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CHAPTER 4

EXPERIMENTAL PLANNING USING EDM MACHINE

4.1 INTRODUCTION

In this chapter existing EDM systems like ELECTRONCIA make and

SPARKONIX make EDM machines, effect of input parameters, dielectric

system, selection of workpiece and tool, machining performance evaluation,

machining parameters selection and design of experiments are described in

detail.

4.2 EXISTING EDM SYSTEMS

There are many manufacturers available worldwide for EDM systems.

But, in India there are very few commercially available EDM manufacturers like

Makino, Electronica, and Sparkonix. In this work, Electronica and Sparkonix

EDM machines have been selected for drilling of micro-holes. At the time of

purchasing the machines, there are many undisclosed parameters which are

effective in the determination of machining characteristics. So this research aims

to study and optimize the process parameters of the machine setup for better

machining characteristics such as MRR, TWR, OC and Taper.

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4.3 ELECTRONCIA MAKE EDM MACHINE

A scheme of experiments is performed using ELECTRONICA make

EDM Machine which is shown in Figure 4.1.

Figure 4.1 EDM Machine (ELECTRONICA) for micro-hole machining of

SS 304

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4.3.1 Technical Specifications

Work Table : 450X300 mm Granite

X &Y Axes Travel : 350 & 250 mm

Z Axis Travel : 350 mm

Maximum electrode length : 400 mm

Electrode pipe diameter : 0.3 - 3.0 mm

Maximum weight of the work piece : 350 kg

Maximum coolant pressure : 6MPa

Maximum drill depth : 300 min

Input Power Supply : 415 VAC, 3 phase, 50 Hz

Connected Load : 3 KVA

Maximum Machining Current : 30 Amps

Machining fluid : DI water

4.4 SPARKONIX MAKE EDM MACHINE

The machine shown in Figure 4.2 consists of an x-y table with

movements in the x-axis as well as y-axis of the horizontal plane. Both

movements are achieved by means of dove-tailed x-y slides driven with lead

screw and are hand wheel operated. The Z- axis comprises the servo- slide

movement on which the mechanism for rotating electrode along with a chuck is

mounted for holding the required tubular copper or tubular brass electrode as

shown in Figure 4.3. The x and y axis movements as well as z- axis movements

of electrode can be set and adjusted using digital read out (DRO) provided at the

top of the control panel. The control panel appears on the right hand slide of the

machine at about eye-level and the electrical controller is housed below the x-y

table in the machine cabinet. The left hand side of the machine cabinet below the

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x-y table is occupied by the coolant tank and the high pressure pump unit along

with filter elements.

An arrangement for holding and adjusting the level of electrode guides

bush is provided on the left hand side of the machine pillar holding the electrode

rotating mechanism and the servo slide assembly. A machine lamp is similarly

located. A pressure setting valve and pressure gauge to monitor correct operating

pressure are provided on the machine pillar centrally on the left hand side.

The work piece or job is secured and clamped at an appropriate

location on the x-y table. The location of small holes or fine deep holes to be

drilled may be marked on the job. The job may be set with the help of dial stand

and DRO. A suitable electrode of particular size as well as an appropriate guide

bush is selected and the electrode inserted into the chuck for holding it. The

electrode is then tested for coolant flow at a pressure of about 100 Kg/sq.cm. The

electrode may now be positional on to job to start drilling operation.

4.4.1 Technical Specifications

Power rating : 3 KVA

Working voltage : 415 VAC, 3 phase, 50 Hz

Maximum Machining Current : 25 Amps

Electrode Movement (Z1) : 150 mm

Guide movement (Z2) : 50 mm

Work table size : 600x400 mm

Table travel : 300x200 mm

Maximum work piece size (LxWxH) : 400x300x200 mm

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Maximum work piece weight : 500 kg

Maximum electrode length : 400 mm

Electrode pipe diameter : 0.3 - 3.0 mm in Steps of 0.1 mm

Machining fluid : DI water

\Fluid tank capacity : 50 liters

Type of electrode : Brass and copper tubes.

