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INTERNATIONAL JOURNAL OF ADVANCED RESEARCH IN
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME
               ENGINEERING AND TECHNOLOGY (IJARET)
ISSN 0976 - 6480 (Print)
ISSN 0976 - 6499 (Online)
Volume 3, Issue 2, July-December (2012), pp. 22-36                  IJARET
© IAEME: www.iaeme.com/ijaret.html
Journal Impact Factor (2012): 2.7078 (Calculated by GISI)
www.jifactor.com
                                                                   ©IAEME




  AN OVERVIEW OF EXPERIMENTAL INVESTIGATION OF NEAR
     DRY ELECTRICAL DISCHARGE MACHINING PROCESS
                                Mane S.G.1, Hargude N.V.2
   1,2
     Department of Mechanical Engineering, PVPIT Budhgaon, Sangli 416416,Maharashtra,
                                          India.
            E-mail: shrikant_mane3665@rediffmail.com; nvhargude@gmail.com .

ABSTRACT

    EDM has achieved a status of being nearly indispensable in the industry because of its
ability to machine any electrically conductive material which is difficult-to-machine
irrespective of its mechanical strength. Out of the three EDM processes viz. wet, dry &
near-dry; near-dry EDM is proved to be most environment-friendly. Further some other
problems like higher discharge energy requirement in wet EDM and the reattachment of
debris to the machined surface in dry EDM can be overcome in near-dry EDM. Also, it is
found that near-dry EDM has the advantage in finish operation with low discharge energy
considering its higher MRR than wet EDM and better surface finish quality than dry EDM.
In view of these factors, near-dry EDM may prove to be the most prominent process
amongst the three EDM processes in near future to finish machine the difficult to machine
materials. Significant work has been done in the parametric optimization of wet EDM
processes. Efforts are also on in the parametric optimization of dry EDM processes.
However, irrespective of its inherent advantages over wet and dry EDM processes, not
much attention has been given towards the parametric optimization of the near-dry EDM
process. It is essential to have information on the optimum operating conditions to make
the near dry EDM process cost effective and economically viable one. If applied as the post
process of direct metal deposition (DMD), the near-dry EDM milling processes can be
targeted to finish the near-net-shape parts produced by DMD. Hence the authors feel that,
there is a wide scope to work in this area to optimize the vital parameters of near-dry EDM
process. The experimental investigations of near dry electrical discharge machining process
carried out by a handful of researchers have been overviewed in present work.

Keywords: Electrical discharge machining, near dry EDM, material removal rate,
  Surface roughness




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International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME


1. INTRODUCTION

      Electrical discharge machining (EDM) is often used to machine difficult-to-machine
materials. EDM has achieved a status of being nearly indispensable in the industry because of
its ability to machine any electrically conductive material irrespective of its mechanical
strength. EDM removes work material by melting and vaporizing it through a series of
discharging electric sparks. The spark removes the work material.
      Conventional EDM processes use liquid dielectric fluid. However, the dielectric fluid,
particularly hydrocarbon oil itself is one of the main sources of pollution in die sinking
electrical discharge machining. Wastes of dielectric oil are very toxic, cannot be recycled and
need to be disposed of appropriately; otherwise, there is a possibility of both the land and
water being polluted[8].The process generates gases and fumes due to the thermal
decomposition of the dielectric. Another main problem of die sink EDM is the high amount
of energy consumed. The energy consumed in the spark gap, which is the effective energy for
the erosion of the material, is usually less than 20% of the total input of electrical energy. On
the other hand, the energy consumed by the dielectric system may represent 50% of the total
input of electrical energy, especially when low values of peak current are used [8].
      Another emerging technology, viz. powder mixed EDM, increases the cost of
machining and also environment-unfriendly like conventional EDM.
      Dry EDM is another technique, which employs gas as a dielectric medium instead of
liquid. Due to the reattachment of debris to the machined surface, dry EDM may have
limitations of meeting the combined material removal rate (MRR) and surface roughness
requirements. The accuracy of surface profile deteriorates with the debris deposition. The
major challenges in dry EDM process are low stability of arc column, low material removal
rate, arcing, poor surface quality as compared to conventional EDM and odor of burning.
However efforts have been made in the experimental investigation and parametric
optimization of dry EDM processes [11-14].
      These problems faced in dry EDM can be reduced & overcome in near- dry EDM by
replacing the gas with the mixture of gas and dielectric liquid. The liquid content in the mist
media helps to solidify and flush away the molten debris and the debris reattachment is
alleviated in near-dry EDM. Compared to the conventional EDM process, near-dry EDM has
another advantage. It does not require a bath of dielectric fluid. Only a small amount of liquid
dielectric fluid is used making the process environment-friendly. Further it has the benefit to
tailor the concentration of liquid and properties of dielectric medium to meet desired
performance targets. Also, it is found that near-dry EDM has the advantage in finish
operation with low discharge energy considering its higher MRR than wet EDM and better
surface finish quality than dry EDM.

2. PRESENT STATUS AND SCOPE

       The metal working fluids (MWFs) are extensively used in conventional machining
processes. The economical, ecological and health impacts of metal working fluids (MWFs)
can be reduced by using minimum quantity lubrication referred to as near dry machining. In
near dry machining (NDM), an air-oil mixture called an aerosol is fed onto the machining
zone [9]. This concept of near dry machining can be well applied in EDM process, the
process being referred to as near-dry EDM process.
       The feasibility of near-dry EDM was explored by Tanimura et. al. in 1989, who
investigated EDM in water mists in air, nitrogen & argon gases. Further investigation of near-
dry EDM was conducted by Kao et. al.(2007) [1], in wire EDM experiments. After the first


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International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME


exploitation by Tanimura (1989), not much study has been conducted on this process until
recently by Kao (2007) in near-dry wire EDM.
         Near dry EDM milling as a super-finishing process to achieve a mirror like surface
finish has been investigated. Near dry EDM exhibits the advantage of good machining
stability and smooth surface finish at low discharge energy input [6]. Advantages of near-dry
EDM were identified as a stable machining process at low discharge energy input because the
presence of liquid phase in the gas environment changes the electric field, making discharge
easier to initiate and thus creating a larger gap distance. In addition, good machined surface
integrity without debris reattachment that occurred in dry EDM was attained since the liquid
in the dielectric fluid enhances debris flushing. Other potential advantages of near-dry EDM
are a broad selection of gases and liquids and flexibility to adjust the concentration of the
liquid in gas. The dielectric properties can thus be tailored in near-dry EDM to meet various
machining needs, such as high MRR or fine surface finish. Also Near dry EDM shows
advantages over the dry EDM in higher material removal rate (MRR), sharp cutting edge and
less debris deposition. Compared to wet EDM, near dry EDM has higher material removal
rate at low discharge energy and generates a smaller gap distance [10]. Also compared with
conventional wet wire EDM, near dry wire EDM consistently produces better Ra values on
PCD coated WC work-pieces, but near dry wire EDM produces lower MRR than wet wire
EDM under some conditions [3]. However, the technical barrier in near-dry EDM lies in the
selection of proper dielectric medium and process parameters.
      From the review of literature it is seen that experimental investigations have been
carried out in order to study the effect of various input parameters like discharge current, gap
voltage, pulse on time, gas pressure, fluid flow rate, electrode orientation and spindle speed
on material removal rate (MRR), surface roughness and tool wear rate and to improve the
performance of near dry EDM process [1-6]. However, it is necessary to optimize the input
parameters for maximum material removal rate (MRR) and minimize the surface roughness
to make the near dry EDM process cost effective and economically viable one.

3. PRESENT WORK

     The experimental investigations of near dry electrical discharge machining process
carried out by a handful of researchers have been overviewed in present work in view of the
following points.
1) Comparative study of wet, dry and near dry EDM in view of the response variables viz.
material removal rate (MRR), surface roughness, gap distance and debris deposition.
2) Study of effect of various electrical input parameters viz. discharge current (ie), gap
voltage (ue), pulse on time (te), pulse interval (to), open circuit voltage (ui) on material
removal rate (MRR), surface finish & tool wear rate (TWR).
3) Study of effect of various machining input parameters viz. gas input pressure, fluid flow
rate and spindle speed on material removal rate (MRR), surface finish & tool wear rate
(TWR).
4) Study of effect of the electrode material and dielectric medium (various liquid-gas
mixtures) on material removal rate (MRR) and surface finish at high and low discharge
currents.
5) Study of effect of fluid flow rate (concentration of the liquid in gas) and discharge current
on gap distance and debris deposition.




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International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME


3.1 Comparative study of wet, dry and near dry EDM in view of the response variables
viz. material removal rate (MRR), surface roughness, gap distance and debris
deposition.

