In addition to the three standard states of matter, namely solids, liquids, and gases, there is also a fourth state called a plasma. Plasmas are particularly interesting for their unique real-world applications, one of which is the plasma cutter. These metal-cutting devices harness the power of thermal plasmas to quickly and accurately slice through sheet metal. In this post, we delve into the world of plasma cutters: First, we define what exactly a plasma state consists of and the different types that exist. Then we investigate how plasma cutting machines work and the components involved. Finally, we spell out the steps taken to operate these metalworking devices.
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What is a Plasma?
We are all well-acquainted with the properties of the classical states of matter, the familiar trio of solid, liquid, and gas. However, most people don't think a whole lot about the fourth state of matter, the plasma. A plasma is essentially a highly-charged gas formed through the infusion of lots of energy. This extra energy strips electrons from the gas atoms. As it does so, the gas turns into a highly-energised mix of positive ions and electrons making it an extremely good conductor of electricity. The external energy source that gives rise to the plasma can take various forms. In the case of man-made plasmas, this is typically heat, electrical discharge, or laser light.
Types of Plasmas
Plasmas come in two distinct flavours: hot thermal plasmas and cold non-thermal ones.
Hot Plasmas
Hot or thermal plasmas are usually generated through the application of very high voltage, resulting in scorching temperatures in the region of 20,000 ºC. This exceptional heat bestows upon a plasma some interesting properties. These include exceptional electrical conductivity, the generation of electric and magnetic fields, and the emission of electromagnetic radiation. Some of these properties have given rise to some useful applications. One of those applications, which we are interested in here, is the cutting of sheet metal with a plasma cutter.
Cold Plasmas
Conversely, cold or non-thermal plasmas present a stark contrast. While their electrons boast high temperatures, their positive ions and neutral particles remain relatively cool. A familiar example to most of us is the fluorescent lighting in our homes and offices (or at least it was until LEDs took over!). This type of lighting works as a result of a cold plasma operating at room temperature which is responsible for the light emitted.
How do Plasma Cutters Work?
The first thing to remember with plasma cutters is that they can only cut through conductive materials, typically a metal, such as steel or aluminium. Through that electrical conductivity, a high-temperature and high-velocity plasma jet is generated that is directed towards the workpiece. When the plasma comes into contact with the metal's surface, it heats it to its melting point almost instantaneously. The high speed plasma rushing out of the nozzle then blows away the molten material severing the sheet metal in the process.
Power Supply
The first key component to any plasma cutter is its power supply. The power supply provides the electrical energy needed to create the plasma arc. However, different plasma cutters can have different power requirements. Some basic machines can be run off residential mains although their maximum cutting depth is limited. More professional machines, which are more capable, usually require higher power electrical systems typical of workshop environments.
The type of technology used in the power supply is important too. Most modern plasma cutters today use inverter electronics (as opposed to bulky transformers) to convert the incoming AC power into high-frequency AC (before it is rectified back to DC power used by most of today's plasma cutters). This technology is superior to the traditional transformer method due to its improved electrical efficiency, and reduced size and weight of the machine required.
In addition, the electrical inverting step itself can be either MOSFET-based or IGBT-based. MOSFET and IGBT refer to the type of transistors used within the power supply. MOSFETs are older technology and are more vulnerable to damage from the extreme electrical conditions of a plasma cutter. IGBT transistors are more robust and therefore are more often used in more powerful and more premium machines. In general, an IGBT-based inverter power supply is the preferred option and is increasingly becoming the de facto standard for plasma cutters.
Control
Next we have the plasma cutter control panel, which allows an operator to set the amperage of the machine. The level of current that can be set determines how deep a cut the plasma cutter can make. In general, the higher the amperage, the greater the depth, and the thicker the workpiece that can be cut.
Traditionally, plasma cutters are operated manually. This is where an operator guides a hand-held plasma torch along the desired cutting path. However, for better precision, modern plasma cutters are increasingly being controlled by computer numerical control (CNC) systems. These CNC plasma cutters use computers to precisely control the movement of the torch over a plasma cutting table and automate the whole cutting process. Consequently, CNC plasma cutting machines are ideal when making complex shapes and patterns in sheet metal, as well as for making repetitive work more accurate.
