Plasma Arc Machining – Working Principle, Construction, Working, Advantages and Disadvantages

Plasma Arc Machining is used to remove material from the workpiece. In this process, a high velocity jet of high-temperature gas is used to melt and remove material from the workpiece. This high velocity of hot gas is also known as plasma jet.
When a gas or air is heated at a temperature of more than 5000 °C, then it will start getting ionized into positive ions, negative ions and neutral ions. When the gas or air is ionized its temperature reaches from 11000 °C to 28000 °C and this ionized gas is called plasma.
The gas or air is heated with arc and the plasma produced by heating gas is used to remove material from the workpiece. So the whole process is called Plasma Arc Machining.
In this process, a high velocity of high temperature air is used to remove material from the workpiece by melting it.

The gas used in plasma arc machining is chosen according to the metal which is used as the workpiece.

The plasma arc machining is used for cutting alloys steels, stainless steel, aluminum, nickel, copper and cast iron.

Plasma Explained:-

When a solid is heated to the melting point, it turns into liquid and when a liquid is heated, it turns into gas.
If a gas is heated to nearly 2000 °C, the molecules will dissociate into separate atoms.
If the temperature is further raised to nearly 3000 °C, electrons of atoms of gas will be displaced and the atoms are ionized and this ionized gas is called plasma.

Working principle of Plasma Arc Machining

PAM process uses ionized plasma to transfer heat. The plasma is obtained by forcing gas through an electric arc generated between cathode and andode.
The high temperature plasma jet melts the metal and remove material from the workpiece.

Components of Plasma Arc Machining:

1 Plasma Gun:
Different gases like nitrogen, hydrogen, argon or mixture of these gases are used to create plasma. This plasma gun has a chamber which has a tungsten electrode. This tungsten electrode is connected to the negative terminal and nozzle of the plasma gun is connected to the positive terminal of the DC power supply. The required mixture of gas is supplied to the gun. A strong arc is produced between the anode and the cathode.
After that, there is a collision between the electron of the arc and the molecules of the gas and due to this collision, gas molecules get ionized and heat is generated.

2 Power Supply:
DC Power Supply is used to develop two terminals in the plasma gun. Heavy potential difference is applied across cathode and anode so that arc produced is strong and is able to ionize the gas mixture and convert it into plasma.

3 Cooling Mechanism:
A cooling mechanism is added to the plasma gun as heat is produced in it as hot gases continuously pass out from the nozzle.
A water jacket is used to cool the nozzle. The nozzle is surrounded by water jet.

4 Workpiece:
Different materials can be worked using this plasma arc machining. Different metals like aluminum, magnesium, carbon, stainless steel and alloy steels can be worked using this process.

Construction of Plasma Arc Machining:E

The plasma arc cutting torch carries a tungsten electrode fitted in the chamber.
This tungsten electrode is connected to the negative terminal of the DC power supply.
For plasma arc machining, a plasma gun is required. This plasma gun has a chamber.
This plasma gun has a tungsten electrode fitted inside the chamber.
This tungsten electrode is connected to the negative terminal of DC Power Supply and acts as a cathode.
At the bottom of the chamber, there is a copper nozzle that is connected to a positive terminal of the DC Power Supply and acts as an anode.
The rest of the chamber is made of insulating material and acts as an insulator.
Gas enters the chamber through a small passage present at the right side of the chamber.
The cathode and the anode remain cool despite the hot gases passing through them as they are water cooled. Water circulation is present around the torch.

Working:

At first, when a D.C power is supplied to the circuit, a strong arc is produced between the cathode ( electrode ) and the anode (nozzle).
After that, gas is supplied to the chamber. This gas can be hydrogen, nitrogen, argon or mixture of these gases chosen according to the metal to be worked.
The gas used in the process is heated using the arc produced between the cathode and the anode. This gas is heated to very high temperatures from 11000 °C to 28000 °C.
As the arc comes into contact with the gas, there is a collision between the electron of the arc and the molecules of the gas and the molecules of the gas will dissociate into separate atoms.
Due to the high high temperature generated from the arc, electrons from some atoms will be displaced and atoms are ionized ( electrically charged ) and the gas turns into plasma. As the gas is ionized, a large amount of thermal energy is liberated.
After the gas is ionized, this high temperature ionized gas is directed towards the workpiece with high velocity.
The electric arc has some other benefits like it increases the temperature of ionized gas, makes the beam almost parallel, and increases the velocity of the gas.
As the plasma jet reaches the workpiece, the plasma melts the workpiece and the high-velocity gas blows away the molten metal.
In this way, plasm arc machining is used to remove material from the workpiece.

Advantages of Plasma Arc Machining:

1) Hard as well as brittle metals can be easily machined with this process.
2) Plasma Arc Machining gives a faster production rate.
3) Small cavities can be machined using this process with good dimensional accuracy.
4) It can be used for rough turning of very hard materials.
5) It is also used in machines that are used to repair jet engine blades.

Disadvantages of Plasma Arc Machining:

1) The equipment used in Plasma Arc Machining are very costly.
2) Metallurgical changes take place on the surface of the workpiece.
3) The consumption of inert gas is high.
4) As oxidation and scale formation takes place, shielding is required.

Applications of PAM:

1) It is used in mill applications.
2) It is also used in the nuclear submarine pipe system.
3) Used in welding rocket motor case.
4) Used in welding of stainless steel tubes.
5) It is used for profile cutting.

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