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Journal of Minerals & Materials Characterization & Engineering, Vol. 9, No.8, pp.709-739, 2010
jmmce.org Printed in the USA. All rights reserved
MRR Improvement in Sinking Electrical Discharge Machining: A Review
1Kuldeep Ojha*, 1 R. K. Garg, 2K. K. Singh
1 Department of Industrial and Production Engineering, Dr B. R. Ambedkar Natio nal Institute of
Technology, Jalandhar-144011, Punjab, India
2 Department of Mechanical Engineering & Mining Machinery Engineering, Indian School of
Mines (ISM), Dhanbad-826004, Jharkhand, India
*Corresponding author: email@example.com
Electrical discharge machining (EDM) is one of the earliest non-traditional machining
processes. EDM process is based on thermoelectric energy between the workpiece and an
electrode. Material removal rate (MRR) is an important performance measure in EDM process.
Since long, EDM researchers have explored a number of ways to improve and optimize the MRR
including some unique experimental concepts that depart from the traditional EDM sparking
phenomenon. Despite a range of different approaches, all the research work in this area shares
the same objectives of achieving more efficient material removal coupled with a reduction in tool
wear and improved surface quality. The paper reports research on EDM relating to
improvement in MRR along with some insight into mechanism of material removal. In the end of
the paper scope for future research work has been outlined.
Keywords: EDM, parameters, MRR, dielectric, powder, variations, milling, ultrasonic,
Electrical discharge machining is basically a non-conventional material removal process which is
widely used to produce dies, punches and moulds, finishing parts for aerospace and automotive
industry, and surgical components . This process can be successfully employed to machine
electrically conductive part s irrespective of their hardness, shape and toughness [2-4].
710 K ul deep Ojha, R. K. Garg, K. K. Singh Vol.9, No.8
The review presented in this paper is on different techniques proposed and investigated by
researchers resulting in improvement in material removal rate in EDM. Being an important
performance measure, MRR improvement has always been a major area of focus for researchers
and scrutiny of the published research work emphasized the need for such a review paper
reporting all the available literature and suggesting the future direction for research. The end of
the paper identifies the major EDM academic research area and suggests future direction for the
EDM research as a novel contribution to the archival literature.
1.1 Working Principle of EDM
The working principle of EDM process is based on the thermoelectric energy. This energy is
created between a workpiece and an electrode submerged in a dielectric fluid with the passage of
electric current. The workpiece and the electrode are separated by a specific small gap called
spark gap. Pulsed arc discharges occur in this gap filled with an insulating medium, preferably a
dielectric liquid like hydrocarbon oil or de-ionized (de-mineralized) water [5-8]. Schumacher 
described the technique of material erosion employed in EDM as still arguable. This is because
ignition of electrical discharges in a dirty, liquid filled gap, when applying EDM, is mostly
interpreted as ion action identical as found by physical research of discharges in air or in
vacuum as well as with investigations on the breakthrough strength of insulating hydrocarbon
The working principle of EDM is shown in Fig. 1. This technique has been developed in the late
1940s . The electrode moves toward the workpiece reducing the spark gap so that the applied
voltage is high enough to ionize the dielectric fluid . Short duration discharges are generated
in a liquid dielectric gap, which separates electrode and workpiece. The material is removed
from tool and workpiece with the erosive effect of the electrical discharges . The dielectric
fluid serves the purpose to concentrate the discharge energy into a channel of very small cross
sectional areas. It also cools the two electrodes, and flushes away the products of machining
from the gap. The electrical resistance of the dielectric influences the discharge energy and the
time of spark initiation . Low resistance results in early discharge. If resistance is large, the
capacitor will attain a higher charge value before initiation of discharge. A servo system is
employed which compares the gap voltage with a reference value and to ensure that the electrode
moves at a proper rate to maintain the right spark gap, and also to retract the electrode if short-
circuiting occurs. When the measured average gap voltage is higher than that of the servo
reference voltage, preset by the operator, the feed speed increases. On the contrary, the feed
speed decreases or the electrode is retracted when the average gap voltage is lower than the
reference voltage, which is the case for smaller gap widths resulting in a smaller ignition delay.
Thus short circuits caused by debris particles and humps of discharge a crater are avoided. Also
quick changes in the working surface area, when tool shapes are complicated, does not result in
hazardous machining. In some cases, the average ignition delay time is used in place of the
Vol.9, No.8 MRR Improvement in Sinking Electrical Discharge Machining 711
average gap voltage to monitor the gap width  The RC circuit employed in EDM did not
give good material removal rate, and higher material removal rate was possible only by
sacrificing surface finish. A major portion of the time of machining was spent on charging the
capacitors as shown in Fig. 2. .
Figure 1: Working principle of EDM
Figure 2: Variation of capacitor voltage with time in RC circuit .