3.1.1 Wire EDM cutting

          MRR envelopes, which illustrate feasible EDM process regions, have been studied
by Miller . MRR envelopes of wet and dry EDM cutting of 1.27- mm- thick Al6061 are
presented as the baseline data for the comparison with two new envelopes of the near dry
EDM. In each envelope, to was varied to find the maximum achievable wire feed rate,
which was then converted to MRR. Four levels of te were selected: 4, 10, 14, and 18 µs.
The upper and lower boundaries of the MRR envelope correspond to the minimum and
maximum values of te (4 and 18 µs). The specific machine limits, maximum and
minimum to (1000 and 6 µs), as well as wire breakage and short- circuit limitations, form
the left and right envelope boundaries of the MRR envelope. The average pulse
current ie is about 25 A. To investigate the relationship between the gap distance and
dielectric fluid properties, the grooves machined at various water flow rates (0, 5, 8,
15, 21, 35, 50, 75 ml/min), as summarized in Table 1 , were studied. The groove quality
and groove width were examined and measured using an optical microscope at 100x
magnification. Three repeated tests were conducted in each experimental setup [1].




 Table 1. Average gap distance in EDM cutting under wet, dry & Near dry conditions

3.1.2 EDM drilling

     Two sets of EDM drilling experiments were conducted. The first set was to
evaluate the drilling speed and hole quality, including the shape variation and debris
deposition, of wet, dry, and near dry EDM. The average pulse current was set at 10 A. The
work-piece used was 1.27- mm-thick Al6061. For wet EDM, the flow rate of de- ionized
water was 107 ml/min. For dry EDM, the air jet pressure was set at 0.62MPa. For near dry
EDM, the water flow rate and the pressure of the carrying air jet were set at 21 ml/min and
0.62MPa, respectively. The hole quality was inspected using an optical microscope at
100x magnification. The second set investigates the effects of water flow rates on EDM
drilling speeds with ie values at 10, 12, and 15 A. Diameters of drilled holes at different water
flow rates were also measured for the investigation of the relationship between the gap
distance and dielectric fluid properties. The water flow rate was varied as 5, 8, 15, 21, and 35
ml/ min as shown in Table 2 [1].




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International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
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Table 2. Average gap distance in EDM drilling under wet, dry and near dry conditions

     Fig.1 shows three MRR envelopes which outline the feasible regions for the wet, dry,
and near dry wire EDM cutting of 1.27- mm- thick Al6061. The average of three
repeated test results is presented. The range of variation of three tests is within 10% of the
nominal value and is consistent for all experimental conditions. For wet and dry wire EDM,




the region of feasible MRR is bounded by the wire breakage, short circuit, and machine limits
of maximum and minimum te (18 and 4µs) and maximum to (1000 µs).

Fig 1.Comparision of boundaries of feasible MRR envelopes for wet, dry and near dry
wire EDM

      The wet EDM has a significantly higher MRR than that of the dry EDM
(21.9mm3/min vs. 0.98mm3/min). At low pulse intervals of to , frequent EDM pulses
generate concentrated heat and lead to wire breakage. The minimum value of to that can
be reached at high level of te without wire breakage, is greatly dependent on the
dielectric fluid used. For wet EDM, due to the higher thermal conductivity of the bulk
water than that of the water–air mixture, to can be as low as 100 µs at te = 18 µs. For the
near dry EDM using water–air mixture at a water flow rate of 5.3 ml/min, the envelope
boundary falls between the wet and dry EDM. The maximum MRR is improved, from
0.98mm3/min in dry EDM, to 2.53mm3/ min. The near dry EDM has a consistently
higher MRR than that of dry EDM for all to and te . However, the wire breakage, due to the

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International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME


lower capability of water–air mixture to relieve the concentrated heat from the wire electrode,
still limits the MRR in near dry EDM at low to. Nevertheless, near dry EDM shows two
advantages. First, there is no short circuit limit at the lower boundary. Second, in the
region of very low- energy input (te= 4 µs and to >150 µs), the MRR in near dry
EDM is higher than that of the wet EDM.
       The close up view of MRR (below 4 mm3/min) vs. to for the wet and near dry EDM is
shown in Fig.2. Three regions, designated as I–III, are identified.
      In Region I (to >650 µs), the near dry EDM has higher MRR than that of wet
EDM because the lower thermal conductivity and heat capacity of the water–air
mixture contribute to less heat dissipation during discharge and a larger portion of
discharge energy for material removal. At the very low discharge energy setup, te= 4 µs,
wet EDM fails to cut due to the short circuit, but near dry EDM still works with fairly low
MRR. The higher dielectric strength of the water medium generates a narrow gap
distance and causes a frequent short circuit in wet EDM.
        In Region II (250 < to <650 µs), the MRR of near dry and wet EDM is roughly the
same. At the highest te ( = 18 µs), the MRR of wet EDM starts to exceed that of near dry
EDM. Under higher energy input, the higher viscosity of the water dielectric fluid in wet
EDM generates larger explosion force, which contributes to the high MRR.
      In Region III (to<250 µs), a significant MRR difference exists between wet and near
dry EDM. The MRR drops in near dry EDM and, wire breakage occurs as to is
further reduced. The dielectric fluid viscosity is critical to the MRR in Region III.




Fig. 2.Comparison of MRR performances of wet and near dry wire EDM under varied t0 and te and
three regions based on near dry and wet EDM performance (ie= 25 A, ue= 45 V).

     Optical micrographs of top and cross-sectional side views of EDM drilled holes and the
drilling time under the wet, dry, and near dry conditions are shown in Fig. 3. The dry
EDM takes 428 s to drill a hole through the 1.27- mm thick Al6061. This is very
long Compared to the 11 and 13 s drilling time for the wet and near dry EDM
respectively. The dry EDM also has a severe debris deposition problem, which
subsequently creates a tapered hole. The taper in wet EDM also exists but is not as
significant as in dry EDM. The smallest taper exists in holes drilled by near dry EDM,
which generates a straight hole with sharp edges.


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International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
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    The electrode wear in near dry EDM is 3.7 mg per hole, which is larger than the 2.7 mg
per hole in wet EDM. The higher thermal load on the electrode in near dry EDM likely
causes the higher electrode wear. The same phenomenon also exists in near dry wire EDM.
As shown in Fig.2, at low to the wire breakage due to electrode wear limits the MRR in near
dry wire EDM.
    The groove width in wire EDM is used to estimate the gap distance. The average gap
distance under the wet, dry, and near dry wire EDM and the associated dielectric strength and
viscosity of the dielectric fluid are listed in Table 1.




Fig.3. Optical micrographs on holes drilled on 1.27mm Al6061: (a) wet, (b) dry, and (c) near
dry EDM conditions (ie= 10 A, te= 10 µs, to= 70 µs, ue= 60 V).

The gap distance of wet EDM is wider than that of near dry EDM. This is likely caused
by the lower viscosity of the water–air mixture. Similarly, in near dry EDM, higher water
flow rate generates larger gap distance.
     No debris deposition is observed for near dry EDM. This occurs because water–
air mixture has a better flushing capability than the air jet in dry EDM [1].

3.1.3 EDM milling

        Fig. 4 illustrates the configuration of the EDM milling process. Grooves of 8 mm in
length and varied depth for different processes were made. To measure the surface roughness
at the bottom of the slot, a Taylor Hobson Form Talysurf profilometer with a 2 µm stylus
radius was used. The cutoff length was set to 0.25 mm for the finished surface and 0.8 mm
for the roughened surface. The measurement length was set to 8 mm. The weight of the part
before and after machining was measured using an Ohaus GA110 electronic scale with a 0.1
mg resolution and converted to the volumetric material removal and MRR.[2].
      The experimental investigation of dry and near-dry EDMs was carried out in three sets
of experiments, marked as Expts. I, II, and III.
      1. Expt. I. Dielectric medium and electrode material selection: Experiments were
conducted to select the dielectric medium and electrode material at high and low discharge
energy levels for roughing and finishing operations, respectively. The depth of cut and the
input pressure were set at 0.1 mm and 480 kPa, respectively, for the roughing operation and
at 0.02 mm and 480 kPa, respectively, for the finishing operation.
     2. Expt. II. Exploratory experiments: Based on the selected dielectric medium and
electrode material, several sets of experiments were conducted to investigate the effects of


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International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME


external air jet, depth of cut, gas input pressure, discharge current, and pulse duration in dry
EDM roughing and near-dry EDM finishing.

