2 shows the configuration of a sinking electrical Generally, machining accuracy of EDM is very high in the
discharge machine. The workpiece can be formed, either order of several micrometers, and achievable surface
by replication of a shaped tool electrode, or by 3D roughness is Rz 0.4µm. On the other hand, the material
movement of a simple electrode like in milling or a removal rate of EDM is low compared to other machining
combination of the above. The electrode material is processes. Hence EDM is mostly used in die and mold
normally copper or graphite. The numerical control making, where complicated shapes in hard materials and
monitors the gap conditions (voltage and current) and with high precision have to be machined. In spite of
synchronously controls the different axes and the pulse decreased opportunities for the EDM process to be
generator. The dielectric liquid is filtrated to remove debris adopted in the manufacturing processes of certain dies and
particles and decomposition products. molds as a result of the recent improvement of high-speed
Figure 3 outlines the wire electrical discharge machining milling technology, EDM of micro-parts has paved the way
(WEDM) method. Complicated shapes can be cut using a for expanding application fields.
wire electrode of 0.02 to 0.33mm in diameter like a wire 2.4 Pulse generator
sawing machine. The wire electrode is usually a plain brass
wire or coated wires, such as zinc coated brass or coated Early EDM equipment used relaxation type pulse
steel wires. Sometimes tungsten or molybdenum wires are generators with capacitor discharges as shown in Figure 4.
used in case of thin wires. Since wire orientation can be This type of equipment has been used especially in WEDM
changed by controlling the horizontal position of the upper until recent years because discharge current with high peak
wire guide relative to the lower guide, all types of ruled values and short duration is needed in WEDM due to the
surfaces can be cut. Since discharge currents with a high reasons described in Section 5.1. With improved capability
of power transistors which can handle large currents with
Gap condition high response, the relaxation type was replaced by the
analyzer transistor type shown in Figure 5. However, the relaxation
type pulse generators are still being used in finishing and
micro-machining because it is difficult to obtain significantly
Z-axis short pulse duration with constant pulse energy using the
NC Tank Filter transistor type pulse generator as described in Section 6.1.
Pulse
In Figure 5, a series of resistances and transistors are
generator
connected in parallel between the direct current power
XY table supply and the discharge gap. Suppose one transistor is
switched on and a discharge occurs, since the discharge
Pumping system voltage is around 20V, the discharge current is 4A in this
Figure 2: Sinking electrical discharge machine. case. Here the discharge voltage is almost constant at 20V
regardless of the discharge current, because a larger
Wire electrode current brings about a higher degree of ionization and
greater diameter of the arc column resulting in increased
electrical conductivity of the plasma. Hence, the discharge
Upper wire Upper feeding brush current increases proportionally to the number of
guide
Workpiece (+)
R
C
Tool electrode
Lower feeding brush
Charge Discharge Workpiece
Lower wire guide

Figure 3: Wire electrical discharge machining (WEDM). Figure 4: Relaxation type pulse generator.
 Gap voltage
Gate control circuit
Long delay Forward

0V
Time
Large gap

Gap voltage Gap voltage

Proper delay

20 Ω
100V Tool electrode Time Proper gap
Short delay
Power supply Workpiece Retract

Time
Figure 5: Transistor type pulse generator. Small gap

Ignition delay time td Figure 7: Principle of servo feed control.

Pulse interval to rate. Advanced electrical discharge machines can control
the discharge interval adaptively by monitoring the gap
Gap voltage Open voltage uo
voltage waveforms.
≒100V
0V 2.5 Servo feed control (Gap control)
Discharge voltage
ue = 20V Time Tool electrode feed is not constant like in conventional
Current Discharge machining methods. The servo feed control shown in
0A current ie Figure 7 keeps the working gap at a proper width. Larger
gap widths cause longer ignition delays, resulting in a
Discharge duration te Time higher average gap voltage. When the measured average
(µs – ms) gap voltage is higher than the servo reference voltage
Figure 6: Gap voltage and current waveforms. preset by the operator, the feed speed increases. On the
contrary, the feed speed decreases or the electrode is
transistors which are switched on at the same time. retracted when the average gap voltage is lower than the
Figure 6 shows voltage and current waveforms measured servo reference voltage, which is the case for smaller gap
at the gap, which are typical in sinking EDM operations. widths resulting in a smaller ignition delay. Thus short
When the transistors are switched on, the open voltage uo, circuits caused by debris particles and humps of discharge
100V in the case of Figure 5, is applied between the tool craters can be avoided. Also quick changes in the working
electrode and workpiece, but discharge does not occur surface area, when tool electrode shapes are complicated,
immediately but occurs after the ignition delay time, td. This does not result in hazardous machining. In some cases, the
is because dielectric breakdown “physically” needs a average ignition delay time is used in place of the average
statistical time lag in which initial electrons should be gap voltage to monitor the gap width [5].
generated by the bombardments of cosmic, radioactive,
ultraviolet, and X rays with the neutral species in the gap, 3 GAP PHENOMENA
and the formative time lag in which the initial electrons are 3.1 Single pulse discharge
accelerated to generate the electron avalanche [3]. In EDM, EDM gap phenomena are usually depicted as shown in
since the dielectric liquid is contaminated with electrically Figure 8. The arc column is depicted with a diameter
conductive debris particles, the movement of debris smaller than the gap width. Debris particles are very fine
particles in the gap should be considered. Bommeli et al. like dust and minute bubbles are suspended in the gap.
[4] statistically investigated the distribution of ignition delay However, this sketch is not entirely correct. As described
time to demonstrate that discharge occurs after an later, the gap is mostly occupied with gas and the diameter
activation time which is “practically” needed in addition to of debris particles is in the same order as the gap width and
the statistical and formative time lags for debris particles to surface roughness of the electrode surfaces as
form bridges between electrodes. Since dielectric schematically shown in Figure 9. Dielectric breakdown in
breakdown strength decreases with the decrease of the consecutive pulse discharges therefore occurs at the
gap width and increase of the density of debris particles, boundary between bubbles, where debris particles are
the ignition delay time reflects the gap conditions. condensed, or inside the bubble.
After the dielectric breakdown, a discharge current, ie, is Figure 10 shows a schematic view of the discharge spot.
passed through the gap. The gate control circuit keeps the The arc column diameter is considered to increase with the
transistors on for the discharge duration, te, after the passage of time [6-8] and equal to the diameter of the
dielectric breakdown, resulting in a uniform discharge generated discharge crater [6]. If this is true, the diameter
crater size independent of the ignition delay time which can of the arc column measures several times larger than the
vary statistically for each discharge. Then after the fixed gap width. Electrode materials and dielectric liquid are
discharge interval, to, the transistors are again switched on evaporated, molecules are dissociated, and atoms are
and open voltage is applied between the electrodes. ionized, resulting in a rapid expansion of the bubble. Since
The discharge energy per single pulse q is expressed as, the expansion is restricted by the inertia and viscosity of the
dielectric liquid, the pressure inside the bubble becomes
q = ue × i e × t e (1) extremely high and the boundary between the bubble and
where ue is the discharge voltage, which is around 20V. liquid expands with the velocity of several tens m/s [9, 10].
Hence discharge current and discharge duration are set by It is still believed that the dielectric liquid plays a significant
operators considering whether the process is for a roughing role in material removal because the high pressure and
or a finishing operation. To obtain stable machining, the velocity field in the bubble may serve as the dynamics of
discharge interval should be sufficiently long so that the the material removal in EDM [11, 12].
plasma is extinguished and hence the dielectric breakdown After the end of the discharge duration, ions and electrons
strength is recovered during the interval. However, too long are recombined and the dielectric breakdown strength is
an interval results in a decrease of the material removal recovered. The evaporated atoms and molecules are
 Discharge column Bubble role in flushing debris particles and cooling of the gap.
3.2 EDM plasma
Initial electrons, which are generated by the ultraviolet ray,
x-ray, cosmic ray and radiation from the earth crust, are
Melted pool accelerated by the electric field and ionize the neutral
Dielectric breakdown Melting and Expansion of bubble
evaporating
species due to collision, resulting in an electron avalanche.
Thus, the electric field is distorted and streamers are
developed toward both the anode and cathode, resulting in
Debris
an established discharge [3]. In EDM, since discharge
Bubble duration is normally over several µs and current density is
Recovery of dielectric Discharge interval
108 - 109A/m2, the established discharge is an arc
strength
?
Figure 8: Model of EDM gap phenomena.
discharge. An arc discharge is sustained by electron
emission from the cathode spot which is due not only to the
secondary emission but also to the thermionic and field
emissions. Since both temperature and electrical field are
Debris particles strong, the emission process is strongly dependent on both
variables (T-F theory) [14]. Thus the plasma is highly
Tool electrode ionized resulting in high current densities with the
comparatively low discharge voltage of about 20 V. The
discharge voltage is composed of an anode, cathode and
Bubble Bubble channel voltage drop, and its value will slightly change
depending on electrode materials, dielectric fluids, gap
width and pulse conditions.
Because of the complicated physical phenomena occurring
in the discharge gap, precise analysis of the EDM plasma
Workpiece is difficult. Eubank et al. [15] analyzed the expansion of the
cylindrical plasma considering the evaporation of water
Dielectric fluid Tar dielectric and enthalpy increase in plasma due to
dissociation and ionization of water. The fraction of energy
Figure 9: Schematic view of discharge gap. distributed to the plasma was obtained by subtracting the
fractions of the energy transferring to the anode and
Tool electrode cathode from 100%. The fractions of the anode and
Melted zone
cathode were obtained from comparison between the
Debris Bubble
measured material removal per pulse discharge and
molten material volume calculated using the point
Liquid heat-source on cathode model [16] and expanding circular
heat-source on anode model [17]. However, the plasma
itself was not analyzed.
Discharge current Discharge plasma
Melted zone
Hayakawa et al. [18-20] first conducted
Workpiece
magnetohydrodynamics analysis in the steady state of a
DC arc between parallel plane copper electrodes. The arc
Figure 10: Discharge phenomena in EDM gap. was assumed to be in air under a constant discharge
Diameter reaches several millimeters
current and gap width as those used in the actual EDM
process. They assumed that the species in
Bubble
high-temperature air which includes copper electrode vapor
Debris are N2, O2, NO, N, O, Cu, NO+, N+, O+, Cu+, N2+, O2+, Cu2+,
Tool electrode
and electrons. Considering the temperature dependence of
the thermophysical properties of the plasma, the
Boundary Discharge spot electromagnetic field, temperature, pressure, and velocity
Workpiece Boundary distributions were calculated for the regions including both
the electrodes and discharge gap. The conservation
Figure 11: Bubble and debris particles generated by
equations of mass, momentum and energy, Ohm’s law and
single pulse discharge.
Maxwell’s equations were solved. The energy equation
solidified or condensed to form debris particles or dielectric contained the Joule heating, conduction, convection, and
liquid, but gases such as hydrogen and methane which are radiation terms. Figure 12 shows an example of the
generated by the dissociation of the working oil are left to calculated temperature fields. It was found that most of the
form a bubble. The diameter of the bubble reaches several discharge power is distributed in the electrodes, and heat
millimeters, several tens of times larger than the gap width transfer due to convection and radiation is negligible. It was
as shown in Figure 11. Since pulse discharge occurs also found that the plasma is extinguished within a few
several thousand times or more per second, obviously the microseconds after the end of the discharge duration as
gap becomes filled with gas in typical EDM processes [12]. shown in Figure 13 [18,20]. However, the arc which they
Hundreds of debris particles are generated per single pulse analyzed was not in dielectric liquid but in air. The arc was
discharge, blown off through the bubble generated by the not in transient but in steady state, and removal of the
discharge and stopped at the boundary of the bubble due electrodes, i.e. mass transfer from the electrode surfaces,
to the viscosity of the dielectric liquid [13]. Melted and was not taken into consideration. Furthermore, the arc was
evaporated materials are cooled by the dielectric liquid and assumed to be in thermoequilibrium, and the equations of
solidified to form spherical debris particles. Thus debris motion of the three species: electrons, ions, and neutral
particles are removed from the gap with the dielectric liquid particles, were not solved separately. The gap phenomena
and not reattached on the electrode surfaces. Furthermore, were simplified to be symmetrical between the anode and
heat convection in the boundary layers of the dielectric the cathode regions. Therefore, the fractions of energy
liquid cools the electrode surfaces, resulting in machining distributed to anode and cathode were equal.
stability. Thus, the dielectric liquid plays another important
 Removal amount, ×10-9 g/pulse
16000

