A plasma cutter is a tool that’s used to cut metal. Plasma cutters range in size from small, hand-held units that an amateur mechanic might keep in his garage to large computer-controlled machine tools that can cut steel up to three inches thick.
A plasma cutter works by melting metal. A high velocity stream of gas then blows the molten metal away. What makes a plasma cutter different to an oxyacetylene cutter is that it burns at a very much higher temperature and the heat is concentrated into a very narrow beam. This means it can cut faster, which results in a smaller heat-affected zone and less warping of the metal being cut. Faster cutting also reduces damage to vulnerable coatings like paint and to electronic components and circuit boards.
In an oxyacetylene torch the gas burns at around 6,000 degrees F (around 3,700 degrees C) but a plasma cutter produces a cutting jet of 30,000 to 40,000 degrees F (around 18,700 to 25,000 degrees C.) This is actually hotter than the surface of the sun, which is a particularly relevant comparison because the sun is also composed of plasma.
Plasma is similar to a gas: it’s a cloud of atoms of the same element or elements, but with one big difference. In plasma the electrons that usually orbit around the nucleus of each atom are free to wander throughout the cloud. In other words, they are no longer tied to an individual atom. This changes the electric balance of the atom, leaving it positively charged, or ionized. The wandering electrons carry a great deal of energy, which is released when they collide with other electrons and ions.
A plasma cutter harnesses this energy by forming a plasma between an electrode in the cutting head and the metal to be cut. For this to happen the workpiece has to be part of the electrical circuit, so a clamp is connected that runs back to the power supply. Next, an inert gas is passed over the electrode and an electrical spark created between electrode and the workpiece. The spark ionizes the gas, raising its temperature and turning it to plasma.
In early plasma cutters the spark was created by touching the electrode to the metal workpiece, then lifting it away. This created an arc that, sustained by the stream of inert gas, bridged the gap. Most modern plasma cutters now use a spark generator to create a pilot plasma within the cutter head. This is then brought into contact with the workpiece and the full plasma is generated.
The heat of the plasma melts the metal it comes into contact with, but for effective cutting that metal must be removed. This is done by blowing high velocity compressed air at the melt pool. For simplicity, this air is also supplied through the head of the plasma cutter, usually being delivered by a separate compressor.
When working with a plasma cutter it’s important to understand the dangers. In addition to the molten metal there is a risk of electrocution if the person doing the cutting holds both the plasma cutter and the workpiece at the same time. However, providing sensible safety precautions are taken, the plasma cutter is an extremely versatile metal cutting tool and a valuable addition to any well-equipped workshop.
