What is Plasma cutting?
Plasma cutting is a melting process in which a jet of ionized gas at temperatures above 20,000°C is used to melt and expel material from the cut. This heat melts the metal and the gas flow ejects it from the cut. Plasma gases are usually argon, argon/hydrogen, or nitrogen.
During the process, an electric arc is struck between an electrode (cathode) and the workpiece (anode). The electrode is recessed in a water- or air-cooled gas nozzle which constricts the arc causing the narrow, high-temperature, high-velocity plasma jet to form.
When the plasma jet hits the workpiece, recombination takes place and the gas reverts to its normal state, emitting intense heat as it does so. Plasma gases are usually argon, argon/hydrogen, or nitrogen. These inert gases can be replaced by air but this requires a special electrode of hafnium or zirconium.
The use of compressed air makes this variant of the plasma process highly competitive with the oxy-fuel process for cutting carbon-manganese and stainless steels up to 20mm thick. Inert gases are preferred for high-quality cuts in reactive alloys.
Plasma arc can cut a very wide range of electrically conductive alloys including plain carbon and stainless steel, aluminum and its alloys, nickel alloys, and titanium. The method was originally developed to cut materials that could not be satisfactorily cut by the oxy-fuel process.
Normally, the component or sheet to be cut remains stationary and the plasma torch is moved. Additionally, because the cost of the plasma torch is low compared with the price of the manipulation equipment, it is common to fit several torches to a cutting table.
How does plasma cutting work?
Plasma cutters send an electric arc through a gas that passes through a narrowed opening. The restricted opening (nozzle) through which the gas flow causes it to pass at high speed, like air flowing through a venturi in a carburetor. This high-speed gas cuts through the molten metal.
You can also think of it as an electrically heated gas stream. I like to think of it as a state where all electrons from each atom flow from atom to atom rather than just orbiting. Regardless of what’s going on in a plasma stream, cutting metals is pretty easy with it.
Take that stream of electricity flowing through gas and narrow it through a small opening. Now this stream is really dense and moving very quickly. The resulting stream can easily melt and blow through most metals. That’s a plasma torch.
Plasma cutting torches typically use a copper nozzle to narrow the gas stream with the arc flowing through it. This arc jumps from one electrode in the torch to something else – usually the conductive material that is being cut. This is a ‘transferred arc’. There are some systems that use a ‘non-transferred arc where it bounces off the electrode back to the nozzle, but these are not normally used for cutting.
This means that plasma cutting is only used for conductive materials, mainly mild steel, stainless steel, and aluminum. But many other metals and alloys are also conductive, such as copper, brass, titanium, Monel, Inconel, cast iron, etc. The problem is that the melting temperature of some of these metals makes it difficult to cut with good edge quality.
The electrode is usually made of copper, but with a metal insert where the arc is attached. This is because the copper would melt too quickly if the arc were to stick directly to it. Tungsten is a great electrode material, so many electrodes have a tungsten insert.
Some smaller flashlights use a pencil-style electrode made entirely of tungsten with a pointed end. The problem with tungsten is that it burns in the presence of oxygen. So if you are using oxygen or compressed air as the cutting gas, the insert is made from a material called hafnium. Hafnium lasts much longer in the presence of oxygen but wears off a little each time the arc is started.
Why use Oxygen in a plasma torch?
For the same reason that you use oxygen in an acetylene torch, the oxygen in the plasma stream reacts with mild steel. Therefore, pure oxygen is only used when cutting mild steel or “carbon steel”. This chemical reaction between the oxygen in the plasma gas and the base metal speeds up the cutting process and improves the edge quality.
However, since oxygen does not react in the same way as stainless steel or aluminum, cheaper gases such as nitrogen or compressed air (which is mainly nitrogen anyway) can be used for these metals.
Other specialty gases are sometimes used for other purposes. Argon gas is used in plasma marking (a completely different topic). A mixture of argon and hydrogen is often used when cutting thicker stainless steel or aluminum. Some people use a mixture of hydrogen and nitrogen or methane and nitrogen when cutting thinner stainless steel. Each mixture has its advantages (improved cut quality) and its disadvantages (costs and handling).
