1. INTRODUCTION

1.1 EDM:

Electro Discharge Machining is considered a non-conventional process. It consists of removing

material from a part by the repetition of electric discharges (produced by electric pulse

generators with very short pulses) produced between an electrode (the tool) and the component,

surrounded by a dielectric fluid that cools and cleans the produced debris (eroded material which

is ejected as small spherical particles).

The Electro Discharge Machining was one of the pioneering non conventional processes applied

by the industry. It is usual to refer the process as EDM, SEDM for sinking EDM and WEDM for

Wire EDM.

Nowadays, the process is usually applied by the industry for high precision machining of

electrical conductive materials like metals, metal alloys, graphite or ceramics (whatever the

hardness value presented).

The negligible value of the process forces and the low unit removal rate per spark (UR) indicate

that the EDM process is suitable for the machining of small details in big parts or the machining

of miniaturized components.

Some of the latest advances in the field of EDM (both sinking and wire EDM) are directed to the

production of details with micrometric dimensions in high precision components. For sinking

EDM at the miniaturized scale, it is usual to use the term microEDM, while the WEDM at micro

scale is referred as “ultra thin WEDM”, “thin WEDM” or “micro WEDM”.

The present document presents basically the EDM principle, the history of EDM, differences

when scaling the process and the latest advances in the field of micro electro discharge

machining.

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1.2 THERE ARE THREE MAIN TYPES OF EDM WHICH ARE

DIFFERENT ACCORDING TO THE TYPE OF ELECTRODE USED.

Wire EDM:

In which the electrode is a brass wire that comes off a spool and is fed through an upper and

lower diamond guide and then discarded after it is used. The wire is controlled by a CNC control

which allows you to program a path for the wire to travel along sort of like a super precision

band saw. The wire can range from 0.0120 to 0.0008 inches in diameter. This form of EDM is

able to hold size well under 0.0001 inches.

Sinker EDM:

In which the electrode is a machined shape made of materials like graphite, copper, or copper-

tungsten and the machine uses this shape to erode the inverse shape in the work piece. This

process is very accurate and commonly used to burn mold cavities where the electrode starts out

the shape of the final molded part and is used to erode a cavity in a mold that is later used to

make thousands or millions of parts.

Small whole EDM drill:

In which the electrode is a brass or copper tube ranging from 0.004 to 0.250 inches in diameter

that is used to blast holes through the work piece. This process is similar to sinker EDM expect

that sinkers have very sensitive power supplies to protect and conserve the electrode and popper

has very aggressive power supply to blast through the work piece very quickly.

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2. HISTORICAL BACKGROUND

The principles of EDM are known since two centuries ago (in 1786, the British Physicist

Priestley observed the presence of small craters in opposite electrodes in between a spark raised)

focusing the first application for the principle: the preparation of colloidal dissolutions of metals.

The first application of the sparks to obtain geometrical shapes was made during the I and II

World War. At the beginning, the possibilities of the technique were not considered due to the

low productivity and lack of process control. In the first designs, the electrode and the part wear

were similar and the presence of no desired arcs dropped the process performance. The gap

between the electrode and the part was controlled by vibrating systems that reduced the electrode

wear, which was still excessive.

The definitive push of the technology (initially in SEDM) was made in Moscow in 1943 by two

married Society Scientists: Dr. Boris and Dr. Natalya Lazarenko. They developed some key

components that made possible to apply the technology in the industry: the spark generator

(Resistance-Capacitance RC generator) and the first servo control circuit to keep a constant

discharge gap. The new developments were presented in a job titled “About the inversion of

wear effect in electric discharges” and published in April 23rd, 1943 by B.N. Zolotych, a

collaborator of the mentioned scientists and one of the most important researchers in the field of

EDM.

The RC circuit (fig. 1) developed by Lazarenkos has been the base for the generators of

commercial machines until recent years, being applied by some companies and newly introduced

for micro EDM.

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The first transistorised spark generators were Developer by 1964. Now they are applied in most

commercial models.

The next big step for the EDM technology was made in 1969 by Prof. Bernd Schumacher with

the development of the wire EDM machine.

Since then, the process improvements have consisted of better control electronics: numerical

controls spark generation, larger number of controlled axes, automatic threading systems, etc.

The improvements in the dimensional control and quality of the applied materials for the

electrodes is also noticeable for both die sinking and wire EDM.