Figure 3: Schematic view of discharge gap 
712 K ul deep Ojha, R. K. Garg, K. K. Singh Vol.9, No.8
Fig. 3 shows a systematic view of EDM spark spot . The arc column diameter increases with
the passage of time [16-18] and equal to the diameter of the generated discharge crater . In
the spark gap electrode materials and dielectric liquid are evaporated, molecules are dissociated,
and atoms are ionized, resulting in a rapid expansion of the bubble from dielectric fluid. Since
the expansion is restricted by the inertia and viscosity of the dielectric, the pressure inside the
bubble becomes extremely high and the boundary between the bubble and liquid expands with
the velocity of several tens m/s [19, 20]. It is still believed that the dielectric liquid plays a
significant role in material removal because the high pressure and velocity field in the bubble
may serve as the dynamics of the material removal in EDM [21, 22]
In this process there is no direct contact between the electrode and the workpiece thus
eliminating mechanical stresses, chatter and vibration problems during machining . Trends on
activities carried out by researchers depend on the interest of the researchers and the availability
of the technology. Rajurkar  has indicated some future trends activities in EDM: machining
advanced materials, mirror surface finish using powder additives, ultrasonic-assisted EDM and
control and automati on.
1.2 Process Parameters and Performance Measures
The process parameters and performance measures are shown in Fig. 4. The process parameters
can be divided into two categories i.e. electrical and non-electrical parameters. Major electrical
parameters are discharge voltage, peak current, pulse duration and pulse interval, electrode gap,
polarity and pulse wave form. The EDM process is of stochastic thermal nature having
complicated discharge mechanism . Therefore, it is difficult to explain all the effect of these
parameters on performance measures. However, researchers now rely on process analysis for
optimization of parameters to identify the effect of operating variables on achieving the desired
machining characteristics. Lin et al.  applied grey relational analysis for solving the
complicated interrelationships between process parameters and the multiple performance
measures. Taguchi approach has also been used by many other researchers to analyze and design
the ideal EDM process [26-28].
Main non-electrical parameters are flushing of dielectric, workpiece rotation and electrode
rotation. These non electrical parameters play a critical role in optimizing performance measures.
Researches on flushing pressure reveal that it affects the surface roughness, tool wear rate, acts
as coolant and also plays a vital role in flushing away the debris from the machining gap [29-31].
Workpiece rotary motion improves the circulation of the dielectric fluid in the spark gap and
temperature distribution of the workpiece yielding better MRR and SR . Similarly, electrode
rotation results in better flushing action and sparking efficiency . Therefore, improvement in
MRR and SR has been reported due to effective gap flushing by electrode rotation [34-36].
Vol.9, No.8 MRR Improvement in Sinking Electrical Discharge Machining 713
Major performance measures in EDM are MRR, TWR and SR. For MRR, research work has
been focused on material removal mechanism [37-39] and methods of improving MRR [40-43].
Similarly for TWR, research work on tool wear process and methods of improvement in TWR
has been reported [44-46]. Though EDM is essentially a material removal process, efforts have
been made to use it as a surface treatment method and/or an additive process also. Many surface
changes have been reported ever since the process established itself in the tool rooms of
manufacturing industry .
Figure 4: Process parameters and performance measures of EDM Process
2. MECHANISM OF MATERIAL REMOVAL
The first serious attempt of providing a physical explanation of the material removal during
electric discharge machining is that of Dijck . In his Ph.D. thesis, he presented physico-
mathematical analysis of the process. He presented a thermal model together with a
computational simulation to explain the phenomena between the electrodes during electric
discharge machining. However, as he himself admitted in his thesis, the number of assumptions
made to overcome the unavailability of experimental data at that time was quite significant.
Further enhanced models were developed in the late eighties and early nineties trying to explain
the phenomena that occur during electric discharge machining in terms of heat transfer theories.
714 K ul deep Ojha, R. K. Garg, K. K. Singh Vol.9, No.8
Probably the most advanced explanation of the EDM process as a thermal process w as developed
during an investigation carried out at Texas A&M University: this stream of research resulted in
a series of three research papers.
In the first paper, a simple cathode erosion model for the process was presented . This point
heat-source model differed from previous conduction models in the way that it accepts power
rather than temperature as the boundary condition at the plasma/cathode interface. Optimum
pulse times were predicted to within an average of 16% over a two-decade range after the model
is tuned to a single experimental point. In this model, a constant fraction of the total power
supplied to the gap was transferred to the cathode over a wide range of currents. A universal,
dimensionless model was then presented which identifies the key parameters of optimum pulse
time factor (g) and erodibility (j) in terms of the thermo physical properties of the cathode
material. Compton's original energy balance for gas discharges was amended for EDM
conditions. Here it was believed that the high density of the liquid dielectric causes plasmas of
higher energy intensity and pressure than those for gas discharges. These differences of
macroscopic dielectric properties affect the microscopic mechanisms for energy transfer at the
cathode. In the very short time frames of EDM, the amended model uses the photoelectric effect
rather than positive-ion bombardment as the dominant source of energy supplied to the cathode
As a second in a series of theoretical models, an erosion model for the anode material was
presented . As with the point heat-source model in the previous article, this model also
accepts power rather than temperature as the boundary condition at the plasma/anode interface. A
constant fraction of the total power supplied to the gap is transferred to the anode. The power
supplied was assumed to produce a Gaussian-distributed heat flux on the surface of the anode
material. Furthermore, the area upon which the flux is incident was assumed to grow with time.
As a third in a series of theoretical models a variable mass cylindrical plasma model (VMCPM)
was developed for the sparks created by electrical discharge in a liquid media . The model
consists of three differential equations-one each from fluid dynamics, an energy balance, and the
radiation equation-combined with a plasma equation of state. A thermo physical property
subroutine allows realistic estimation of plasma enthalpy, mass density, and particle fractions by
inclusion of the heats of dissociation and ionization for a plasma created from demonized water.
Problems with the zero-time boundary conditions are overcome by an electron balance
procedure. Numerical solution of the model provides plasma radius, temperature, pressure, and
mass as a function of pulse time for fixed current, electrode gap, and power fraction remaining in
However, from a careful reading of these three papers, it emerges that for small discharge
energies the presented models are quite inadequate to explain the experimental data. Also, all
Vol.9, No.8 MRR Improvement in Sinking Electrical Discharge Machining 715
these models are based on a number of assumptions. Later, many alternative models have been
reported in the literature for material removal mechanism in the EDM process. Singh and Ghosh
 re-connected the removal of material from the electrode to the presence of an electrical
force on the surface of the electrode that would be able to mechanically remove material and
create the craters. They proposed thermo-electric model as a general method of calculating the
electrostatic force on the surface of the cathode and the stress distribution inside the metal during
the discharge. The result obtained for the stress distribution deep inside the metal, where the
surface stress acts as a point force, can be extended for any kind of discharge. The model can
explain the experimental results for short pulses. The model proposes that the electrostatic forces
are the major cause of metal removal for short pulses and melting becomes the dominant
phenomenon for long pulses. The model explains the reason for constant crater depth with
varying discharge duration, for short pulses.
Erden  proposed that material removal mechanism relating to three phases of sparking,
namely breakdown, discharge and erosion. Also, it was found that reversing the polarity of
sparking alters the material removal phenomenon with an appreciable amount of electrode
material depositing on the workpiece surface . Gadalla and Tsai  investigated the
material removal of WC–Co composite. They attributed the material removal to the melting and
evaporation of disintegrated Co followed by the dislodging of WC gains, which have a lower
electrical conductivity. However, Lee and Lau  indicated that thermal spalling contributes to
the material removal mechanism during the sparking of composite ceramics. This is because the
physical and mechanical properties promote abrupt temperature gradients from normal melting
A number of researchers have reported that the surface material of eroded electrode differ
considerably from initial one in composition and properties. This surface material consists of
dielectric pyrolysis products and an alloy between matrix and electrode. The workpiece material
may diffuse into electrode surface and influence its wear resistance [35, 56-59]. Several other
researchers have also reported presence of considerable quantity of opposite electrode material in
the surface treated and debris produced [38, 60, 61]. Roethel  proposed a mechanism of mass
transfer of electrode material and determined the change in thermally influenced zone. Pandey
and Jilani  proposed a thermal model on plasma channel growth and thermally damaged
surface layer. The change in chemical composition of material was found to be confined within
re-solidified layer [38, 39, 50, 63-65].
Although several researches have tried to explain the material removal mechanism (MRM) but in
the light of the many available models, it appears that this mechanism is not yet well understood.
Further investigations are necessary to clarify the MRM. Especially the available models are
based on several assumptions and also there is lack of enough experimental scientific evidence to
716 K ul deep Ojha, R. K. Garg, K. K. Singh Vol.9, No.8
build and validate them. This is due to stochastic thermal nature having complicated discharge
3. METHODS OF IMPROVING MRR
Material removal rate is an important performance measure and several researchers explored
several ways to improve it. Although in a major bulk of research papers, researchers tried to
optimize process parameters to get optimum combination of performance measures for different
work-tool interfaces but several researchers tried innovative ways of MRR improvement as well.
While going through the available literature on the process, a need is felt to summarize all the
results and conclusions made by different researchers. This paper presents review of such work
in following six sub-sections.
3.1 By Electrode Design
Several researchers have tried to improve MRR using alternate types of tooling design. The
research work in this area can be classified into following categories.
Investigating suitable electrode material for a particular workpiece material to improve
and optimize the MRR.
Trying different kind of electrode geometries and designs to improve the MRR
In a number of published papers, investigations related to suitable electrode materials have been
reported for machining a particular material and improvement in MRR. In this paper,
investigations related to electrode geometries and designs have been discussed. Generally 3-D
profile electrodes are employed in EDM process which are costly and take more time for their
manufacturing . However, computer numerical controlled EDM machines are now available.
These machines are able to generate complicated shapes without using complex shaped
electrodes . Bayramoglu and Duffill [40, 41] have investigated on frame type copper cutting
tool for machining mild steel workpieces. They have discussed the advantages of using frame
type electrodes. Saito et al.  have conducted a comparative investigation between solid
electrode and wire frame electrode for producing cubic cavities. The authors have reported
improvement in flushing conditions, material removal rate and machining efficiency. They
recommended application of frame type tooling for the workpiece shapes having linear or axi-
symmetrical swept surfaces when high material removal rate is desired. Fig. 5 shows differences
in electrode designs: 3D form tool, frame tool and plate tool. It is reported that the plate type tool
gave better material removal rate in comparison to a 3-D form tool .
Several electrode geometries have also been tried to find improvement in material removal rate.
Researchers have found that hollow tube electrodes and electrodes with eccentric drilling results
Vol.9, No.8 MRR Improvement in Sinking Electrical Discharge Machining 717
in better material removal rate [33, 67-69]. This improvement takes place due to improved
flushing condition arrangement for such designs. Research on 3D form tool with different
geometries revealed that best tool shape for higher MRR and lower TWR is circular in shape,
followed by triangular, rectangular, and square cross sections . Limitation of frame type and
plate type tool is that these tools are applicable only for basic (spheres, conics and simple 2 D
sweeps) and intermediate (complex 2D sweeps, ruled surfaces, and fillets) shapes .
Generally, tool makers use thumb rule or trial and error method for EDM tool design. Therefore,
scientific investigation of electrode design is needed to be developed for industrial application.
Plate and Frame type of tools was are used to produce four categories of 3D shaped workpiece
cavities by CNC EDM. With CNC EDM machines the machining can take place in the X, Y, Z
and rotational C directions. But a market survey showed that many of the CNC capabilities had
not being fully utilized by ‘toolmakers’ . The design and manufacture of 3D shaped
718 K ul deep Ojha, R. K. Garg, K. K. Singh Vol.9, No.8
workpiece was still largely dependent on plunging a formed tool along the z-axis rather than
using the multi-axis CNC capabilities. Plate and frame type tool may find wide industrial
application in near future.
3.2 By Controlling Process P a r am e ters
The material removal rate can be controlled and improved by controlling process parameters.
The first parameter affecting the MRR is discharge voltage. This parameter is basically related to
spark gap and dielectric strength . Before the current flows, the voltage increases causing
ionization path in dielectric fluid. This voltage is called open gap voltage. Once current starts
flowing, the voltage drops and stabilizes the working gap level. Thus higher voltage setting
results in higher spark gap. Due to higher spark gap, flushing conditions improves resulting in
higher MRR and rough surface. Electric field strength increases by increasing open circuit
voltage resulting in higher MRR.
The amount of power used in machining depends upon peak current. During each pulse on-time
the current increases up to preset value called peak current. The peak current is governed by
surface area of cut. Higher amperage improves material removal rate but at a cost of tool wear
and surface quality. Higher peak current setting is applied for roughing operation.
The amount of energy applied during machining is controlled by peak current and pulse duration
. Longer pulse duration results in higher material removal resulting in broader and deeper
crater formation. However, too much pulse duration is counter productive and once optimal
value for a particular workpiece- electrode combination is exceeded, material removal rate starts
decreasing. Pulse interval influences the speed and stability of the cut. In theory, the shorter
interval results in faster machining operation. But if the interval is too short, the ejected
workpiece material will not be swept away by the flow and the fluid will not be deionized
resulting in unstable next spark.
Material removal rate is highly affected by types of dielectric and method of flushing .
Flushing is a useful procedure to remove debris from discharge zone even if it is difficult to
avoid concentration gradient and inaccuracy [73, 74]. The influence of flushing on MRR and
electrode wear has been studied by mathematical models and experimentally and many flushing
methods have been proposed . The dielectric fluid is flushed through the spark gap to remove
gaseous and solid debris during machining and to maintain the dielectric temperature well below
flash point. Better flushing conditions results in increase in both increase in material removal rate
and tool wear. Better flushing conditions is reported by introducing electrode rotation, workpiece
rotation, increasing flushing pressure, and tube electrode design.
