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How a plasma cutter works

 Plasma processPlasma cutters work by sending an electric arc through a gas that is passing through a constricted opening.  The gas can be shop air, nitrogen, argon, oxygen. etc. This elevates the temperature of the gas to the point that it enters a 4th state of matter.  We all are familiar with the first three: i.e., solid, liquid, and gas.   Scientists call this additional state plasma.  As the metal being cut is part of the circuit, the electrical conductivity of the plasma causes the arc to transfer to the work.

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

In many of today's better plasma cutters, a pilot arc between the electrode and nozzle is used to ionize the gas and initially generate the plasma prior to the arc transfer.

Other methods that have been used are touching the torch tip to the work to create a spark, and the use of a high-frequency starting circuit (like a spark plug).   Neither of these latter two methods is compatible with CNC (automated) cutting.

 Consumables from Hypertherm Powermax 900 plasma cutterThis photo shows consumables from a Hypertherm Powermax 900 plasma cutter.   The electrode is at the center, and the nozzle just below it.  The orange piece above the electrode is the swirl ring, which causes the plasma to turn rapidly as it passes.

While these parts are all referred to as consumables, it is the electrode and nozzle that wear and require periodic replacement.

CNC (computer numerically controlled) plasma cutters fully automate the shape production process.  Early CNC machines used a tape with small holes punched through to provide instructions to a primitive (by today's standards) computer.

Today's CNC units use either expensive limited production computers made specifically for running burning machines, or personal computers adapted to run the machines.  Both provide the same cut quality and production speed. In CNC cutting, you arrange your shapes on the computer screen and cut them automatically, without having to touch the material. CNC software lets you program pauses for piercing, scale up or down in seconds, set acceleration and deceleration at corners, and other functions not possible with electric eye units.

 screen shot of CorelDrawThese shapes were created in CorelDraw, and then turned and nested together to minimize scrap material.   The group of shapes was then exported as a single HPGL (Hewlett Packard Graphics Language) file. The file was then imported into a separate driver program.

The shapes are cut in a sequence that minimizes the distance the torch has to travel before returning to the home position.  Here, all 21 pieces are completed in 3/16" mild steel in under 10 minutes, at a cutting speed of approximately 80 inches per minute.

Note that while 21 pieces were produced, there are actually 49 separate shapes that had to be cut, some within other shapes.  This is all done automatically, with a 2 second pause to pierce the steel for each new cut. Lead-ins that were used to prevent blemishes in the finished part due to piercing.