Erosion transformer : Input- 440 VAC Output- 60 VAC

Figure 4.2 EDM Machine (Sparkonix) (Disintegrator MICRO DRILL DSH-

II) for micro-hole drilling of SS 316

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Figure 4.3 Tool setup

4.4.2 Spark Generator/Controller

The controller cabinet is located on the right side of the machine and can be

pulled out completely if required for repairs or maintenance. This can be done by

disconnecting main supply, the job cable, control panel socket and the four bolts

holding the cabinet.

The control cabinet houses the following:-

1) Erosion transformer : Converts 415 v input to 80v i.e. steps down

transformer.

2) Rectifier Unit : Convert the stepped down AC voltage to DC.

3) Control circuit : Fitted on a chassis, it consists of control

transformers, contactors for mains, pump and

erosion, and rotating motor control, power

MOSFET circuit and indicators circuit.

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The following controls and indicators are located on the front panel:-

Master main ON (MCB) : For providing 415 VAC three phase supply

to machine.

Mains ON : For switching ON mains.

Emergency OFF : For switching OFF mains

Pump ON : For Switching ON pump.

Pump OFF : For Switching OFF pump and Erosion.

Erosion ON : For switching ON Erosion

Down Indicator (Green LED) : Indicator forward (towards job) movement

of electrode.

Up indicator (Red LED) : Indicator reverse (away from job)

Movements of electrode.

Up/Down Switch : In forward mode, this control helps to bring

down the electrode during setting of job

without switching ON pump erosion. This

should always be kept at up position except

when it is required for setting of the job.

Current : This is a set of six switches and will provide

a total of 25Amps of current these 25 Amps

are divided into six steps of 1A, 2A, 4A, 6A,

6A and 6A. So any current between 1A to

25A can be selected with the help of these

switches.

Rotating ON/OFF : Used for switching ON rotating (Erosion

will not start unless rotating is ON).

Motor ON/OFF : Used for switching ON or OFF a servo

motor.

Pump socket : Connects supply to coolant pump via cable

and socket.

Control panel socket : The control panel cables are connected here

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by 48 pin plug and socket arrangement. The

other end of the cable is terminated on

control panel fitted on the column.

Electrode and job : Electrode and job terminals constitute to the

o/p and the connected respectively to the

electrode and the x-y table. Both

connections should be tight and proper and

job should be clamped on x-y table as far as

possible.

ON/OFF time switches : With the help of these two rotary switches,

we can change ON time OFF time of the

output current in 10 steps. These can be

selected with reference to the trial chart.

Buzzer : This is for job setting purpose. If this switch

is ON and the electrode touches the job, the

buzzer will sound and be down. movement

of the motor will be stopped.

Capacitor : This switch is used for carbide job with

copper electrodes. With increasing steps

value of capacitor increases.

4.5 EFFECT OF INPUT PARAMETERS

Based on the discharge phenomena discussed above, the effect of

various input parameters on MRR and surface roughness is discussed below.

4.5.1 Discharge Current

The discharge current is a measure of the power supplied to the

discharge gap. A higher current leads to a higher pulse energy and formation of

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deeper discharge craters. This increases the MRR and the surface roughness (Ra)

value. Similar effect on MRR and Ra is produced when the gap voltage is

increased. The current density is the most important parameter which determines

the MRR and surface condition. The current density is affected by either

changing the current or changing the electrode (tool) – work piece gap. When the

current is increased, each individual sparks removes a larger crater of metal from

the work piece. But it also increases surface roughness. Increasing spark

frequency results in decrease of surface roughness and reduces the removal of

crater of metal from the work piece. The gap between the electrode (tool) and

work piece is determined by the spark voltage and current. A small gap produces

more accuracy with a better surface finish and slower MRR.