Fig. 4 Configuration of EDM milling:                  Table 3. Roughing and finishing DOE
(a) Overview and (b) close-up view of                Roughing process     Finishing process
   the electrode and cutting region                    experiments            experiments
                                                     ________________________________________
                                                     Run te ie ui ue to       Run te ie ui ue to
                                                        (µs) (A) (V) (V) (µs)  (µs) (A) (V) (V) (µs)
                                                     _____________________________________________
                                                    1     4 20 160 40 20       1    2 1 160 80 12
                                                    2    12 20 160 40 8        2    4 1 160 40 4
                                                    3     4 30 160 40 8        3    2 3 160 40 12
                                                    4    12 30 160 40 20       4    4 3 160 80 4
                                                    5     4 20 260 40 8        5    2 1 260 80 4
                                                    6    12 20 260 40 20       6    4 1 260 40 12
                                                    7     4 30 260 40 20       7    2 3 260 40 4
                                                    8    12 30 260 40 8        8    4 3 260 80 12
                                                    9     4 20 160 80 8        9    4 3 260 40 4
                                                    10 12 20 160 80 20 10            2 3 260 80 12
                                                    11     4 30 160 80 20 11         4 1 260 80 4
                                                    12 12 30 160 80 8 12             2 1 260 40 12
                                                    13     4 20 260 80 20 13         4 3 160 40 12
                                                    14 12 20 260 80 8 14             2 3 160 80 4
                                                    15     4 30 260 80 8 15          4 1 160 80 12
                                                    16 12 30 260 80 20 16            2 1 160 40 4
                                                    17     8 25 210 60 14 17         3 2 210 60 8
                                                    18     8 25 210 60 14 18         3 2 210 60 8
                                                    19     8 25 210 60 14 19         3 2 210 60 8
                                                    20     8 25 210 60 14 20         3 2 210 60 8
                                                      _____________________________________________

    3. Expt. III. DOE: Two DOE tests based on the 25-1 fractional factorial design were
performed to study the effect of five process parameters (ie, te, ue, to, and ui ) and their
interactions. Four center points were used in the design to test the curvature effect of the
model. The design matrices are listed in Table 3. Analysis of variance (ANOVA) was applied
to analyze the main effects and interactions of input parameters. The DOE results can identify
directions for further process optimization.

3.2 Study of effect of various electrical input parameters viz. discharge current, gap
voltage, pulse on time, pulse interval, open circuit voltage on material removal rate
(MRR), surface finish & tool wear rate (TWR).

     The electrical parameters are among the most important factors in EDM. The discharge
current (ie ), pulse duration (te) and gap voltage (ue) determine the discharge energy per pulse;
the pulse interval (to) decides the time available for gap reconditioning between two
consecutive discharges; the open circuit voltage (ui ) controls the discharge gap distance; and
the polarity influences the material removal ratio between the electrode and work-piece. In
this study, different levels of these electrical parameters are selected to study both the
roughing and finishing, and dry and near dry EDM processes.
     Figure 5 shows the effect of discharge current, ie, in the roughing operation. Higher
discharge current increases the discharge energy, removes more work material, and generates
a rougher surface. The increase of MRR and surface roughness with ie is significant.
Experiments with higher ie was limited due to the maximum current limit of the rotary
spindle.


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International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
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Fig. 5 Effect of the discharge current on high         Fig. 6 Effect of the discharge current on the
energy input dry EDM with oxygen                      finishing EDM (t =4 µs, t0 =8µs, ue =60 V,
                                                                        e


( te =4µs, t0 =8µs, ue=60 V, and ui =200 V)           and ui =200 V copper electrode)

     Figure 6 shows the effect of discharge current using water-nitrogen mixture in near-dry
EDM finishing. The surface finish was improved from 2.5 µm to 0.8 µm Ra by reducing the
discharge current from 20 A to 1 A. The reduced discharge current lowered the discharge
energy per pulse and generated finer craters and lower surface roughness. However, the MRR
also dropped quickly, from 0.81 mm3 /min to 0.13 mm3 /min.
     Statistical analysis using ANOVA for dry EDM drilling reveals that discharge current , ie
is the most significant parameter due to the highest F value. With a variation in current from
12 to 15 A, and further increase up to 18 A, a linear increase in average MRR has been
observed . From ANOVA table for MRR, a very higher F value (248.5) indicates that
discharge current ie is more significant than gap voltage V. The gap voltage (V) is also a
significant parameter at 95 % confidence level. An increase in voltage appears to cause
a decrease in MRR. An increase in gap voltage from 50 to 65 V causes a decrease in
average MRR by 1.69 % . As the voltage changes from 65 to 80 V, further reduction in MRR
by 18.26 % has been observed.[7].

3.3 Study of effect of various machining input parameters viz. gas input pressure, fluid
flow rate & depth of cut on material removal rate (MRR), surface finish .

     The effect of the gas pressure input to the spray generator on surface finish and MRR in
near-dry EDM finishing with graphite electrode and kerosene-air mixture is shown in Fig. 7.
As seen in the figure, the surface roughness is nearly unaffected. Under the stable discharge
conditions, the surface roughness mostly depends on the discharge energy. The MRR
gradually increases until the gas pressure reaches 480 kPa. The enhanced gas flow provided
better debris flushing as well as more oxygen content. In the following DOE of the finishing
EDM, the gas pressure was set at 480 kPa.
     Figure 8 shows the effect of the depth of cut in oxygen assisted dry EDM roughing. The
MRR reached the maximum, 22 mm3 /min, at a 500 µm depth of cut. When the depth of cut
is beyond 500 µm, the increase of MRR is limited due to the debris removal problem. The
debris can bridge between the electrode sidewall and work-piece, resulting in arcing or short
circuit. This was confirmed by observing frequent servo retraction of the electrode to regulate
the discharge condition. The surface roughness was generally not affected by the depth of cut
because it does not influence the discharge condition at the bottom of the electrode. In the
following DOE roughing experiments with oxygen, the depth of cut was set at 500 µm.



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International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME




Fig. 7: Effect of the input pressure of the spray     Fig. 8: Effect of the depth of cut on dry EDM
 delivery device to the finishing process            rough cutting with oxygen (ie=30 A, te=4 µs,
(ie=1 A, te=2 µs, to=16 µs, ue=20 V&ui=200 V         to=8 µs, ue=60 V, and ui=200 V)
graphite electrode with kerosene-air mixture)

      The MRR in near dry EDM under 5.3 and 75 ml/min water flow rates is shown in Fig.9.
In near dry EDM, high water flow rates increases the MRR because of improved cooling,
more efficient debris flushing, and higher dielectric fluid viscosity due to the higher
concentration of water. It improves the MRR at low to (below 500 µs) for all values of te, and
is particularly beneficial when te is high (= 18 µs). The peak MRR rises to 3.9mm3/min at 75
ml/ min flow rate. A much higher flow rate is required to increase the MRR because the
nozzle is set near the discharge gap and thus not all water droplets are successfully delivered
into the gap.




Fig. 9. MRR envelopes of near dry wire EDM cutting at two de-ionized water flow rates (5.3 and 5
ml/min, ie = 25 A, ue = 45 V).


3.4 Study of effect of the electrode material and dielectric medium (various liquid-gas
mixtures) on material removal rate (MRR) and surface finish at high and low discharge
currents.

      Experiments were conducted at high and low discharge energies to study effects of the
electrode material and dielectric medium for roughing and finishing operations, respectively.
Figure 10(a) shows the results on MRR and surface roughness at high discharge energy input.
The copper electrode was successful at removing the work-material in nearly all dry and

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International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
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near-dry EDM cases (except the near-dry EDM with kerosene-nitrogen and kerosene-helium
mixtures). However, the graphite electrode failed in a high discharge energy setting due to
severe arcing. The deposited workpiece material, similar to that in arc welding, was observed
at the outer circumference of the machined spot, as shown in Fig.11(a). The severe arcing
causes discharge localization and large scale material melting, while ideal sparks should
uniformly distribute over the machining area and erode the material. The arcing was likely
stimulated by the excessive amount of graphite powder chipped off from the electrode tip, as
shown in Fig.11(b). The high thermal load, due to lower cooling efficiency in dry and near-
dry EDMs, cracked the brittle graphite electrode. The resultant graphite powder bridged the
work-piece and electrode, causing discharge localization and, thus, arcing. For the effect of
dielectric medium, oxygen, water-oxygen mixture, and kerosene-air mixture are found to
achieve comparable MRRs and better surface finish than liquid kerosene in wet EDM. The
lower viscosity of the liquid-gas mixture resulted in shallower craters on the machined
surface and, thus, better surface finish.
     Since oxygen was confirmed to have the highest MRR, its potential is further exploited
in this study. Water-oxygen mixture is another good candidate for roughing since it provided
high MRR close to that of oxygen and had good surface finish. The flushing of water-oxygen
mixture is helpful in high discharge energy to solidify and remove the molten debris.
However, the water combined with oxygen induces severe electrolysis corrosion on a
machined surface. Hence, copper electrode and oxygen gas are selected for further DOE
study of high MRR roughing EDM
     Figure 10(b) shows the results of the MRR and surface roughness at low discharge
energy input. The graphite electrode exhibited its advantage over copper electrode with
higher MRR and comparable surface roughness. In near-dry EDM using water mixture with
nitrogen or helium, the graphite electrode achieved a similar quality of the surface finish
(0.87−0.95 µm Ra) and twice the MRR as that of copper electrode. At low discharge energy
input, the graphite powder, which exists in much smaller amounts than that at high discharge
energy, assisted the machining process to improve the discharge transitivity and,
consequently, the MRR. It is hypothesized that the carbon powder plays a role in assisting the
discharge ignition and evenly distribute the sparks, as identified by Yang and Cao.




