How Does a Plasma Arc Machining Works ?

Plasma-arc machining (PAM) employs a high-velocity jet of high-temperature gas to melt and displace material in its path which is also called as PAM, this is a method of cutting metal with a plasma-arc, or tungsten inert-gas-arc, torch. When a gas or air heated at high temperatures, the number of collisions between atoms increases. When you heat the gas above 5500ºC, it partially ionises into positive ions, negative ions and neutral ions. When you further heat the gas above 11000ºC then, it completely ionises. Such a completely ionised gas is called as Plasma. Temperatures in the plasma zone range from 20,000° to 50,000° F (11,000° to 28,000° C).

What is Plasma Arc Machining?

The plasma arc process has always been seen as an alternative to the oxy-fuel process.Generally, Plasma Arc Machining (PAM) is a metal cutting process where metals are cut with plasma arc, tungsten-inert-gas arc or a torch.These method is usually used according to the work piece.If the work piece may be made up of aluminium, iron or steel. For example, for aluminium nitrogen is used, for argon hydrogen is used. In most of the cases, nitrogen and hydrogen are used. Plasma Arc Welding employs a high-velocity jet of high-temperature gas to melt and displace material in its path.

Construction :-
Plasma arc machining consists of a Plasma gun. Plasma gun has an electrode made up of tungsten situated in the chamber. Here, this tungsten electrode is connected to the negative terminal of DC power supply. Thus, the tungsten acts as a cathode. While the positive terminal of DC power supply is connected to the nozzle. Thus, nozzle of the plasma gun acts as an anode.

Working Principle of PAM :-

The basic principle is that the arc formed between the electrode and the work piece is constricted by a fine bore, copper nozzle. This increases the temperature and velocity of the plasma emanating from the nozzle. As the gas comes in contact with the plasma, there is a collision between the atoms of a gas and electrons of an electric arc and as a result, we get an ionised gas. That, means we get the plasma state that we wanted for Plasma Arc machining.When used for cutting, the plasma gas flow is increased so that the deeply penetrating plasma jet cuts through the material and molten material is removed in the efflux plasma.

In the whole process, high temperature conditions are required. As hot gases come out of nozzle there are chances of overheating. In order to prevent this overheating, a water jacket is used.

It is mostly used for the metals that cannot be cut by an oxyacetylene torch. The process totally different from the oxy-fuel process because in the plasma process operates by using the arc to melt the metal whereas in the oxy-fuel process, the oxygen oxidises the metal and the heat from the exothermic reaction melts the metal. Thus, unlike the oxy-fuel process, the plasma process can be applied to cutting metals which form refractory oxides such as stainless steel, aluminium, cast iron and non-ferrous alloys.

Application:-

• It is used to weld pipes and tubes of stainless steel or titanium.
• It is mostly used in electronic industries.
• It is used to repair tools, die and mold.
• It is used to welding or coating on turbine blade.

Advantages of PAM Process :-

• It gives faster production rate..It is the main advantage of PAM. For example, mild steel of 6mm thick can be cut at 3m/min .
• Very hard and brittle metals can be machined.The plasma arc can be used to cut any metal or even to nonconducting materials like concrete etc., since it is primarily a melting process.
• Small cavities can be machined with good dimensional accuracy.
• Due to high speed of cutting the deformation of sheet metals is reduced while the width of the cut is minimum.
• Owing to the high productivity of the plasma arc cutting coupled with the tendency to use cheap and easily available plasma-forming media (air, water, ammonia etc.,), PAC is finding ever increasing application. 
• Smooth cuts free from contaminants are obtained in the process.
• Profile cutting of metals especially of stainless steel and aluminium can be very easily done by PAM.
• Operating costs are less when compared to oxy-fuel torch 8.Can be automated.

Disadvantages of PAM Process :-

• The main disadvantage of PAC is the high initial cost of the equipment. However, it can be made economical, if the quantity involved is large and the thickness is up to 50mm. 
• The process requires over safety precautions which further enhance the initial cost of the setup.
• This entire machining process consumes high amount of inert gases.
• Sharp corners are difficult to produce because of the wide diameter of the plasma stream.
• Production of narrower surfaces takes place which is unnecessary.
• Some of the workpiece materials are very much prone to metallurgical changes on excessive heating so this fact imposes limitations to this process.
• The operator or person handling whole process must take proper precautions. This process can affect human eyes so a proper googles or helmet must be wear by an operator.