PLASMA ARC MACHINING (PAM)

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

• Because of the high temperature involved, the process

can be used on almost all materials including those which are resistant to oxy-fuel gas cutting.

• Plasma is a mixture, of free electrons, positively charged ions and

neutral atoms. It can be obtained by heating a gas to a very high

temperature so that it is partially ionized.

• The central zone of the plasma, reaches a temperature of 15.000°C

and is completely ionized. Much of the heating of the gas

takes place in the constricted region of the nozzle duct, resulting in high velocity gas exit.

Direct current plasme generator

Transferred Arc

Non-transferred Arc

• A high frequency spark is used to initiate a pilot arc between the tungsten electrode (cathode) and the copper nozzle (anode) both of which are water cooled.

• The pilot arc is then cut off and the external arc generates a plasma jet which exits from the nozzle at near sonic velocity.

• Water injection is sometimes used to assist in confining the arc, to blast away the scale and to reduce smoke.

• Greater nozzle life is also reported for torches of the water injection type.

• The plasma jet heats the workpiece by bombardment with electrons and by transfer of energy from the high-temperature, high-energy gas.

• The heat is effective in cutting the workpiece is up to a thickness of 50 mm.

Water-injected plasma system

Air Plasma

• Compressed air is used as plasma gas.• When subjected to high temperature in the

electric arc, the air breaks down into its constituent gases.

• Since the oxygen in the resulting plasma is very reactive, cutting speeds are increased by 25%

Selection of Gas

• Any gas that does not attack the tungsten electrodes or the workpiece can be used.

• Carbon and alloy steels and cast iron are usually cut with a mixture of nitrogen-hydrogen, or with compressed air.

• Stainless steel, aluminium and other non-ferrous metals are cut with mixtures of argon-hydrogen, or nitrogen-hydrogen.

• Typical flow rate of the gas is 2-11 m3/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 race of 250-1700 mm/min.

Process Characteristics• The obtainable cutting rates in PAM are 250-1700

mm/min depending on the thickness and material of the workpiece.

For example, a 25 mm thick aluminium plate can be cut at a speed of 750 mm/min; while a 6 mm carbon steel sheet can be cut at 4000 mm/min.

The-use of water injection can increase the cutting rate in carbon steel to 6000 mm/min for a 5 mm thick plate.

• Surfaces cut by plasma torch are smoother than surfaces cut by the oxy-acetylene, but the edges are rounded.

• The corner radius is a minimum of 4 mm on thinner plates.

• In addition, the wails of the cut nave a 'V shape with an included angle of 5-10°.

• Multiport nozzles under proper operating conditions reduce this beveling to 1-2°, The kerf width is usually 2.5-9 mm.

• Accuracy on the width of slots and diameter of holes is ordinarily from ± 0.8 mm on 6-30 mm thick plates and ± 3.0 mm on 100-150 mm thick plates.

• The depth of heat affected zone depends on the work material, its thickness and cutting speed.

• On a workpiece of 25mm thickness the heat affected zone is about 4 mm, and it is less at high cutting speeds.

Applications

• PAM is chiefly used to cut stainless steel and alumium alloys. It is preferred to oxy-fuel cutting because it produces comparatively smoother cuts, and is free from contamination.

• Another reason is that the aluminium alloys cannot be cut by oxy-fuel gas method.

• Heavy-duty plasma torches can cut stainless steel of thickness up to 100-125 mm and aluminium alloy of thickness up to 150mm.

• Other metals which are resistant to oxy-fuel cutting and hence cut by plasma arc method are magnesium, titanium, copper, nickel and alloys of copper, and nickel.

• profile cutting of metals, particularly stainless steel and aluminium, has been the most prominent commercial application of PAM.

• The PAM equipment are, therefore, offered with numerical control table and often with multiple-torch capability to increase productivity.

• PAM has also been considered for lathe turning, milling and planing.

• A typical set-up for turning with plasma torch• Plasma arc is also used successfully in

conventional turning and milling machines to machine very hard materials.

• The plasma are torch mounted just ahead of the cutting tool heats up the workpiece and a conventional turning tool or a milling cutter does the machining.

Effect of surface speed on removal rate

• The principal advantage of PAM is that it is almost equally effective on any metal, regardless of its hardness or refractory nature.

• As there is no contact between the tool and the workpiece, only a simple workpiece supporting structure is adequate.

• The cutting rates in this process are high enough to facilitate this method to be used on almost all materials.

• The main disadvantages of this process are the metallurgical alteration of the surface.

• Thus, a secondary machining needs to be performed to remove this surface by 1.5 mm or more, unless it can withstand the hardened and uneven surface.

• Eye shielding and noise protection are necessary for the operator.