Electro Discharge Machining

Introduction ................................................................................................................................................. 1

Impulses generation ..................................................................................................................................... 3

Lazarenko’s circuit .................................................................................................................................... 3

Power transistor circuit ............................................................................................................................ 3

Process parameters ...................................................................................................................................... 4

System .......................................................................................................................................................... 6

Plunge EDM .............................................................................................................................................. 7

Wire EDM ................................................................................................................................................. 8

Micro EDM................................................................................................................................................ 8

Introduction

This category can be classified as non-conventional thermal working process, in which a conductive material

is eroded by a sequence of fast electrical discharges – intensities of tens or hundreds of A.

The system basically

consists in two

electrodes separated by

a dielectric liquid; one

electrode is

continuously micro-

melted. Materials

involved are generally

metal and their alloys

and a great advantage of the process is the possibility to achieve complex geometries. The principal

application is fabrication of metallic moulds.

 Possibility to realize every 2D and 3D complex geometry, exception made of undercut;

 Possibility to work on hard metals and alloys that cannot be processed easily (and cheaply) with

conventional processes;

 Hardness and resilience are not influent on removal rate or required energy;

 No contact between workpiece and electrode (no shear stresses): possibility to process fragile parts

without provoking damages.

Material removal on the workpiece is achieved through combination of thermal, electrical and mechanical

effects: electrical discharges provoke erosion of the piece, shaping it. Plunge or die sinking EDM reproduce

the negative shape of the tool. 1

Both the workpiece and the tool are electrodes, maintained at a precise constant gap in which there is a

dielectric liquid. The liquid has the function to increase discharge efficiency,

because it decreases the arc section and makes possible the voltage control. Typical

frequencies of the sequence are 1000-3000 Hz. Erosion is homogeneous and

uniform, and happens also on the tool even if more slowly. Parameters, polarity

1 2

and materials must be correctly chosen to reduce MRR . A spark is triggered in the

point of minimum electrical resistance, thus a

low tool-workpiece distance and a high

conductivity of the fluid favour it. Frequency of

discharge determines surface finishing and MRR. Each spark cause melting or vaporization of the material.

After the impulse, vapour metal bubbles collapse and the material is rapidly removed, leaving a hole on both

the electrodes. The dielectric fluid takes re-solidified material away. Obviously, the process cannot be used

with electrical insulators. 1. Voltage increase, electric

field formation at the point of min

distance;

2. Negative particle bridge

formation (emitted by the

3

workpiece ) and partial ionization

of the dielectric; 4

3. Dielectric breakdown ;

4. Current increase and

voltage decrease, particles

emigration and vapour formation.

Start of the melting process;

5. P (up to 20 MPa) and T

increase (4000-12000 °C), channel

expansion;

6. Vapour bubble expansion:

max intensity of electrical

discharge and heat, opening of

electrical circuit and immediate

vaporization of the material in the

discharge point;

7. Current and heat

generation lowering, disappearing

of the discharge channel,

1 Material Removal Rate.

2 T ≈ 3000 °C. Only one spark per interval is produced.

3 The workpiece is the cathode, the negative electrode. Electron migrate from the cathode to the tool, creating the

discharge channel. The cathode can reach 99% of the total erosion because cations produce a greater erosion [on the

workpiece] than electrons [on the tool].

4 Electron and cations are accelerated by the electric field, reaching in few seconds a great velocity, generating a ionized

channel that is a conductor and no more a dielectric. 2

solidification into hollow spheres of the vaporized material and into filled spheres of the melted

materials into the dielectric;

8. Vapour bubble implosion: projection of the molten material outside the crater (“cavitation effect”);

5

9. Closure of the circuit: ready for the next pulse .

This cycle is repeated at elevated frequency and the conductor channel forms in different points, producing

a uniform erosion on the entire surface. If the discharge is concentrated in the same points, a degenerative

phenomenon occurs. The gap must be kept constant in order to have a gradual erosion, starting from

asperities and then creating the desired shape. Number of sparks increases with wider surfaces. The material

removal is continuous.

The machine structure is rigid in order to avoid vibrations thus allowing to obtain tight tolerances (0.02 – 0.12

3

mm); 0.2 – 12 μm of roughness are generally reached and typical MRR at 400 A is up to 4.5 cm /min.

