How a plasma cutter works?

The plasma cutting machine works by feeding an arc into a gas that passes through a narrow opening. This gas can be store air, nitrogen, argon, oxygen, etc. This raises the temperature of the gas to the point where it enters a fourth state of matter. We are all familiar with the first three states: i.e., solid, liquid and gas.  Scientists refer to this additional state as plasma. Since the metal being cut is part of a circuit, the conductivity of the plasma causes the arc to transfer to the workpiece.

The restricted opening (nozzle) through which the gas passes causes it to squeeze through at high speed, much like air passing through the venturi of a carburetor. This high velocity gas cuts through molten metal. The gas is also directed to the perimeter of the cutting area to protect the cut.

In many of today's better plasma cutters, a test arc between the electrode and nozzle is used to ionize the gas and initially create a plasma before the arc is transferred.

Other methods that have been used are touching the torch tip to the workpiece to create a spark, and using a high frequency starting circuit (such as a spark plug).  Neither of these latter two methods meets the requirements for CNC (automatic) cutting.