An Overview of Experimental Investigation of Near Dry Electrical Discharge Machining Process

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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

22

AN OVERVIEW OF EXPERIMENTAL INVESTIGATION OF NEAR

DRY ELECTRICAL DISCHARGE MACHINING PROCESS

Mane S.G.1, Hargude N.V.

2

1,2Department of Mechanical Engineering, PVPIT Budhgaon, Sangli 416416,Maharashtra,

India.

E-mail:[email protected]; [email protected] .

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 processamongst 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

INTERNATIONAL JOURNAL OF ADVANCED RESEARCH IN

ENGINEERING AND TECHNOLOGY (IJARET)ISSN 0976 - 6480 (Print)

ISSN 0976 - 6499 (Online)

Volume 3, Issue 2, July-December (2012), pp. 22-36

IAEME: www.iaeme.com/ijaret.html

Journal Impact Factor (2012): 2.7078 (Calculated by GISI)

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IJARET

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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 ofdischarging 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 ofmachining 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 isalleviated 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 machiningprocesses. 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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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 thepresence 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 withconventional 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 wireEDM 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 (i e), 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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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 s.

The upper and lower boundaries of the MRR envelope correspond to the minimum and

maximum values of te (4 and 18 s). The specific machine limits, maximum and

minimum to (1000 and 6 s), 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 qualityand 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 EDMdrilling 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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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 4s) and maximum to (1000 s).

Fig 1.Comparision of boundaries of feasible MRR envelopes for wet, dry and near drywire 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 pulsesgenerate 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 waterair mixture, to can be as low as 100 s at te = 18 s. For the

near dry EDM using waterair 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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lower capability of waterair mixture to relieve the concentrated heat from the wire electrode,

still limits the MRR in near dry EDM at low t o. 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 s and to >150 s), 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 isshown in Fig.2. Three regions, designated as IIII, are identified.

In Region I (to >650 s), the near dry EDM has higher MRR than that of wet

EDM because the lower thermal conductivity and heat capacity of the waterair

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 s,

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


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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 s, to= 70 s, 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 waterair 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 m 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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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

(s) (A) (V) (V) (s) (s) (A) (V) (V) (s)_____________________________________________

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 40 124 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 40 127 4 30 260 40 20 7 2 3 260 40 4

8 12 30 260 40 8 8 4 3 260 80 129 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 80 412 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 40 417 8 25 210 60 14 17 3 2 210 60 8

18 8 25 210 60 14 18 3 2 210 60 819 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 appliedto 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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Fig. 5 Effect of the discharge current on high Fig. 6 Effect of the discharge current on theenergy input dry EDM with oxygen finishing EDM (te=4 s,t0=8s, ue=60 V,

(te=4s,t0=8s, 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-dryEDM finishing. The surface finish was improved from 2.5 m to 0.8 m Ra by reducing the

discharge current from 20 A to 1 A. The reduced discharge current lowered the dischargeenergy 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 Fvalue. 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 m depth of cut. When the depth of cutis beyond 500 m, 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 m.


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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 s,(ie=1 A,te=2 s,to=16 s, ue=20 V&ui=200 V to=8 s, 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 s) for all values ofte, and

is particularly beneficial when te is high (= 18 s). 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 5ml/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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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 observedat 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 likelystimulated by the excessive amount of graphite powder chipped off from the electrode tip, asshown 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

(0.870.95 m 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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Fig.10. MRR andRa results of different dielectric fluids for copper and graphite electrode materials:

(a) at high discharge energy input (ie=20 A,te=4s,to=8s, ue=60 V, and ui=200 V) and(b) at low discharge energy input (ie=1 A,te=4s,to=8s, ue=60 V, and ui=200 V)


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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 s, to=8 s, ue=60 V, and ui=200 V)

The copper electrode produced slightly better surface finish, 0.80m and 0.85m 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 copperelectrode 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 m 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 m and 0.87 mRa for graphite electrode and 0.85 m and 0.80

mRa for copper electrode) than the water-air and water-oxygen mixtures (1.68m and 1.62mRa for graphite electrode and 0.98 m and 1.25m 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.


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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 EDMdrilling 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


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36

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, EcologicalMachining : 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 dischargemachining 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.

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An Overview of Experimental Investigation of Near Dry Electrical Discharge Machining Process

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