Plasma cutters are becoming the tool of choice for cutting metal. They're quick, clean and easy, requiring less skill to operate than an oxyacetylene torch. However, if you don't make tons of cuts and don't want to invest in a plasma or oxy setup, you can adapt your arc welder to make plasma-like cuts. It's not technically a plasma cut, but the air arc cut it produces mimics the plasma cut.
Attach a brass plumbing valve to one end of a one-fourth-inch copper tube.
Attach a brass cap on the other end of the copper tube.
Drill a hole in the cap and tap threads into the hole that will match a small brass carburetor jet. This will allow you to use interchangeable jets to adjust the air spray. The plumbing valve will allow you to adjust the air volume.
Attach the tubing to your arc welder handle. For temporary use, use zip ties to secure it.
Insert a carbon electrode instead of a welding stick. Make sure the tip of the electrode is a sharp, conical form, like a sharpened pencil.
Flex the nozzle toward the the tip of the carbon electrode.
Attach a fast-release compressor hose coupling to the brass plumbing valve, then attach your air compressor's hose to the coupling.
Turn the amperage on your arc welder up 25 to 50 amps. This will provide a very hot arc and excessive oxidation, giving you controlled blow-through in plate metal up to one-inch thick, similar to a plasma cutter.
Like welders and plasma cutters, optimal performance requires precise tuning and a bit of experimentation.
Use caution with the air stream so it doesn't blow molten metal towards you or anyone else. Wear proper skin, eye and face protection.