Temperature [K] [K]

ie = 25 A
Temperature 12000

8000 anode (Cu)

cathode (Cu)
4000

0
0 AnoAnode
de
100
Gap
RadRiu
adius 200 Gap
s [μ[μm] 300 Catho
m] Cathode
de
Pulse duration, te µs
Figure 12: Temperature distribution in electrodes and
Figure 14: Relationship between removal amount and
gap obtained by magnetohydrodynamics analysis.
pulse duration.
10000

0.1mm
Temperature [K] [K]

Calculation model
Temperature

Heat flux q”
5000 Heat source area A
Measuring point
Fe
Fe
Foil electrode
Cu
Cu
Rod electrode
Foil electrode
0 Experimental Constantan wire
0 1 2 3 4 5
setup (φ0.1mm)
Time [µs] Thermocouple
Time [µs]
Figure 13: Calculated plasma temperature decrease
during discharge interval when material used for both Amplifier
anode and cathode is steel or copper. Figure 15: Method to obtain energy distributed
into electrode in single pulse discharge.
3.3 Energy distribution
The anode and cathode removal amounts are greatly
Energy distribution ratio

dependent on the discharge duration, and they are quite

different even when the materials of the two electrodes are
the same. Figure 14, for example, shows the relationship
between removal amount per pulse and discharge duration
obtained when copper was used both for anode and
cathode [21]. The anode removal amount is greater than
the cathode one for discharge duration shorter than 20µs, anode ie = 25 A
while it is smaller with the discharge duration longer than cathode
20µs. Motoki and Hashiguchi [22], and Van Dijck [23] Gap = 40 μm
explained that such a phenomenon is caused by the
variation of the energies distributed into the anode and
Pulse duration, te µs
cathode with the discharge duration, on the basis of the T-F
electron emission theory [14]. Figure 16: Relationship between energy distribution
Hence it is important to measure the energy distribution ratio and pulse duration.
between the anode, cathode and dielectric fluid. Koenig et It was concluded that the difference in removal amount
al. [24] measured the energy distribution by measuring the between anode and cathode is mainly caused by the
temperatures of the electrodes and dielectric fluid. Xia et al. phenomenon of carbon adhesion onto the anode surface,
[21,25,26] measured the energy distribution by comparing not by energy distribution, when hydrocarbon dielectrics
the measured temperatures of the electrodes with the are used and both anode and cathode are made of copper.
calculated results obtained under the assumed ratio of the Motoki et al. [28] and Ikai et al. [29] also explained that the
energy distributed in electrodes, using the calculation electrode wear amount is reduced due to the protective
model and experimental setup shown in Figure 15. When effects of carbon layer with high boiling temperature and
the calculated temperature agreed with the measured one, high thermal resistance. Mohri et al. [30] examined the
they found that the estimated energy distribution was catalysis and structure of the carbon layer generated in the
correct. Figure 16 shows the energy distribution obtained EDM process. Kunieda et al. [31] measured copper vapor
by Xia et al. when copper was used for both anode and density in the arc plasma using spectroscopic analysis and
cathode under the same pulse conditions as in Figure 14. proved that the carbon layer protects the copper anode
The energy distribution to anode and cathode is about 40% from wear based on the fact that the deposition of thicker
and 25%, respectively. They reported that the energy carbon layer on the copper anode results in lower copper
distributed to anode is always greater than that to cathode, vapor density.
and is rarely affected by the discharge duration, in both
single discharge [21] and continuous pulse discharges [26]. Figure 17 shows the energy distribution in the gap
measured by Xia et al. [26] in the continuous pulse
It is obvious that the difference in amount of material discharge. Ratio of energy used for removal of electrodes
removal cannot be fully explained by the energy distribution. is significantly low (1%), and most of the heat is lost by
Xia et al. [21,25] and Natsu et al. [27] investigated the conduction into the electrodes. Van Dijck [23] also indicated
influence of carbon adhesion to the anode on the removal that more than 90% of the heat is conducted into the
amount under different machining conditions, such as electrodes and that the ratio of the volume of metal ejected
discharge duration, polarity, and dielectric flushing flow rate. to the volume of metal melted is only 1 to 10%.
 Energy into tool electrode required to obtain stable machining.
Energy into workpiece
Energy 34% 3.5 Thermal aspects
Loss due to
carried away convection To obtain the relationship between pulse conditions and
48% by debris and radiation material removal rate, many attempts have been made to
Loss into 1%
Total discharge 3% calculate temperature distribution in the electrodes caused
gap
energy
18% by a single pulse discharge by solving time-dependent heat
30% transfer equations assuming various heat source models.
34%
Loss due to By integrating the solution for a point source which is
conduction liberated instantaneously at a given point and time with
Energy into workpiece
regard to appropriate space and time variables, one can
(a) Distribution of discharge (b) Energy distribution in
energy workpiece
obtain solutions for instantaneous and continuous sources
of any spatial configuration (Carslaw and Jaeger [36]). The
Figure 17: Energy distribution in EDM process analytical solutions of the temperature distributions at time t
(Ie:16A, te:100μs , Cu(+) – Cu(-)). produced by stationary Gaussian energy distribution
sources in a semi-infinite solid were given by Pittaway [37].
It is also found from Figure 17 that 18% of the energy goes With the advent of powerful computers and numerical
to the discharge gap. In the magnetohydrodynamics methods, it is nowadays not difficult to take into account the
analysis of the steady state arc, however, Hayakawa et al. temperature dependence of thermophysical properties of
[20] found that almost all the discharge power is conducted electrode materials and latent heat of melting and
into the electrodes and the heat dissipated by convection vaporization. It is also possible to consider the time
and radiation is negligible. This is because the arc column dependent radius of the circular heat source and time
is established in steady state and both convection and dependent heat flux, using the principle of superposition
radiation are insignificant in the narrow gap between based on the linearity of the heat conduction problem.
parallel plane electrodes. This result agrees with the
distribution measured when discharge duration was several It should be noted, however, that boundary conditions, such
thousand times longer than the duration actually used in as the time dependent function of plasma radius, spatial
EDM [20]. The power distributed into the gap with the configuration of the heat flux, distribution of heat flux in the
actual discharge duration is considerably higher than that in heat source, and ratios of energy distributed to anode and
the steady arc. Thus it was concluded that the gap cathode, also exert significant influence on the calculation
condition is not in equilibrium during the actual discharge results. Snoeys and Van Dijck [7,8,23] systematically
duration in EDM, and a large fraction (18%) of the analyzed the temperature distribution assuming a circular
discharge power is consumed in the formation of plasma heat source with time dependent radius and time
through ionization, excitation, dissociation, and dependent heat flux on a semi-infinite cylinder. The heat
polymerization. So it means the plasma formation is still source growth function was obtained from an iterative
going on. calculation of the heat conduction equation by assuming
that the temperature at the center of the heat source
3.4 Measurement of plasma temperature corresponds to the metal boiling temperature at a pressure
The arc plasma temperature in EDM was measured using equal to an average pressure in the gas bubble calculated
spectroscopic analysis, and it was found that the from the thermodynamical model. Good agreement was
temperature reaches 6000 to 7000K [32-35]. Figure 18 found between melting point isothermals calculated and
shows the change in measured temperatures during and those measured from the pictures of the cross sections of
after a discharge when a copper electrode of 2mm in craters cut perpendicular to the surface. Since boiling of
diameter was used both for anode and cathode under the superheated metal is suppressed by the bubble pressure
following pulse conditions: discharge current: 40A; during the discharge duration, the most significant removal
discharge duration: 300µs. It is found that the deionization was assumed to be associated with ejection of liquid metal
time for a 0.5mm gap width is considerably shorter than for by bulk boiling inside the normal boiling point isothermal
1.5mm. This is because a wide gap results in a small surface, at the end of the discharge. The calculated
temperature gradient and a great heat capacity of the superheated volume agreed well with the measured
plasma. On the other hand, Hashimoto et al. [34] found that material removed per pulse. It was also found that the
the ignition delay time of the following pulse discharge metal removal efficiency, which was defined as the ratio of
becomes zero when the discharge interval after the the ejected to melted volume, was 0.01 to 0.1.
preceding pulse discharge was shorter than 5µs. They also However, with regard to the heat source configuration,
found that the plasma temperature measured at 5µs after some papers assume a point heat source [16], and many
the cease of discharge current was below 5000K. The other papers consider that the diameter of the heat source
calculated temperature decrease during the discharge is equal to that of the discharge crater generated by the
interval shown in Figure 13 also supports their results. This discharge [6,21,22,28,29]. Although, in most cases, it is
means that a plasma temperature of 5000K is considered assumed that the heat source is stationary and uniform in
as the threshold of deionization. This temperature drop its heat flux, Kunieda et al. [38] measured the arc column
during the pulse interval shows the minimum interval movement in a single pulse discharge and found that both
10000 anode and cathode spots move in the same way and the
Gap distance 1.5mm area of movement becomes larger with the progress of the
Temperature K