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International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME




Fig.10. MRR and Ra results of different dielectric fluids for copper and graphite electrode materials:
(a) at high discharge energy input (ie=20 A, te=4µs, to=8µs, ue=60 V, and ui=200 V) and
(b) at low discharge energy input (ie=1 A, te=4µs, to=8µs, ue=60 V, and ui=200 V)




                                                  33
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME




Fig. 11 Graphite electrode in near-dry EDM at high discharge current: (a) Damaged workpiece
surface due to arcing and (b) damaged tool (ie=20 A, te=4 µs, to=8 µs, ue=60 V, and ui =200 V)

     The copper electrode produced slightly better surface finish, 0.80µm and 0.85µm Ra,
using water-helium and water-nitrogen mixtures, respectively, but its MRR was low
compared with graphite. The frequent servo retraction was observed when using the copper
electrode at low discharge energy, probably because the discharge is difficult to initiate.
When kerosene or kerosene based mixtures were used as dielectric fluids, the copper
electrode cannot maintain stable discharges because of the narrow gap distance in the low
discharge energy EDM.
     Considering the effect of the dielectric medium, near-dry EDM outperformed both dry
and wet EDMs to generate better surface finish and higher MRR. The best surface finish of
0.8 µm was achieved using the water-nitrogen mixture. The highest MRR of 1.8 mm3 /min
was obtained using the kerosene-air mixture. In dry EDM at low energy input, the MRR was
low, using an oxygen medium, and the surface was rough.
     The water based mixture generally provided better surface finish than the kerosene based
mixture with the sacrifice of MRR due to its lower viscosity and correspondingly smoother
and shallower crater for each discharge. Water-nitrogen and water-helium mixtures yielded
better surface finishes(0.95 µm and 0.87 µm Ra for graphite electrode and 0.85 µm and 0.80
µm Ra for copper electrode) than the water-air and water-oxygen mixtures (1.68µm and 1.62
µm Ra for graphite electrode and 0.98 µm and 1.25µm Ra for copper electrode). A possible
reason is that nitrogen and helium shielded the process from oxygen and thus reduce the
corrosion caused by water electrolysis. The mixture with helium produced a slightly better
surface finish over that of nitrogen. Nitrogen has the potential to form a hard nitride surface
layer by alloying with elements in the work-material.
      Kerosene-air mixture produced higher MRR than that of kerosene with nitrogen or
helium. The oxygen content in the air generates more heat for material
removal through an exothermic reaction, but the surface finish was adversely affected. When
kerosene was used as dielectric media, the deterioration caused by electrolysis corrosion was
not observed. For further DOE study of finishing EDM, the graphite electrode and water-
nitrogen mixture are selected. Nitrogen is selected over helium because of the comparable
performance, lower cost, and potential to form a hard nitride surface layer on the machined
surface for better wear resistance.




                                                34
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME


 3.5 Study of effect of fluid flow rate (concentration of the liquid in gas) and discharge
current on gap distance and debris deposition.




Fig. 12. The effect of de-ionized water flow rate and discharge current on the MRR of EDM drilling
(te = 10 ms, to= 70 ms, ue = 60 V).

    Effects of water flow rate and pulse current ie on the MRR in near dry EDM
drilling are shown in Fig. 12 . The efficiency of near dry EDM drilling improves with a
higher water flow rate under all three levels of ie. The MRR is low at ie = 10A due to the
low-energy input. The highest energy input (ie = 15 A), however, does not generate the
highest MRR as expected. This is caused by the debris flushing problem at high-energy input.
The medium level of ie(=12A) has the highest MRR by balancing the debris flushing and
power input. The measured average gap distance is calculated using the difference between
the average hole diameter and electrode diameter. Table 2 lists the average gap distance in
wet, dry, and near dry EDM at five water flow rates. Following the same trend observed in
Table 1 for the wire EDM, higher water flow rate corresponds to larger gap distance. A
model is developed to investigate the effect of dielectric strength and dynamic viscosity on
the gap distance.

4. CONCLUSION
        Advantages of near-dry EDM can be identified as a stable machining process at low
discharge energy input because the presence of liquid phase in the gas environment changes
the electric field, making discharge easier to initiate and thus creating a larger gap distance.
In addition, good machined surface integrity without debris reattachment that occurred in dry
EDM can be attained since the liquid in the dielectric fluid enhances debris flushing. Other
potential advantages of near-dry EDM are a broad selection of gases and liquids and
flexibility to adjust the concentration of the liquid in gas. The dielectric properties can thus be
tailored in near-dry EDM to meet various machining needs, such as high MRR or fine surface
finish. However, the technical barrier in near-dry EDM lies in the selection of proper
dielectric medium and process parameters.
      From the review of literature it is seen that experimental investigations have been
carried out in order to study the effect of various input parameters like discharge current, gap
voltage, pulse on time, gas pressure, fluid flow rate and spindle speed on material removal
rate (MRR), surface roughness and tool wear rate and to improve the performance of near dry
EDM process.
     However, irrespective of its inherent advantages over wet and dry EDM processes, not
much attention has been given towards the parametric optimization of the near-dry EDM


                                                35
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME


process. It is essential to have information on the optimum operating conditions to make the
near dry EDM process cost effective and economically viable one.
     Authors conclude that, there is a wide scope to work in this area to optimize the vital
parameters of near-dry EDM process.

5. REFERENCES
[1] C.C. Kao,Jia Tao, Albert J. Shih,”Near Dry Electrical Discharge Machining”,
International Journal of Machine Tools & Manufacture 47 (2007), 2273-2281.
[2] Jia Tao, Albert J. Shih,Jun Ni,”Experimental study of the Dry & Near-Dry Electrical
Discharge Milling Processes”, Journal of Manufacturing Science & Engineering (Feb2008),
Vol.130 / 011002-1- 011002-8.
[3]Y Jia, B.S. Kim, D.J. Hu & J Ni, “ Parametric study on near-dry wire electro-discharge
machining of polycrystalline diamond-coated tungsten carbide material”, Proceedings of the
Institution of Mechanical Engineers, Part B : Journal of Engineering Manufacture (2010) Vol.
224, 185-193.
[4] M. Fujiki, Gap-Yong Kim, Jun Ni, Albert J. Shih,”Gap control for near-dry EDM milling
with lead angle”, International Journal of Machine Tools & Manufacture 51 (2011), 77-83.
[5] M. Fujiki, Jun Ni, Albert J. Shih,” Investigation of the effect of electrode orientation &
fluid flow rate in near-dry EDM milling”, International Journal of Machine Tools &
Manufacture 49 (2009), 749-758.
[6] Jia Tao, Albert J. Shih, Jun Ni,” Near-Dry EDM Milling of Mirror-Like Surface Finish”,
International Journal of Electrical Machining 13 ( January 2008). 29-33.
[7] P. Govindan, Suhas S. Joshi, “ Experimental characterization of material removal in dry
electrical discharge drilling” , International Journal of Machine Tools & Manufacture 50
(2010), 431-443.
[8] Fabio N. Leao , Ian R. Pashby , “ A review on the use of environmentally-friendly
dielectric fluids in electrical discharge machining”, Journal of Materials Processing
Technology 149 (2004), 341-346.
[9]Viktor P. Astakhov, General Motors Business Unit of PSMI, USA, “ Ecological
Machining : Near Dry Machining”.
[10] B.C. Routara, B.K. Nanda, D.R. Patra,” Parametric optimization of CNC wire cut EDM
using Grey Relational Analysis”, Proceedings of the International Conference on Mechanical
Engineering ( Dec.2009),RT-24,1-6.
 [11] S. Abdulkareem, A.A. Khan & Z.M. Zain,”Effect of Machining Parameters on Surface
Roughness during Wet & Dry Wire EDM of Stainless Steel”, Journal of Applied Sciences 11
(10), 1867-1871, (2011).
 [12] Jia Tao, “Investigation of Dry & Near Dry Electrical Discharge Milling Process”, A
dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of
Philosophy (Mechanical Engineering) in The University of Michigan, (2008).
[13] Sourabh K. Saha, S.K.Choudhury, Department of Mechanical Engineering, Indian
Institute of Technology Kanpur, ”Multi-objective optimization of the dry electric discharge
machining process”, (Jan. 2009).
[14] Grzegorz Skrabalak, Jerzy Kozak, “ Study on Dry Electrical Discharge machining”,
Proceedings of the World Congress on Engineering, London UK, (2010) Vol. III.




