electro discharge machining of aisi 304 using ...workpiece. die casting, injection moulding,...

5th International & 26th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12th–14th, 2014, IIT

Guwahati, Assam, India

296-1

ELECTRO DISCHARGE MACHINING OF AISI 304 USING SOLID AND

BUNDLED ELECTRODES

Harshit K. Dave1*, Sudhanshu Kumar2, Nipul C. Rana3, Harit K. Raval4

1,2,3,4Department of Mechanical Engineering, S.V.N.I.T, Surat, Gujarat-395007 *[email protected], [email protected], [email protected],

[email protected]

Abstract

This paper presents the investigation on generation of square shape cavities with solid and bundled tool electrodes.

The workpiece and tools material are AISI 304 and electrolytic copper respectively. Effect of peak current and pulse

on time has been observed on machining performance in terms of material removal rate and tool wear rate. Three

different levels of peak current and five levels of pulse on time have been selected for investigation. To remove the

un-machined pinned shape structure of workpiece, 1mm orbital radius has been given to both the electrodes.

Experiments have been designed using fractional factorial design. The experimental results indicate that solid tool

yields 45% more material removal rate than bundled tool electrode and TWRfor both tool is decreasing with pulse

on time.

Keywords: Orbit radius, solid tool, bundled tool, factorial design

1 Introduction

Electro discharge machining process is non contact

type machining process that uses the electric spark to

erode the electrical conductive materials. The

electrically conductive tool electrode, which has the

male shape of the die cavity, is prepared to machine the

die cavity. The method is especially effective in

machining hard die steels, complex cavities and small

workpiece. Die casting, injection moulding, forging,

extrusion, upset forging and powder compaction dies

are manufactured using EDM technology (Rajurkar et

al., 2013).

Electro discharge machining process is the un-

conventional machining process that is mostly used in

die making industries. Die making industries require

complicated geometry with high dimensional accuracy.

The machining of these complex shapes is difficult with

conventional machine tools (Ekmekci et al., 2006). The

possibilities of generating non-circular geometry with

EDM technology is much more than any other machine

tool system. The hardness, brittleness or melting points

of the material do not affect the machining and tool does

not need to be harder (Ferreira, J. C., 2007). The

application of EDM process is simple and economical

than any other un-conventional machining process.

EDM is basically electro-thermal process in which

electric energy is transferred to the thermal energy. So,

the application of thermal energy is controlled by the

input of electrical parameters such as discharge current,

open circuit voltage, pulse duration, gap voltage and

duty cycle (Jain V.K., 2002). The performance of EDM

may be influenced with the tool material, design and

manufacturing method of electrodes. Tool electrode

design also affects on the cost of machining. Excessive

tool wear during EDM process leads to unpredictable

tool life and inconsistent component dimensions. In

EDM, tool geometry is very important because

electrode is employed to produce its replica on the

workpiece.

Several attempts have been made to find out the

effect of tool electrode designs on machining of

complex geometries. Mehra et al. (2012) compared the

performance of a hollow electrode with a solid electrode

in blind hole drilling of ductile iron at 5-15A peak

current. MRR was compared using Taguchi design.

Higher MRR was obtained with hollow electrode

compare to solid electrode at constant pulse on time and

current. At constant flushing pressure authors found

higher MRR with hollow electrode.Gu et al. (2012)

compared the performance of bundled electrode with a

solid die-sinking electrode.Experiments were conducted

at very high peak current (40-127A). Besides inner

flushing with different flow rates, a 0.5 mm radius

orbital motion was adopted. Orbital motion was not

given to the solid electrode. It was obtained that bundled

electrodes can endure a much higher peak current than

solid electrode which results in a substantially higher

MRR and a comparably lower TWR. The benefit of the

bundled electrode makes it much more feasible for

ELECTRO DISCHARGE MACHINING OF AISI 304 USING SOLID AND BUNDLED ELECTRODES

296-2

application in large area rough machining.Murugesan et

al. (2012) compared multi-hole electrode with a solid

electrode in blind hole drilling of Al-15% SiC metal

matrix composite. A 12 mm copper rod with an array of

48 holes of 1mm diameter drilled in it has been used as

a multi-hole electrode. From experiments, it was found

lower machining time in case multi-hole electrode

compare to solid electrode. Forcing the dielectric

through a number of small holes increases the velocity

of the dielectric, which in turn increases the debris

removal.Ojha et al. (2011) studied the effect of different

tool angles on EDM machining performance. Three

different electrodes of constant cross-section area of 50

mm2 and varying angles of 50, 90 and 130

degreewereused in the investigations. The significance

of effective flushing was found on response

characteristics. It was found that MRR increases with

increase in tool angle owing to increase in current. After

certain level, the MRR tends to decrease due to

inefficient flushing. Yilmaz et al. (2010) compared

experimental investigation of EDM fast hole drilling of

aerospace alloys (Inconel 718 and Ti-6Al-4V). Different

electrode type and material, viz. single and multi-

channel tubular electrodes made of brass and copper

materials were selected. It was concluded that the

single-channel electrode has comparatively better MRR

and lower electrode wear ratio (EWR). Mohan et al.

(2004) studied the machining characteristics of

SiC/6025 Al composite using a tube electrode with

rotation. MRR, TWR and Ra were adopted to evaluate

the machinability at 5, 8 and 11A peak current. The

effect of EDM drilling was found with the rotating tube

electrode has produced higher MRR than the rotating

solid electrode. The decrease in hole diameter has

produced a better MRR, Ra and higher TWR. Dave et

al. (2012) studied the effect of different flushing

conditions i.e. through flushing and jet flushing in

drilling of deep holes. Higher range of peak current (13,

21 and 28A) was considered for investigation. The

result indicates the higher MRR with through flushing

(tubular) tool electrode.

From the past research works, it has been observed

that different tool designs have been used to improve

the machining performance of EDM. Solid and tubular

or tool with multi holes are used for generation of

cavities. Peak current and pulse duration are most

affecting electrical parameters (Kansal et al., 2006; Tasi

et al., 2007). Most of the work has been reported at

higher range of peak current and pulse duration. In the

present paper, square shape cavity has been generated

using two different types of tool electrodes i.e. solid and

bundled tool electrode.Identical values of flushing

pressure have been maintained for both types of tool.

Effect of electrical parameters like peak current and

pulse on time at relatively lower values have been

investigated contrary to that reported in the literature to

check its feasibility.

2 Experimental Plan and Procedure

Experimental investigations were carried out on die

sinker EDM “JOEMARS AZ50R”. The workpiece

material used for the experiments is AISI 304 (0.08% C,

18% Cr, 2% Mn, 8% Ni). The tool electrode was made

of pure electrolytic copper. Two types of electrodes i.e.,

solid and bundled (group of copper tubes) have been

used in this investigation. The dimension of solid tool is

8mm x 8mm (square) and the bundled electrode has

been made of 9 tubular copper cell electrodes having

inner and outer diameter of the individual tubes 1.77

mm and 2.66 mm respectively. The experimental set up

has been shown in Fig 1.

(a)

(b)

Figure 1 Experimental set up with (a) bundled tool

(b) Solid tool

Commercially available dielectric fluid is used

during the experiments. Thedielectric fluid being used

in present study has flash point of 200°C and viscosity

of 50SUS at 100°F. Two different types of flushing, jet

flushing and through flushing, have been used.

2.1 Selection of tool geometry

Cylindrical tool shape is more common geometry

that is being used in electro discharge machining. So

circular shape was selected and trial experiments were

performed to check the feasibility of the circular

shape.In the case of bundled electrode some of the

material in the form of pin shaped could not be removed

from the cavity (Fig 2a). So, orbital motion (radius 1

mm) was adopted for bundled electrode to remove the

material. But by doing this, proper shape was not

generated though unwanted material removed from the



296-3

machined cavity (Fig 2b). To improve this shape, more

orbital radius (2.5 mm) was given to same size circular

tool. The cavity generated was much improved shape

than the previous shape but was not perfectly circular

(Fig 2c). So it was not possible to select circular shape

tool for the experiments. Then square shape bundled

electrodes were prepared and trial experiments were

performed on it. The observed cavity was perfect square

shape with the bundled electrode and so square shape

tool was selected (Fig 2d)

(a)

(b)

(c)

(d)

Figure 2 Machined cavity (a) without orbital

(bundled circular electrode) (b) with orbital radius 1

mm (bundled circular electrode) (c) with orbital

radius 2.5 mm (bundled circular electrode) (d) with

orbital motion (bundled square electrode)

2.2 Selection of parameters

In the present work current and pulse on time has

been selected as input machining parameters because

they can potentially affect MRR and TWR during EDM

operation (Kansal et al., 2006; Tasi et al.,

2007).Experiments have been carried out on Z

Numerically Controlled (ZNC) die-sinking Electro

Discharge Machine (EDM). In this machine minimum

current 0.3A and maximum current 50A can be

set.Some trials experiments have been conducted with

both types of tool electrodes at higher and lower range

of peak current. The machining depth was decided as 3

mm. After trial experiments, it has been observed that at

higher peak current, bundled electrode bear heavy wear

and failed to achieve the required depth. Hence, lower

range of peak current (2, 3 and 4A) has been selected

for investigation. Pulse on time is from lower to

medium range i.e. 31 to 155 µs. The machining

conditions and number of levels of the parameters are

selected as given in table 1.The other parameters are

kept constant throughout the experiments. In similar

type of work (Dave et al., 2011), MRR is reported to be

maximum at duty factor of 0.7. Hence, in present work

duty factor is kept constant at 0.7. All cavities have been

generated by providing orbital motion to the tool

electrodes.Both tool electrodes have been given 1mm

orbital radius in order to get identical shape of square

cavity.

Table 1 Parameters and their levels

Parameter Unit Values

Peak current (Ip) A 2, 3, 4

Pulse on time (ton) µs 31, 61,

93, 121,

155

Duty factor - 0.7

Open circuit voltage V 170

Working gap voltage V 62

Polarity - +

Flushing pressure Kg/cm2 0.3

In this investigation, three levels of peak current (Ip) and

five levels of pulse on time (ton) has been taken. Using

the factorial design method, total 15 trials have been

decided. Each experiment has been replicated twice and

average of both results has been used for analysis. The

experimental design is such that each level of every

factor comes at equal number of times.

Table 2 Experimental table and observation

Exp.

No.

Ip

ton

MRR

Solid

(Vws)

MRR

Bundled

(Vwb)

TWR

Solid

(Vts)

TWR

Bundled

(Vtb)

1 4 31 1.383 0.713 0.0160 0.0110

2 2 31 0.411 0.401 0.0055 0.0080

3 4 121 0.868 0.802 0.0018 0.0032

4 4 155 0.604 0.201 0.0007 0.0010

5 2 121 0.278 0.100 0.0011 0.0008

6 3 61 0.924 0.661 0.0052 0.0057

7 4 61 1.277 0.909 0.0078 0.0083

8 2 61 0.332 0.467 0.0028 0.0037

9 2 155 0.229 0.080 0.0007 0.0002

10 3 121 0.596 0.165 0.0011 0.0008

11 4 93 1.123 0.991 0.0036 0.0066

12 3 93 0.786 0.675 0.0023 0.0028

13 2 93 0.295 0.166 0.0016 0.0021


296-4

14 3 31 0.992 0.503 0.0108 0.0064

15 3 155 0.433 0.121 0.0004 0.0004

3 Results and Discussions

All experiments have been carried out using

factorial design as mentioned in table 1. The final depth

of cavity is fixed to 3 mm for each experiment. As

mentioned earlier, solid and bundled types tool

electrode have been used. Therefore, two types of

flushing methods have been applied i.e. jet and through

flushing for solid and bundled respectively. Machining

has been done with 1 mm orbit radius to avoid the

unmachined pin shape with bundled electrode.

Machining time has been carefully recorded for each

experiment. The response parameters MRR and TWR

have been calculated using weight difference method as

shown in equations 1 and 2.

wb wa

w

W WMRR=

tρ

−

× (1)

tb ta

t

W WTWR=

tρ

−

× (2)

where,

Wwb=weight of workpiece before machining, Wwa =

weight of workpiece after machining, Wtb = weight of

tool before machining, Wta = weight of tool after

machining, w

ρ = density of AISI304 (8gm/mm3),

tρ =

density of Copper (8.94gm/mm3), t = machining time

3.1 Effect of peak current on MRR and TWR

Effect of peak current (Ip) on MRR (mm3/min) has

been plotted using average of all MRR at particular

peak current.

Figure 3 Effect of peak current on MRR with both

tools

From Fig. 3,MRR can be observed increasing with peak

current. Highest MRR has been calculated with 4A peak

current for both type of tools. With increase in peak

current, the thermal energy at the machining area

increases due to this, higher MRR can be expected.In

comparison with bundled tool electrode, solid tool

electrode results higher MRR. Highest MRR obtained

with solid tool electrode is 45% more than that of

bundled tool electrode. The effective area of machining

is larger in solid tool than that of bundled tool electrode.

This may leads to the more stable machining in case of

solid tool, results higher MRR. The effective area of

both tool electrodes has been shown in Fig. 4.

(a) (b)

Figure 4 Effective area (a) solid tool (b) bundled tool

Effective area of solid tool = side side× (3)

= 64 mm2

Effective area of bundled tool = (Outer dia.-Inner dia.)4

π

(4)

= 2 2

(2.66 1.77 )4

π−

= 5.599 mm2

As seen in Fig. 5, TWR for both types of tool increases

with increase in peak current. This is due to the fact that

an increase in dischargecurrent increases the pulse

energy that leads to an increasein heat energy rate,

which is subjected to both of theelectrodes, and in the

rate of melting and evaporation.

0.15

0.25

0.35

0.45

0.55

0.65

0.75

0.85

0.95

1.05

1 2 3 4

MR

R (

mm

3 /

min

)

Peak current (A)

Solid Bundled



296-5

Figure5 Effect of peak current on TWR with

bothtools

From Fig 5, one can observe that highest tool wear

rate occurs at 4A peak current and approx. same for

both types of tool. At the lower peak current (2A),

bundled tool electrode bears higher wear rate than solid

tool electrode. However, at 3A peak current lower

electrode wear has been observed in bundled tool than

solid tool electrode.

