PLASMA ARC MACHINING(PAM),ELECTRON BEAM MACHINING AND ELECTRIC DISCHARGE MACHINING


PLASMA ARC MACHINING (PAM)

what is plasma? 


A Plasma is state of gases which occurs when gases are heated up to very high temperature of approx 5000 degrees. The gases become super heated and ionized (electrically). This can be used to cut metals and other materials.

BASIC IDEA:-
                   
PLASMA ARC MACHINING is the process in which the plasma (super heated gases) at very high velocity is directed on the work piece. This causes deformation of work piece to required shape. The above figure shows basic set up of PAM process.

A tungsten electrode is inserted to the gun and made cathode. Nozzle of the gun is made anode.
Figure below shows tungsten as cathode and nozzle as anode.

Heavy potential difference in developed between cathode and anode to generate plasma.

HEAT GENERATED REGION IN PLASMA ARC MACHINING AS SHOWN IN FIGURE BELOW:-

WORKING PROCESS:-

1. STEP 1

PLASMA GUN:- 


Plasma can be created from many gases like nitrogen, hydrogen, argon etc. The plasma gun has tungsten electrode which is negatively charged and nozzle as positively charged.

High potential difference is applied between them to generate plasma. A strong arc is established between the two terminals anode and cathode. There is a collision between molecules of gas and electrons of the established arc. As a result of this collision gas molecules get ionized and heat is evolved. 

2. STEP 2


This super heated gas is directed towards work piece and such high temperature melts the material easily. Water jacket is provided to cope up with high temperature (to escape the excess heat). 
 
Advantages:-
  • Harder and brittle material can be easily cut. 
  • Good production rate. 
  • Intricate shapes can be machined nicely. 
Disadvantages:-
  • Costly process. 
  • High heat generation. 
ELECTRON BEAM MACHINING(EBM)


BASIC IDEA :-

As we know that non conventional machining means there is no direct contact between tool and the work piece. In EBM process, high velocity electrons are directed towards the work piece which generates high heat resulting in vaporization of work piece.

WORKING PROCESS:-

VOLTAGE REQUIRED-150-200 KV.
SPEED OF ELECTRONS-200000 m/s.


      




                           fig:-electron beam machining

Electron beam machining as proposed is projection of high velocity electron on work piece which is capable of eroding the material by thermal effect and causing cutting/drilling action.

1. The question may arise in your mind that what is the role of temperature when electron strikes the work piece?
The answer is very simple, when the electron at very high speed (up to 200000 m/s) strikes the work piece,the kinetic energy is converted to thermal energy on impact. This thermal energy is basic reason of melting the work piece.

2. Now you all would be thinking how the electrons achieve such high velocity of 200000 m/s?

A typical EBM gun consists of an electron quot which is generated by a superheated tungsten filament, which also acts as the cathode. A combination of forces from the negative cathode and the attracting forces from the positive anode causes the free electrons to be accelerated and directed toward the work piece. Before passing through the anode, the beam travels through a bias electrode, which controls the flow of electrons and acts as a switch for generating pulses. After passing through the anode, the electron beam is diverging rapidly and traveling at approximately 150000 m/s. A magnetic coil, which functions as a magnetic lens, repels and shapes the electron beam into a converging beam. The beam is then passed through a variable aperture.

By EBM machining we can obtain machining diameter up to 0.025 mm. The vacuum chamber prevents the electrons to get scattered, which may occur due to collision of gas molecule with the electrons. This may reduce the machining efficiency, hence vacuum is provided.

APPLICATIONS:-

  • Aircraft engine parts. 
  • Intricate and minute holes. 
  • Metals such as steel etc can be machined. 

ELECTRIC DISCHARGE MACHINING(EDM)

POINTS TO KNOW:-
  • First observation made by Joseph Preistly (1770).
  • Consists two electrodes, one as tool and other as work piece with a dielectric medium.
  • Material removal is by electric discharge.
  • Temperature of spark ranges 8000 degree to 12000 degree Celsius. The spark is very effectively controlled over the surface only.

BASIC IDEA OF EDM:-

It is also a non traditional machining process where machining takes place without direct contact between tool and work piece. EDM is carried by process of electric spark (discharge) between tool and work piece.

There are two electrodes:-
Both the electrodes are aught to be conductive for EDM to take place. Tool electrode is called cathode (steel) Work piece electrode is called anode made up of graphite or copper.


The electrolyte flows between the two electrode. This electrolyte is dielectric fluid which completes the overall circuit.


PROCESS OF EDM:-

The two electrode i.e. tool and work piece is connected with DC pulse supply.
The servo motor is connected to tool head to provide required feed to tool post.
The tool and work piece is separated by di-electric fluid.

USE OF DI ELECTRIC FLUID:-

1. Removes burr (machined particles) from machining area.
2. Removes heat produced due to spark.
3. Act as lubricant.
4. Act as coolent.

When the spark is induced due to potential difference between the electrode, it is directed on work region. The repeated spark generates temperature of 8000-10000 degree Celsius. This temperature can melt any material.

ADVANTAGES:-

  • Very intricate shapes can be machined.
  • Very hard material can be easily machined.
  • Minute holes are drilled.
  • Accuracy is high.
DISADVANTAGES:-
  • High power consumption.
  • Tool wear is problem.
  • Slow material removal rate.
  • Limitation occurs when work piece is non conductive.



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