                                             36

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An overview of experimental investigation of near dry electrical discharge machining process

  • 1. INTERNATIONAL JOURNAL OF ADVANCED RESEARCH IN International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME ENGINEERING AND TECHNOLOGY (IJARET) ISSN 0976 - 6480 (Print) ISSN 0976 - 6499 (Online) Volume 3, Issue 2, July-December (2012), pp. 22-36 IJARET © IAEME: www.iaeme.com/ijaret.html Journal Impact Factor (2012): 2.7078 (Calculated by GISI) www.jifactor.com ©IAEME AN OVERVIEW OF EXPERIMENTAL INVESTIGATION OF NEAR DRY ELECTRICAL DISCHARGE MACHINING PROCESS Mane S.G.1, Hargude N.V.2 1,2 Department of Mechanical Engineering, PVPIT Budhgaon, Sangli 416416,Maharashtra, India. E-mail: shrikant_mane3665@rediffmail.com; nvhargude@gmail.com . ABSTRACT EDM has achieved a status of being nearly indispensable in the industry because of its ability to machine any electrically conductive material which is difficult-to-machine irrespective of its mechanical strength. Out of the three EDM processes viz. wet, dry & near-dry; near-dry EDM is proved to be most environment-friendly. Further some other problems like higher discharge energy requirement in wet EDM and the reattachment of debris to the machined surface in dry EDM can be overcome in near-dry EDM. Also, it is found that near-dry EDM has the advantage in finish operation with low discharge energy considering its higher MRR than wet EDM and better surface finish quality than dry EDM. In view of these factors, near-dry EDM may prove to be the most prominent process amongst the three EDM processes in near future to finish machine the difficult to machine materials. Significant work has been done in the parametric optimization of wet EDM processes. Efforts are also on in the parametric optimization of dry EDM processes. However, irrespective of its inherent advantages over wet and dry EDM processes, not much attention has been given towards the parametric optimization of the near-dry EDM process. It is essential to have information on the optimum operating conditions to make the near dry EDM process cost effective and economically viable one. If applied as the post process of direct metal deposition (DMD), the near-dry EDM milling processes can be targeted to finish the near-net-shape parts produced by DMD. Hence the authors feel that, there is a wide scope to work in this area to optimize the vital parameters of near-dry EDM process. The experimental investigations of near dry electrical discharge machining process carried out by a handful of researchers have been overviewed in present work. Keywords: Electrical discharge machining, near dry EDM, material removal rate, Surface roughness 22
  • 2. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME 1. INTRODUCTION Electrical discharge machining (EDM) is often used to machine difficult-to-machine materials. EDM has achieved a status of being nearly indispensable in the industry because of its ability to machine any electrically conductive material irrespective of its mechanical strength. EDM removes work material by melting and vaporizing it through a series of discharging electric sparks. The spark removes the work material. Conventional EDM processes use liquid dielectric fluid. However, the dielectric fluid, particularly hydrocarbon oil itself is one of the main sources of pollution in die sinking electrical discharge machining. Wastes of dielectric oil are very toxic, cannot be recycled and need to be disposed of appropriately; otherwise, there is a possibility of both the land and water being polluted[8].The process generates gases and fumes due to the thermal decomposition of the dielectric. Another main problem of die sink EDM is the high amount of energy consumed. The energy consumed in the spark gap, which is the effective energy for the erosion of the material, is usually less than 20% of the total input of electrical energy. On the other hand, the energy consumed by the dielectric system may represent 50% of the total input of electrical energy, especially when low values of peak current are used [8]. Another emerging technology, viz. powder mixed EDM, increases the cost of machining and also environment-unfriendly like conventional EDM. Dry EDM is another technique, which employs gas as a dielectric medium instead of liquid. Due to the reattachment of debris to the machined surface, dry EDM may have limitations of meeting the combined material removal rate (MRR) and surface roughness requirements. The accuracy of surface profile deteriorates with the debris deposition. The major challenges in dry EDM process are low stability of arc column, low material removal rate, arcing, poor surface quality as compared to conventional EDM and odor of burning. However efforts have been made in the experimental investigation and parametric optimization of dry EDM processes [11-14]. These problems faced in dry EDM can be reduced & overcome in near- dry EDM by replacing the gas with the mixture of gas and dielectric liquid. The liquid content in the mist media helps to solidify and flush away the molten debris and the debris reattachment is alleviated in near-dry EDM. Compared to the conventional EDM process, near-dry EDM has another advantage. It does not require a bath of dielectric fluid. Only a small amount of liquid dielectric fluid is used making the process environment-friendly. Further it has the benefit to tailor the concentration of liquid and properties of dielectric medium to meet desired performance targets. Also, it is found that near-dry EDM has the advantage in finish operation with low discharge energy considering its higher MRR than wet EDM and better surface finish quality than dry EDM. 2. PRESENT STATUS AND SCOPE The metal working fluids (MWFs) are extensively used in conventional machining processes. The economical, ecological and health impacts of metal working fluids (MWFs) can be reduced by using minimum quantity lubrication referred to as near dry machining. In near dry machining (NDM), an air-oil mixture called an aerosol is fed onto the machining zone [9]. This concept of near dry machining can be well applied in EDM process, the process being referred to as near-dry EDM process. The feasibility of near-dry EDM was explored by Tanimura et. al. in 1989, who investigated EDM in water mists in air, nitrogen & argon gases. Further investigation of near- dry EDM was conducted by Kao et. al.(2007) [1], in wire EDM experiments. After the first 23
  • 3. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME exploitation by Tanimura (1989), not much study has been conducted on this process until recently by Kao (2007) in near-dry wire EDM. Near dry EDM milling as a super-finishing process to achieve a mirror like surface finish has been investigated. Near dry EDM exhibits the advantage of good machining stability and smooth surface finish at low discharge energy input [6]. Advantages of near-dry EDM were identified as a stable machining process at low discharge energy input because the presence of liquid phase in the gas environment changes the electric field, making discharge easier to initiate and thus creating a larger gap distance. In addition, good machined surface integrity without debris reattachment that occurred in dry EDM was attained since the liquid in the dielectric fluid enhances debris flushing. Other potential advantages of near-dry EDM are a broad selection of gases and liquids and flexibility to adjust the concentration of the liquid in gas. The dielectric properties can thus be tailored in near-dry EDM to meet various machining needs, such as high MRR or fine surface finish. Also Near dry EDM shows advantages over the dry EDM in higher material removal rate (MRR), sharp cutting edge and less debris deposition. Compared to wet EDM, near dry EDM has higher material removal rate at low discharge energy and generates a smaller gap distance [10]. Also compared with conventional wet wire EDM, near dry wire EDM consistently produces better Ra values on PCD coated WC work-pieces, but near dry wire EDM produces lower MRR than wet wire EDM under some conditions [3]. However, the technical barrier in near-dry EDM lies in the selection of proper dielectric medium and process parameters. From the review of literature it is seen that experimental investigations have been carried out in order to study the effect of various input parameters like discharge current, gap voltage, pulse on time, gas pressure, fluid flow rate, electrode orientation and spindle speed on material removal rate (MRR), surface roughness and tool wear rate and to improve the performance of near dry EDM process [1-6]. However, it is necessary to optimize the input parameters for maximum material removal rate (MRR) and minimize the surface roughness to make the near dry EDM process cost effective and economically viable one. 3. PRESENT WORK The experimental investigations of near dry electrical discharge machining process carried out by a handful of researchers have been overviewed in present work in view of the following points. 1) Comparative study of wet, dry and near dry EDM in view of the response variables viz. material removal rate (MRR), surface roughness, gap distance and debris deposition. 2) Study of effect of various electrical input parameters viz. discharge current (ie), gap voltage (ue), pulse on time (te), pulse interval (to), open circuit voltage (ui) on material removal rate (MRR), surface finish & tool wear rate (TWR). 3) Study of effect of various machining input parameters viz. gas input pressure, fluid flow rate and spindle speed on material removal rate (MRR), surface finish & tool wear rate (TWR). 4) Study of effect of the electrode material and dielectric medium (various liquid-gas mixtures) on material removal rate (MRR) and surface finish at high and low discharge currents. 5) Study of effect of fluid flow rate (concentration of the liquid in gas) and discharge current on gap distance and debris deposition. 24