4.5.2 Voltage

In EDM process with the given input voltage, a spark can be generated

only at or below certain gap between the tool and work piece. The EDM power

supply sense the voltage between the electrodes and then sends the relevant

signal to the servo system, which maintains the desired gap value between the

electrodes. It is a potential that can be measured by volt it also effects to the

MRR and allowed to per cycle. The Arc gap is distance between the electrode

and work piece during the process of EDM. It may be called as spark gap. The

spark gap can be maintained by servo system.

4.5.3 Pulse-on Time

The duration of time the current is allowed to flow per cycle is known

as Pulse-on time. An illustration of on-time, off-time, pulse-time and duty cycle

is shown in Figure 3.4. Material removal is directly proportional to the amount of

energy applied during this on-time. This energy is really controlled by the peak

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current and the length of the on-time. Machining takes place only during the

pulse-on time. When the tool electrode is at negative potential, material removal

from the anode (workpiece) takes place by bombardment of high energy

electrons ejected from the tool surface. At the same time positive ions move

towards the cathode. When pulses with small on times are used, material removal

by electron bombardment is predominant due to the higher response rate of the

less massive electrons. However, when longer pulses are used, energy sharing by

the positive ions is predominant and the MRR decreases. When the electrode

polarities are reversed, longer pulses are found to produce higher MRR.

4.5.4 Pulse-off Time

The duration of time between the sparks (i.e., on-time) is known as

Pulse-off time. This time allows the molten material to solidify and to be washed

out of the arc gap. This parameter is to affect the speed and the stability of the

cut. Thus, if the off-time is too short, it will cause sparks to be unstable. A non-

zero pulse off time is a necessary requirement for EDM operation. Discharge

between the electrodes leads to ionization of the spark gap. Before another spark

can take place, the medium must de-ionize and regain its dielectric strength. This

takes some finite time and power must be switched off during this time. Too low

values of pulse-off time may lead to short-circuits and arcing. A large value on

the other hand increases the overall machining time since no machining can take

place during the off-time. The surface roughness is found to depend strongly on

the spark frequency. When high frequency sparks are used lower values of Ra are

observed. It is so because the energy available in a given amount of time is

shared by a larger number of sparks leading to shallower discharge craters.

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4.5.5 Duty Cycle

Duty cycle is a percentage of the on-time relative to the total cycle

time. This parameter is calculated by dividing the on-time by the total cycle time

(on-time plus off-time). The result is multiplied by 100 for the percentage of

efficiency or the so called duty cycle.

Figure 4.4 An illustration of on-time, off-time, pulse-time and duty cycle.

4.5.6 Inter Electrode Gap

Inter Electrode Gap (IEG) is the distance between the electrode and the

workpiece during the process of EDM. It may be called as spark gap. Since,

during operation, both the work piece and electrode are eroded, the feed control

must maintain a movement of the electrode towards the work piece at such a

speed that the working gap, and hence, the sparking voltage remains unaltered.

Since the gap width is so small, any tendency of the control mechanism to hunt is

highly undesirable. Rapid response of the mechanism to hunt is highly

undesirable. Rapid response of the mechanism is essential and this implies a low

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inertia drive. Overshooting may completely close the gap cause a short circuit;

hence, it is essential to have rapid reversing speed with no backlash.

Actuation of the control drive is derived from an error indication signal

obtained from an electrical sensing device responsive to either the gap voltage or

the working current or both. Servo mechanisms affecting the movement of the

electrode may be either electric-motor-driven, solenoid operated or hydraulically

operated or a combination of these.

4.5.7 Tool Rotation

Tool electrode rotation is commonly used in small-hole EDM drilling operations.

Tool rotation improves flushing and leads to a more uniform electrode wear. The

effects of improved flushing are increased MRR and lower Ra values. At the

same time, process stability increases because tool rotation makes it easier to

introduce fresh dielectric into discharge gap as the used up dielectric is thrown

out due to the centrifugal force. Thus, even with low pulse off times and poor

flushing conditions good machining performance is obtained.