3.2 Effect of pulse on time (ton) on MRR and TWR

Pulse on-time is the time during which current

flows in the electrodes. Since all the work is done

during on-time, the duration of these pulses and the

number of cycles per second (frequency) are important.

Metal removal is directly proportional to the amount of

energy applied during the on-time (Singh et al., 2005).

This energy is controlled by the peak amperage and the

length of the on-time. With longer pulse duration, more

work piecematerial will be melted away. The produced

crater will be broader and deeper than a crater produced

by shorter pulse duration. But extra large pulse duration

can reduce material removal rate. This is because of

expanding of plasma channel that distributes the energy

concentration (Dave et al, 2012). But if the interval is

too short, the ejected work piece material will not be

swept away by the flow of the dielectric and the fluid

will not be de-ionized. This will cause the next spark to

be unstable. This slows the machining rate.

The effect of pulse on time on MRR has been

shown in Fig. 6. The MRR is found decreasing with

increase in pulse on time for both solid as well as

bundled tool electrode. In case of solid tool, the MRR

decreases linearly with pulse on time. This is due to the

fact that higher pulse on time expends the plasma

channel and energy got distributed which results lower

MRR (Dave et al., 2012). However in case of bundled

electrode, MRR first increases then decreases with pulse

on time. This may be because of short pulse duration

causes less vaporization whereas long pulse on-time

durationcauses the plasma channel to expand, resulting

in lessenergy density on workpiece, which is

insufficient to meltand/or vaporize the workpiece

material(Wang and Yan, 2000). Thus MRR exhibits

increasing (with bundled tool) trend in beginning but

further increase in pulse on time decreases the MRR.

Highest MRR has been observed at 31µs pulse

duration (lowest ton) with solid electrode while highest

MRR has been obtained at 61µs pulse duration with

bundled tool electrode. Highest MRR with solid tool is

36% more than that with bundled tool electrode.

The MRR with solid tool is higher than bundled tool for

everypulse on time setting. This may be due to the fact

that solid tool has more effective area than bundled tool

as discussed in equations (3) and (4). So, due to large

effective area more sparks can be generated during

pulse duration. These many sparks may be responsible

for the higher MRR with solid tool than bundled tool

electrode.

Figure6 Effect of pulse duration on MRR with both

tools

The effect of pulse on time on TWR has been

studied through Fig. 7. TWR for both of the tools (solid

and bundled) has been decreasing with increase in pulse

on time. Highest wear rate has been obtained at 31µs

and lowest TWR at 155µs pulse on time. In comparison

with bundled tool, solid tool electrode showsless wear

rate except at 31µs pulse on time. Lower TWR of solid

tool electrode may because of larger effective area of

solid tool electrode. Tool having larger effective area

will conduct heat energy more rapidly and large portion

of heat at tool face is conducted to the dielectric fluid

present around the tool surface. Hence, solid tool

electrode has been reported with lower TWR. This

result is an agreement with researchers (Sohani et al.,

2009), they have reported decrease in TWR with

increase in surface area of tool electrode.

0.001

0.002

0.003

0.004

0.005

0.006

1 2 3 4

TW

R(m

m3

/m

in)

Peak current (A)

Solid Bundled

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 31 62 93 124 155

MR

R (

mm

3 /

min

)

Pulse on time(µs)

Solid Bundled


296-6

Figure7 Effect of pulse duration on TWR with both

tools

4Conclusions

An attempt has been made to study the effect of peak

current and pulse on time on MRR and TWR in

generation of square shaped cavity with solid and

bundled tool electrodes. Experiments have been

performed according to fractional factorial design. The

following conclusions have been drawn through this

investigation:

1. Square shaped cavity has been successfully generated

with both types of tool electrodes i.e. solid and bundled

tool.

2. MRR and TWR for both of tools have been

increasing with increase in peak current.

3. Solid tool gives 45% higher MRR than bundled tool

electrode at 4A peak current.

4. MRR with solid tool has been observed linearly

decreasing with increase in pulse on time but MRR with

bundled tool electrode shows first increasing than

decreasing trend with pulse on time.

5. At lower pulse on time (31µs), TWR for solid tool is

more than bundled tool and is decreasing with

increasing in pulse on time but at highest pulse on time

(155µs) both tool electrodes show approximately same

wear rate.

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of the orbital tool movement on the material removal

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TW

R (

mm

3 /

min

)

Pulse on time(µs)

Solid Bundled



296-7

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electro discharge machining of aisi 304 using ...workpiece. die casting, injection moulding,...

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