In this article we will discuss about the meaning and characteristics of Plasma Arc Machining (PAM) in industries.
Meaning of Plasma Arc Machining (PAM):
When a flowing gas is heated to a sufficiently high temperature of the order of 16,500°C to become partially ionised, it is known as ‘plasma’. In this process heat for machining is produced by convection from the high temperature plasma as well as due to direct electron bombardment. This process is best suited for machining hard- to-cut metals such as super alloys, particularly during cutoff or rough slitting operations.
In this process a gas (usually H2 or N2) is heated by being subjected to the electron bombardment of an electric arc produced between a cathodic electrode and anodic nozzle. The diatomic molecules of gas get dissociated due to their collision with the electrons generated by the arc and thus result in ionisation of the atoms.
The flow of gas should be such that the arc can be stabilised. This ionised stream of gas is impinged on the work surface to cause it to melt and erode. The metal removal rate can be increased by increasing the gas flow rates.
There are two types of plasma arc systems:
i. Transferred arc type;
ii. Non-Transferred arc type.
In the transferred arc type process the arc is struck between the work and the torch electrode. In effect the work is the anode and the electrode is the cathode. In this case more of the electrical energy is transferred to the work, thus giving the work more heat. In the non-transferred arc type process the arc is struck between two parts of the torch.
In effect the torch case is the anode and the torch electrode is the cathode for the purpose of striking the arc. The plasma jet or flame feeds the heat to the work. In both cases a copper alloy nozzle is used on the torch, both the nozzle and the electrode being water cooled.
Plasma systems are used for metal cutting and gauging in difficult materials such as stainless steel. In addition they cut plain carbon steel plate four times faster than the ordinary flame cutting torches. On the machine side plasma has been used for rough turning of very difficult materials.
The torch is held near the rotating work, tangentially to and below the work at an angle of 30° to the horizontal. This is a roughening operation to an accuracy of about 1.5 mm with a corresponding surface finish. Energy output per unit volume of metal removed per second is of the order of 140 J/mm3.
Characteristics of PAM Process:
i. Technique of machining – Heating of workpiece by high temperature ionised gas (plasma) and causing quick melting
ii. Tool – Plasma jet
iii. Velocity of plasma jet – 500 m/sec.
iv. Material removal rate – 150 cm3/min
v. Specific energy – 1000 W/cm3-min
vi. Power range – 2 to 200 kW
vii. Material of workpiece – All materials which conduct electricity
viii. Voltage – 30—250 V
ix. Current – Upto 600 amp.
x. Cutting speed – 0.1—7.5 m/min
xi. Applications – Profile cutting of stainless steel, monel, and superalloy plates
xii. Plate thickness – 200 mm (max)
xiii. Limitation – Low accuracy.