Impulses generation

Lazarenko’s circuit

Lazarenko brothers understood that low duration discharges at high frequency into a dielectric liquid would

concentrate the machining energy onto a reduced area, avoiding waste and improving quality of the final

product.

Power transistor circuit

The new trend improves machining process: it is possible to control the impulse shape, t , t and current in

on off

a more precise way with respect to Lazarenko’s circuit. Square pulses are generally used. With this process,

MRR is higher but the surface finishing quality is lower.

5 Residuals are made of metal particles, C (coming from the decomposition of the dielectric) and gas. 3

Process parameters

Choice of parameters is fundamental to define accuracy and surface finishing of the process. Current and

frequency variations provoke visible effects on the workpiece. Principal parameters are:

 Interval between subsequent discharges

 Impulse duration

 Polarity

 Current

 Open Circuit voltage

 Frontal and lateral gaps

 Throughput of the dielectric

Cycle time. It is the sum of three phases: dielectric

ionization, discharge (t ) and pause. It begins with the voltage application (up to 80 V) through a gap in which

on

the dielectric is present. After dielectric breakdown, current passes and is stabilized at the peak during the

discharge, while voltage decreases. The cycle ends with opening of the circuit: the dielectric liquid gets

4

deionized and returns an insulator. During the pause (t ) the molten/vaporized metal is taken away and the

off

dielectric fluid is recycled. t is in μs and measures that interval between the discharge end and the next

off

spark. t = t + t . Low pause means high erosion; if the spark isn’t interrupted, a continuous electric arc

tot on off

cause the uncontrolled local fusion of both the tool and the workpiece. Increasing t on the other hand

off

means to reduce frequency and so erosion velocity. Erosion time is not t : the former is the time in which

on

EDM occurs, constituted by a series of discontinue discharges; the latter is the spark duration only.

3

MRR (2 – 400 mm /min) depends on duration and intensity of discharges, melting

temperature of metals, polarity.

Tool erosion decreases with

increasing t because at the

on

beginning there are only

electrons to hit the tool:

increasing that time, the

contribution of cations hitting

the workpiece increases, so it is

possible to complete the erosion

with lower number of cycles,

keeping the tool for more time

(lower cycles of electrons hitting

its surface).

Increasing current intensity,

wider craters are formed: MRR and roughness increase if all the other parameters are kept constant. Keeping

the same impulse energy, increasing the current and decreasing t means to decrease MRR and increase

on

surface finishing quality. On the other hand, decreasing the current and increasing t , MRR increases but

on

surface finishing quality decreases. For this reason, Lazarenko’s circuit is used for finishing while power

transistor circuit is used to obtain max MRR.

Also increasing discharge voltage means to increase material removal.

Polarity is chosen as a function of the

materials, on order to reduce wear

and to maximize velocity.

Discharge frequency is the inverse of

the cycle time. Generally frequency is

20 kHz. Keeping the same generated

energy, increasing this value means to

increase surface finishing quality of

the part: each spark will have lower

erosion capacity, leading to smaller

craters. 5

The gap or discharge distance allows the process avoiding direct contact between

the counterparts. It generally varies between 0,025 mm and 0,5 mm. The lower is

the value of the gap, the higher is current intensity, thus MRR and tool erosion

increase.

Discharges are responsible also of metallurgical modification on the surface,

because of the high temperatures involved. Heat affected zone is adjacent to the recast layer (the one that

melted and solidified), whereas a deeper layer, the conversion zone, is characterized by an apparent

modification of the grain boundaries but

there is still an effect due to thermal history.

The recast layer is very brittle; a white coat

here can be present due to chemical

reactions between material and dielectric

fluid at high temperatures. Furthermore, the

larger the machined surface, the more

probable is the presence of cracks.

System

The control of tool feed is necessary to keep constant tool-

workpiece distance. Implementation of a closed-loop is based

on pulse shape analysis (excessive break-down or short-circuits) allows the adjusting of the tool feed, thus of

the gap.

The power generator system is fundamental: it transforms DC into AC in order to form the required pulses.

Dielectric fluid must have high viscosity and electrical resistance in order to remain electrically neutral up to

discharge voltage. Good

6

ionization and deionization

capacity is also required to

guarantee high frequency

sparks. It must resist