Advancements in ElectricalDischarge Machining

BY

VISHAL BORKAR

M. Tech(CAD/CAM)

I-Semester

Most extensively used non-conventional, controlled metal

removal process that is used to remove metal by means of

electrical spark erosion.

The first EDM system was developed by Lazarenko in 1943.

The metal removal process is performed by applying a

pulsating (on/off) electrical charge of high frequency current

through the electrode to the work piece immersed in dielectric

medium.

INTRODUCTION

A tool

Work piece

Servo mechanism

Dielectric fluid

MAIN COMPONENTS OF EDM

EDM SYSTEM

It consist of a table with x,y axis and z axis is vertical.

A NC system, a tank in which there is dielectric fluid.

• Tool electrode is to provide electrical energy to the work material.

• Basic desirable characters of tool material:-

High thermal conductivity

High electrical conductivity

High melting point to reduce its wear rate

Ease to manufacture

EDM TOOLS

TYPES OF ELECTRODES

Metallic electrodes

Non metallic electrodes

Combined metallic and non metallic

Metallic coating on insulators

Most commonly used: Brass, Copper, Graphite, Al alloys.

It acts as an insulator until sufficiently high potential is reached .

Acts as a coolant medium and reduces the extremely high temp. in the arc gap.

It is pumped through the arc gap to flush away the eroded particles between the work-piece and the electrode which is critical to high metal removal rates and good machining conditions.

Basic requirements are:

o Low viscosity

o Absence of toxic vapours

o Chemical neutrality

o Low cost.

Methods of fluid application

• Normal flow

• Reverse flow

• Jet flushing

DIELECTRIC FLUIDS

• Melting and vaporization

BASIC PRINCIPLE OF EDM

Electrical energy turned into thermal energy through a series of discrete

electrical discharges .

The thermal energy generates a channel of plasma between the cathode

and the anode.

workpiece+anode

-cathode

(a) Tool and work piece immersed in

dielectric liquid.

(b) A spark is generated between tool and

work piece.

(c) The high temperature causes the melting

and vaporization of electrodes.

(d) At the end of the pulse, the molten

material is ejected from surface, leaving a

shallow crater.

electrode

EXAMPLES OF EDM

Stepped cavities High speed turbine and mould for the

screw thread of PET bottles

Intricately detailed automobile part Turbine blades

Conventional EDM Or Die Sinking

Wire EDM

TYPES OF EDM

One of the most widely used non-traditional processes.

Sparks occur across a small gap between tool and work.

Requires dielectric fluid, which creates a path for each discharge as

fluid becomes ionized in the gap.

Conventional EDM Or Die Sinking

ELECTRODE/ TOOL MATERIAL:

Graphite,copper, Brass

Dielectric Fluid:

Paraffin or transformer oil, mineral oils, kerosene, distilled and deionized water etc.

Work material:

Machining materials like: - steels, super alloys, refractory’s.

Applications

Production parts

Delicate parts

Hole drilling

Machining of hard metals.

Tooling for many mechanical processes

moulds for plastic injection moulding

extrusion dies

wire drawing dies

forging and heading dies

Work is fed slowly past wire along desired cutting path, like a band

saw operation.

CNC used for motion control

While cutting, wire is continuously advanced between supply

spool/reel and take up spool to maintain a constant diameter.

Dielectric required, using nozzles directed at tool work interface or

submerging work part

Wire is made up of brass, copper or tungsten. (0.25 mm in diameter)

WIRE EDM

The wire is kept in tension using a mechanical tensioning

device reducing the tendency of producing inaccurate parts.

The material is eroded ahead of the wire and there is no direct

contact between the work piece and the wire, eliminating the

mechanical stresses during machining.

Wire EDM Applications:

Ideal for stamping die components.

Other tools and parts with intricate outlines shapes, such as lathe

form tools, extrusion dies and flat templates

EDM DEVELOPMENT

Dry EDM EDG Hole Drilling EDM

ECDM:

It is an emerging hybrid machining process used in precision

machining of hard and brittle non-conducting materials.

Comprising the techniques of electrochemical machining

(ECM) and electro discharge machining (EDM).

Also referred as electrochemical spark machining (ECSM)

process.