8000 Gap distance 0.5mm discharge. Their detection method, however, does not yet
6000 allow the measurement of the diameter of the arc column.
About the timing of material removal, there are still different
4000 findings. Zolotykh [11] reported that high-speed
Discharge duration: 300µs 5 µs photographs showed that most of the metal is ejected due
2000
to boiling of the superheated crater at the end of discharge
0 when the bubble pressure drops sharply, promoting the
-50 0 50 100 150 200 250 300 350 discharge of the gas dissolved in the molten metal. On the
other hand, Yoshida et al. [13] found that metal removed
Time µs
per pulse in air is almost equal to that in liquid, indicating
Figure 18: Plasma temperature at gap center during that metal removal can occur without a sharp drop of
and after discharge measured by spectroscopy.
bubble pressure. Observation of the gap of single pulse the plasma to be extinguished is short, leading to stable
discharge in dielectric liquid using X-ray also shows that machining. Difference in the plasma temperature decrease
85% of material removal occurs during the discharge calculated, shown in Figure 13, is a result of the difference
duration [39]. in the thermal diffusivity of the electrode material. Uno et al.
Xia [25] examined the influence of the assumptions used in [42] and Suzuki et al. [43] used CVD-carbon and
the boundary conditions: heat source diameter, heat flux electrically conductive CVD diamond as tool electrode
distribution in the heat source, and timing of metal removal, materials, respectively. Due to the high thermal diffusivity of
on the calculation results of temperature distributions in the these materials, EDM could be performed with almost no
electrodes. It was found that these assumptions exert a wear of the tool electrode.
significant influence on the calculation results of boiling Zingerman [44] states that Joule heating does not play a
point and melting point isothermal surfaces. Unfortunately, substantial role in the machining of metallic materials,
however, determining the boundary conditions is still a because electrical potential drops in metallic workpieces
controversial issue. Hence fundamental studies on the are negligibly small. According to the calculated results of
issue are highly looked forward to. Rich [45], however, the Joule heating is extremely
To show the influence of thermophysical properties of concentrated in a hemispherical volume directly under the
electrode materials on temperature distribution inside the discharge spot, and for high-resistivity metals, e.g., Hg, Sb,
electrode, the heat transfer equation was solved using a Fe, and Bi, the Joule heating is comparable to the energy
simplified model assuming an expanding circular heat input from the arc plasma. Saeki et al. [46] calculated the
source with a uniform heat flux and ignoring the metal workpiece temperature distribution considering the Joule
removal and temperature dependence of thermophysical heating as well as the heat flux from the arc column and
properties [40]. found that the removal of high-electric-resistivity material
(Si3N4-30wt%SiC) in a single discharge is greater than that
Figure 19 and Figure 20 show the calculated results when of steel due to Joule heating under the same discharge
steel and copper are used as the electrode material, current. However, it was found that a longer discharge
respectively, assuming the same energy distribution ratio of interval is necessary to deionize the arc plasma and
25%. Based on the formula obtained by Saito et al. [41] recover the insulation of the gap after each pulse, which
assuming that the heat source diameter is equal to the results in a lower material removal rate of high-resistivity
discharge crater diameter, the time dependent function of materials in a practical EDM.
the heat source diameter d(t) was determined as:
As a thermal machining process, EDM provides a means of
0.4
d (t ) = 2.4 × 10 −3 × t 0.4 × i e [m] (2) machining ceramic materials, irrespective of their hardness
and strength, provided that their electrical conductivity
Here, t is the time after ignition. Although the melting point values are of the order of 0.01 S/cm (100Ωcm) [47]. Owing
of copper (1357K) is lower than that of steel (1808K), the to the sometimes very different physical properties of the
melted zone of copper is considerably smaller than that of materials, electrical discharge processing produces a
steel due to its higher thermal diffusivity (1.1×10-4m2/s) than porous surface on which the binder phase has been
that of steel (2.0×10-5m2/s). With the expansion of the preferentially removed. Processing of thermo-shock
plasma radius, the melted zone is even resolidified during sensitive materials causes material to be removed by
the discharge duration in the case of copper. During the spalling.
discharge interval to, since the surface temperature at the
discharge spot drops rapidly in copper, the time needed for
Temperature, K

In-d r,
e
po pth l n
μmsition adia sitio
z, R po m
μ
t =10μs, t =100μs, 20μs after
d(t) = 50.8μm d(t)=121μm discharge

Figure 19: Temperature distribution in steel workpiece (cathode) (ie: 30A, te: 100μs, energy distribution to cathode: 25%)
Temperature, K

In-d
e r,
po pth l n
μmsition dia sitio
z,
Ra po m
μ
t =10μs, t =100μs, 20μs after
d(t) = 50.8μm d(t)=121μm discharge

Figure 20: Temperature distribution in copper workpiece (cathode) (ie: 30A, te: 100μs, energy distribution to cathode: 25%)
3.6 Chemical aspects liquid, and electrode materials with low melting point, at
EDM is undeniably a thermal process. However, by least at the surface. Thus, deterioration of surface quality
acknowledging that EDM is also a chemical process, new due to electrolysis in de-ionized water occurs [64]. When
findings and creative ideas can unexpectedly be obtained. ED-machining on high alloy steels in the commonly used
One of the applications of arc discharge was originally de-ionised water, toxic ions of heavy metals become
production of fine particles of metals [48]. Even now the dissolved in the dielectric. These are examples which
Reactive-Electrode Submerged-Arc (RESA) method [49] is demonstrate the importance of chemical aspects.
under investigation for producing ultrafine powders based 3.7 Role of dielectric liquid
on reaction between electrode materials and dielectric fluid. As described in Section 3.5, Zolotykh [11] and Snoeys and
It is well known that diamond-like carbon [50] and carbon Van Dijck [7, 8, 23] reported that most of the metal removal
nanotubes [51] can be produced using arc discharge occurs due to boiling of the superheated molten mass in
evaporation methods. the crater at the end of discharge because boiling of that
The EDM arc plasma temperature is high enough to not superheated metal is prevented by the bubble pressure
only evaporate the electrode materials and dielectric liquid during the discharge duration. Hockenberry et al. [12]
but dissociate and ionize their molecules and atoms. As stated that after the discharge termination when the
Hayakawa et al. [20] concluded, the EDM gap condition is expanded gas bubble starts to collapse due to its pressure
not in equilibrium during the discharge duration, and a large drop below atmospheric pressure, the gas bubble becomes
fraction of the discharge power is consumed in chemical fragmented and the liquid jets which penetrate the bubble
reactions. Eubank et al. [15] analyzed the enthalpy and impinge upon the electrode crater from all directions
increase in plasma involving dissociation and ionization of will strike the discharge crater and eject the molten metal.
water. Lauwers et al. [52, 53] investigated the cutting rate of On the other hand, Yoshida et al. [13] observed debris
WEDM of ceramics using de-ionized water, and pointed out formation and their distribution on the working surface due
that besides typical material removal mechanisms like to a single discharge. Figure 22(a) and 22(b) show the
melting and evaporation, other mechanisms such as distribution of debris obtained from single pulse discharge
decomposition and oxidation were identified. in air and liquid, respectively. In the case of discharge in
Toenshoff et al. [54] states that using a dielectric such as liquid, a solution of gelatin (25wt%) of 303K was used as
mineral oil or an organic fluid for ED sinking, hazardous dielectric liquid, and after the discharge the temperature of
fumes like polycyclic aromatic hydrocarbons, benzene, the experimental setup was lowered to freeze the gelatin to
vapour of mineral oil, mineral aerosols and various
by-products are generated by the dissociation of oil and its 8mm
additives. Kusano et al. [55] analyzed the components of Bubble boundary
the bubble generated using a hydrocarbon dielectric by gas 1.5mm
chromatography and showed that the volume fractions of - 8mm 8 -3mm
8 3mm
hydrogen gas and hydrocarbon gases, which involve
molecules with carbon atoms less than seven, are 56% and 8mm
36%, respectively, under a medium finishing condition. -1.5mm
Another example is the deposition of carbon on the anode - 8mm (b) In liquid
surface by the thermal decomposition of the working oil
(a) In air
(Figure 21). The white layer generated on workpieces
machined in hydrocarbon oils contains more carbon than Ie 32A, te 120μs
the base material, while that in a water dielectric contains Under 5μm 10 15 20 Over 30
less carbon than the base material [56]. Material removal Figure 22: Size and location of debris particles scattered
rates can be improved by supplying oxygen gas into the on electrode surface due to single pulse discharge.
discharge gaps filled with water-based dielectric liquid [57] h
or air [58]. Material removal rate using water based h
Distribution of debris diameter

Distribution of debris diameter

dielectrics compared with that of hydrocarbons is 100 100h

controversial. Although the material removal rate was 80 80h
based on volume, %

Over 30μm
based on volume, %

Over 30μm
improved by mixing organic compounds into water, it did 60 60h
h
not compete with that of hydrocarbon oil [59, 60]. Some 40 40
5 h
publications, however, showed higher material removal rate 20
15
20μm 20
h
20μm
2 10 15μm
of water-based dielectrics [52, 53, 61-63], because water is 0 0h 10
5
dissociated by discharge to hydrogen and oxygen, resulting 20 60 120 200 360
h20 60 120 200 360

in oxidation of workpiece materials [52, 53, 62]. Discharge duration, μs Discharge duration, μs
In order to reach stable machining conditions in WEDM In air In liquid
(large gap width for the vibration of the wire) and to obtain a
high rising speed of the discharge current, it is necessary to Figure 23: Debris diameter distribution based on volume.
use a relatively high open voltage, less insulating dielectric
Total volume of generated

In liquid
10-4
In air
10-5
Ａｎｏｄｅ（＋）
debris, mm3

10-6

Carbon 10-7
deposition
10-8
0 100 200 300
Discharge duration, μs
Cathode （－）
Figure 21: Deposition of pyrolytic carbon on anode in Figure 24: Comparison of total volume of
hydrocarbon dielectric (SEIBU ELECTRIC & MACHINERY). generated debris between air and liquid.
keep the debris from scattering. Then distribution of the found that 80 to 90% of the working area was occupied by
size of debris generated by the single discharge was gas bubbles during the process as illustrated in Figure 27.
observed by an optical microscope. Figure 23 shows the Imai et al. [66] measured the bubble quantity in the working
debris diameter distribution in volume. Figure 24 shows the gap utilizing ultrasonic waves passed through the working
measured total volume of debris generated in air and in gap. The ultrasonic wave transmitted through the electrode
liquid. It is found that there are differences in the total and incident normal to the working surface was mostly
volume of the generated debris and the distribution of the reflected at the interface when the gap was not filled with
debris size between single pulse discharges in liquid and in dielectric liquid. They found that more than half of the
air when the discharge duration is short. No difference working area is occupied by bubbles at higher discharge
exists however for discharge durations longer than 90µs. currents, shorter discharge intervals and longer machining
The debris volume difference is very small when the periods between jump flushing motion of the tool electrode.
discharge duration te is long. This is because the discharge Thus Tanimura et al. [67] proposed a new EDM method in
column is located at the center of the bubble generated by which air mixed with water mist is supplied to the discharge
the discharge itself and the diameter of the bubble grows gap and found that the EDM in mist shows almost the same
up to several millimeters as shown later in Figure 29. machining abilities as conventional EDM. Karasawa et al.
Hence, as time elapses, the arc column environment [68] showed that the material removal rate with a dielectric
becomes equivalent to that of the discharge which occurs liquid poured into the gap in air is higher than that with the
in gas with the increase in the diameter of the bubble. gap submerged in a tub of the dielectric liquid because the
The above results indicate that metal removal can occur ability of flushing debris in the pouring method is higher
without a liquid dielectric. In air, however, most of debris than that in the submerging method. All the above results
particles were reattached to the workpiece surface, as suggest that removal itself does not require the gap to be
shown in Figure 25(a) [13]. This is because the melted entirely filled with liquid. It should be noted that findings
debris particles move with constant velocity in air and do obtained from single pulse discharges in a gap filled with a
not solidify until they hit the electrode surface. In the case dielectric liquid cannot always be extrapolated in the actual
of liquid, in contrast, the debris particles proceed straight EDM process. Natsu et al. [69] found that removal in a
through the gas bubble and penetrate the bubble wall, and single pulse discharge is greater than the material removal
as a result decelerate. They then solidify into a spherical per pulse in consecutive pulse discharges. They measured
shape under the influence of surface tension [12] as shown the change in the removal amount at anode and cathode
in Figure 25(b), confirming that the dielectric liquid is with increasing number of repetition of pulse discharges in
important for the cooling and flushing of debris particles but an originally fresh inter-electrode atmosphere which is
not for material removal. achieved by the jump action of the tool electrode, and
found that the removal amount per pulse decreases
Miyajima et al. [65] found that the working gap is mostly strongly with repetition.
occupied by bubbles although the working gap is
submerged in dielectric liquid. Figure 26 shows the bubble 3.8 Forces applied to electrodes
conglomerated in the discharge gap when machining was The force accompanying the discharge is caused mainly by
interrupted and the tool electrode was lifted to widen the the expansion and contraction of the bubble generated by
gap. From the volume of the conglomerated bubble it was the evaporation, dissociation and ionization of the dielectric
liquid and electrode materials. Figure 28 shows a model to
(a) In air (b) In iquid calculate the bubble oscillation in the gap between
parallel plane electrodes [9]. The model illustrates a case
that a bubble with a high pressure is generated at the
center of the gap between cylindrical electrodes. The
bubble expands and contracts periodically. The reaction
force applied to the tool electrode can be calculated by
integrating both the pressure in the bubble and that in
Figure 25: Shapes of debris generated in air and liquid. dielectric liquid over the working surface [70].
Photos in Figure 29 taken by Ikeda [71] show the oscillation
of the bubble generated by a single pulse discharge in a
Liquid surface gap between parallel plane electrodes. The reaction force
in Figure 29 was measured by Kunieda et al. [72] using the
Split Hopkinson Bar method [73], with which the influence
Tool electrode (+)
of the natural frequency of the measurement system was
(Copperφ20mm） successfully eliminated. At the initial state in which the
bubble is compressed, the force indicates the highest peak.
Workpiece (-) With the expansion of the bubble, the force decreases
(carbon steel) according to the decrease of the bubble pressure. The
P(r)
Pressure distribution

Figure 26: Bubble unified in discharge gap when

machining is interrupted （ie 9A, te 120μs, to 40μs）.
r
Bubbles

Tool electrode
Discharge location
Atmospheric
Gap width Bubble Dielectric
Dielectric pressure
contaminated Workpiece
with debris
Initial diameter of bubble
Figure 28: Pressure distribution in bubble generated by
Figure 27: Schematic view of working surface. single pulse discharge in gap filled with dielectric liquid.
 Photographed

Dielectric reakdown
by Ikeda [71]
Reaction force, N 50

0
Dielectric breakdown
- 50 Discharge duration
0 Dielectric 2breakdown 4
Time, ms
Figure 29: Oscilation of bubble observed by Ikeda [71] and reaction force measured by Split Hopkinson
Bar method (ie 30A, te 150μs, gap width 0.15mm, anode: copper φ20mm, cathode: steel φ20mm).
force even becomes negative because the bubble distribution of the debris. On the other hand, due to the
continues to expand even after the bubble pressure falls extended gap width, servo gap control becomes easy.
below the atmospheric pressure due to the inertia of the Otherwise it is difficult to keep gap widths of several µm
dielectric liquid which is moving radially away from the constant.
discharge spot. When the diameter of the bubble reaches A simplest explanation for the decrease in the dielectric
the maximum, the absolute value of the negative force breakdown strength is that the electrically conductive
becomes maximum. Then, the bubble starts contracting, debris particles reduce the apparent gap width by a
and the force increases showing a dumping oscillation due distance equal to the diameter of the particles. However,
to the viscosity of the dielectric liquid. the measured gap width is occasionally larger than 100µm,
Since the natural frequencies of EDM machine structures whereas the measured average diameter of debris particles
are much lower than the frequency components included in is only 25µm under a certain pulse condition. This result
the reaction force waveform, the gap width cannot respond can be explained in terms of the movement of debris
to the change in the force. Furthermore, the reaction force particles in the gap, which was observed by Suda et al. [78],
in a series of pulse discharges decreases with time while Bommeli et al [4], Schumacher [79] and Kunieda et al. [80].
the working gap fills with bubbles generated by each They found that the debris particles move between the
discharge [70, 72]. Hence influence of the force caused by anode and cathode in a direction perpendicular to the
each pulse discharge is negligibly small in sinking EDM. electrode surfaces due to electrophoresis. They also
However, it causes vibration and deflection of electrodes in observed that some particles are linked in series to form
the cases of wire EDM and micro EDM. chains parallel to the electric fields.
It is also known that an electrostatic force occurs between From the observed results Bommeli et al. [4] and Kunieda
the tool electrode and workpiece [74, 75] mainly during the et al. [80] hypothesized that in the gap there are numerous
discharge delay time in which an open voltage is applied chains of particles which almost bridge the gap and that
between them. In WEDM, when the discharge energy is discharge occurs at the site at which the end of a chain is
small, the influence of the electrostatic force cannot be closest to the opposite electrode surface. Bommeli et al. [4]
ignored in comparison with the above mentioned reaction statistically investigated the distribution of ignition delay
force. Although the electrostatic force per unit length along time to demonstrate that discharge occurs after an
the wire electrode is small compared with the reaction force, activation time which is necessary for debris particles to
the total electrostatic force may not be negligible, because form bridges between the electrodes. Kunieda et al.
the force can be applied at any point on the wire which is calculated the electrophoresis-induced motion of a particle
facing with the workpiece surface. Electromagnetic force and found that the time interval for 30µm diameter steel
can also be applied to the wire electrode at any location particle to reciprocate between the parallel plane
where the workpiece is facing the wire electrode, because electrodes and the average velocity were 1.32ms and
high frequency current pulses generate an electric current 0.136m/s, respectively showing a good agreement with the
flow on the workpiece surface in parallel with the wire results of observation using a high-speed video camera.
electrode due to skin-effects [5, 75, 76]. The velocity is high enough for the last particle to enter
small gaps between the end of the bridge already present
4 PROCESS STABILITY and the opposite electrode surface and ignite the discharge
4.1 Factors determining discharge location during the ignition delay time.
Basically discharge occurs where the gap width is shortest. In order to further study the influence of debris particles, a
This principle guarantees the replicating accuracy of EDM. debris particle with a diameter of 5µm is placed in a gap of
To obtain an electric discharge in clean oil the gap width 20µm in width as shown in Figure 30. If it is true that
must be less than several µm. However, since the dielectric discharge occurs at a point where the gap is shortest, the
liquid is contaminated with electrically conductive debris discharge will probably occur at the point where the debris
particles, whose average diameter is one third to half of the particle is placed. In this experiment [81], the gap was filled
gap width, discharge occurs through the debris particles, with a clean EDM working oil and the surface roughness of
resulting in gap width much larger than that for an the electrodes was 0.8µm Rz. However, the experimental
uncontaminated dielectric [4, 77-79]. This fact sometimes results showed that in most cases, the discharge crater
causes reduced replicating accuracy due to the uneven could not be found at the point where the debris particle
 50mm ie= i1+ i2

i1 i2

ro ro

20μm
Discharge Tool electrode
rc rc
5μm R1 R2

Debris particle
Workpiece
Figure 30: Discharge location is determined in
not deterministic but probabilistic way. Figure 31: Principle of measuring discharge locations.
was placed. This is because occurrence of discharge is a Feed of tool electrode
probabilistic phenomenon and the probability of occurrence
of discharge per unit area is not zero even if the gap width Elede

at the place is longer than other places. Hence the Workpiece

Electro Electrode Electrode Elde
probability of occurrence of discharge on a surface is (a) (b) (c) Electrode
(d) (e)
de
determined by the product between the probability of
occurrence of discharge per unit area and its surface area. Figure 33: Mechanism of movement of localized discharge.
Since the projected area of the debris particle in the
direction normal to the electrode surface is negligible direction of the working surface. The dots indicate the
compared with the area of the electrodes (50mm×50mm), location of each discharge. It was observed that when the
the probability of occurrence of discharge at a point where depth of erosion was shallow, the process was stable. The
the debris is not placed is greater than that at the point distribution of discharge location was uniform, and no
where the debris is placed. It is still true that the probability relationship between consecutive discharges was visible.
of discharge per unit area is higher where the gap width is The tool electrode feed rate was constant and the
shorter and debris number density is higher, but the distribution of gap width was uniform. With the increase of
discharge location is determined not in a deterministic way the eroded depth however, discharge became localized.
but in a probabilistic way. The area of localized discharges moved periodically with a
4.2 Method of discharge location measurement period of several minutes. When the machining depth
increased further, the localization became prominent, and
To obtain stable conditions in EDM, it is essential for the the area of localization narrowed. The movement of the
discharge locations to be dispersed over the working localized area was perfectly periodical. In this phase, the
surface. Figure 31 shows the equivalent circuit to detect tool electrode feed was not smooth and the distribution of
discharge locations developed by Kunieda et al. [82] and gap width was not uniform. Ultimately, discharge was
Obara et al. [83]. The electric feeder is intentionally divided, concentrated at a certain location. In this hazardous
and each divided feeder is connected to the tool electrode situation, it was no longer possible to continue the process
at each end. While the resistance of the feeder, ro, and and thermal damage was left on the workpiece surface.
contact resistance at the feeding point, rc, are constant, the
resistance in the tool electrode from each end of the tool The mechanism of the periodical movement of localized
electrode R1, R2, respectively, varies depending on the discharge is shown in Figure 33. If the flushing of sludge is
discharge location. Consequently, by measuring the ratio difficult, local distribution cannot be eliminated once it is
between currents i1 and i2 through each divided feeder, the created at some area, even if the gap width of the other
discharge location can be obtained in process. regions is narrower, since the dielectric strength is
considerably decreased due to contamination in the
Figure 32 shows the distribution of discharge locations localized area. Meanwhile, the localized area moves to
measured with the extension of the depth of erosion. The other regions, where the gap width is sufficiently short. At
working area of the tool electrode was a rectangle of 14mm this moment, the position of the tool electrode is retracted
by 105mm. The abscissa shows the number of discharges resulting in the fluctuation of the feed speed.
sampled every fifty pulse discharges, and the ordinate
shows the discharge location along the longitudinal
Electrode
105mm

Electrode
location
Electrode

Electrode

105mm
location
105mm

Spark
location

Electrode
Spark
Spark

105

105

0 4000 8000 0 4000 8000 0 4000 8000

Sampling number Sampling number Sampling number
Servo feed
Servo feed

Servo feed
20μm

0 4000 8000 0 4000 8000 0 4000 8000

Electrode Sampling number Sampling number
Electrode Sampling number Electrode
20μm
20μm

20μm

10mm 10mm 10mm

Workpiece Workpiece Workpiece

(a) Stable machining （0.1mm in depth) (b) Periodical movement of localized area （2.2mm) (c) Instable machining （3.5mm)
Figure 32: Distribution of discharge locations measured with extension of depth of erosion.
4.3 Relation between distribution of discharge dimension (Figure 34), and thus the system is not random
location and process instability but chaotic. The higher the attractor dimensions, the more
Localization and concentration of discharges result from stable will machining be.
locally weakened dielectric breakdown strength in the gap. 4.5 Discharge location measurement for preventing
There seems to be two causes for the decrease of the wire breakage in WEDM
dielectric breakdown strength. Figure 35 shows the temperature distribution along the wire
One of the causes is thermal. It was found both analytically electrode in WEDM obtained from the heat transfer
and experimentally that it takes less than five microseconds analysis using the finite difference method [86, 87]. Zone
for the temperature at the center of the plasma generated 3 indicates the part of the wire electrode where discharge
by the preceding discharge to drop under 5000K, which is occurs. Zones 2 and 4 show the part from the upper and
the temperature where the dielectric strength of the gap is lower feeding points to the upper and lower surfaces of the
considered to recover sufficiently [18, 19, 34, 35]. When workpiece, respectively. Although the average temperature
deionization is insufficient during the discharge interval, is around the boiling point of water, which is used as the
subsequent discharges occur at the same location, working fluid, the temperature at the point where the
resulting in the further increase in the temperature at the preceding discharge occurred is significantly high so that
discharge spot, which accelerates the concentration. the tensile strength of the wire weakens at this point.
Formation of debris particles in the gap can be another Hence, in the case that subsequent discharges occur at the
reason. If flushing of the debris particles is insufficient, same location, the temperature on the wire at the discharge
localization of discharge causes increased concentration of spot increases further, causing wire breakage.
debris particles locally, leading to the acceleration of Consideration of this led to the development of adaptive
discharge localization. If discharge is localized, the control systems in which the pulse energy is reduced or
distribution of the gap width becomes uneven, resulting in stopped based on the distribution of discharge locations
deteriorated machining accuracy. measured in process [88-90]. Based on a thermal model,
the temperature distribution and the maximum temperature
4.4 Chaos found in distribution of discharge locations can be estimated. The control strategy is to reduce or stop
The distribution of discharge locations looks random when the pulse energy. The duration of pulse termination
the process is stable, as shown in Figure 32(a). Han et al. depends on several parameters such as the maximum
[84] however found that the distribution of discharge temperature value, the number of times the maximum
locations in EDM is not random but chaotic using the chaos temperature exceeded the limitation of temperature setting,
theory. Chaos is a deterministic development with chaotic and the moving average of the maximum temperature [89].
outcome. In other words, although the system is evolving 4.6 Flushing
with a simple rule in a deterministic way from moment to
moment, forecasting the future of the system is To maintain stable machining, it is critical to flush debris
substantially impossible because the initial condition of the particles and cool the working gap in order to prevent the
system affects the subsequent events dramatically [85]. localization and concentration of discharge locations.
Although the EDM discharge location is determined by a Normally, the tool electrode is lifted periodically to replace
simple rule that the probability of discharge is higher where the contaminated dielectric fluid in the gap with fresh one.
the gap width is shorter and density of debris particle is Masuzawa et al. [91] proposed a special rotary electrode
higher, the movement of discharge location appears movement to enhance the pumping action of dielectric fluid
complicated and irregular. The attractor dimension during the lifting motion. Cetin et al. [92] calculated the
calculated from the correlation integral of the vector defined three dimensional fluid flow in the gap considering the
using the time series of discharge locations indicates that suspended debris particles and obtained the relationship
the dynamics of this system can be described with a low between the tool lifting velocity and height and the flushing
capability. Linear motor equipped sinking EDM machines
[93] realized increased removal rate especially in the
Discharge location

Correlation dimension dm

machining of narrow and deep grooves due to the

100mm

Random high-speed jump flushing operation of tool electrode which

Discharge location

can prevent short circuiting. Machining accuracy was also

100mm

30
improved because the lateral gaps of the groove became
25
narrower and more uniform.
20
0 500 1000 15 The working gap can be flushed by a fresh dielectric fluid
0 500 1000
No. of dischages Discharge locations jetted from nozzles placed adjacent to the discharge gap.
10
No. of discharge Masuzawa et al. [94] however reported that jetting of
5
0
dielectric fluid merely from one direction causes increased
Attractor dimension of 0 5 10 15 20 25 30 density of debris particles in the downstream, resulting in
discharge locations Embedding dimensions m uneven distribution of gap width deteriorating the
Figure 34: Chaos found in distribution of discharge locations. machining accuracy. Hence they demonstrated that
flushing from both sides, alternate flushing, and sweeping
flushing is preferable. Pressure or suction flushing through
400
holes in the electrode or workpiece remains one of the
most efficient flushing methods at least if those holes have
Temperature, ℃

to be provided anyway or does not harm the workpiece.

Both in pressure and suction flushing, one can observe
200 lower electrode wear and larger gap width at the outlet
point in comparison with the inlet point [95].
Tsukahara et al. [96] found that when an oxidized dielectric
oil is used, the metal elements in the tool electrode and
0 workpiece react with the oxidized oil, and organometallic
1 2 Zone 3 4 5 compounds are consequently formed. Thus they proposed
a dielectric liquid which is added with a fatty acid containing
Figure 35: Calculated temperature distribution along wire carboxylic acid. Use of this dielectric liquid results in a more
electrode (discharge current: 90A, discharge frequency: stable process since debris particles are partly dissolved as
18kHz, wire diameter: 0.25mm, workpiece thickness: 100mm).
organometallic compounds and not suspended in the gap hand, Figure 37 shows the basic circuit of the newly
anymore. developed Multi-spark EDM method [103], in which two
4.7 Control of discharge locations discharge points can be obtained for each pulse. The
circuit is comprised of a conventional pulse generator, a
As described in Section 4.3, it is essential to disperse workpiece and two tool electrodes, both of which are fixed
discharge locations uniformly over the working surface. To onto the quill of the same EDM machine. The pulse
disperse the discharge locations, Narumiya et al. [97] generator is connected with one tool electrode, the
added electrically conductive fine powder into the working workpiece, and the other tool electrode serially in this order.
fluid. Now this method is widely used in the finishing stage Accordingly, for each pulse, one discharge occurs in the
to obtain a mirror-like surface. Nishikawa et al. [98] found gap between one tool electrode and the workpiece, and
from experiment that the discharge locations can be another discharge occurs at the same time in the gap
dispersed with this method because the gap width is between the other tool electrode and workpiece. Hence the
increased when powder is added to the working fluid. They removal rate and energy efficiency of the Multi-spark EDM
concluded that the dispersion of the discharge locations is method are considerably higher than those of conventional
one of the reasons why mirror-like surface can be obtained EDM. To balance the removal rates in both gaps, the
using the powder suspended working fluid. They reported, polarity of the pulse generator is changed adaptively to
incidentally, that another reason is the decrease of equalize the gap voltages measured at both gaps.
discharge energy due to the decrease of the inherent
capacity of the discharge gap, because the surface electric 4.8 Gap monitoring and control systems
charge on the electrode surfaces may increase the In order to improve the machining rate, stability and quality,
discharge current over the current supplied from the pulse extensive efforts have been made to develop advanced gap
generator if the gap is narrow especially when the monitoring and control systems. Discharge states can be
discharging surface area is wide. classified into five types, open circuit, normal discharge,
Certain attempts to control discharge location have been abnormal discharges (arcing 1 and arcing 2) and short
made. Mohri et al. [99] controlled the discharge location by circuit as shown in Figure 38 [104, 105]. The ignition delay
sequentially applying one voltage pulse to each needle of a time, td, is an important indicator of the isolation condition of
bundled electrode. The bundled electrode is made of the discharge gap. A gap monitoring circuit was developed
copper needles which are tied together and electrically to identify the five discharge states and output the ratios of
insulated from each other. Luo et al. [100] and Kunieda et gap open, normal discharge, transient arcing, harmful
al. [101] controlled the discharge location by applying a arcing and short circuit [106]. These ratios are used as input
high voltage pulse with a very short rise time to an arbitrary parameters for EDM on-line control based on various
point on the tool electrode around which discharge is control strategies.
intended. Due to the internally existing inductance of the Adaptive control arose with the success of fundamental and
tool electrode, which is small but not negligible, it takes a mathematical adaptive control theory and the development
finite time for the electric potential to transmit in the tool of computer technology. A typical example is the steepest
electrode. Discharge is therefore forced to occur around ascent path algorithm as shown in Figure 39 [107]. In this
the place where the high voltage pulse is applied, because system, the pulse efficiency, i.e. the ratio of normal
the gap voltage is locally risen around there even though in discharge time to total pulse duration, is used as the
an extremely short period of time. Han et al. [102] utilized controlled parameters. The system adjusts on-line the
this principle to WEDM as shown in Figure 36. The high discharge interval, t0, and the control reference voltage
voltage pulse is applied either to the upper or to the lower which is compared to the feedback gap voltage, while the
feeding brush. With the use of the discharge location difference value of the comparison drives the servo
measurement, they demonstrated that the discharge mechanism to maintain the average gap voltage at the level
location can be controlled in the actual WEDM process. 80
Gap voltage [V]