Gas Supply
A stream of gas is another central component of any plasma cutter. The gas is needed for both the plasma jet itself and as a shielding gas (more on that below). Common gases used include compressed air, nitrogen, argon, or a mixture of gases. The choice of gas depends on the type of material being cut and the specific requirements of the plasma cutter.
The gas flow itself is controlled by a gas regulator. It ensures that the right amount of gas is delivered to create and stabilize the plasma arc. Most plasma cutters have built-in gas regulators to precisely control the gas flow. Some plasma cutters also come with a built-in air compressor providing their own source of gas flow.
Arc Initiation within a Plasma torch
Cross-section of a plasma torch head showing arc initiation
When the trigger is pressed on the plasma torch, the gas begins to flow through it. As the gas moves through the plasma torch, a component called a swirl ring helps create a vortex in the gas flow. This circular movement of gas enhances plasma cutting performance.
At the same time as the gas starts flowing, a negatively-charged cathode, colloquially known as the 'electrode', generates an electric arc discharge between it and the nozzle of the torch. This discharge ionizes the gas vortex passing through it, transforming it into a plasma.
Constriction and Acceleration
The newly created plasma flows through the plasma torch nozzle which has a small opening at its far end. As the plasma flows through this constriction, it accelerates to very high speeds turning it into a fine plasma jet as it exits the torch.
Shield Gas
Cross-section of a plasma torch head showing shield gas and cap function
At the same time the plasma jet exits the nozzle, a parallel ejection of gas called the "shield gas" is blown out in the form of a protective ring around the super-heated plasma. As it does so, it further shapes the plasma jet enhancing its precision.
As the metal workpiece is liquified at the cutting interface, the shield gas also helps to blow away the molten metal. The shield gas flow is directed by a shield cap or shield diffuser which sits around the nozzle. This shield cap also helps protect the torch nozzle from damage by physically keeping it away from the superheated cutting area.
Ground Clamp
Finally, we have the ground clamp, which is connected to the workpiece and serves to complete the plasma cutter's electrical circuit. It is because of this completed electric circuit that the plasma flows from the torch into the material being cut.
How to Use a Plasma cuttter
Using a plasma cutter safely requires following a series of steps:
Safety
Safety Gear: Before starting, put on the appropriate safety gear, including a welding helmet with a shaded lens, flame-resistant clothing, gloves, and safety goggles.
Ventilation: Ensure you're working in a well-ventilated area or use a fume extractor system because plasma cutting generates fumes that can be harmful to health.
Setup
Select the Correct Gas: Choose the appropriate gas based on the material you're cutting. Common choices include:
Connect the Ground Clamp: Attach the ground clamp securely to the workpiece or the metal surface you intend to cut. This completes the electrical circuit that is required for plasma cutting.
Set Parameters: Adjust the cutting parameters on the plasma cutter's control panel, specifically the amperage and gas flow rate. Refer to the manufacturer's recommendations to set them for the specific material and thickness you're working with.
Operation
Ignite the Arc: Turn the plasma cutter on and initiate the plasma arc by pressing the trigger on the torch.
Maintain Proper Torch Height: Keep the torch at the recommended distance from the workpiece, typically around 1/8 to 1/4 inch (3 - 6 mm) above the material's surface. Maintaining the correct height ensures a stable arc and precise cutting.
Control Torch Movement: Move the torch steadily along the desired cutting path, following your marked guidelines. Avoid stopping during the cut to prevent the overheating of a single location or creating uneven cuts.
Control Slag: As you cut, you may notice slag (molten metal residue) accumulating on the underside of the cut. Some plasma cutters have a slag removal feature, but if not, you can use a slag hammer or chipping tool to remove it after the cut is complete.
Conclusion
Plasma cutters represent an ingenious fusion of physics and engineering, enabling the transformation of electrical energy into a formidable tool for efficient sheet metal-cutting. In this post, we've examined how a plasma cutter works, exploring its key components and the role each plays in this metal-cutting tool. With this theoretical knowledge in hand, we have also looked at what it takes to actually operate a plasma cutter. Given its usefulness, if you too find yourself often working with sheet metal, then you should consider adding a plasma cutter to your own tool inventory. In this way, you can harness its power in your next creative endeavor.