Vol.9, No.8 MRR Improvement in Sinking Electrical Discharge Machining 719
Scrutiny of the published literature reveals that most of the published work in EDM relates to
improvement in performance measures, parametric optimization and selection of tool and
workpiece combination. So a lot of empirical data is available in handbooks and published
papers regarding optimum parameters settings for a work-tool interface. These published data are
widely used in industry in application of EDM technology.
3.3 By EDM Variations
A number of EDM variations have been developed to improve performance measures.
Development of hybrid machining processes involving combined operation of other machining
processes with EDM is an important variation. Introduction of ultrasonic vibration to the tool is
one of the methods applied to expand the application of EDM and to improve the machining
performance on difficult to machine materials. The investigation on the effects on ultrasonic
vibration of the electrode has been undertaken since mid 1980s. The higher efficiency gained by
the employment of ultrasonic vibration is mainly due to improvement in dielectric circulation.
Better dielectric circulation facilitates the debris removal and the creation of a large pressure
change between the electrode and the workpiece, as an enhancement of molten metal ejection
from the surface of the workpiece . Ghoreishi and Atkinson  compared the effects of
high and low frequency forced axial vibration of the electrode, rotation of the electrode and
combinations of the methods (vibro-rotary) in respect of MRR, tool wear ratio and surface
quality in die sinking EDM with flat electrode. It is reported that the combination of ultrasonic
vibration and electrode rotation leads to increase in MRR and TWR. The vibro-rotary increases
MRR by up to 35% compared with vibration EDM and by up to 100% compared with rotary
EDM in semi finishing. Fig. 6 shows difference in Vibratory, rotary and vibro-rotary electrodes.
Zhang et al.  investigated the ultrasonic EDM in gas. The gas was applied through the
internal hole of a thin-walled pipe electrode. The result showed an increase in MRR with respect
to the increase of open voltage, pulse duration, amplitude of ultrasonic actuation, discharge
current and the decrease of the wall thickness of electrode pipe. Gunawan et al.  studied the
effect of vibrated workpiece. They reported that when the vibration was introduced on the
workpiece the flushing effect increased. They have found that high amplitude combined with
high frequency resulted in increase in MRR. Researchers have investigated combinations of
workpiece vibration, tool vibration with electrode rotation.
Han et al.  proposed a novel high speed EDM milling method using moving arc. They
connected a copper electrode rotating rapidly around its axis and a workpiece to a DC power
supply to generate a moving electric arc. The electrode was shaped like a pipe in order to ensure
a high relative speed of any point on the electrode with respect to the workpiece. It was found
that the MRR of EDM milling is almost four times greater than that of traditional EDM without
720 K ul deep Ojha, R. K. Garg, K. K. Singh Vol.9, No.8
any deterioration in surface roughness. The increase in MRR is due to enhanced duty cycle
during EDM milling.
A hybrid machining process (HMP) involving high-speed machining (HSM) was proposed by
researchers [81-83]. An increase in material removal rate was reported but success of such
machining was found to be dependent to a large extent on the availability and performance of a
single cutting / dielectric fluid. Xu et al.  introduced a new kind of electrical discharge
machining technology named tool electrode ultrasonic vibration assisted electrical discharge
machining in gas medium. Experimental results showed that material removal rate could be
increased greatly by introducing ultrasonic vibration. The comparison of MRR in traditional
EDM in gas and ultrasonic vibration assisted gas medium EDM for machining cemented
carbides workpiece was reported. MRR was found considerably higher for a particular discharge
pulse-on time for ultrasonic vibration assisted machining.
Figure 6: Vibratory, r o tary and vibro-rotary electrode
There is widespread academic and industrial interest in the development and use of hybrid
machining process involving high-speed machining (HSM), grinding and EDM assisted by
ultrasonic vibration. Ultrasonic vibration EDM is suitable to produce deep and small holes
products. Ultrasonic EDM in gas and high speed EDM milling method using moving arc
technology are at research and development stage and requires through investigation before
3.4 By Powder Mixed Dielectric
Powder mixed electric discharge machining (PMEDM) is one of the new innovations for the
enhancement of capabilities of electric discharge machining process. In this process, a suitable
material in fine powder is properly mixed into the dielectric fluid. The added powder improves
the breakdown characteristics of the dielectric fluid. The insulating strength of the dielectric fluid
Vol.9, No.8 MRR Improvement in Sinking Electrical Discharge Machining 721
decreases and as a result, the spark gap distance between the electrode and workpiece increases
[85-87]. Enlarged spark gap distance makes the flushing of debris uniform. This results in much
stable process thereby improving material removal rate and surface finish. Fig. 7 show the
principle of powder mixed EDM.
