Plasma Arc Machining (PAM): Working Principle, Application and Advantages

In this post, you learn what is plasma arc machining, how it’s done, working principle, advantages, limitations and more

Plasma Arc Machining

When a flowing gas is heated to a sufficiently high temperature to become partially ionized, it is known as ‘plasma’. This is virtually a mixture of free electrons, positively charged ions and neutral atoms.

What is plasma arc machining?

Plasma arc machining is a metal removal process in which the metal is removed by focusing a high-velocity jet of high temperature (11,000°C to 30,000°C) ionized gas on the workpiece.

Working Principle of Plasma Arc Machining

The principle of plasma arc machining is shown in the figure.

In a plasma torch, known as the gun or plasmatron, a volume of gas such as H2, N2, 02, etc. is passed through a small chamber in which a high-frequency spark (arc) is maintained between the tungsten electrode (cathode) and the copper nozzle (anode), both of which are water-cooled.

• In certain torches, an inert gas-flow surrounding the main flame is provided to shield the gas from the atmosphere.
• The high-velocity electrons generated by arc collide with the gas molecules and produce dissociation of diatomic molecules of the gas resulting in ionization of the atoms and causing large amounts of thermal energy to be liberated.

• The plasma forming gas is forced through a nozzle duct of the torch in such a manner as to stabilise the arc.
• The heating of the gas takes place in the compressed zone of the nozzle duct resulting in almost high exit gas velocity and high core temperature up to 16,000 °C.
• The relative plasma jet melts the workpiece material and the high-velocity gas stream effectively blows the molten metal away.
• The depth of heat affected zone depends on the work material, its thickness and cutting speed. On a workpiece of 25 mm thickness, the heat-affected zone is about 4 mm and it is less at high cutting speeds.
• A typical flow rate of the gas is 2 to 11 m/hr. Direct current, rated at about 400 V and 200 kW output is normally required.
• Arc current ranges between 150 and 1000 A for a cutting rate of 250 to 1700 mm/min.

Accuracy

This is a roughing operation to an accuracy of about 1.5 mm with a corresponding surface finish. Accuracy on the width of slots and diameter of holes is ordinarily from +0.8 mm on 6 to 30 mm thick plates, and + 3,0 mm on 100 to 150 mm thick plates.

Applications of Plasma Arc Machining

• This is chiefly used to cut stainless steel and aluminium alloys.
• Profile cutting of metals, especially of these metals and alloys, has been the common prominent commercial application of PAM.
• On the machining side, plasma has been used successfully in conventional turning and milling of very difficult materials.

Advantages and limitations

• The principal advantage of this process is that it is almost equally effective on any metal, regardless of its hardness or refractory nature.
• There being no contact between the tool and workpiece, only a simply supported workpiece structure is enough.
• The main disadvantages of this process are the metallurgical change of the surface.
• Safety precautions are necessary for the operator and those in nearby areas. This adds an additional cost.

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Read also:

• Ultrasonic Machining (USM): Parts, Working Principle, Advantages, Application and More
• Abrasive Jet Machining (AJM): Parts, Working Principle, Advantages & Applications
• Laser Beam Machining (LBM): Parts, Working Principle, Applications, Limitations and more

External resources:

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