It is common knowledge that, in conventional EDM, there

exists only one discharge point for each pulse. On the other
td te to
40 ui
Superimposed to lower brush
ti
Superimposed to upper brush ue
Probability

Without superimposition 0
800 1600 2400 Time [μs]
Gap current [A]

4
2 ie is

0
0 40 80 100 160 Open Normal Arcing Arcing Short Time [μs]
circuit spark 1 2 circuit
Discharge location, mm
Wire electrode Figure 38: Voltage and current waveforms of
the five basic EDM gap states [104, 105].
Workpiece
Lower brush Upper brush
Discharge interval to [μs]

50 Start point
Figure 36: Control of discharge locations by
Arcing and short-circuit zone

30 Ridge Pulse efficiency: 20%

superimposing high voltage to feeding points in WEDM.
20 30%
40%
Discharges 50%
60%
10 Steepest ascent path
70%
L R
5 80%

3 Arcing zone
-2.0 -1.0 0 1.0 2.0
Conventional Multi spark Servo reference voltage us [V]

Figure 37: Principle of multi-spark EDM method. Figure 39: Algorithm of steepest ascent path [107].
determined by the servo reference voltage, to seek the

Discharge
current
highest value of the pulse efficiency in order to achieve the
maximum machining rate. Removal rate
To reflect the dynamic behavior of the EDM process, a
dynamic modeling approach so called self-tuning control
Time
was proposed [108]. In this model, the outputs which are
used to adjust the servo reference voltage and discharge
interval are based on the detection of the type of pulses that Surface
Tool wear ratio
occur in the gap (see Figure 38). During machining, the roughness
outputs are maintained at the level of reference values
which are manually set according to given machining
conditions. To improve machining productivity, this model
has been simplified by linearizing the output value with the Figure 40: Relation between discharge current
machining gap size. During machining, the difference waveforms and machining characteristics.
between the output of the process and the output of the discharge duration result in higher material removal rates.
reference model is controlled at a minimum value [109]. To At the same time, low tool electrode wear can also be
address the internal dynamic characteristics of the closed satisfied because the carbon layer deposited on the anode
loop servo system for stabilizing the machining process, the tool electrode is thicker when longer discharge durations
adaptive servo system was modified into a self-tuning are used [21, 25-31]. However, surface roughness is not
regulator system [110]. good because the crater generated by each pulse
When a deep and narrow cavity is machined, electrode discharge is large. Thus this pulse condition is normally
jumping operation is always used to retract the tool from the used in rough machining.
machining area to improve the flushing condition. In the On the other hand, the pulse condition with longer
adaptive jumping control system, the system output is discharge duration and lower peak current brings about
calculated based on the ratio of the sum of abnormal both lower tool electrode wear ratio and better surface
discharge time to the total pulse on-time. When the roughness. However, the lower heat flux due to the lower
machining depth increases, the ratio of the jump-down time current density at the discharge spot results in lower energy
in one jumping cycle is reduced to avoid harmful arcing [111, efficiency of material removal compared with the pulse
112]. condition with shorter discharge duration and higher peak
EDM lacks a direct link between the theoretical models of current even though the same pulse energy is applied. In
material removal mechanism and industrial applications. In this situation, the melted and evaporated regions in the
many cases, empirical knowledge is helpful for deciding workpiece are small and most of the energy distributed to
machining parameters without explicit mathematical models. the workpiece is dissipated inside the workpiece due to
Such expert knowledge is simply expressed as IF-THEN heat conduction. Obviously, the material removal per pulse
rules to describe the relationship between the input and is small. In addition, longer discharge duration results in
output of system. Therefore, they are applied in on-line lower discharge frequency per unit time. Thus this pulse
control, off-line process planning and operator assistance. condition is suitable for finishing.
There are many research results published on fuzzy logic Finally, pulse conditions with shorter discharge duration
control either on sinking EDM or on wire EDM [113-118]. and higher peak current provide better surface roughness
An artificial neural network consists of many neurons due to smaller discharge crater. Moreover, although
arranged in different layers. Each neuron is connected to material removal per pulse is not large, it is large enough
every neuron in the previous and following layers with a compared with the pulse condition with longer discharge
variable weight. The self-learning algorithm automatically duration and lower peak current under the same discharge
adjusts these weights based on training input-output sets. energy per pulse because energy efficiency is higher due to
Neural networks have been used to optimize EDM sinking larger heat flux. Higher repetition rate of discharge per unit
process to improve gap width control, harmful arc time due to shorter discharge duration further increases the
prevention and high machining accuracy [119, 120]. They material removal rate. However, the tool electrode wear
are also used in WEDM to estimate the workpiece height ratio is high because the carbon layer deposited on the tool
and maintain optimal and stable machining [121]. electrode is thin. This is why extremely short discharge
To integrate EDM within the next generation agile duration and high peak current compared with the case of
manufacturing environment, a system with communication sinking EDM, for example 1µs and 100A, are used in
function through internet has been developed. The Web WEDM, where the problem of tool electrode wear is
Home Page Browser window is used to browse information insignificant.
on the Internet. The complete data during monitoring can be 5.2 Factors influencing tool electrode wear ratio
recorded and displayed. The recorded data can then be Influence of polarity
used for off-line analysis and comparison of the effects of
different machining conditions [122, 123]. As shown in Figure 16, the energy dissipation into the
anode is greater than into the cathode. Nevertheless, in
5 MACHINING CHARACTERISTICS sinking EDM, polarity of the tool electrode is normally
positive except when a very short discharge duration is
5.1 Influence of pulse conditions used. This is because the carbon layer which is deposited
Figure 40 shows three machining characteristics which are on the anode surface due to thermal dissociation of the
most important in EDM practice: material removal rate, hydrocarbon oil protects the anode surface from wear.
surface roughness and tool electrode wear ratio. Here the Since the carbon layer is thick when the discharge duration
tool electrode wear ratio is defined as the ratio of volume of is long, the tool electrode wear ratio is low with the polarity
tool electrode wear to the volume of workpiece removal. of positive tool electrode under the pulse condition of
Requirements for any two of the characteristics can be longer discharge durations. On the contrary, a negative tool
satisfied if the remaining one is sacrificed using the electrode is used considering the energy distribution in the
discharge current waveforms indicated in Figure 40. cases of finish machining and micromachining where
However, there is no current waveform which can satisfy all deposition of carbon layer is scarce and of WEDM in which
the requirements. deionized water is normally used as dielectric liquid.
Current waveforms with larger peak current and longer
Influence of thermal properties of electrode materials tool electrode is almost zero because a micro rod is first
Since EDM is a thermal process, the influence of thermal made on the same machine by WEDG, followed by
properties of electrode materials on the removal volume is reversing its polarity to use the micro rod as tool electrode
significant. When the heat flux from the arc column is equal, to machine micro holes without loosening the chuck.
higher heat conductivity results in lower temperature on the Tool electrode wear
electrode surface. Hence materials with higher heat On-machine forming of micro rods is also possible using a
conductivity are suitable as tool electrodes. On the other block tool electrode as shown in Figure 42. However, wear
hand, under the same temperature distribution inside the of the block deteriorates the machining accuracy. In WEDG,
electrode, when the surface temperature is not over the there is no problem of wear, because a renewable wire is
boiling point or melting point, removal does not occur. used as tool electrode. Since the wire is guided by the
Hence materials with higher melting point and boiling point groove of the wire guide, machining accuracy is not
are also suitable as tool electrodes. The results of heat deteriorated by the wire vibration unlike the WEDM
conduction analysis shown in Figure 19 and 20 indicate process.
that although both melting point and boiling point of copper
are lower than those of steel, copper is more suitable as a Reaction force
tool electrode because copper has a much higher heat As shown in Figure 29, the peak of the reaction force
conductivity than steel. Thus, considering the higher generated by a single pulse discharge is remarkably large.
energy distribution to the anode, carbon layer deposition on Use of a block electrode causes discharge even at the end
the anode, and the thermal properties of electrode of the micro rod which is being formed, thus exciting
materials, it should be possible to select appropriate vibration of the micro rod. In WEDG, however, the bending
discharge current waveforms, polarity, and tool electrode moment applied to the almost finished micro rod is
materials to obtain minimum tool electrode wear ratio even insignificant because discharge only occurs at the foot of
lower than several %. the rod being formed.
Relaxation type pulse generator
6 CHALLENGES IN MINIATURIZATION
In micro EDM, the relaxation type pulse generator shown in
6.1 Strategy found in WEDG Figure 4 is still being used. If the transistor type is used, it
EDM processes are widely used for the micro-machining of takes at least several tens of ns for the discharge current to
fuel injection nozzles, spinneret holes for synthetic fibers, diminish to zero after detecting the occurrence of discharge
electronics and optical devices, micro-mechanical parts, because the electric circuit for detecting the occurrence of
and micro-tools for producing these devices. Figure 41 discharge, the circuit for generating an output signal to
shows the Wire Electro-Discharge Grinding (WEDG) switch off the power transistor and the power transistor
method which was developed by Masuzawa et al. [124]. itself have a certain amount of delay time. Hence it is
Using a wire electrode traveling along the groove of a wire difficult to keep the constant discharge duration shorter
guide, a micro rod can be machined like in a turning than several tens of ns using the transistor type pulse
process. Then, on the same machine, by reversing the generator.
polarity of the machined micro rod, micro holes of less than Stray capacitance
10 µm in diameter can be drilled by EDM. The following are
When the relaxation type pulse generator in Figure 4 is
the essential problems of micro EDM which were
used, capacitance of the capacitor should be decreased to
elucidated and solved by Masuzawa [125] through the
obtain smaller discharge energy per pulse. In the actual
development of the WEDG method.
EDM machine, however, stray capacitance exists between
On-machine forming of tool electrodes the electric feeders, between the tool electrode holder and
When a micro tool electrode is purchased or made using work table, and between the tool electrode and workpiece.
another machine in the machine shop and held by the Hence all the charge stored in the stray capacitance is
chuck of the micro EDM machine, the micro tool electrode discharged to the working gap together with the charge
is always off-centered more or less from the rotation axis of stored in the capacitor wired to the circuit as shown in
the spindle. In the WEDG machine, however, run-out of the Figure 43. This means the minimum discharge energy per
pulse is determined by the stray capacitance. Hence
Workpiece shortening of electric feeders and use of electrical insulator
for the tool electrode holder and work table materials are
effective for reducing discharge energy. In the final finishing,
Dielectric liquid when minimum discharge energy is necessary, the
capacitor is not wired and machining is conducted with the
stray capacitance only. Since the working surface area
Wire electrode between the tool electrode and workpiece in WEDG is
narrower than that in the block electrode method, smaller
stray capacitance of the working gap is advantageous to
Wire guide WEDG for the limit of miniaturization.
Some attempts [126, 127] were made to replace the
Figure 41: Wire electro-discharge grinding
relaxation type pulse generator with the transistor type
(WEDG) method [124]. pulse generator in micro EDM, and a minimum discharge

