Download - WIRE-EDM_AK, Wire Electro Discharge Machining guide

7/30/2019 WIRE-EDM_AK, Wire Electro Discharge Machining guide

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WIRE-ELECTRO DISCHARGE MACHINE

-By Ankush kapoor

INTRODUCTION

Electro Discharge Machining is one of the new Non Conventional Machining Process that

has come into being in the last part of twentieth century. E.D.M. uses the electrical spark to

create plasma in the dielectric between the tool and the work piece which further leads to

material removal. The tool and work piece are always placed inside the dielectric. The E.D.M

requires both the work piece and tool to be the good conductor of electricity. In the

material removal process, the alternating voltage pulse creates Potential difference which is

used to create electrical energy in form of moving electrons and the thermal energy

produced by its collision to work piece is main reason for removal of material from the work

piece. The tool in the electro discharge machine can be of various types. It is recommended

that one uses the type of tool which can be used to do different type of machining and

machine intricate geometries. The type of machine we are studying about is Wire-Electro

Discharge Machine in which a conductive wire is used as a tool in the machining process. It

is one of the most important method of E.D.M.

SPECIALITY & APPLICATION

Wire E.D.M. can be used to machine any work piece which is a good conductor ofelectricity because its machining operation does not depends on hardness of work

piece but on the thermal properties of the work piece.

In E.D.M, we can do machining on intricate geometries depending on the type andgeometry of tool we use in the machine.

In E.D.M. machining, we can do very precise machining because of highly localizedheating and material removal of the work piece.

W-EDM SHEMATIC DIAGRAMS

Schematic Diagram of Wire E.D.M. Principle.

Source: www.ncpre.iitb.ac.in


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Path of wire in Wire-EDM

Source: www.sciencedirect.com

MECHANISM OF MATERIAL REMOVAL

In the Wire E.D.M. process, the cycles of pulses of voltage are applied to work piece and tool

in which the work piece is made positively charged and the tool is made negatively charged

and both these pieces are immersed in the flowing dielectric. When the voltage exists

between the tool and the work piece, the electric field is created in between the two and

this electric field exceeds the work function of electron in the tool material and accelerates

the electron to the work piece. When the electron travels to the work piece, it collides with

the particles of dielectric in the spark gap and ionizes them into electrons and ions. As this

happens many times, a large pool of electrons and ions is created which now can be called

as plasma. Since the electric field is still there, these large number of electrons are now

accelerated to the work piece which was positively charge and ions are accelerated to the

tool which was negatively charged. As, these electrons collides with work piece, all there

kinetic energy is converted into thermal energy and they are heated up to very high

temperatures but this heating is very localized up to few micro metres only. This high

temperature either leads to the instant vaporization or melting of metal, some part of which

is removed by flowing dielectric. But whole of the molten metal is still not removed. Now as

the voltage pulse comes to an end, the plasma collapses sending the shock and pressure

waves into the spark gap. When these shock waves reach the work piece, they completely

remove the molten metal from the work piece. The ions which impinged on the tool made

tool wear.

QUES. & ANSWER

Ans.1

INITIATION OF SPARK


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In wire E.D.M. for each cycle there is some time ton when voltage is at value Vw and in the

time toff, voltage is either zero or some lower value V0. So, when at the start of ton, voltage

becomes Vw, potential difference is created between tool which is negatively charged and

work piece which is positively charged. This electric field from work piece to tool passing

through the spark gap causes some of the electrons to eject from tool and acceleratetowards work piece. As, these electrons accelerates towards work piece they collide with

dielectric particles and ionizes them. Since this happens with many electrons, a large

number of ions and electrons are formed and this state can be categorized as plasma. So,

when these electrons and ions moves to the work piece and tool respectively, they

constitute a spark.

MATERIAL REMOVAL PRINCIPLE

As described above, when the electrons of the plasma reach the tool, there is a lot of

localized heat produced at that point which takes the localized temperature to the excess of

10000 degrees Celsius. This causes some of the metal to vaporize instantly and some of the

metal is melted. This melted metal is partially removed. When the pulse stops, the plasma

formed collapses and it sends the shock wave. As this shock wave reaches the work piece is

causes complete metal removal from the localized part. During the toff time, work piece is

completely flushed with dielectric to completely remove the remaining unattached metal

and make it ready for next cycle.

Ans.2

The Controllable parameters of the Wire EDM process are

Time when voltage pulse exists in each cycle-ton. Time when voltage pulse does not exists in each cycle-toff. Value of voltage applied-Vw. The maximum value of current in each cycle-I0. The Properties of dielectric. The flushing rate of dielectric. Feed rate of wire.

Ans.3The main purposes of dielectric are listed below.

To Prevent Oxidation- If the work piece is exposed to the air, when the temperaturereaches higher value and the process is going on, there are the chances that the

work piece may be oxidized leading to formation of its oxides. These oxides reduces

the electrical conductivity of the work piece which hinders the wire E.D.M. process.

Formation of Plasma- The dielectric aids the formation of plasma as the workfunction of their molecules is less than air. But is should be made sure that the

dielectric should not break down due to applied electric field.

Flushing of metals from work piece- The dielectric is continuously flushed on thework piece when the pulse is on as well as when it is off. The dielectric helps to


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remove the material of metal which is now unbounded or unattached from metal

may have not been removed till now.

Ans.4

Normally we use deionised water or kerosene for dielectric in wire E.D.M. but if we use Tapwater as dielectric, it has too many impurities and thus it ionizes at very less voltage. Further

its breakdown voltage is also very low. So there are chances that if we use tap water it may

completely breakdown within the required voltage and process stops in between.

Ans.5

The requirements for the work piece are

The work piece material should be a good conductor of electricity. The work piece should not be already oxidized. If its surface is oxidized, use file to

remove the upper layer of surface.

The work piece should have lower melting point as the machining takes place bymelting of material. The less the melting point, less is the energy requires in

machining

The hardness and stiffness has of work piece has no effect on this process as themachining takes place by thermal removal of metal.

The requirements for the wire material are

High Electrical conductivity- It should be a good conductor of electricity High Melting Point- The melting point of wire material should be high so that it does

not melts during the process.

Low Cost- The wire must be as cheap as possible. High thermal conductivity- This will lead to less rise in local temperature and thus

less tool wear.

Ans.6

The possible reasons for overcut are:

The unequal cross-section of wire and work piece- The wire we use is never perfect.There are some cross sections whose area is larger than the mean area of wire. So,

when these cross sections act as tools, the spark gap decreases momentarily and it

leads to overcut.

Over cut at sharp corners- As the wire tries to make a cut at a corner, it dwells longerat the inner side of the corner and moves fast at the outer side. So, it creates an

overcut at the inner side and under cut at the outer side.

Ans.7

The Advantages and disadvantages of Wire E.D.M process are given below:Advantages:


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