Figure 7: Principle of powder mixed EDM 
A voltage of 80–320V is applied between the spark gap of 25–50 µm and an electric field in
the range of 105–107 V/m is created . The powder particles become energized and behave in
a zigzag fashion. These charged particles are accelerated due to the electric field and act as
conductors promoting breakdown in the gap. This increases the spark gap between tool and the
workpiece. Under the sparking area, these particles come close to each other and arrange
themselves in the form of chain like structures. The interlocking between the powder particles
occurs in the direction of flow of current. The chain formation helps in bridging the discharge
gap between the electrodes. Because of bridging effect, the insulating strength of the dielectric
fluid decreases resulting in easy short circuit. This causes early explosion in the gap and ‘series
discharge’ starts under the electrode area. The faster sparking within a discharge causes faster
erosion from the workpiece surface and hence the material remo v al r a te i n cr ea ses.
A number of research works have been reported for different combinations of materials, powders
and operating conditions. Erden and Bilgin  investigated mixing of copper, aluminum, iron
and carbon powders in kerosene oil as dielectric for machining of brass–steel and copper–steel
pairs. The machining rate was found to increase with powder particle concentration obtained due
to the decrease in time lags at high impurity concentrations. Jeswani  investigated the effect
of the addition of fine graphite powder into kerosene oil as dielectric. The experimentation
resulted in 60% increase in MRR and 28% reduction in wear ratio.
722 K ul deep Ojha, R. K. Garg, K. K. Singh Vol.9, No.8
Yan and Chen [91-93] investigated the effect of suspended aluminum and silicon carbide
powders on EDM of SKD11 and Ti–6Al–4V. It was reported that the MRR improves
considerably whereas the SR increases. Ming and He  investigated conductive and inorganic
particles as powder and increase in MRR, decrease in the TWR and improvement in the surface
quality of the workpiece was reported. Yu et al.  investigated the effects of aluminum
powder on EDM of tungsten carbide. The aluminum powder allowed both higher discharge gap
and MRR. Wang et al.  investigated the effect of mixing Al and Cr powder mixture in
kerosene. It was found that machining parameters have remarkable influence on the machining
characteristics. The results indicate that Al and Cr mixture in kerosene fluid reduces the isolation
and increases the spark gap. With this, the process gets stabilized and the MRR is enhanced
considerably. The effect of various powder characteristics on machining of SKD-11 material was
reported by Tzeng and Lee . The various additives mixed in the working fluid were Al, Cr,
Cu and SiC. It was found that the concentration, size, density, electrical resistivity and thermal
conductivity of powders significantly affect the machining performance. Addition of appropriate
amount of powders to the dielectric fluid resulted in increased MRR and decreased TWR. For a
fixed concentration of particles, the smallest size of the particle led to highest MRR and lowest
TWR. The machining characteristics of insulating Si3N4 ceramics by mixing the various powders
into the dielectric fluid were investigated by Tani et al. . It was reported that MRR increased
considerably while the surface finish was not improved so much by using the powder suspended
dielectric fluid. Rozenek et al.  compared machining characteristics by using kerosene
dielectric and mixture of deionized water with different abrasive powders at different
concentrations on hard material. It was reported that the addition of powder in the dielectric
enhances both MRR and TWR. Kansal et al.  established optimum process conditions for in
rough machining phase using the Taguchi method with graphite powder. They reported that
addition of an appropriate amount of the powder into the dielectric causes considerable
improvement in MRR.
Despite the promising results, powder mixed EDM process is applied in industry at very slow
pace. One of the key reasons is that many fundamental issues of this new development, including
the machining mechanism are still not well understood . The complexity of this process,
especially in context with thermo physical properties of the suspended particles deserves a
thorough investigation. Secondly, the difficulty in operation of dielectric interchange, the higher
amounts of powder consumption, the environmental requirements of fluid disposal and its higher
initial cost (two to three times higher than the one required for a conventional EDM system) have
restricted its frequent use .
3.4 Dry EDM and EDM with Water
Kunieda and Yoshida  have discussed the principle of dry EDM and compared its
performance with EDM in oil as dielectric. Dry electrical discharge machining is a process that
Vol.9, No.8 MRR Improvement in Sinking Electrical Discharge Machining 723
uses gas as dielectric medium. The principle of dry EDM is shown in Fig. 8 .This dry
technique has been firstly presented by Kunieda et al.  for environmental preservation,
human health and prevention of fire hazards. The authors found that the material removal rate is
increased due to the enlarged volume of discharged crater and more frequent occurrence of
discharge. Kunieda et al.  have made improvement in dry EDM technique by introducing
high speed 3D milling. The MRR increased when the discharge power density on the working
surface exceeded a certain threshold limit due to thermally activated chemical reaction between
the gas and workpiece material. The maximum removal rate obtained was almost equal to that of
high speed milling of quenched steel workpiece on a milling machine. Yu et al.  reported a
comparison in machining characteristics of oil EDM milling and dry EDM milling. According to
the results, the material removal rate of dry EDM milling is about 6 times larger than that of oil
Figure8: Principle of dry EDM 
Other improvement in dry EDM technique is ultrasonic vibration assisted dry EDM. Research
developments in this variation have been discussed in previous section 3.3.