Support a variety of materials including metals, ceramics,

composites, alumina, glass, etc.

inert

material copper

Dilute HCl

Application of voltage to the cell results in reduction of electrolyte with liberation

of hydrogen gas at the cathode tip.

When voltage exceeds beyond a threshold value, hydrogen gas bubbles grow in

size.

Discharge occurs at the tip of the cathode and Machining takes place on the work

piece

Suitable material (aluminium, chromium, copper, silicon carbide, etc.) in

powder form is mixed into the dielectric fluid used in EDM.

When a voltage of 80 –320 V is applied between the tool electrode and the

work piece placed close to each other, an electric field is generated

The additive particles fill up the spark gap. The high electric field energies

powder particles which act as conductors.

PMEDM (Powder Mixed Electrical Discharge Machining)

These conductive particles form chains at different places under

sparking area, which bridges the gap between tool electrode and work

piece material.

Due to bridging effect, the gap voltage and insulting strength of

dielectric fluid reduces which result in easy short circuiting.

Due to this, electric density decreases and hence uniform distribution

of sparking takes place.

This leads to uniform erosion on work piece which results in

improvement in surface finish.

Dry EDM is a 'green' environment friendly EDM technique in which instead of

mineral oil based liquid dielectric, gas at high pressure is used as the dielectric

medium.

A high velocity gas flow is supplied through a thin walled pipe electrode.

The role of the gas is to remove the debris from the gap and to cool the inter

electrode gap.

During the pulse interval, the gas jet blows off the plasma formed by previous

pulse discharge and decreases the temperatures of the discharge spots.

DRY EDM

INNOVATIVE EDM TECHNOLOGIES

Factors favouring dry EDM:

i .No fire hazard

ii. No toxic fumes generated

iii. No need for special treatment for disposal of sludge, dielectric waste, etc

Other Advantages:

a) Very low tool wear.

b) lower residual stresses

c) Higher Precision.

d) Smaller discharge gap length.

f) No electrolytic corrosion of w/p (as compared to water based dielectrics).

Major Disadvantages:

i. Low material removal rate (MRR)

ii. Poor process stability

Removes conductive materials by rapid spark discharges between a rotating tool and work piece that are separated by a flowing dielectric fluid.

The spark gap is normally held at 0.013 to 0.075 mm by the servomechanism that controls the motion of the work piece

The conductive wheel, usually made of graphite, rotates at 30 to 180 m/min in a dielectric bath of filtered hydrocarbon oil.

The work piece is usually connected to the positive terminal of the dc power supply.

EDG (Electro Discharge Grinding)

The work piece is machined using a stream of electric sparks. Each

spark discharge melts or vaporizes a small amount of metal from the

work piece surface

Material removal rates range from 0.16 to 2.54 cm3/min.

1. Steel and carbide

2. Thin sections (abrasive wheel where pressure might cause distortion)

3. Brittle materials or fragile parts on which abrasive materials might

cause fracturing.

APPLICATIONS

EDM HOLE DRILLING

It was designed especially for fast, accurate small and deep-hole drilling

applications.

A precision tubular electrode (generally brass or copper) mounted into the

drill chuck located on the "Z" axis and held in location on top of the work piece

by the ceramic guide.

The top of the work piece is located and the drilling depth is set.

The electrode rotation is turned on and the de-ionized water solution,

which is pressurized between 50 -100 kg/cm2, is sent through the

tubular electrode as a flushing agent.

The power supply parameters are set-which normally consist of on-

time, off-time, peak current and amount of capacitance.

At this point, the discharge is turned on to begin the drilling cycle.

At the end of the drilling cycle, the discharge is turned off and the

"Z" axis is retracted above the work piece.

Applications:-

Mould and part manufacturing-injection nozzles

Oil drain holes, hydraulic cylinders and dowel pins,

turbine blades, ball bearings and safety wire holes in hex nuts.

www.wikipedia.com

(http://www.atlantaedm.com/articles/a8-history-of-edm.php)

(http://www.jobshop.com/techinfo/papers/whatiswireedm.shtml)

(http://www.britannica.com/EBchecked/topic/354662/machine-

tool/39176/Nonconventional-methods-ofmachining)

Research performed on the EDM process (http://www.fs.uni-

lj.si/lat/edm/index.html)

REFERENCES: