Plasma Arc Machining – Process, Diagrams, Working Principle

What is Plasma Arc Machining Process ?

Plasma arc machining is a non traditional process of metal removal that involves focusing a high-velocity jet of high-temperature (11,000°C to 30,000°C) ionized gas on a workpiece.

Plasma Arc Machining is a technique for removing material from a workpiece. A high-velocity jet of high-temperature gas is used in this process to melt and remove material from the workpiece. This fast-moving hot gas is also known as a plasma jet.

When a gas or air is heated to temperatures greater than 5000 °C, it begins to ionize into positive ions, negative ions, and neutral ions.

When a gas or air is ionized, its temperature rises to between 11000 and 28000 degrees Celsius, and the ionized gas is known as plasma.

The arc heats the gas or air, and the plasma produced by the heating gas is used to remove material from the workpiece. As a result, the entire procedure is known as Plasma Arc Machining.

A high velocity of high temperature air is used in this process to melt material from the workpiece. The gas used in plasma arc machining is selected based on the metal used as the workpiece.

Plasma arc machining is used to cut steel alloys, stainless steel, aluminum, nickel, copper, and cast iron.

What is Plasma ?

When a solid is heated to its melting point, it becomes liquid, and when a liquid is heated, it becomes gas. When a gas is heated to nearly 2000 °C, the molecules dissociate into individual atoms.

When the temperature is raised to nearly 3000 °C, electrons in gas atoms are displaced, and the atoms are ionized, this ionized gas is known as plasma.

Working Principle of Plasma Arc Machining

Working principle of plasma Arc Machining is based on use of ionized plasma to transfer heat and this high temperature plasma jet melts the metal and remove the material from the workpiece.

The plasma is produced by forcing gas through an electric arc created between the cathode and the anode.

It based on the principle of using high temperature plasma jet to remove the material by melting the contents of the workpiece.

The material is removed in the plasma arc machining process by directing a high-velocity jet of ionized gas at high temperatures (11000 ° C to 28,000 ° C) onto the workpiece.

Components of Plasma Arc Machining

1. Plasma gun

To make plasma, Different gases such as nitrogen, argon, hydrogen, or a mixture of these gases are used. A tungsten electrode is installed in the chamber of the plasma gun. Negative polarity is applied to the electrode, while positive polarity is applied to the gun’s nozzle.

The gun’s supply of gases is never depleted. Between the two terminals, anode and cathode, a strong arc is formed. There is a collision between gas molecules and the established arc’s electrons. Gas molecules are ionized and heat is released as a result of this collision. Plasma is a hot, ionized gas that is directed at a high velocity to the workpiece. The supply rate of gases determines the established arc.

2. Power supply

In the plasma gun, a power supply (DC) is used to create two terminals. A tungsten electrode is inserted into the gun and serves as the cathode, while the gun’s nozzle serves as the anode. A large potential difference is applied across the electrodes to create a plasma state of gases.

3. Cooling Mechanism

Because hot gases continuously exit the nozzle, there is a risk of overheating. To prevent the nozzle from overheating, it is surrounded by a water jacket.

4. Tooling

In PAM, there is no visible tool. A focused spray of hot, plasma-state gases is used as a cutting tool.

5. Workpiece

The PAM process can be used to process workpieces made of a variety of materials. These materials include aluminum, magnesium, stainless steel, and carbon and alloy steels. All materials that can be processed by LBM can also be processed by the PAM process.

Construction of Plasma Arc Machining

The plasma arc cutting torch is equipped with a tungsten electrode that is installed in the chamber. This tungsten electrode is connected to the DC power supply’s negative terminal.

A plasma gun is required for plasma arc machining. There is a chamber in this plasma gun. Inside the chamber of this plasma gun, tungsten electrode is fitted. This tungsten electrode is connected to the negative terminal of the DC Power Supply and serves as a cathode.

A copper nozzle at the bottom of the chamber is connected to a positive terminal of the DC Power Supply and serves as an anode. The rest of the chamber is made of insulating material and serves as an insulator.

Gas enters the chamber through a small passage on the right side of the chamber. Despite the hot gases passing through them, the cathode and anode remain cool because they are water cooled. Water circulates around the torch.