Since 70s, the WEDM process has increased its productivity 20 times, the process costs are now

one third and the achievable surface roughness is 15 times better.

The latest advances in the field of EDM are driven towards two clear tendencies: obtain higher

process productivity by increasing the spark power (while obtaining a higher reliability);

reducing the spark energy to reduce the spark removal rate and produce smaller geometric shapes

(microEDM).

In Japan, the developments by Prof. Kunieda and, special Prof. Masuzawa, have constituted

some of the most important advances in the field of EDM and microEDM.

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3. PROCESS DESCRIPTION

The process has been explained by different theories, being the thermo-electric model the one

that best fits the practical experiments. The next figures represent the different steps described by

the mentioned theory for a spark cycle Werner, Houman y Poco Graphite.  

The charge loaded electrode approaches the surface of the component, which is loaded with the

opposite charge. In between both electrodes there is an isolating fluid, referred as dielectric fluid.

Despite being an electric insulator, a large voltage difference can produce the dielectric

breakage, producing ionic fragments that make possible the electric current to jump between the

electrode and the work piece. The presence of metallic particles suspended in the dielectric fluid

can be good for the electricity transfer in two different ways: on one side, the particles are good

to ionise the dielectric and, what is more, they can provide the electric charge; on the other side,

the particles can catalyze the dielectric breakage. For that reason, the electric field is larger in

that position in which the electrode and the work piece are closer. In this process stage the

situation can be represented in fig. 3.

 Figure 3. First stage of the EDM cycle

In the next stage (figure 4), as the number of ionic particles in the dielectric increases, the

isolating capabilities of the dielectric fluid drop in a narrow channel that appears in that position

in which the electric field is larger. At the same time, the voltage difference presents the highest

value, being the current still zero.

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Figure 4. 2nd stage of the EDM cycle

Just as represented in fig. 5, the current passes by the dielectric fluid that doesn’t play as

insulator. As the current passes, the voltage decays.

Figure 5. 3rd phase of the EDM cycle

Fig. 6 depicts the next stage, the heat produced in the area increases fast as the current increases

and the voltage difference decreases. The generated heat ablates part of the dielectric fluid, the

part and the electrode, creating a discharge channel between the electrode and the part.

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 Figure 6. 4th phase of the EDM cycle

A vapour bubble (fig. 7) is produced and expands against the ions entering the discharge

channel. The lions are attracted by the intense electromagnetic field arising during the discharge.

At the same time, the current increases as the voltage drops.

 

 Figure 7. 5th phase of the EDM cycle

By the end of the pulse (fig. 8), the electric current and the voltage achieve equilibrium, while

the produced heat and pressure reach their maximum value. At the same time, the part material is

removed. The material layer just under the discharge is melted but remains in the same position

due to the bubble pressure. The discharge channel consists of plasma formed out of part,

dielectric and electrode material.

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Figure 8. 6th phase of the EDM cycle

When the pause time between consecutive discharges starts (fig. 9), the electric current and the

voltage drop to zero. The temperature decreases fast, collapsing the vapour bubble and producing

the ejection of the melted material.

Figure 9. 7th phase of the EDM cycle

In the next phase (fig. 10), new dielectric flushes into the area, cleaning and cooling the part

surface. The material which was melted but was not ejected is solidified producing a recast layer.

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Figure 10. 8th phase of the EDM cycle

In the last phase (fig. 11), the ejected material creates small spheres dispersed in the dielectric,

some electrode particles and vapour that go to the dielectric surface. For a short pause time the

melted material and electrode would accumulate making the spark to become unstable and

producing electric arcs that would damage the electrode and the part. Concerning the dimensions

of the produced particles, the experimental studies indicate that they follow a normal statistical

distribution, with values fitting the theoretical model proposed by Rajurkar.

Some other jobs, like Schumacher, propose that gap contamination has influence on the process

ignition, spark location and gap width.

All the sequence is repeated at a rate circa 250000 times per second but, in an instance only one

cycle can be present.

 

Figure 11. 9th phase of the EDM cycle

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4. EDM PROCESS PARAMETERS

In theory, we can say that the process parameters of EDM and the process parameters of Micro-

EDM are quite similar. This is because the working principal is the same which that both of the

machining uses Electric Discharge Machining where electrodes discharges pulses and cut away

the metal with help of dielectric fluid for better machining accuracy. The dielectric fluid also acts

as a lubricant to ensure the machining is accurate and running smooth. We can assume that the

process parameters needed in EDM and micro-EDM is similar due to the similarity explained

above. It is also states that the micro EDM is similar to the principal of macro EDM where the

process mechanism is based on an electro-thermal process that relies on a discharge through a

dielectric in order to supply heat to the surface of the work piece

4.1 DISCHARGE VOLTAGE:

The spark gap and the breakdown strength of the dielectric are related to the discharge voltage in

EDM processes. Current will flow into the system and before it happen the open gap voltage

increases until it has created a path that will go through the dielectric. The path that is mentioned

before is called the ionization path. When the current is flowing, voltage drops and stabilizes at

the working gap level. The preset voltage determines the width of the spark gap between the

leading edge of the electrode and the work piece (Sanjeev Kumar, Rupinder Singh). If we set the

voltage to a high value then the gap will increase, increasing the gap will improve the flushing

conditions and helps to stabilize the cut. The open circuit voltage also have an impact to the

system, as we increase the open circuit voltage tool wear rate (TWR) and surface roughness

increases because the field strength increases.

4.2 PEAK CURRENT:

Peak current is known as the amount of power used in discharge machining which this parameter

is measured in amperage and above all this is the most important parameter in EDM machining.

During each on-time pulse, the current increases until it reaches a preset level which is express as

the peak current. In roughing operations or cavities in large surface areas higher amperage is

used. Using higher currents will definitely improve material removal rate (MRR) but it will give

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an impact on the surface finish and tool wear. Despite the machine cavity is a replica of tool

electrode and excessive wear will hamper the accuracy of machining and as a result, all of the

above statements is important in EDM. New improved electrode materials, especially graphite,

can work on high currents without much damage

4.3 PULSE DURATION AND PULSE INTERVAL:

Expressed in units of microseconds the cycle has an on-time and off –time. On the on-time all

the work is produced and as a result the duration of these pulses and the number of cycles per

second are important. Metal removal is directly proportional to the amount of energy applied

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

amperage and the length of the on-time. Pulse duration and pulse off-time is called pulse

interval. If the pulse duration is longer, then more work piece material will be melted away.

Then, it will have a broader and deeper hole than using shorter pulse duration. Even though the

hole has rough surface finish, the extended pulse duration will allow more heat sink into the

work piece and in the mean time it will spread which means the recast layer will be larger and

the heat affected zone will be deeper. However, exceeding the pulse duration will also have its

benefits. Whereas, when the optimum pulse duration for each electrode and work material

combination is exceeded, the material removal rate will start to decrease. The longer the duration

will have effect on the wear of the work material where when the duration of the pulse is longer,

then there will be a no-wear situation. But there are a certain limits for that point to be reached.

But if that point is reached, increasing the duration will cause the electrode to grow from plating

build-up. To complete the cycle sufficient pulse interval is needed before the next cycle can be

started. Other than that, the pulse interval also affects the speed and the stability of the cut.

4.4 PULSE WAVEFORM:The normal pulse waveform that we always see is rectangle, but now new shapes have been

developed. Pulse wave is a non-sinusoidal waveform that is similar to square wave. By using

trapezoidal wave generators the relative tool wear can be reduced to a very low value. Other

types of generators introduce an initial pulse of high voltage but low current and a few

microseconds duration, before the main pulse, which facilitates ignition.

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4.5 POLARITY:

Can be either positive or negative. The current will pass through the gap and create high

temperature that will cause the material to evaporate at both the electrode spots. The plasma

channel is made of ion and electron flows. Electrons have mass smaller than anions and as the

electrons processes it shows quicker reaction, the anode material is worn out predominantly. As a

result it causes minimum effect to the tool electrode and becomes important for finishing

operations with a shorter on-time.

