An Introduction to Plasma Cutters

This "Magic Wand of Metalworking" can easily slice through a variety of metal types and thicknesses.

If you routinely work with metal and need to make numerous cuts, want to cut intricate patterns in sheet metal or crave a fast, easy and precise way to size metal stock before welding, a plasma cutter may be the tool of your dreams. This simple-to-operate electric cutting torch enables you to make long, straight precision cuts in all kinds and thicknesses of metal, slice expanded metal mesh and create curving or complex designs. Having one of these tools at your disposal can help you save time and radically expand your metalworking repertoire.

How plasma cutters work
A plasma cutter uses a high-velocity jet of ionized gas to cut metal. The gas is fed through a constricting orifice, where it conducts electricity from the torch of the cutter to the workpiece. The resulting plasma heats the workpiece so the material melts, and the high-velocity stream of ionized gas blows the molten metal away, severing the material. Though several different gases can be used with plasma cutters, the most common is compressed air, delivered either by an external compressor or by a small compressor that’s built into the tool.

A small plasma cutter, such as this Lincoln Electric Tomahawk 375, is ideal for cutting sheet and thin plate metal. This unit has a built-in air compressor; some models require an external compressor.

Because typical plasma cutting does not rely on the oxidation process, it can be performed on any type of conductive metal such as carbon steel, aluminum and stainless steel. Oxyfuel cutters such as an oxyacetylene torch, however, rely on oxidation to burn through metal, and as a result they can only be used to cut steel and other ferrous metals.

A plasma cutter’s efficiency depends on the torch’s nozzle design and how it handles the airflow to the workpiece. Design improvements (as in Lincoln Electric’s Tomahawk line of plasma cutters) cause the high-velocity air to swirl, divide and recombine within the torch, resulting in a more concentrated arc, faster cutting, a thinner kerf and greater thickness-cutting capacity than older models offered.

Is plasma cutting right for you?
Compared with other methods of cutting metal (such as oxyfuel systems), plasma cutting is typically not only easier for the first-timer to master but also more accurate. This is not to say that the tools don’t have drawbacks. Heavy steel (1 in. thick and greater) usually isn’t cut well by them because they would require a very high-capacity power supply to do the job. They are typically more expensive than oxyfuel torches, and unless you have a unit with a built-in air compressor, you’ll need to have an external compressor to run it.

A plasma cutter’s torch is simple, but the consumables (electrode and nozzle) are subject to wear and occasionally need to be replaced.

Nevertheless, a plasma cutter is a great investment if you plan to regularly work with conductive metals that are less than 1 in. thick. When shopping for a plasma cutter, consider these guidelines:

  • Choose a machine that can handle cutting metal that is approximately twice as thick as your typical workpieces. Most plasma-cutting power sources are rated by amperage, which can range from light-duty (about 25 amps, suitable for cutting 1/4-in.-thick material) to heavy-duty (80 amps, capable of cutting 1-in.-thick metal). If you most often cut very thin metal, you may be OK with a lower-amperage 1/4- or 1/2-in.-class tool. If you frequently cut metal that is thicker, look for a higher-amperage 1-in.-class machine.
  • Check the unit’s duty cycle, which is the amount of time you can continuously cut before the machine or torch will require cooling. Duty cycle is rated as a percentage of a 10-minute period. For example, a 60 percent duty cycle at 50 amps means you can cut with 50 amps of output power continuously for six minutes in a 10-minute period.
  • Look for a cutter with the fewest consumable parts, and compare how long those parts will last. Some manufacturers rate consumables by number of cuts; others use the number of starts as the measurement.
  • Check how visible your workpiece will be when you’re cutting. Visibility is especially important when cutting an intricate pattern. A smaller, less bulky torch will enable you to better see where you are cutting, as will an extended nozzle.
  • If you plan to transport the cutter to various job sites, make sure to purchase one that has a built-in air compressor.
  • For optimum safety, look for a machine that offers a pre-use warning so you can ensure that all body parts are clear of the nozzle before the arc initiates. Other safety systems (such as Lincoln Electric’s Nozzle-in-Place safety sensor) ensure that the cutter will not start an arc unless the nozzle is attached. (Warning: Some safety systems can be fooled into thinking the nozzle is in place even when it is not. If the cutter is then activated, you could be exposed to as much as 300 volts of direct current.)

Hold the torch at a slight angle when making or ending cuts. Once the cut is underway, hold the torch perpendicular to the workpiece for the best results.

Operation technique and tips
Before you start cutting, make sure you have a clean compressed-air supply that’s not contaminated by water or oil, as either substance will prematurely wear out the consumables. Check that the air pressure is set to the level required by the tool, and ensure that the nozzle and the electrode of the cutter are correctly installed. Wear proper safety gear (long sleeves, gloves, dark goggles or a welding shield or helmet). To get the best results when cutting, follow these tips:

  • Pierce the workpiece by approaching the metal with your torch at an angle (60 degrees from horizontal, 30 degrees from vertical); then rotate the torch to the vertical position. This way, the molten metal is blown away from the torch.
  • Maintain a constant work distance, somewhere from 3/16 to 1/8 in. between the nozzle of the torch and the workpiece. Until you’re used to working with the tool, consider using an insulated drag cup that that snaps over the nozzle and allows you to rest the torch directly on the workpiece.
  • Do not touch the nozzle to the workpiece when using current levels of 45 amps or more. Doing so will drastically reduce the nozzle life, as the cutting will double arc through the nozzle.
  • When the tool is moving at the right cutting speed, the molten metal spray will blow out the bottom of the workpiece at a 15- to 20-degree angle. Moving too slowly will create slow-speed dross, an accumulation of molten metal on the bottom edge of the cut. Moving too fast will cause high-speed dross to accumulate on the top surface.
  • Set the current to maximum as you begin; then turn it down as needed. More power is usually better, except when you’re doing precision cutting or when you need a small kerf.
  • Travel in the direction that will produce the best finished work. As you push the torch away from you, the right-hand side of the workpiece will tend to have a better, squarer edge. If you are making a circular cut and plan to keep the round piece as your finished work, move in a clockwise direction. If you plan to keep the piece from which the circle was cut, move in a counterclockwise direction.

This photo of a 1/16-in.-thick steel workpiece shows how smooth a plasma cut can be. The dross (see photo below) on the back of this cut was caused by the torch moving too slowly.

After finding the right machine and learning some tricks of the trade, you’ll be ready to put your new tool to use. With experience, you’ll work faster, make better cuts and be able to accomplish more than you might have imagined possible.