Water as an alternative to hydrocarbon oil has been taken to promote green EDM process
because hydrocarbon oil as dielectric decomposes and releases harmful vapors. Jeswani 
compared the performances of kerosene and distilled water over the pulse energy range of 72–
288 mJ. Machining in distilled water resulted in a higher material removal rate and a lower wear
ratio than in kerosene for high pulse energy range. Jilani and Pandey  have investigated
water as dielectric using distilled water, tap water and a mixture of tap and distilled water in 25%
and 75% ratio. The best machining rates was obtained with tap water. Konig and Siebers 
explained the effect of working medium on material removal process. It was found that erosion
process in water-based medium possesses higher thermal stability and much higher power input
can be achieved especially under critical conditions, resulting in much greater increases in the
724 K ul deep Ojha, R. K. Garg, K. K. Singh Vol.9, No.8
removal rate. Chen et al.  found that carbide is formed on the workpiece surface while using
kerosene while oxide is formed on the workpiece surface while using distilled water for Ti–6Al–
4V alloy. The MRR is greater and the relative wear ratio is lower when machining in distilled
water rather than in kerosene.
The applied sinking EDM technology in industry still use paraffin as dielectric. Distilled water is
being used as dielectric in wire EDM technology. The technologies of dry EDM and sinking
EDM with water are at research and development stage. However, due to increasing focus on
green machining concept, these technologies are likely to gain much focus and industrial
application in near future.
3.6 Some Other Techniques Used
Kunieda and Muto  proposed the multi-spark EDM method which was newly developed to
obtain higher removal rates and lower energy consumption compared with conventional EDM.
This technique is a modification of the basic EDM principle because in basic EDM, only single
discharge is delivered for each electrical pulse. To set up multiple discharge points for each pulse
two types of electrode design has been proposed by researchers. Mohri et al.  divided a tool
electrode into multiple electrically insulated electrodes connected to the pulse generator through
a resistor. In this case, after a discharge occurs in the gap between one of the divided electrodes
and the workpiece, the gap voltages at other electrodes are maintained at the open circuit voltage
level for a certain period of time until the surface electric charge over these electrodes is
redistributed or another discharge occurs. Therefore, finally discharge can occur at different
electrodes simultaneously. Suzuki et al.  and Kubota et al.  proposed a twin electrode
discharge system for the electric discharge dressing of metal bonded grinding wheels. The
discharge circuit was formed by connecting the pulse generator to one of the two twin electrodes,
the grinding wheel, and the other twin electrode serially. In this system, for each pulse, two
discharge points can be obtained simultaneously at both the gaps between the twin electrodes and
the grinding wheel using only one pulse generator. Kunieda and Muto  used twin electrode
discharge system as shown in Fig. 9. The removal rate and energy efficiency were found higher
than those of conventional EDM in which there is only on e di sc ha rg e point for each pulse.
Vol.9, No.8 MRR Improvement in Sinking Electrical Discharge Machining 725
Figure 9: Principle of Multi-spark EDM 
Chen et al.  designed a new mechanism of cutting pipe combined with electrical discharge
machining. This new mechanism was designed with multi electrode system controlled by planet
gear system in die sinking EDM as shown in Fig. 10. The newly developed mechanism resulted
in both increase in material removal rate and relative electrode wear ratio for SUS 304 workpiece
material. Influence of process parameters have been investigated on MRR for developed
mechanism. The largest removal rate and the lowest electrode wear ratio were reported when
workpiece rotates at 8 rpm.
Figure 10: Difference between conventional and multi electrode EDM 
The industrial/ commercial relevance of these approaches are mentioned in section 4.
726 K ul deep Ojha, R. K. Garg, K. K. Singh Vol.9, No.8
4. REMARKS AND FUTURE TRENDS
The objective of the review article has been aimed to report the work of various researchers for
improving material removal rate during EDM and to bridge the gap between the untouched
areas. After an elaborate scrutiny of the published work, the following remarks emerge from the
existing published work.
Various theoretical models describing material removal mechanism have been proposed by
the researchers from time to time. Major limitation of these models is that the models are
based on several assumptions. Therefore, these models cannot be universalized/ applicable to
all conditions. Case to case empirical models is better suitable for quantifying material
removal rate. So there is a scope to develop more of such models in future research works
and also to find solutions to the assumptions made by researchers.
Most of research work in EDM relates to use of 3D form tool. Alternate types of tools like
frame type and plate type are yet to be tried for more work-tool interfaces.