4.6 DIELECTRIC SYSTEM

It consists of dielectric fluid, reservoir, filters, pump, and delivery devices.

A good dielectric fluid should possess certain properties, viz as shown below, it

should:

i. Have high dielectric strength (i.e. remain electrically non-

conductive until the required breakdown voltage between the

electrodes is attained),

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ii. Take minimum possible time possible time to breakdown (i.e.

ignition delay time) once the breakdown voltage is reached,

iii. Deionize the gap immediately after the spark has occurred,

iv. Serve as an effective cooling medium.

The fluids commonly used as dielectric are transformer oil, paraffin oil,

kerosene, lubricating oils, and de-ionized water which give higher MRR and

functions as more effective cooling media but they also cause high electrode

wear rates. Further, they suffer from the drawback of causing corrosion. To

overcome this problem, corrosion inhibitors are used but they result in increased

electrical conductivity to an unacceptable level.

De-ionized water is commonly used as a dielectric in wire-EDM and

drilling of small diameter holes. The filtration of dielectric fluid before

recirculation is highly essential so that a change in its insulation qualities during

the process is minimal. The concentration of the debris particles in the gap

increases rapidly as the machining progresses. These wear particles should be

removed from the gap so that the fresh dielectric enters the IEG for spark

discharges. Increase in the pollution of dielectric results in decrease of the

breakdown intensity of the field. Hence, IEG at the entrance of the clean fluid is

much narrower than at the exit of the flow. This significantly affects the

reproduction accuracy of the process. It is possible to correct such errors only if it

could be feasible to predict quantitatively the error of the shape.

Effective flushing of dielectric removes by-products from the gap.

Ineffective flushing results in lower MRR and poorer surface finish. The

effective flushing may increase MRR as much as by a factor of 10. Poor flushing

ends up with stagnation of dielectric and building-up of machining residues

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which are apart from low MRR also lead to short circuits and arcs. A good

flushing system is the one that shoots the dielectric to the place where the

sparking occurs. It is felt that adequate flushing in case of blind cavities is

difficult. Various methods have been proposed by different researchers to ensure

proper and adequate flushing of the gap and thereby obtaining a better process

output.

i. Suction through electrodes

ii. Pressure through electrodes

iii. Jet flushing

iv. Alternating forced flushing

v. Ultrasonic vibration of electrodes

vi. Rotating electrode flushing

These are the some of the dielectric flushing techniques. In some cases,

hypodermic needles have been used as nozzles for flushing in tight quarters. In

case of machining of blind cavities, flushing through a hole in the tool is most

effective but it rises to protruding bump (or spike). This is the situation similar to

the one faced during ECM. To avoid this problem, a rotating tool with a eccentric

hole (off-centred holes) for dielectric supply can be used. Jet flushing is less

effective hence it should be used only when none of the other methods can be

used due to tool or workpiece configuration. In case of inflammable dielectric

fluids, the workpiece should always be immersed in the dielectric fluid to

minimize any chance of accidental fire. As machining continues, the dielectric

gets contaminated with more and more amount of debris. Presence of such debris

in the IEG, results in bridging the gap. It yields more number of arcs. Arcs are

undesirable in EDM because they damage both tool and workpiece. Occurrence

of such arcs can be eliminated by proper filtration of the dielectric as well as

appropriate flushing of the IEG.

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4.6.1Functions of Dielectric

It acts as an insulation medium.

It cools the spark region and helps in keeping the tool and work piece

cool.

It carries away the eroded metal particles along with it

It maintains a constant resistance across the gab.

It remains electrically non-conducting until the required breakdown

voltage is reached.

It cools the machining zone by carrying away excess heat from the tool

electrode and the workpiece.

4.6.2 Properties of Dielectric

The most important properties of dielectric are its dielectric strength,

viscosity, thermal conductivity and thermal capacity. Dielectric strength

characterizes the fluid’s ability to maintain high resistivity before spark discharge

and the ability to recover rapidly after the discharge. High dielectric strength

leads to a lower discharge gap which in turn leads to a low gap resistance.

