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How Plasma Cutters Work ?
 Photo courtesy Torchmate CNC Cutting Systems Plasma cutter at work. |
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 usWhere 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. In the next section, we'll examine the science behind this system.
Photo courtesy Torchmate CNC Cutting Systems
This is what plasma cutting looked like in 1980.
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.
Click on Cold, Warm and Hot to watch the H2O in this glass of water
move through three states of matter.
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 (see How Atoms Work), 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.
 Photo courtesy National Oceanic and Atmospheric Administration/Department of Commerce |
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. Check out How Fluorescent Lamps Work to learn more.
Inside a Plasma Cutter
 Photo courtesy Torchmate CNC Cutting Systems Inside a plasma cutter: The electrode is at the center, and the nozzle is just below it. The orange piece is the swirl ring, which causes the plasma to turn rapidly as it passes. |
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.
Click the On/Off switch to watch a plasma cutter in action.
Plasma on the Job
Plasma cutters are now a staple of industry. They are used largely in custom auto shops as well as by car manufacturers to customize and create chassis and frames. Construction companies use plasma cutters in large-scale projects to cut and fabricate huge beams or metal-sheet goods. Locksmiths use plasma cutters to bore into safes and vaults when customers have been locked out.
Photo courtesy Torchmate CNC Cutting Systems
In CNC (computer numerically controlled) cutting, you arrange your shapes on the computer screen and cut them automatically, without having to touch the material.
Plasma Art
 Photo courtesy Torchmate CNC Cutting Systems Powermax 600 plasma cutter |
The plasma cutter is one of the most interesting and powerful tools developed in the 20th century. Using basic principles of physics to harness the fourth state of matter, the plasma cutter performs with nearly magical results. One can only imagine, as our understanding of plasma grows, how many more tools and applications will utilize this fascinating force of nature.