4.6 ELECTRODE GAP:

The tool servo-mechanism is one of the most important in the efficient working of EDM process,

and the servo-mechanism function is to control the working gap to the set value. An electro-

mechanical and hydraulic systems are used and normally designed to respond to average gap

voltage. In order to obtain good performance, gap stability and the reaction speed of the system

needs to be account for where the presence of backlash is particularly undesirable. For the

reaction speed, it must obtain a high speed so that it can respond to short circuits or even open

gap circuits. Gap width is not measured directly, but can be inferred from the average gap

voltage

4.7 MATERIAL REMOVAL RATE:

Based from the journal “Influence of pulsed power conditions on the machining properties in

micro-EDM” shows that the source energy of electro discharge between the tool electrode and

the work piece is an electric one which power can be determined by the supplied voltage and

current

4.8 TOOL WEAR RATE:

The ratio of amount of electrode to the amount of work piece removal is defined as the wear ratio

(Yao Yang Tsai, Takahisa Masuzawa). There are four methods that are known to evaluate the

electrode wear ratio by means of measuring weight, shape, length, and total volume respectively.

A common one is by calculating the volumetric wear ratio method is unsuitable for micro-EDM

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5. CHARACTERISTIC, ADVANTAGES AND

DISADVANTAGES

5.1 CHARACTERISTICS:

(a) The process can be used to machine any work material if it is electrically conductive

(b) Material removal depends on mainly thermal properties of the work material rather than

its strength, hardness etc

(c) In EDM there is a physical tool and geometry of the tool is the positive impression of the

whole or geometric feature machined

(d) The tool has to be electrically conductive as well. The tool wear once again depends on

the thermal properties of the tool material

(e) Though the local temperature rise is rather high, still due to very small pulse on time,

there is not enough time for the heat to diffuse and thus almost no increase in bulk

temperature takes place. Thus the heat affected zone is limited to 2 – 4 μm of the spark

crater

(f) However rapid heating and cooling and local high temperature leads to surface hardening

which may be desirable in some applications

(g) Though there is a possibility of taper cut and overcut in EDM, they can be controlled and

compensated

5.2 ADVANTAGES OF EDM:

Conventional EDM machines can be programmed for vertical machining, orbital, vectorial,

directional, helical, conical, rotational, spin and indexing machining cycles. This versality gives

electrical discharge machines many advantages over conventional machine tools.

Any material that is electrically conductive can be cut using the EDM process.

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Hardened work pieces can be machines eliminating the deformation caused by heat

treatment.

X, Y, AND Z axes movements allow for the programming of complex profiles using

simple electrodes.

Complex dies sections and molds can be produced accurately, faster, and at lower costs.

The EDM process is burr-free.

Thin fragile sections such as webs or fins can be easily machined without deforming the

part.

Micro EDM is a better alternative to micro fabrication techniques for production of some

of the 3dD microstructure in silicon.

5.3 DISADVANTAGES OF EDM:

Relatively slow rate of material removal.

Additional lead time and cost used for creating electrodes for ram/sinker EDM.

Reproducing sharp corners on the work piece is difficult due to electrode wear.

Electrical power consumption is high.

Material must be electrically conductive.

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6. MATERIALS

6.1 MACHINABLE MATERIALS:

The only condition for a material to be machined by EDM is to conduct electrical current. Most

of materials applied by the metal processing industry can be machined by this technique:

aluminium, brass, bronze, copper, titanium, etc. Some materials like Silicon or Germanium are

semi-conductors (methaloids) and depending on their structure or whether they are doped, can

also be machined. For ceramics, there are some techniques that make them conductive for EDM. 

Given the low process productivity, it is only used for hard materials (tempered steels, tungsten

carbide, etc.) or in those cases in which accuracy or small edge radius in ruled surfaces (wire

EDM) or sharp edges (using shaped electrodes) are needed.

6.2 ELACRODE MATERIAL:

WEDM machines use a thin metal string to cut the parts. The wire presents some erosion (lower

than the component) but it is continuously circulating to provide a constant geometry for the tool

machining the part and obtain a higher precision. Excessive wire erosion produces the breakage,

marks on the part, time consumption and higher costs.

For microWEDM, the wire must present a higher tensile strength to avoid its rupture (tungsten

wire is the most usual for straight cuts while molybdenum is used for taper cuts). The most usual

dielectric is oil or keroxene (deionised water in some machines) to provide a smaller gap and

lower unitary material removal rates.