Hence, high discharge currents may flow leading to a higher MRR. Also, fluids

with high dielectric strength need lower time for the recovery of dielectric

strength. Thus, low pulse-off times are sufficient. This not only improves the

MRR but also provides better cutting efficiency because of a reduced probability

of arcing. Liquids with low viscosity generally provide better accuracies because

of a better flow ability of the oil leading to improved flushing. Also, the sideward

expansion of the discharge plasma channel is restricted by high viscosity fluids.

This focuses the discharge energy over a small region and leads to a deeper crater

which reduces the surface finish. Dielectric fluids with high thermal conductivity

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and thermal heat capacity can easily carry away excess heat from the discharge

spot and lead to a lower thermal damage.

4.7 SELECTION OF WORKPIECE AND TOOL

The brass electrode of diameter 500 m is used as a tool electrode.

The melting point of the tool material should be high for machining difficult-to-

cut materials. Normally copper, brass, and tungsten tools are used for this

purpose. However, brass tool is more economical than copper and tungsten tool

and mostly used in industrial applications. Also, brass tools can withstand high

tensile stress compared to pure copper tools. The workpiece used is SS 304 plate

of size 50×50×5 mm and SS 316. In the case of SS, its mechanical strength and

chemical resistance can easily be controlled by changing the composition of the

alloy. Therefore, SS is one of the most widely used materials for micro

mechanical structures. The composition of SS is given in Table 4.1 and the

physical properties of workpiece and tool materials are presented in Table 4.2

Table 4.1 Chemical composition of Stainless steel

component Fe C Mn Si P S Cr Ni

Weight %(SS 304)

74 0.08 2.0 1.0 0.04 0.03 19.0 10.5

Weight %(SS 316)

74 0.055 1.5 0.75 0.02 0.004 16.4 13.9

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Table 4.2 Physical properties of workpiece and tool materials

Material Stainless SteelGrade 304

Stainless SteelGrade 316

Brass

ThermalConductivity(W/m-K)

21.5 14.6 115

Density (kg/m3) 8060 8027 8700

Poisson's Ratio 0.33 0.33 0.31

Specific Heat (J/kg-K) 503 450 0.378

Yield Strength (MPa) 290 205 -

Melting point (°C) 1400-1455 1390-1440 885-900

The properties of De-Ionized water at room temperature are given in Table 4.3

Table 4.3 Electrical, thermal, and mechanical properties of De-ionizedWater at room temperature

De-ionized Water

Dielectric strength (MV/m) 13

Dielectric constant 80.4

Dynamic viscosity (g/m s) 0.92

Thermal conductivity (W/m K) 0.606

Heat capacity (J/g K) 4.19

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The positive terminal is connected to the workpiece and made as

anode and the negative terminal is connected to the tool and made as cathode.

When the negative terminal is given to the tool, the tool wear is less due to low

sparking energy distribution at the tool, and this helps in improving the

micromachining accuracy. De-ionized water is used as the dielectric medium in

EDM and is an alternative to traditional hydrocarbon oil to promote a better

working environment from the perspective of health and safety, as it does not

decompose and release harmful vapours (CO and CH4).

The Experimental condition for micro-hole machining on SS 304 are

given in Table 4.4

Table 4.4 Experimental conditions for micro-hole machining on SS 304

Workpiece 50×15×5 mm SS 304 plate

Tool electrode Brass tool of diameter 500 m

Dielectric fluid De-Ionized water

Polarity Positive (workpiece ‘+ve’ and tool ‘-ve’)