  • 4. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME 3.1 Comparative study of wet, dry and near dry EDM in view of the response variables viz. material removal rate (MRR), surface roughness, gap distance and debris deposition. 3.1.1 Wire EDM cutting MRR envelopes, which illustrate feasible EDM process regions, have been studied by Miller . MRR envelopes of wet and dry EDM cutting of 1.27- mm- thick Al6061 are presented as the baseline data for the comparison with two new envelopes of the near dry EDM. In each envelope, to was varied to find the maximum achievable wire feed rate, which was then converted to MRR. Four levels of te were selected: 4, 10, 14, and 18 µs. The upper and lower boundaries of the MRR envelope correspond to the minimum and maximum values of te (4 and 18 µs). The specific machine limits, maximum and minimum to (1000 and 6 µs), as well as wire breakage and short- circuit limitations, form the left and right envelope boundaries of the MRR envelope. The average pulse current ie is about 25 A. To investigate the relationship between the gap distance and dielectric fluid properties, the grooves machined at various water flow rates (0, 5, 8, 15, 21, 35, 50, 75 ml/min), as summarized in Table 1 , were studied. The groove quality and groove width were examined and measured using an optical microscope at 100x magnification. Three repeated tests were conducted in each experimental setup [1]. Table 1. Average gap distance in EDM cutting under wet, dry & Near dry conditions 3.1.2 EDM drilling Two sets of EDM drilling experiments were conducted. The first set was to evaluate the drilling speed and hole quality, including the shape variation and debris deposition, of wet, dry, and near dry EDM. The average pulse current was set at 10 A. The work-piece used was 1.27- mm-thick Al6061. For wet EDM, the flow rate of de- ionized water was 107 ml/min. For dry EDM, the air jet pressure was set at 0.62MPa. For near dry EDM, the water flow rate and the pressure of the carrying air jet were set at 21 ml/min and 0.62MPa, respectively. The hole quality was inspected using an optical microscope at 100x magnification. The second set investigates the effects of water flow rates on EDM drilling speeds with ie values at 10, 12, and 15 A. Diameters of drilled holes at different water flow rates were also measured for the investigation of the relationship between the gap distance and dielectric fluid properties. The water flow rate was varied as 5, 8, 15, 21, and 35 ml/ min as shown in Table 2 [1]. 25
  • 5. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME Table 2. Average gap distance in EDM drilling under wet, dry and near dry conditions Fig.1 shows three MRR envelopes which outline the feasible regions for the wet, dry, and near dry wire EDM cutting of 1.27- mm- thick Al6061. The average of three repeated test results is presented. The range of variation of three tests is within 10% of the nominal value and is consistent for all experimental conditions. For wet and dry wire EDM, the region of feasible MRR is bounded by the wire breakage, short circuit, and machine limits of maximum and minimum te (18 and 4µs) and maximum to (1000 µs). Fig 1.Comparision of boundaries of feasible MRR envelopes for wet, dry and near dry wire EDM The wet EDM has a significantly higher MRR than that of the dry EDM (21.9mm3/min vs. 0.98mm3/min). At low pulse intervals of to , frequent EDM pulses generate concentrated heat and lead to wire breakage. The minimum value of to that can be reached at high level of te without wire breakage, is greatly dependent on the dielectric fluid used. For wet EDM, due to the higher thermal conductivity of the bulk water than that of the water–air mixture, to can be as low as 100 µs at te = 18 µs. For the near dry EDM using water–air mixture at a water flow rate of 5.3 ml/min, the envelope boundary falls between the wet and dry EDM. The maximum MRR is improved, from 0.98mm3/min in dry EDM, to 2.53mm3/ min. The near dry EDM has a consistently higher MRR than that of dry EDM for all to and te . However, the wire breakage, due to the 26
  • 6. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME lower capability of water–air mixture to relieve the concentrated heat from the wire electrode, still limits the MRR in near dry EDM at low to. Nevertheless, near dry EDM shows two advantages. First, there is no short circuit limit at the lower boundary. Second, in the region of very low- energy input (te= 4 µs and to >150 µs), the MRR in near dry EDM is higher than that of the wet EDM. The close up view of MRR (below 4 mm3/min) vs. to for the wet and near dry EDM is shown in Fig.2. Three regions, designated as I–III, are identified. In Region I (to >650 µs), the near dry EDM has higher MRR than that of wet EDM because the lower thermal conductivity and heat capacity of the water–air mixture contribute to less heat dissipation during discharge and a larger portion of discharge energy for material removal. At the very low discharge energy setup, te= 4 µs, wet EDM fails to cut due to the short circuit, but near dry EDM still works with fairly low MRR. The higher dielectric strength of the water medium generates a narrow gap distance and causes a frequent short circuit in wet EDM. In Region II (250 < to <650 µs), the MRR of near dry and wet EDM is roughly the same. At the highest te ( = 18 µs), the MRR of wet EDM starts to exceed that of near dry EDM. Under higher energy input, the higher viscosity of the water dielectric fluid in wet EDM generates larger explosion force, which contributes to the high MRR. In Region III (to<250 µs), a significant MRR difference exists between wet and near dry EDM. The MRR drops in near dry EDM and, wire breakage occurs as to is further reduced. The dielectric fluid viscosity is critical to the MRR in Region III. Fig. 2.Comparison of MRR performances of wet and near dry wire EDM under varied t0 and te and three regions based on near dry and wet EDM performance (ie= 25 A, ue= 45 V). Optical micrographs of top and cross-sectional side views of EDM drilled holes and the drilling time under the wet, dry, and near dry conditions are shown in Fig. 3. The dry EDM takes 428 s to drill a hole through the 1.27- mm thick Al6061. This is very long Compared to the 11 and 13 s drilling time for the wet and near dry EDM respectively. The dry EDM also has a severe debris deposition problem, which subsequently creates a tapered hole. The taper in wet EDM also exists but is not as significant as in dry EDM. The smallest taper exists in holes drilled by near dry EDM, which generates a straight hole with sharp edges. 27
  • 7. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME The electrode wear in near dry EDM is 3.7 mg per hole, which is larger than the 2.7 mg per hole in wet EDM. The higher thermal load on the electrode in near dry EDM likely causes the higher electrode wear. The same phenomenon also exists in near dry wire EDM. As shown in Fig.2, at low to the wire breakage due to electrode wear limits the MRR in near dry wire EDM. The groove width in wire EDM is used to estimate the gap distance. The average gap distance under the wet, dry, and near dry wire EDM and the associated dielectric strength and viscosity of the dielectric fluid are listed in Table 1. Fig.3. Optical micrographs on holes drilled on 1.27mm Al6061: (a) wet, (b) dry, and (c) near dry EDM conditions (ie= 10 A, te= 10 µs, to= 70 µs, ue= 60 V). The gap distance of wet EDM is wider than that of near dry EDM. This is likely caused by the lower viscosity of the water–air mixture. Similarly, in near dry EDM, higher water flow rate generates larger gap distance. No debris deposition is observed for near dry EDM. This occurs because water– air mixture has a better flushing capability than the air jet in dry EDM [1]. 3.1.3 EDM milling Fig. 4 illustrates the configuration of the EDM milling process. Grooves of 8 mm in length and varied depth for different processes were made. To measure the surface roughness at the bottom of the slot, a Taylor Hobson Form Talysurf profilometer with a 2 µm stylus radius was used. The cutoff length was set to 0.25 mm for the finished surface and 0.8 mm for the roughened surface. The measurement length was set to 8 mm. The weight of the part before and after machining was measured using an Ohaus GA110 electronic scale with a 0.1 mg resolution and converted to the volumetric material removal and MRR.[2]. The experimental investigation of dry and near-dry EDMs was carried out in three sets of experiments, marked as Expts. I, II, and III. 1. Expt. I. Dielectric medium and electrode material selection: Experiments were conducted to select the dielectric medium and electrode material at high and low discharge energy levels for roughing and finishing operations, respectively. The depth of cut and the input pressure were set at 0.1 mm and 480 kPa, respectively, for the roughing operation and at 0.02 mm and 480 kPa, respectively, for the finishing operation. 2. Expt. II. Exploratory experiments: Based on the selected dielectric medium and electrode material, several sets of experiments were conducted to investigate the effects of 28
