Electro Discharge Machining

Electro Discharge Machining

Index

Electro Discharge Machining ............................................................................................................................ 1

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