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How Plasma Cutters Work

Modern industry depends on the manipulation of heavy metal and alloys: We need metals to build the tools and transportation necessary for day-to-day business. For example, we build cranes, cars, skyscrapers, robots, and suspension bridges out of precisely formed metal components. The reason is simple: Metals are extremely strong and durable, so they're the logical choice for most things that need to be especially big, especially sturdy, or both.

The funny thing is that metal's strength is also a weakness: Because metal is so good at resisting damage, it's very difficult to manipulate and form into specialized pieces. So how do people precisely cut and manipulate the metals needed to build something as large and as strong as an airplane wing? In most cases, the answer is the plasma cutter. It may sound like something out of a sci-fi novel, but the plasma cutter is actually a common tool that has been around since World War II.

Conceptually, a plasma cutter is extremely simple. It gets the job done by harnessing one of the most prevalent states of matter in the visible universe. In this article, we'll cut through the mystery surrounding the plasma cutter and see how one of the most fascinating tools has shaped the world around us.

Where Saws Failed

In World War II, U.S. factories were cranking out armor, ordnance, and aircraft almost five times faster than the Axis powers. This was largely thanks to private industry's tremendous innovations in the field of mass production.

One area of innovation rose out of the need to cut and join aircraft parts more efficiently. Many factories working on military aircraft adopted a new method of welding that involved the use of an inert gas fed through an electric arc. The breakthrough discovery was that charging the gas with an electric current formed a barrier around the weld, which protected it from oxidation. This new method made for much cleaner lines at the joints and much sturdier construction.

In the early 1960s, engineers made a new discovery. They figured out that they could boost temperatures by speeding up the flow of gas and shrinking the release hole. The new system could reach higher temperatures than any other commercial welder. In fact, at these high temperatures, the tool no longer acted as a welder. Instead, it worked like a saw, cutting through tough metals like a hot knife through butter.

This introduction of the plasma arc revolutionized the speed, accuracy and types of cuts manufacturers could make in all types of metals.

States of Matter

A plasma cutter can pass through metals with little or no resistance thanks to the unique properties of plasma. So what is plasma?

There are four states of matter in the world. Most things we deal with in our daily lives are in the form of solids, liquids, or gases. These states are divided based on the way that molecules behave within each one. Take water as an example:

• As a solid, water takes the form of ice. Ice is made up of neutrally charged atoms arranged in a hexagonal pattern that forms a solid. Because the molecules stay fairly still relative to each other, they form a solid -- something that holds its shape.

• As a liquid, water takes its drinkable form. The molecules are still bound to each other, but they move relative to each other at slow speeds. The liquid has a fixed volume, but no constant shape. It changes shape to fit whatever container you put it in.

• As a gas, water takes the form of steam. In steam, molecules move around at high speeds, independently of each other. Because the molecules are not bound to each other, a gas has no fixed shape or fixed volume.

The amount of heat (which translates to the amount of energy) applied to water molecules determines their behavior and therefore their state. Simply put, more heat (more energy) excites molecules to the point that they break free of bonds that bind them together. With minimal heat, the molecules are tightly bound, and you get a solid. With more heat, the molecules escape the rigid bonds, and you get a liquid. With even more heat, the molecules escape the loose bonds, and you get a gas.

So what happens if you were to heat gas even more? This brings us to the fourth state: plasma.

What is Plasma?

If you boost a gas to extremely high temperatures, you get plasma. The energy begins to break apart the gas molecules, and the atoms begin to split. Normal atoms are made up of protons and neutrons in the nucleus, surrounded by a cloud of electrons. In plasma, the electrons separate from the nucleus. Once the energy of heat releases the electrons from the atom, the electrons begin to move around quickly. The electrons are negatively charged, and they leave behind their positively charged nuclei. These positively charged nuclei are known as ions.

When the fast-moving electrons collide with other electrons and ions, they release vast amounts of energy. This energy is what gives plasma its unique status and unbelievable cutting power.

Commonplace Plasma

Almost 99 percent of all matter in the universe is plasma. It's not common on Earth because of its extremely high temperatures; but somewhere like the sun, it's the norm. On Earth, you find it in lightning, among other places.

Plasma cutters are not the only devices to harness the power of plasma. Neon signs, fluorescent lighting and plasma displays, just to name a few, all rely on it to get the job done. These devices use "cool" plasma. Though cool plasma cannot be used to cut metals, it has tons of other useful applications.

Inside a Plasma Cutter

Plasma cutters come in all shapes and sizes. There are monstrous plasma cutters that use robotic arms to make precise incisions. There are also compact, handheld units that you might find in a handyman's shop. Regardless of size, all plasma cutters function on the same principle and are constructed around roughly the same design.

Plasma cutters work by sending a pressurized gas, such as nitrogen, argon, or oxygen, through a small channel. In the center of this channel, you'll find a negatively charged electrode. When you apply power to the negative electrode, and you touch the tip of the nozzle to the metal, the connection creates a circuit. A powerful spark is generated between the electrode and the metal. As the inert gas passes through the channel, the spark heats the gas until it reaches the fourth state of matter. This reaction creates a stream of directed plasma, approximately 30,000 F (16,649 C) and moving at 20,000 feet per second (6,096 m/sec), that reduces metal to molten slag.

The plasma itself conducts electrical current. The cycle of creating the arc is continuous as long as power is supplied to the electrode and the plasma stays in contact with the metal that is being cut. In order to ensure this contact, protect the cut from oxidation and regulate the unpredictable nature of plasma, the cutter nozzle has a second set of channels. These channels release a constant flow of shielding gas around the cutting area. The pressure of this gas flow effectively controls the radius of the plasma beam.