  • 8. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME external air jet, depth of cut, gas input pressure, discharge current, and pulse duration in dry EDM roughing and near-dry EDM finishing. Fig. 4 Configuration of EDM milling: Table 3. Roughing and finishing DOE (a) Overview and (b) close-up view of Roughing process Finishing process the electrode and cutting region experiments experiments ________________________________________ Run te ie ui ue to Run te ie ui ue to (µs) (A) (V) (V) (µs) (µs) (A) (V) (V) (µs) _____________________________________________ 1 4 20 160 40 20 1 2 1 160 80 12 2 12 20 160 40 8 2 4 1 160 40 4 3 4 30 160 40 8 3 2 3 160 40 12 4 12 30 160 40 20 4 4 3 160 80 4 5 4 20 260 40 8 5 2 1 260 80 4 6 12 20 260 40 20 6 4 1 260 40 12 7 4 30 260 40 20 7 2 3 260 40 4 8 12 30 260 40 8 8 4 3 260 80 12 9 4 20 160 80 8 9 4 3 260 40 4 10 12 20 160 80 20 10 2 3 260 80 12 11 4 30 160 80 20 11 4 1 260 80 4 12 12 30 160 80 8 12 2 1 260 40 12 13 4 20 260 80 20 13 4 3 160 40 12 14 12 20 260 80 8 14 2 3 160 80 4 15 4 30 260 80 8 15 4 1 160 80 12 16 12 30 260 80 20 16 2 1 160 40 4 17 8 25 210 60 14 17 3 2 210 60 8 18 8 25 210 60 14 18 3 2 210 60 8 19 8 25 210 60 14 19 3 2 210 60 8 20 8 25 210 60 14 20 3 2 210 60 8 _____________________________________________ 3. Expt. III. DOE: Two DOE tests based on the 25-1 fractional factorial design were performed to study the effect of five process parameters (ie, te, ue, to, and ui ) and their interactions. Four center points were used in the design to test the curvature effect of the model. The design matrices are listed in Table 3. Analysis of variance (ANOVA) was applied to analyze the main effects and interactions of input parameters. The DOE results can identify directions for further process optimization. 3.2 Study of effect of various electrical input parameters viz. discharge current, gap voltage, pulse on time, pulse interval, open circuit voltage on material removal rate (MRR), surface finish & tool wear rate (TWR). The electrical parameters are among the most important factors in EDM. The discharge current (ie ), pulse duration (te) and gap voltage (ue) determine the discharge energy per pulse; the pulse interval (to) decides the time available for gap reconditioning between two consecutive discharges; the open circuit voltage (ui ) controls the discharge gap distance; and the polarity influences the material removal ratio between the electrode and work-piece. In this study, different levels of these electrical parameters are selected to study both the roughing and finishing, and dry and near dry EDM processes. Figure 5 shows the effect of discharge current, ie, in the roughing operation. Higher discharge current increases the discharge energy, removes more work material, and generates a rougher surface. The increase of MRR and surface roughness with ie is significant. Experiments with higher ie was limited due to the maximum current limit of the rotary spindle. 29
  • 9. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME Fig. 5 Effect of the discharge current on high Fig. 6 Effect of the discharge current on the energy input dry EDM with oxygen finishing EDM (t =4 µs, t0 =8µs, ue =60 V, e ( te =4µs, t0 =8µs, ue=60 V, and ui =200 V) and ui =200 V copper electrode) Figure 6 shows the effect of discharge current using water-nitrogen mixture in near-dry EDM finishing. The surface finish was improved from 2.5 µm to 0.8 µm Ra by reducing the discharge current from 20 A to 1 A. The reduced discharge current lowered the discharge energy per pulse and generated finer craters and lower surface roughness. However, the MRR also dropped quickly, from 0.81 mm3 /min to 0.13 mm3 /min. Statistical analysis using ANOVA for dry EDM drilling reveals that discharge current , ie is the most significant parameter due to the highest F value. With a variation in current from 12 to 15 A, and further increase up to 18 A, a linear increase in average MRR has been observed . From ANOVA table for MRR, a very higher F value (248.5) indicates that discharge current ie is more significant than gap voltage V. The gap voltage (V) is also a significant parameter at 95 % confidence level. An increase in voltage appears to cause a decrease in MRR. An increase in gap voltage from 50 to 65 V causes a decrease in average MRR by 1.69 % . As the voltage changes from 65 to 80 V, further reduction in MRR by 18.26 % has been observed.[7]. 3.3 Study of effect of various machining input parameters viz. gas input pressure, fluid flow rate & depth of cut on material removal rate (MRR), surface finish . The effect of the gas pressure input to the spray generator on surface finish and MRR in near-dry EDM finishing with graphite electrode and kerosene-air mixture is shown in Fig. 7. As seen in the figure, the surface roughness is nearly unaffected. Under the stable discharge conditions, the surface roughness mostly depends on the discharge energy. The MRR gradually increases until the gas pressure reaches 480 kPa. The enhanced gas flow provided better debris flushing as well as more oxygen content. In the following DOE of the finishing EDM, the gas pressure was set at 480 kPa. Figure 8 shows the effect of the depth of cut in oxygen assisted dry EDM roughing. The MRR reached the maximum, 22 mm3 /min, at a 500 µm depth of cut. When the depth of cut is beyond 500 µm, the increase of MRR is limited due to the debris removal problem. The debris can bridge between the electrode sidewall and work-piece, resulting in arcing or short circuit. This was confirmed by observing frequent servo retraction of the electrode to regulate the discharge condition. The surface roughness was generally not affected by the depth of cut because it does not influence the discharge condition at the bottom of the electrode. In the following DOE roughing experiments with oxygen, the depth of cut was set at 500 µm. 30
  • 10. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME Fig. 7: Effect of the input pressure of the spray Fig. 8: Effect of the depth of cut on dry EDM delivery device to the finishing process rough cutting with oxygen (ie=30 A, te=4 µs, (ie=1 A, te=2 µs, to=16 µs, ue=20 V&ui=200 V to=8 µs, ue=60 V, and ui=200 V) graphite electrode with kerosene-air mixture) The MRR in near dry EDM under 5.3 and 75 ml/min water flow rates is shown in Fig.9. In near dry EDM, high water flow rates increases the MRR because of improved cooling, more efficient debris flushing, and higher dielectric fluid viscosity due to the higher concentration of water. It improves the MRR at low to (below 500 µs) for all values of te, and is particularly beneficial when te is high (= 18 µs). The peak MRR rises to 3.9mm3/min at 75 ml/ min flow rate. A much higher flow rate is required to increase the MRR because the nozzle is set near the discharge gap and thus not all water droplets are successfully delivered into the gap. Fig. 9. MRR envelopes of near dry wire EDM cutting at two de-ionized water flow rates (5.3 and 5 ml/min, ie = 25 A, ue = 45 V). 3.4 Study of effect of the electrode material and dielectric medium (various liquid-gas mixtures) on material removal rate (MRR) and surface finish at high and low discharge currents. Experiments were conducted at high and low discharge energies to study effects of the electrode material and dielectric medium for roughing and finishing operations, respectively. Figure 10(a) shows the results on MRR and surface roughness at high discharge energy input. The copper electrode was successful at removing the work-material in nearly all dry and 31
  • 11. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME near-dry EDM cases (except the near-dry EDM with kerosene-nitrogen and kerosene-helium mixtures). However, the graphite electrode failed in a high discharge energy setting due to severe arcing. The deposited workpiece material, similar to that in arc welding, was observed at the outer circumference of the machined spot, as shown in Fig.11(a). The severe arcing causes discharge localization and large scale material melting, while ideal sparks should uniformly distribute over the machining area and erode the material. The arcing was likely stimulated by the excessive amount of graphite powder chipped off from the electrode tip, as shown in Fig.11(b). The high thermal load, due to lower cooling efficiency in dry and near- dry EDMs, cracked the brittle graphite electrode. The resultant graphite powder bridged the work-piece and electrode, causing discharge localization and, thus, arcing. For the effect of dielectric medium, oxygen, water-oxygen mixture, and kerosene-air mixture are found to achieve comparable MRRs and better surface finish than liquid kerosene in wet EDM. The lower viscosity of the liquid-gas mixture resulted in shallower craters on the machined surface and, thus, better surface finish. Since oxygen was confirmed to have the highest MRR, its potential is further exploited in this study. Water-oxygen mixture is another good candidate for roughing since it provided high MRR close to that of oxygen and had good surface finish. The flushing of water-oxygen mixture is helpful in high discharge energy to solidify and remove the molten debris. However, the water combined with oxygen induces severe electrolysis corrosion on a machined surface. Hence, copper electrode and oxygen gas are selected for further DOE study of high MRR roughing EDM Figure 10(b) shows the results of the MRR and surface roughness at low discharge energy input. The graphite electrode exhibited its advantage over copper electrode with higher MRR and comparable surface roughness. In near-dry EDM using water mixture with nitrogen or helium, the graphite electrode achieved a similar quality of the surface finish (0.87−0.95 µm Ra) and twice the MRR as that of copper electrode. At low discharge energy input, the graphite powder, which exists in much smaller amounts than that at high discharge energy, assisted the machining process to improve the discharge transitivity and, consequently, the MRR. It is hypothesized that the carbon powder plays a role in assisting the discharge ignition and evenly distribute the sparks, as identified by Yang and Cao. 32