How to Use a Plasma Cutter?
Using a plasma cutter is very convenient and fairly easy. The benefit is that metal-based “free form” cuts can be made based on the routing of the router. Because this machine conducts plasma by creating a circuit, a grounding clamp is required, much like welding.
Step 1: Choose Work Location. Since we plan to cut metal, it is important to place the metal on a surface that is secure and allows freedom of movement. A “grid” or similar surface that acts as a table is perfect.
Step 2: Plugin Unit. Ensure that the unit is off and plug it in.
Step 3: Connect the Air. Connect the external air compressor to the plasma cutter. This is to ensure that the plasma stream remains under high pressure. To secure the connections, pull back the outer flange of the socket connection and insert the plug connection.
Step 4: Turn the Air On. Turn on the airflow. In this case, rotate the lever 90 degrees from perpendicular to the air duct to inline.
Step 5: Attach the Ground Clamp. Place the metal you are using on the table and attach the grounding clamp close to where you want to cut.
Step 6: Turn on the Machine. Switch on the machine by setting the switch behind the device to ON.
Step 7: Set the Current. In this case we set it to 25 for 18 ga sheet.
Step 8: Cut the Metal. Use the trigger on the gun to activate the plasma cutter. Note that the trigger has security that must be lifted before you can pull the trigger. Hold the router (nozzle end) close to the metal and use the guidelines surrounding the nozzle to trace templates, if any.
Step 9: Turn Off the Machine. When you’re done cutting your metal, turn off the machine.
Step 10: Disconnect the Ground Clamp. Disconnect the ground clamp from the metal you are working on.
Step 11: Turn Off Air. In this case, turn off the air by rotating the lever 90 degrees from inline to perpendicular to the line.
Step 12: Wind Up All Hoses. Wrap up the plasma gun line, airline, and ground line.
Plasma Cutting Tips to Improve Results
If you consider a few tips and best practices when choosing and using a plasma cutter, you can improve results.
Tip 1: Choose the right plasma cutter
Some of the key factors to consider when choosing a plasma cutter are output power, cutting speed, input power, duty cycle, and weight and size. When choosing your machine, think about the tasks you perform most often.
- Output power: The power output required depends mainly on the thickness and type of material to be cut. Two standards: nominal and sectional cuts. A nominal cut is the thickness of soft metal that an operator can manually cut at a speed of 15 inches per minute (IPM). A heavy cut is a maximum thickness a plasma cutter can handle. The travel speed is slower and the cut may need cleaning.
- Cutting speed: This is usually reported as inches per minute (IPM). One machine that cuts 1/2-inch material can take five minutes while another machine takes one. The cutting speed makes a significant difference in production time.
- Input power: Do you always use the plasma cutter in the same place or do you need portability and the ability to use a variety of power sources? Look for plasma cutters that offer a range of performance options. Some can switch from 120 volts to 240 volts.
- Duty cycle: The duty cycle is the time a machine can cut off in a 10-minute cycle without overheating. If a machine’s duty cycle is 60%, the machine can run six out of ten minutes continuously and then has to cool down for the remaining four minutes. A larger duty cycle is important when making long cuts, when running high productivity applications, or when using the machine in a hot environment.
- Weight and size: If portability is your thing, there are many portable devices available that weigh less than 45 pounds.
Tip 2: Read the manual
Read the instruction manual carefully to become familiar with the safe and correct operation of your plasma cutter. This will help you optimize the capabilities of your plasma cutter and promote the safe use of the machine.
Tip 3: Pay Attention to setup
Attach the grounding clamp only to clean metal. If necessary, grind off any rust or paint, as these obstruct the flow of electricity.
Also, place the ground clamp as close as possible to the cut or, if possible, to the workpiece itself. Check your cables for worn spots, loose connections, or anything that could unnecessarily interfere with the flow of current.
To adjust the amperage or heat of the cutting unit to the correct level, perform some practice cuts with high amperage. You can then reduce the current according to your driving speed. If the amperage is too high or your driving speed is too slow, the material being cut can get hot and build up scabies.
