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Thermadyne S.A. Welding Information 
Bulletin #16

Plasma ARC Cutting

The Plasma Arc Cutting system employs the use of an electric arc and a pressurized volume of ionized air forced through a small orifice (TIP) fitted between the electrode in the plasma torch head and the workpiece to be cut. This constricted, high speed and high temperature plasma arc stream cuts through metal in a concentrated localized area and the molten metal is blown away by the high velocity arc and air steam.

The Plasma Arc Cutting process is capable of cutting and gouging most Ferrous (steel) and Non-Ferrous (Aluminium, Copper etc) metals producing a clean narrow cut width (determined by the tip orifice size) and smoother surfaces. The process is more portable, cuts faster and cleaner than the gas cutting (oxy/fuel) process, requiring no pre-heat, and with the added advantage of being able to cut Stainless Steel and Non-Ferrous metals (oxy/fuel cannot) cleanly and effectively, using less cumbersome equipment. The process can be used with either a hand held torch or on automated systems.

The Plasma Power source has drooping current characteristics, using higher voltages than standard welding machines and the torches are well insulated to protect the operator against the high voltages present.

The majority of Plasma Arc Cutters today have high frequency arc starting features meaning that non base metal contact is used as opposed to machines without high frequency that require scratch start to initiate the arc.

Hand cutting torches using stand-off guides (determines tip distance from workpiece) enables the operator to rest the torch on the workpiece and by using a template or straight edge is able to cut straight or profile edges cleanly and accurately. The torch can also be used for gouging by changing the tip to a gouging tip that enables the operator to angle the torch to + - 30 degrees as opposed to the 90 degree used when cutting.

Long (extended) tips can be fitted for maximum visibility giving a clear view of the cut to be made either in the stand-off or drag position. The tips contain an orifice which constricts the plasma arc. The tips come in various orifice sizes and also gradually wear with use and must be replaced when the arc cut becomes too wide.

Due to the alloying denseness and higher electrical resistance of Stainless Steel the thickness of cut achieved per cutting machine capacity is considerably lower than steel. All manufactures of Plasma Arc Cutting equipment indicate the cutting capacity when cutting various metals.

When cutting a circle the cut is started at the plate leading edge towards the circle edge or a hole is pierced in the centre of the circle and cut towards the circle edge. On piecing the hole through the plate the torch is angled at + - 30º and once pieced the torch is raised to a 90º position.

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