The basic characteristics of electrode materials:

• High electrical conductivity – electrons are cold emitted more easily and there is less bulk

electrical heating

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• High thermal conductivity – for the same heat load, the local temperature rise would be

less due to faster heat conducted to the bulk of the tool and thus less tool wear

• Higher density – for the same heat load and same tool wear by weight there would be less

volume removal or tool wear and thus less dimensional loss or inaccuracy

• High melting point – high melting point leads to fewer tools wear due to less tool material

melting for the same heat load

• Easy manufacturability

• Cost – cheap

The followings are the different electrode materials which are used commonly in the industry:

• Graphite

• Electrolytic oxygen free copper

• Tellurium copper – 99% Cu + 0.5% tellurium

• Brass.

6.3 DIELECTRIC FLUID:

The most usual dielectric fluids are deionised water and hydrocarbon based oils that can reduce

the gap, stabilises the wire vibrations (when applied in WEDM) and produce ramified discharges

to reduce the crater depth and obtain better surface finishing. Smaller gap is lined to more

difficult debris elimination and, usually more arcing and slower feed.

For WEDM machines deionised water is the most usual dielectric fluid while oil is commonly

applied in SEDM. For microEDM, both fluids are currently applied in both processes depending

on the machine producer and the material of the machined part: SODICK, MAKINO apply oil in

their machines, AGIE-Charmless use water in the WEDM machines and SARIX uses water to

machine deep holes with die sinking micro EDM machines.

Deionised water can provide 10 times higher productivity than oil in WEMD machines [12], but

the gap is slightly larger (5~10 mm). This is especially interesting when cutting thick

components

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7. APPLICATIONS

For microEDM, the most important applications are: small precision cutting punches and dies,

precision moulds, components for medical engineering, sensors and optics, watch making (gears

and holes) or the production of small multi-component electrodes.

The first applications of micro electro discharge machining were implemented in the 90s, when

thin electrodes were used to develop micromechanics systems and micro fluidic components for

non-invasive medics, micro mechanisms for pharmaceutics and micro-chemistry. The process

was slow by then, reason for which the application was limited to universities and research

centres.

At present, micro EDM is a key technology for the production of micro tooling for

microinjection and micro punching because it can machine hard alloys and tempered steels that

can withstand the wear produced by the processed materials.

Micro EDM serves many industries including medical, aerospace, automotive, appliance, and

HVAC industries. Micro EDM does prototypes as well as long term production for all of the

industries listed above.

DIFFERENCE NETWEEN MACRO AND MICRO EDM

Micro Electro Discharge Machining is a market growing processing technology due to the

industrial interest and the increasing number of applications. The most important difference

between microEDM and EDM (for both wire and die sinking EDM) is the dimension of the

plasma channel radius that arises during the spark: in conventional EDM is much smaller than

the electrode but the size is comparable for microEDM.

The higher precision can be achieved only if electrode vibrations and wear are contained. This

implies an important limitation for conventional EDM that turns out to be more restrictive in

micro EDM. For each discharge, the electrode wear in micro EDM is proportionally higher than

conventional EDM. The electrode is softened, depending the section reduction on the spark

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energy.  Some key aspects to machine with small electrodes can be extracted from the presented

ideas :

- Control the pulse energy

- Control the wire traction force (for WEDM)

- Increase the gap stability obtained by the control (avoid discharge fluctuations)

- Increase the machine positioning accuracy For microEDM, the entire machine, the

electrodes, the programme, the control, the measuring instruments and the operators play

an important role in the process

FUTURE CHALLENGES:

In the field of EDM and, especially in microEDM, there are many fields of research which will

be important in the future.

Although it can sound strange, despite being an industrially well established process and its

broad application by many sectors, there are many theories about what happens during the

discharge but none of them can explain completely the process. The process conditions:

machining inside a dielectric fluid with a gap of just a few microns, gas generation with material

particles projected at high speed and high temperature at a repetition rate circa 1 MHz All these

aspects make the process hard to model and difficult to characterise.

For micro EDM, the process lacks reliability. The applied wires and electrodes are far from their

yield strength to avoid ruptures; this reduces the process productivity and the maximum

reachable accuracy. For micro EDM, most of the machines are still limited in terms of

interpolation, etc. Especially for micro EDM milling, the electrode wear is a field of research,

together with CAM generation to consider the electrode wear compensation within the electrode

trajectory.

The high costs of machinery, components, electrodes, machine tuning, technical assistance. At

present, there is a lot of job to do until obtaining technology tables for both microEDM and thin

WEDM to make introduce these machines as production systems in industrial companies.

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