Pulse generator
R
C Stray capacitance
Micro rod workpiece Tool electrode

Workpiece

Block tool electrode

Figure 42: On-the-machine forming of micro rod Figure 43: Charge stored in both stray capacitance
using block electrode. and condenser is discharged.
duration of 30ns was achieved [126]. However, even if Discharge crater size
future developments of electronics devices can further Figure 45(a) shows a discharge crater generated on the
reduce the delay time existent in the transistor type pulse tungsten anode surface when using a capacitance of
generator, the discharge energy can never be smaller than 3300pF. Figure 45(b) shows a discharge crater generated
the energy stored in the stray capacitance. with stray capacitance only. Even with the minimum
Masuzawa et al. conducted assembly [128], milling [129], discharge energy, diameter of the discharge crater
blanking [130], electrochemical discharge machining [131], measures 2µm, indicating that the minimum discharge
and ultrasonic machining [132], based on the crater size may influence on the limit of miniaturization. As
on-the-machine manufacturing of micro jigs, cutting tools, described in Section 5.2, under the pulse conditions used
blanking tools, tool electrodes, and horns using the WEDG in micro EDM, since the carbon layer is scarcely deposited
method. on the anode surface due to the extremely short discharge
6.2 Factors influencing limits of miniaturization in duration, polarity of workpiece is normally positive because
micro EDM energy distribution into anode is larger than that into
cathode. Thus Han et al. [134] compared the diameter of
Kawakami et al. [133] examined the limits of minimum craters generated on an anode and a cathode workpiece
machinable size using the WEDG method. Cemented and found that the diameter was 2µm and 0.8µm,
tungsten carbide rods of 0.3mm in diameter with a WC respectively (Figure 46). However, the minimum diameter
particle size of 0.4µm were used as the workpiece. The of the micro rod machined by WEDG using the reversed
relationship between the target rod diameter and machined polarity was almost the same as that using the normal
rod diameter was obtained as shown in Figure 44. Red polarity. This was probably because the machining time
marks in Figure 44 indicate machining failure due to was about eight times longer than the normal polarity.
breakage of the micro rod. It should be noted that micro
rods smaller than 2.3µm in diameter were not obtained. Since discharge energy is 1/2(Cu02), decreasing the open
Deviation in the machined rod diameter from the target rod voltage u0 is effective for reducing the crater size. Thus
diameter was obviously caused by the positioning error and Egashira et al. [135] decreased the voltage of the power
thermal deformation of the equipment, and by the source to 20V and obtained the minimum rod diameter of
inaccuracy of wire electrode diameter used in WEDG. 1µm as shown in Figure 47.
However, these are not the reasons why rod diameters Residual stress
under 2.3µm were never obtained. If they were, one would There are two kinds of residual stress which may influence
have obtained successful results in the vacant zone since the limit of miniaturization. One is residual stress already
machining was repeated in great numbers. Therefore, in present prior to machining, and the other is residual stress
this case, plausible reasons would be the size of the caused by EDM.
discharge crater, sub-surface layer damaged by EDM,
residual stress, and material structure of the workpiece. Residual stress already present prior to machining, for
example, can be found in tungsten rods which are
produced by plastic deformation using the wire drawing
Machined rod diameter, μm

process. The residual stress in the axial direction is not

15
15 distributed uniformly in the radial direction. When this
material rod is held by the chuck of the spindle of an EDM
20μm
10
10
Success
5
5 Failure
Vacant Target value
zone
0
0
0 5 10 15
0 5 10 15
Target rod diameter, μm

Figure 44: Relationship between target rod diameter Figure 47: Tungsten rod of 1μm diameter fabricated by
and machined rod diameter (WC grain size : 0.4μm). WEDG (Open voltage: 20V, stray capacitance) [135].

300µm 300µm

11.5μm
2μm

Before annealing
Without After annealing at 1000K
(a) C=3300 pF, open (b) Stray capacitance
voltage 110V only, open voltage 40V Figure 48: Influence of residual stress already present prior to
machining (tungsten rod, C=3300pF, open voltage 110V).
Figure 45: Discharge craters generated on tungsten anode.

10µm 10µm

Normal polarity: Reversed polarity:

workpiece positive workpiece negative
Figure 46: Surface of micro rods obtained by
WEDG (Cemented tungsten carbide, open Figure 49: Tooth edge deviation due to
voltage 40V, stray capacitance only). tensile residual stress [136].
machine, the center of rotation does not always coincide (Figure 50). The machining time was about 5 minutes,
with the center of the material rod. Therefore the micro rod which is 600 times less than that required for serial
is generally formed off-centered from the center of the machining by a single electrode. Arrayed electrodes of
material rod, and this residual stress causes deformation of even complicated cross section shapes, like hexagonal and
the micro rod machined as shown in Figure 48 [133]. gear, can be fabricated using LIGA with a high aspect ratio.
Since EDM is a thermal process, tensile residual stress is Figure 51 shows another novel approach to improve the
generated in the re-solidified layer. Spur et al.[136], Kruth et throughput in micro EDM [142]. In the first step (n=1), a
al.[137], and Klocke et al. [138] reported that thin fins cut by single micro cylindrical electrode is made by WEDG. In the
wire EDM may bend due to the residual stress caused by second step (n=2), a plate electrode is perforated to have a
the EDM process when fin thickness is less than 0.1mm in pattern of holes using the cylindrical electrode made in the
rough cutting (Figure 49). first step. In the third step (n=3), using the plate electrode
Influence of material structure as tool electrode, the pattern is replicated to a block
workpiece. In the next step (n=4), the workpiece is used as
Kawakami et al. [133] examined the influence of the grain tool electrode to make many patterns of holes precisely and
size of tungsten carbide (WC) in cemented tungsten efficiently. After this, steps of 3 and 4 may be repeated to
carbide material on the limit of miniaturization. Since the obtain numerous numbers of holes.
minimum diameter obtained using cemented tungsten
carbide with WC grain size of 0.4µm was 2.3µm as shown Figure 52 shows the simultaneous processing method for
in Figure 44, change in the limit was examined using grain micro rods and holes developed by Yamazaki et al. [143].
size larger than 0.4µm. The results showed that vacant With the polarity of the rod electrode positive, which is
zone with WC grain size of 2µm was wider than that in reverse to the normal micro EDM, the rod electrode is
Figure 44 and the minimum machinable rod diameter rotated and fed into a plate electrode at a feed rate of ten
increased to 3µm. times higher than the normal one. Since the circumferential
region of the rod electrode wears at a much higher rate
Almond et al. [139] examined the influence of voids existing than the center, the end of the rod electrode is sharpened
in the workpiece material prior to machining on micro EDM. in just a few minutes. At the same time, a tapered micro
Han et al. [134] examined the sub-surface layer damaged hole is generated in the plate electrode corresponding to
by micro EDM. Kawakami et al. [133] compared the the conical shape of the rod electrode. Thus a pair of micro
machining characteristics of micro EDM between
poly-crystal and mono-crystal tungsten. The minimum rod
machinable thickness of a micro wall was thinner when （＋）
mono-crystal tungsten was used compared with plate 50μm
poly-crystal tungsten. However, since cracks were （－） rod rod plate
０ ２ ４ ６ ８
generated parallel to the {100} planes, it is not always true min ＜before＞ ＜after＞
that mono-crystal is more suitable for miniaturization than Time
poly-crystal. They also found that the machining speed Figure 52: Simultaneous processing of micro rod and hole.
depends on the crystal orientation of mono-crystal tungsten,
indicating that micro EDM may not be a pure thermal
process.
6.3 Advancements in micro EDM
The use of a single tool electrode has limits in throughput
and precision because of positioning error and tool wear.
Thus high aspect ratio electrode arrays were fabricated by
the LIGA process [140, 141]. A LIGA-fabricated array of Figure 53: EDMing micro rods by self-drilled holes.
400 Cu electrodes with 20µm diameter was used to
machine through-holes in 50µm thick stainless steel

(a) φ4μm cemented (b) φ5μm hole machined

carbide micro rod by formed rod in Cu plate
100μm in thickness
Figure 54: Micro rod formed by self-drilled hole
method and micro hole machined by formed rod
(Open voltage:70V，C: stray capacitance).
Figure 50: LIGA fabricated electrodes [141].
Wire electrode Cylindrical electrode
Inlet Outlet
n=1： WEDG Plate electrode (a) Self-formed slots

