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What is Plasma Arc Cutting?

May 6, 2014

And how does it work?

Plasma is defined in Websters as a ”collection of charged particles ... containing about equal numbers of positive ions and electrons and exhibiting some properties of a gas but differing from a gas in being a good conductor of electricity ...”. You can also think of it as an electrically heated gas stream. I like to think of it as a condition where all of the electrons from every atom are flowing from atom-to-atom, instead of just orbiting. Regardless of what’s going on inside a plasma stream, the way to cut metals with it is pretty straightforward. Take that stream of electricity flowing through a gas, and constrict it through a small orifice. Now that stream is really dense and moving very fast. The resulting stream can melt and blow through most metals quite easily. That’s a plasma torch.

Plasma cutting torches usually use a copper nozzle to constrict the gas stream with the arc flowing through it. That arc jumps from an electrode in the torch to something else – usually the conductive material being cut. That’s a ‘transferred arc’. There are some systems that use a ‘non-transferred’ arc where it jumps from the electrode back to the nozzle, but those are not usually used for cutting. So that means that plasma cutting is only used for materials that are conductive, primarily mild steel, stainless steel, and aluminum. But lots of other metals and alloys are conductive too, such as copper, brass, titanium, monel, inconel, cast iron, etc. The problem is that the melting temperature of some of those metals makes them difficult to cut with a good quality edge.

How a-plasma torch works - Click for larger view

The electrode is usually made of copper, but with a metal insert at the point where the arc attaches. That’s because the copper would melt too fast if the arc attached directly to it. Tungsten makes a great electrode material, so lots of electrodes have a tungsten insert. Some smaller torches use a ‘pencil’ style electrode made entirely out of tungsten with a sharpened end. The problem with tungsten is that it burns up in the presence of Oxygen. So when using Oxygen or compressed air as the cut gas, the insert is made of a material called Hafnium. Hafnium lasts a lot longer in the presence of Oxygen, but it still wears a little bit with each start of the arc.

So why use Oxygen in a plasma torch? The same reason you use Oxygen in an acetylene torch – the Oxygen in the plasma stream reacts with mild steel. That is why pure Oxygen is only used when cutting mild steel, or ‘carbon steel’. That chemical reaction between the Oxygen in the plasma gas and the base metal helps to speed up the cutting process and improve the edge quality. But since Oxygen doesn’t have the same reaction with Stainless Steel or Aluminum, less expensive gases can be used for those metals, like Nitrogen or compressed air (which is mostly Nitrogen anyway).

Other specialty gases are sometimes used for other purposes. Argon gas is used when plasma marking (a whole other subject). A mixture of Argon and Hydrogen is often used when cutting thicker Stainless Steel or Aluminum. Some people use a mixture of Hydrogen and Nitrogen, or Methane and Nitrogen when cutting thinner Stainless Steel. Each mixture has its advantages (improved cut quality) and its disadvantages (cost & handling).

So those are some of the basics of plasma cutting – an arc carried in a stream of gas from an electrode through an orifice and then into the conductive metal that is being cut. There are lots of other considerations that I will address later, such as swirling gases, kerf, height control, arc starting, shield gas, etc. But whether it’s hand-held or mounted on a CNC machine, the basics are the same.

Posted in Cutting Systems , Tagged with Plasma

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