Tip 4: Trace the path before cutting
Trace the path you want to cut without pulling the trigger. For long cuts, practice your movements before pulling the trigger to make sure you have enough freedom of movement to make a continuous cut. Stopping and restarting in the same place is difficult and usually results in irregularities in the cut edge.
You can also do a pattern cut with the same material that you will be working with. This way you will ensure that you are using the correct settings and cruising speed.
Tip 5: Use proper technique
Use your non-cutting hand as a support for your other hand. This stabilizes your cutting hand, allows freedom of movement in all directions, and helps maintain a constant 1/16 “to 1/8” spacing. Note that most people find it easier to pull a flashlight towards their body than to push it away.
Maintaining a gap of 1/16 “to 1/8” increases the cutting capacity of smaller machines and extends the life of consumables.
Use a tow guard if your machine is equipped with one. This allows you to rest the torch on the workpiece while maintaining an optimal distance without touching the metal with the tip, which negatively affects the quality of the cut and the life of the consumable.
Start cutting by placing the torch as close to the edge of the base metal as possible. Press the trigger to start the bias current. The pilot arc lights up, followed by the cutting arc. Once the cutting arc begins, slowly move the torch over the metal.
Adjust your speed so that the cutting sparks emerge from the bottom of the metal. At the end of a cut, angle the torch slightly towards the end of the cut or pause briefly to complete the cut. The wake air remains shortly after the trigger is released to cool the burner and the consumable parts.
Tip 6: Check Consumables
If the tip or electrode is worn or damaged, the quality of the cut will be affected. So check your consumables regularly. If the tip hole is irregular and/or covered with splashes, discard it. If the electrode tip forms a pit, discard it.
Consumables wear out with every cut, but factors such as moisture in the air supply, cutting excessively thick materials, or poor technology increase consumable deterioration. It is recommended that the tip and electrode be swapped together for the best possible cut quality.
Do not overtighten the consumable holder. The parts inside actually need to move (separate) to create an arc. So just use your finger to tighten the cup.
Tip 7: Watch the travel speed
The faster you go (especially with aluminum), the cleaner your cut will be. When cutting thicker material, set the machine to full power and vary your driving speed. With thinner material, reduce the amperage and switch to a tip with a lower amperage to create a narrow kerf.
With a reasonable travel speed, the arc should emerge from the material at an angle of 15 to 20 degrees against the direction of travel. If it goes straight down, it means you are moving too slowly. If it sprays back, it means you are moving too fast.
If you travel at the right speed and use the right amount of heat, you will get a very clean cut with less scabies on the underside of the cut and little or no distortion of the metal.
Tip 8: Follow safety procedures
For proper plasma safety, exposed skin must be protected. You will need welding gloves and a welding coat or other flame-retardant clothing. Button the shirt cuffs, pockets, and collar to prevent them from catching sparks.
Protect your eyes with the correct shadow lens for the plasma cutter you plan to use. The operating instructions indicate the required color for the current intensity. You can use conventional plasma cutting/oxy-fuel goggles or a welding helmet with cutting mode.
Safety procedures must be followed closely in every plasma cutting application.
Advantages of Plasma Cutting
- Able to cut all conductive materials. Flame cutting, though also suitable for cutting thick metals, is limited to ferrous metals only.
- Great quality for thickness up to 50 mm.
- Maximum thickness up to 150 mm.
- Comparatively cheap for medium-thickness cuts.
- Best way to cut medium-thickness stainless steel and aluminum.
- CNC machines are available to provide high precision and repeatability.
- Can cut in water, resulting in smaller HAZ. Also reduces noise levels.
- Smaller cutting kerf compared to flame cutting.
- Quicker cutting speed than oxyfuel.
Disadvantages of Plasma Cutting
- Larger HAZ compared to laser cutting.
- Quality with thinner sheets and plates not as good as laser cutting.
- Tolerances not as precise as laser cutting.
- Does not reach thicknesses like waterjet or flame cutting.
- Leaves a HAZ which waterjet does not.
- Wider kerf than laser cutting.