n=2： Pattern machining 150 μm

150 μm
(b) Formed rod
n=3： Machining multi-electrode 150 μm

Inlet Outlet
(c) Hole machined by formed rod

n=4： Multi-hole machining Figure 55: Irregular hole made by self-drilled hole
Figure 51: μEDMn method [142] method (open voltage: 110V，capacitance: 100pF).
rod and micro hole can be machined in a single process corrosion resistance, and tribological properties can be
without positioning control in the radial direction. obtained when the polarity of the workpiece is positive.
Figure 53 shows a simple but novel idea proposed by Mohri et al. [147] and Goto et al. [148] found that alloying
Yamazaki et al. [144] to machine micro rods and holes speed can be improved significantly by using partially
without using any special equipment like WEDG. With the sintered or green compacted materials such as Cu, Al,
polarity of a rod electrode negative, the rod electrode is WC-Co, and Ti as tool electrode. This is because the wear
rotated and fed into a plate electrode to make a hole. After ratio of these materials is high due to the low thermal
the rod electrode returns to its initial position, the rod conductivity of these materials. As shown in Figure 56 [148],
electrode is moved from the center of the hole at a certain a hard material (TiC) layer is coated on the workpiece
distance. Next the polarity of the rod electrode is reversed, surface since Ti is carbonized in the hydrocarbon oil. Using
and then the rod electrode is fed into the plate electrode the same setup as the usual EDM process, harder and
either with or without rotation. Positioning of the rod thicker layer coating with a higher bonding strength can be
electrode in reference to the tool electrode is not necessary conducted in a shorter time compared with the
because self-drilled holes are utilized as the tool electrodes. conventional methods like CVD and PVD. Figure 57 shows
Figure 54 shows a micro rod of 4µm in diameter formed by a cross-section of a TiC layer formed on carbon steel in
this method. Using this rod electrode as tool electrode, eight minutes [149]. Applications of this method to cutting
micro holes with a diameter of 5µm was perforated on a tools and blanking tools show that tool life becomes
copper plate. In Figure 55, a cross-shaped right-angled slot significantly longer [148, 150]. Various types of powder and
was formed using a rod electrode. Positioning the rod compact were proposed depending on workpiece materials
electrode on each corner of the cross-shaped slot without a and alloyed layers for the applications to steel rolls, molds,
rotation, EDM was performed to form the cross-shaped rod dies, cutting tools, turbines and aeroengine components
with reversed polarity. Using this cross-shaped electrode, [151-153]. To make this process possible, a negative
cross-shaped holes were formed on a plate. polarity of the tool electrode and a long discharge duration
is used. Under these conditions, as described in Section
7 INNOVATIVE EDM TECHNOLOGIES 3.3, the material removal rate of the cathode (tool
electrode) is higher than that of the anode and carbon
7.1 Electrical discharge coating and alloying coming from the hydrocarbon dielectric is deposited on the
EDM is undeniably classified as a removal process. anode (workpiece). The worn tool electrode material (Ti)
However, the EDM process can also be used as a surface and the deposited carbon yields the TiC coating.
treatment method and/or an additive process. White layers Hayakawa et al. [154] fabricated a micro structure shown in
generated on a steel workpiece machined by EDM using Figure 58 using EDM deposition in air. The suitable
hydrocarbon dielectric have a higher carbon content than discharge conditions for this process were predicted from
the base material and hence shows increased resistance to the transient temperature analysis of the tool electrode and
abrasion and corrosion [56, 64, 97, 145]. Under the high workpiece. To enhance the wear of the tool electrode, its
temperature of the discharge column, the white layer can polarity was set positive, which is opposite to the above
absorb carbon from the gases formed in the discharge coating and alloying methods, because removal rate of the
column from the hydrocarbon dielectric. Since WEDM anode is higher than that of the cathode in air.
normally takes place in deionized water, a discharge
column with a high oxygen and hydrogen content is built up. 7.2 Coloring of Titanium alloy using EDM process
In particular the oxygen causes decarbonization of the Minami et al. [155] developed a new method of coloring
material metal surface, thus increasing the density of alloys, titanium alloys using the WEDM process. Since deionized
which are generally chromium and nickel. This also water is normally used in WEDM, an oxide layer is formed
increases the corrosion-resistance of the surface [64]. over the surface of the anode workpiece due to electrolysis.
Narumiya et al. [97] reported that the workpiece machined It is known that the surface of titanium alloy and stainless
using a powder (Si, SiC or Al) suspended oil shows not only steel can be colored by anodic oxidation due to the
improved surface roughness but also increased corrosion interference of light in the oxide film formed by electrolysis.
resistance compared to the surface machined without
powder. Masui et al. [146] added fine tungsten powder in
the dielectric fluid to form an alloyed layer and found that Tool electrode (+)
great improvements in surface roughness, hardness,

Servo deposit

Hydrocarbon
Ti electrode
dielectric Power Work (-) Work (-) Work (-)
supply
Hard layer
Figure 58: Fabrication of micro-structure using EDM deposition
(Tool and workpiece: carbon steel, medium: air) [154].
－
Workpiece W ire e le c tro d e
Incident beam Interfering beams
W o rk p ie c e
(T ita n iu m a llo y)
Figure 56: Principle of EDM surface modification [148]. ＋
air P ro c e s s in g
d ire c tio n
h TiO2 film
Ti
absorption 1 s t-c u t s u rfa c e C o lo re d s u rfa c e
TiC layer (6µm)
Interference of light

S45C

Figure 57: Surface modification of carbon steel

by TiC semi-sintered electrode [149]. Figure 59: Coloring of titanium alloy with EDM [155].
Hence an arbitrary color is given to the WEDMed surface material,whereas diamond was evaporated due to the heat
during the finish cutting process as shown in Figure 59. The flux from the discharge column generated between the tip
color can be changed by controlling the feed rate of the of the needle electrode and the diamond surface because
wire electrode or varying the open voltage because the the dielectric hysteresis loss of diamond was insignificant.
color is determined by the oxide layer thickness. Using a Uno et al. [163], Kawada et al. [164] and Luo et al. [165]
simple electrode controlled by an NC servo system in reported that one of the difficulties encountered in the EDM
sink-EDM, fine and multicolored pattern can be drawn on slicing of silicon wafers is the large voltage drop caused by
titanium products [156]. the high electric resistivity of the silicon single crystal, while,
7.3 EDM of non-conducting materials Kawada et al. reported that this large voltage drop also
It has been believed that only electrically conductive results from the high contact resistance at the interface
materials can be machined by EDM. Diamond however can between the silicon wafer and metal electric feeder.
be machined by means of EDM by providing diamond with Generally, even at the interface between contacting smooth
a conducting coating of graphite by heating the diamond in plane plates, the area of real contact is very narrow and the
a non-oxidizing flame or in a pyrolitic atmosphere voltage drop is significant due to the highly concentrated
comprising a carbon compound until the temperature of electric current density in the area of real contact. Hence,
conversion to graphite is reached. When discharge occurs the voltage drop at the interface between metal and
between the tool electrode and the graphite coating, the high-electric-resistivity material is enormous, whereas that
temperature at the discharge spot increases above the in a metal-to-metal contact can be ignored compared with
conversion temperature of diamond-graphite, the bottom of the voltage drop in the discharge channel. They have
the crater is coated by newly formed graphite, hereafter the therefore demonstrated that the machining rate can be
process is repeated [157, 158]. improved by forming a low electric resistivity layer over the
surface of the silicon wafer. On the other hand, the contact
Fukuzawa et al. [159-161] found a method for EDMing resistance is caused not only by the small area of real
completely nonconductive ceramics. As shown in Figure 60, contact but by the difference in the work function between
a metal plate or mesh is placed on the ceramics to be materials in contact which creates the Schottky barrier. Luo
machined. In the first stage, discharges generate between et al. [165] and Kunieda et al. [166] eliminated the effects of
the metal plate and tool electrode. While the metal plate is the Schottky barrier in metal-silicon contact by
being eroded, the working oil is thermally decomposed and electroplating the silicon surface with metal like Ni, Al and
the pyrolitic carbon is deposited on the workpiece whose Sb-Au to convert the rectifying contact to an ohmic one and
polarity is positive. This is the same phenomenon of carbon reduce the contact resistance in the EDM of the silicon
deposition on the tool electrode observed in the usual die single crystal.
sinking, which was described in Section 3.6. Even after the
metal plate has eroded out, the surface of the ceramics is 7.4 Dry EDM
covered with the carbon layer which is electrically Based on the consideration described in Section 3.7,
conductive. Hence, discharge continues to occur over the Kunieda et al. [58, 167-169] developed the dry EDM
ceramics as long as the deposition of carbon due to the method shown in Figure 62. A high velocity gas flow is
discharge is maintained. This method enables EDM supplied through a thin-walled pipe electrode, causing
machining of even 3D shapes of Si3N4, ZrO2, SiC, Al2O3, molten workpiece material to be removed and flushed out
AlN, glass, old ceramics, and diamond. Figure 61 [161] of the working gap without being reattached to the
shows an example of machining a chair-shaped product of electrode surfaces. During the pulse interval, the gas jet
Si3N4 using WEDM. blows off the plasma formed by the previous pulse
Taniguchi et al. [162] used a micro wave to machine discharge and decreases the temperatures of the
alumina ceramics and diamond. These workpieces were discharge spots. Environmentally, this process is clean
placed in the gap between a pair of needle electrodes because there is no generation of hazardous gas nor waste
whose axis was oriented in parallel to the electric field of from the dielectric liquid. There is no risk of fire hazards
the micro wave standing in a micro wave tube. Alumina because no flammable dielectric is used. NTIS Tech Note
ceramic was melted due to the dielectric heating inside the [170] reports briefly that in NASA holes in complicated
hardware were drilled successfully by a dry EDM process
Linear variable differential that uses a tubular copper electrode and argon or helium
transformer gas as the dielectric medium. However, NASA does not
disclose any further information. Advantages of the dry
Recorder EDM are as follows:
Tool electrode
EDM power - Using oxygen gas for the machining of steel, even
(-)
supply higher material removal rate than the conventional
EDM can be obtained due to the heat generation by
Digital
the oxidation of steel.
oscilloscope
Ceramics - Tool electrode wear is almost negligible for any
Assisting
electrode (+)
discharge duration.

Figure 60: Schematic illustration of EDM for insulating

ceramics with an assisting electrode method [159]. Pipe tool electrode Oxygen gas

Guide
Debris

Workpiece
Figure 61: Machining of insulating ceramics by
wire EDM (workpiece: Si3N4 ) [161]. Figure 62: Dry EDM milling.
- Heat affected zones such as the white layer are thin microscopes, high-speed imaging devices, and software
and micro cracks are few. tools for numerical analysis, fundamental studies are
- Process reaction force is negligible. progressing step by step, and some of the common EDM
knowledge which have been accepted for a long time are
- Gap is narrower than conventional EDM. being modified. On the other hand, given that numerous
- There is no corrosion which is caused by electrolysis discharge gap phenomena have yet to be fully understood,
when deionized water is used as dielectric fluid. the potentials of EDM technology may not be fully realized.
Hence dry EDM is also suitable for WEDM and micro EDM Undoubtedly, continuous efforts are expected to lead to
owing to the negligible process reaction force and the further developments of the EDM technology in the future.
minimized surface damage. Furudate et al. [171,172] found
that, when it is applied to finish-cutting in WEDM (Figure 9 ACKNOWLEDGEMENTS
63), straightness and geometrical accuracy of the finished The authors would like to acknowledge the following
surface are excellent owing to the significantly reduced colleagues who contributed to this paper: Prof. D. Allen, Dr.
vibration of the wire electrode due to the considerably H. Altena, Dr. P. Bleys, Dr. A. Goto, Dr. S. Hayakawa, Dr. T.
smaller process force compared with that in conventional Ishida, Prof. F. Klocke, Prof. J. Kozak, Prof. J.P. Kruth, Prof.
WEDM. G. N. Levy, Dr. D. Lung, Mr. T. Masaki, Dr. K. Masui, Prof. T.
Masuzawa, Prof. T. Matsuo, Dr. H. Minami, Prof. N. Mohri,
Prof. W. Natsu, Prof. Y. Takeuchi, Prof. Y. Uno, Prof. Z.Y.
Wire electrode Yu.

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