Pulse-on time 100 to 200 s

Peak current 2 to 4 amps

Gap voltage 20 to 40 volts

4.8 MACHINING PERFORMANCE EVALUATION

To observe the surface quality and dimensions of micro holes a

Scanning Electron Microscope (SEM; JEOL-6390) having 3, 00,000X

magnification and an accelerating voltage of 30KV is used. In this research

study, EDM characteristics such as MRR, TWR and OC are considered as the

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output responses for through micro-hole machining. The MRR is calculated as

the workpiece weight loss over the machining time, which is expressed as

milligrams per minute. As during the EDM process both the workpiece and tool

electrode is eroded simultaneously, TWR is expressed as the ratio of tool weight

loss to the machining time. A digital vernier caliper with 0.01 mm accuracy is

used to measure the length of the tools before and after machining. The loss in

length of the tools is calculated by taking the difference between initial and final

length measurements.

Measurement of MRR:

MRR = Work piece weight loss (g)Machining Time (min) (3.1)

Measurement of TWR:

TWR = Tool weight loss (g)Machining Time (min) (3.2)

OC of the machined micro-hole is calculated by subtracting the diameter of the

tool electrode from the diameter of the machined micro hole.

Due to the debris movement and secondary discharge occurring on the

side of hole in addition to front end tool wear, the machined hole is not ideally

cylindrical but tapered one, which is undesirable in precision machining

processes. The typical shape of micro-drilled hole resulting from electro-

discharge machining is shown in Figure 4.5. The diameter of the drilled hole is

larger at the top, i.e., at the entry and decreases along the depth and is minimum

at the bottom (exit). The diameters of machined micro-hole at the entry, Dt and at

the exit, Db are measured with the help of optical microscope at the magnification

of 10X and the taper of the machined micro-holes are calculated as follows.

Taper =LDD bt

2 (3.3)

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Figure 4.5 Schematic cross-sectional view of EDMed micro-drilled hole

4.9 MACHINING PARAMETERS SELECTION

In this study, the experimental plan has three controllable variables

namely current, pulse-on time, and gap voltage. The range of the current, pulse-

on time, and gap voltage are selected as 2 to 4 A, 100 to 200 s and 20 to 40 V

respectively based on the available settings in the machine set-up as shown in

Table 4.5.

Table 4.5 - Machining parameters and their levels for micro-hole machining

of SS 304

Symbol Parameters Level 1 Level 2 Level 3

A Pulse-on time, s 100 150 200

B Current, A 2 3 4

C Voltage, V 20 30 40

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The selection of machining parameters for SS 316 is presented in Table

4.6.

Table 4.6 - Machining parameters and their levels for micro-hole machining

of SS 316

Symbol Parameters Level 1 Level 2 Level 3

A1 Current, A 10 12 14

B1 Voltage, V 30 40 50

C1 Pulse -on time, s 100 150 200

D1 Pulse -off time, s 20 30 40

4.10 DESIGN OF EXPERIMENTS

Design of Experiments (DoE) refers to planning, designing and

analyzing an experiment so that valid and objective conclusions can be drawn

effectively and efficiently. To perform a designed experiment, changes are made

to the input variables and the corresponding changes in the output variables are

observed. The input variables are called factors and the output variables are

called response. Factors may be either qualitative or quantitative. Qualitative

factors are discrete in nature (such as type of material, color of sample). Each

factor can take several values during the experiment. Each such value of the

factor is called a level. A trial or run is a certain combination of factor levels

whose effect on the output is of interest. It is essential to incorporate statistical

data analysis methods in the experimental design in order to draw statistically

sound conclusions from the experiment. Some of the advantages of DoE over

One-Variable-At-a-Time approach (OVAT) are that a DoE approach enables to

separate the important factors from the unimportant ones by comparing the factor

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effects. Also, interaction effects among different factors can be studied through

designed experiments.

4.11 CONCLUDING REMARKS

In this chapter constructional features and specifications of the

existing EDM systems like ELECTRONCIA make and SPARKONIX make

EDM machines are explained. Furthermore the effect of input parameters,

dielectric system, selection of workpiece and tool, machining performance

evaluation, machining parameters selection have been explained. Design of

experiments is described in detail.