  • 12. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME Fig.10. MRR and Ra results of different dielectric fluids for copper and graphite electrode materials: (a) at high discharge energy input (ie=20 A, te=4µs, to=8µs, ue=60 V, and ui=200 V) and (b) at low discharge energy input (ie=1 A, te=4µs, to=8µs, ue=60 V, and ui=200 V) 33
  • 13. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME Fig. 11 Graphite electrode in near-dry EDM at high discharge current: (a) Damaged workpiece surface due to arcing and (b) damaged tool (ie=20 A, te=4 µs, to=8 µs, ue=60 V, and ui =200 V) The copper electrode produced slightly better surface finish, 0.80µm and 0.85µm Ra, using water-helium and water-nitrogen mixtures, respectively, but its MRR was low compared with graphite. The frequent servo retraction was observed when using the copper electrode at low discharge energy, probably because the discharge is difficult to initiate. When kerosene or kerosene based mixtures were used as dielectric fluids, the copper electrode cannot maintain stable discharges because of the narrow gap distance in the low discharge energy EDM. Considering the effect of the dielectric medium, near-dry EDM outperformed both dry and wet EDMs to generate better surface finish and higher MRR. The best surface finish of 0.8 µm was achieved using the water-nitrogen mixture. The highest MRR of 1.8 mm3 /min was obtained using the kerosene-air mixture. In dry EDM at low energy input, the MRR was low, using an oxygen medium, and the surface was rough. The water based mixture generally provided better surface finish than the kerosene based mixture with the sacrifice of MRR due to its lower viscosity and correspondingly smoother and shallower crater for each discharge. Water-nitrogen and water-helium mixtures yielded better surface finishes(0.95 µm and 0.87 µm Ra for graphite electrode and 0.85 µm and 0.80 µm Ra for copper electrode) than the water-air and water-oxygen mixtures (1.68µm and 1.62 µm Ra for graphite electrode and 0.98 µm and 1.25µm Ra for copper electrode). A possible reason is that nitrogen and helium shielded the process from oxygen and thus reduce the corrosion caused by water electrolysis. The mixture with helium produced a slightly better surface finish over that of nitrogen. Nitrogen has the potential to form a hard nitride surface layer by alloying with elements in the work-material. Kerosene-air mixture produced higher MRR than that of kerosene with nitrogen or helium. The oxygen content in the air generates more heat for material removal through an exothermic reaction, but the surface finish was adversely affected. When kerosene was used as dielectric media, the deterioration caused by electrolysis corrosion was not observed. For further DOE study of finishing EDM, the graphite electrode and water- nitrogen mixture are selected. Nitrogen is selected over helium because of the comparable performance, lower cost, and potential to form a hard nitride surface layer on the machined surface for better wear resistance. 34
  • 14. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME 3.5 Study of effect of fluid flow rate (concentration of the liquid in gas) and discharge current on gap distance and debris deposition. Fig. 12. The effect of de-ionized water flow rate and discharge current on the MRR of EDM drilling (te = 10 ms, to= 70 ms, ue = 60 V). Effects of water flow rate and pulse current ie on the MRR in near dry EDM drilling are shown in Fig. 12 . The efficiency of near dry EDM drilling improves with a higher water flow rate under all three levels of ie. The MRR is low at ie = 10A due to the low-energy input. The highest energy input (ie = 15 A), however, does not generate the highest MRR as expected. This is caused by the debris flushing problem at high-energy input. The medium level of ie(=12A) has the highest MRR by balancing the debris flushing and power input. The measured average gap distance is calculated using the difference between the average hole diameter and electrode diameter. Table 2 lists the average gap distance in wet, dry, and near dry EDM at five water flow rates. Following the same trend observed in Table 1 for the wire EDM, higher water flow rate corresponds to larger gap distance. A model is developed to investigate the effect of dielectric strength and dynamic viscosity on the gap distance. 4. CONCLUSION Advantages of near-dry EDM can be identified as a stable machining process at low discharge energy input because the presence of liquid phase in the gas environment changes the electric field, making discharge easier to initiate and thus creating a larger gap distance. In addition, good machined surface integrity without debris reattachment that occurred in dry EDM can be attained since the liquid in the dielectric fluid enhances debris flushing. Other potential advantages of near-dry EDM are a broad selection of gases and liquids and flexibility to adjust the concentration of the liquid in gas. The dielectric properties can thus be tailored in near-dry EDM to meet various machining needs, such as high MRR or fine surface finish. However, the technical barrier in near-dry EDM lies in the selection of proper dielectric medium and process parameters. From the review of literature it is seen that experimental investigations have been carried out in order to study the effect of various input parameters like discharge current, gap voltage, pulse on time, gas pressure, fluid flow rate and spindle speed on material removal rate (MRR), surface roughness and tool wear rate and to improve the performance of near dry EDM process. However, irrespective of its inherent advantages over wet and dry EDM processes, not much attention has been given towards the parametric optimization of the near-dry EDM 35
  • 15. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME process. It is essential to have information on the optimum operating conditions to make the near dry EDM process cost effective and economically viable one. Authors conclude that, there is a wide scope to work in this area to optimize the vital parameters of near-dry EDM process. 5. REFERENCES [1] C.C. Kao,Jia Tao, Albert J. Shih,”Near Dry Electrical Discharge Machining”, International Journal of Machine Tools & Manufacture 47 (2007), 2273-2281. [2] Jia Tao, Albert J. Shih,Jun Ni,”Experimental study of the Dry & Near-Dry Electrical Discharge Milling Processes”, Journal of Manufacturing Science & Engineering (Feb2008), Vol.130 / 011002-1- 011002-8. [3]Y Jia, B.S. Kim, D.J. Hu & J Ni, “ Parametric study on near-dry wire electro-discharge machining of polycrystalline diamond-coated tungsten carbide material”, Proceedings of the Institution of Mechanical Engineers, Part B : Journal of Engineering Manufacture (2010) Vol. 224, 185-193. [4] M. Fujiki, Gap-Yong Kim, Jun Ni, Albert J. Shih,”Gap control for near-dry EDM milling with lead angle”, International Journal of Machine Tools & Manufacture 51 (2011), 77-83. [5] M. Fujiki, Jun Ni, Albert J. Shih,” Investigation of the effect of electrode orientation & fluid flow rate in near-dry EDM milling”, International Journal of Machine Tools & Manufacture 49 (2009), 749-758. [6] Jia Tao, Albert J. Shih, Jun Ni,” Near-Dry EDM Milling of Mirror-Like Surface Finish”, International Journal of Electrical Machining 13 ( January 2008). 29-33. [7] P. Govindan, Suhas S. Joshi, “ Experimental characterization of material removal in dry electrical discharge drilling” , International Journal of Machine Tools & Manufacture 50 (2010), 431-443. [8] Fabio N. Leao , Ian R. Pashby , “ A review on the use of environmentally-friendly dielectric fluids in electrical discharge machining”, Journal of Materials Processing Technology 149 (2004), 341-346. [9]Viktor P. Astakhov, General Motors Business Unit of PSMI, USA, “ Ecological Machining : Near Dry Machining”. [10] B.C. Routara, B.K. Nanda, D.R. Patra,” Parametric optimization of CNC wire cut EDM using Grey Relational Analysis”, Proceedings of the International Conference on Mechanical Engineering ( Dec.2009),RT-24,1-6. [11] S. Abdulkareem, A.A. Khan & Z.M. Zain,”Effect of Machining Parameters on Surface Roughness during Wet & Dry Wire EDM of Stainless Steel”, Journal of Applied Sciences 11 (10), 1867-1871, (2011). [12] Jia Tao, “Investigation of Dry & Near Dry Electrical Discharge Milling Process”, A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy (Mechanical Engineering) in The University of Michigan, (2008). [13] Sourabh K. Saha, S.K.Choudhury, Department of Mechanical Engineering, Indian Institute of Technology Kanpur, ”Multi-objective optimization of the dry electric discharge machining process”, (Jan. 2009). [14] Grzegorz Skrabalak, Jerzy Kozak, “ Study on Dry Electrical Discharge machining”, Proceedings of the World Congress on Engineering, London UK, (2010) Vol. III. 36