Even in 3D form tools, not much published work is available corresponding to use of
different tool cross-sectional geometries like rectangular, triangular etc. on performance
measure like MRR, EWR etc. Therefore, effect of different tool geometries on MRR, EWR,
surface roughness etc. has to be explored for more work materials.
Hollow tube and eccentric drilled holes type electrodes are reported to have a positive impact
on MRR due to improved flushing conditions. Such designs need investigations for more
work materials to evaluate their cas e to ca se ef fe ct s.
Some non-electrical parameters like electrode rotation and workpiece rotation while
machining improve the flushing conditions and thus may improve MRR. Case to case impact
of these parameters while machining may be evaluated for more work materials.
A lot of research works using hybrid technique of ultrasonic vibration assisted EDM has been
carried out on steel based materials due to their wide industrial applications. Not so much
published work is reported on composites and harder materials like alumina and ceramics.
This hybrid technique can be tried for new material combinations. Further, there are only a
few published papers on vibro-rotary EDM for different work materials.
Copper electrode has frequently been used as electrode material in ultrasonic vibration
assisted EDM. Other electrode materials need to be investigated thoroughly.
Very less work has been reported on MRR improvement using powders of important alloying
elements like chromium and vanadium. Also, many materials like water hardened die steel,
molybdenum high speed tool steel have not been tried as work material in powder mixed
electric discharge machining. The same may be tried in future works.
The dry EDM technique in combination with sinking EDM, wire EDM, ultrasonic assisted
EDM and EDM milling may be tried for optimization of MRR, EWR, surface roughness etc.
in future works.
Vol.9, No.8 MRR Improvement in Sinking Electrical Discharge Machining 727
Performance of water based dielectric is yet to be investigated for machining materials like
composites and carbides.
Multi spark EDM and multi electrode EDM are relatively new techniques for MRR
improvement and are still in experimental stage. More empirical validation using different
workpiece materials is required before the method may be recommended for commercial
There is negligible published work available on comparative analysis of various EDM
techniques of MRR improvement with same/different work materials in EDM.
In this paper, review of EDM research work related to MRR improvement has been presented
along with some insight into the basic EDM process and material removal mechanism. The
major research development resulting in improvement in material removal rate is summarized in
Table 1in chronological order. It is found that the basis of controlling and improving MRR
mostly relies on empirical methods. This is largely due to stochastic nature of the sparking
phenomenon involving both electrical and non-electrical process parameters along with their
complicated interrelationship. Being an important performance measure, the MRR has been
getting overwhelming research potential since the invention of EDM process, and requires more
study/experimentat ion /mod eli ng i n f utu re.
Table 1: Major research development in chronological ord er r e su lt ing in MRR improvement
researchers Year Contribution Workpiece
Parameters taken into
Mild steel Copper Voltage,
Ming and He
 1995 Investigated
the effect of
steel and high
728 K ul deep Ojha, R. K. Garg, K. K. Singh Vol.9, No.8
Wong et al.
 1995 Investigated
of flushing on
and stability of
AISI 01 tool
steel Copper Pulse
Yoshida  1997 Investigated
with EDM in
Steel (S45C) Copper Voltage,
Wong et al.
 1998 Investigated
in EDM with
steel copper Spark
Chen et al.
 1999 Investigated
water as the
Yan  2000 Compared the
composite Copper polarity,
Vol.9, No.8 MRR Improvement in Sinking Electrical Discharge Machining 729
Muto  2000 Investigated
steel SUJ2 Copper Voltage,
 2001 Investigated
Steel Graphite Voltage Electrode
Lee  2001 Investigated
the effects of
SKD11 Copper Spark
Zhao et al.
 2002 Performed
Steel Copper Current,
730 K ul deep Ojha, R. K. Garg, K. K. Singh Vol.9, No.8
Atkinson  2002 Investigated
the effect of
high and low
Tool and die
steel A1S1 01Copper Open
Mohan et al.
 2004 Investigated
effect of tube
plate type tool
with 3 D form
Steel Copper Voltage,
Singh et al.
 2005 Optimize the
EN 31 tool
steel Copper Pulse on
of the added
Vol.9, No.8 MRR Improvement in Sinking Electrical Discharge Machining 731
Chen et al.
 2008 Introduced a
SUS 304 Copper Peak
Han et al.
 2009 Proposed a
Mold steel Copper Open
Xu et al.  2009 Proposed the
in gas medium
Zhang et al.
 2002 Proposed and
Steel Copper Voltage,
effects of gas
Kunieda et al.
 2003 Investigated
of 3D cavities
(SS400) Copper Discharge
732 K ul deep Ojha, R. K. Garg, K. K. Singh Vol.9, No.8
using gas as
Yu et al.  2004 Compared
milling and oil
Zhang et al.
 2006 Applied
EDM in gas
steel Copper Open
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