Working of Plasma Arc Machining

When DC power is given to the circuit, a strong arc forms between the cathode (electrodes) and anode (nozzle).

Following that, the chambers are filled with gas. This gas may be a mixture of hydrogen, nitrogen, argon, or other gases selected based on the metal being worked.

The gas used in the process is heated by an arc formed between the cathode and the anode. This gas heats at extremely high temperatures ranging from 11000 ° C to 28000 ° C.

When the arc makes contact with the gas, the electrons of the arc collide with the molecules of the gas, causing the gas molecules to separate into different atoms.

Because of the high temperatures produced by the arc, electrons from some atoms are displaced, the atoms are ionized (electrically charged), and the gas is converted into plasma. A significant amount of thermal energy is released as the gas is ionized.

After the gas has been ionized, the high temperature ionized gas is directed with high velocity towards the workpiece.

Other advantages of using an electric arc include increasing the temperature of the ionized gas, making the beam approximately parallel, and increasing the velocity of the gas.

As the plasma jet approaches the workpiece, the plasma melts it and the molten metal is blown away by the high-velocity gas. Plasma arc machining is used in this manner to remove material from the workpiece.

Plasma Arc Machining working Principle
Plasma Arc Machining

Applications

Applications of Plasma Arc Machining is in mill applications, nuclear submarine pipe system, in welding rocket motor case , in welding of stainless steel tubes and used for profile cutting.

1. PAM is an attractive turning method for difficult-to-machine materials by conventional methods. In this regard, cutting speeds of 2 m/min and a feed rate of 5 mm per revolution produced a surface finish of 0.5 mm Rt. The depth of cut can be controlled through the machining power or surface speed.

2. Computer numerical controlled PBM is used for profile cutting of metals that are difficult to tackle by oxyacetylene gas technique such as stainless steel and aluminum. A large number of parts can also be produced from one large sheet thus eliminating shearing operations.

3. PBM can cut 1.5-mm-deep,12.5-mm-wide grooves in stainless steel at 80 mm3 /min, using 50 kW as the cutting power. Such a high machining rate is 10 times the rate of grinding and chipping meth- ods. Lower machining rates are obtainable when these grooves are cut in nonconductive materials. The groove dimension however depends on the traverse speed, arc power, and the angle and height of the plasma arc.

4. The process is recommended for parts that have subsequent welding operations.

5. A plasma arc can cut tubes of wall thickness of up to 50 mm. In this
case no deburring is required before tube welding.

6. Underwater NC plasma cutting can achieve machining accuracy of
±0.2 mm in 9 m at low cutting speeds.

The plasma Arc Machining is used for metal extraction process. The PAM is used for cutting alloy steel, stainless steel, cast iron,copper,nickel, titanium, and aluminium, etc.

Advantages and Disadvantages of Plasma Arc Machining

Advantages

1. Requires no complicated chemical analysis or maintenance
2. Uses no harmful chlorinated fluorocarbons, solvents, or acid cleaning
chemicals
3. Operates cleanly, often eliminating the need for vapor degreasing, sol-
vent wiping, ultrasonic cleaning, and grit blasting
4. Requires no worker exposure to harmful chemicals
5. Needs less energy to operate.

6. This method can be used to machine both hard and brittle metals.
7. Plasma Arc Machining provides a faster rate of production.
8. This process can machine small cavities with high dimensional accuracy.
9. It is suitable for rough turning of extremely hard materials.
10. It’s also found in machines that repair jet engine blades.

Disadvantages

Limitations of plasma Arc Machining are :-

1. The large power supplies needed (220 kW) are required to cut through
12-mm-thick mild steel plate at 2.5 m/min.
2. The process also produces heat that could spoil the workpiece and pro-duce toxic fumes.

3. The equipment used in Plasma Arc Machining is very expensive ( costly ).
4. Metallurgical changes occur on the surface of the workpiece.
5. Inert gas consumption is high.
6. Shielding is required as oxidation and scale formation occur.

Frequently Asked Questions

What is plasma arc machining process ?

Plasma arc machining is a non traditional process of metal removal that involves focusing a high-velocity jet of high-temperature (11,000°C to 30,000°C) ionized gas on a workpiece.

The metal is removed in plasma Arc Machining due to

A. erosion
B. chemical reaction
C. melting of metal
D grinding

Answer is B

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