Plasma Cutting Overview and Tips
Plasma arc cutting is a process where an open arc, much like in TIG welding, can be constricted by passing through a small nozzle, or orifice, from the electrode to the work-piece. The gas used, typically air, combines with an electrical current to create a high temperature plasma arc. When placed in contact with an electrically conductive material, the arc passes through the metal, melting a thin area. The force of the arc pushes the molten metal through the work-piece and severs the metal.
Plasma cutters are used for cutting a wide variety of metals and are extremely popular for cutting operations and automotive work. The following guide will review the applications for plasma cutting as well as suggest some tips.
Why Use a Plasma Cutter
Plasma cutting provides numerous advantages over oxy-fuel. It cuts faster; does not require a pre-heat cycle; produces a small and more precise kerf (the width of the cut), and affects a smaller section of the metal being cut, which prevents the surrounding area from warping or damaging the paint. The arc for plasma cutting is around 20,000 degrees F, while the flame from oxyacetylene is a typically around 6000 degrees F.
The Plasma process also cuts any type of electrically conductive metal (the oxy-fuel process cannot cut stainless steel or aluminum). Plasma cutting is a cleaner, more convenient method of metal cutting because dry air is used for most plasma cutting applications. Some high end plasma cutters use nitrogen gas instead of oxygen.
Projects such as hole piercing and cutting stacked materials are also ideal for plasma cutters because the heat is concentrated in such a compact area.
Safety: Greater Heat Means Greater Hazards
With the amount heat produced by a plasma cutter, the risk of electric shock is much greater. While cutting, wear gloves and handle the torch according to the manufacturer's guidelines. Sparks from plasma cutting can do a lot of damage very quickly, causing severe burns. Therefore, make sure you wear appropriate clothing to cover your skin, as well as a helmet that protects your eyes from harmful rays and sparks.
Before starting, make sure you have a good, solid grounding according to the National Electrical Code (NFPA 70) and to OSHA standards. Make sure you are not wearing anything synthetic and create a 35-foot safety zone around your cutting area that is free of flammable materials. If possible, stand on a rubber mat or piece of plywood to keep yourself grounded--keeping in mind that either could catch a spark and begin to burn. In addition, any damaged wires or other parts must be repaired immediately on a plasma cutter.
Some users prefer plasma cutters over oxy-fuel cutting setups because oxy-fuel is highly flammable, and there is also a chance of a flashback while cutting. Though plasma cutters carry the risk of a fatal electric shock, the chance of an explosion is far less likely. Each user will have to weigh the pros and cons of plasma vs. oxy-fuel cutting since both present their own hazards and challenges.
Practical Concerns with a Plasma Cutter
Electrical Concerns: There's no use in buying a tool that you can't use at home or in your shop. Plasma cutters with a 27 amp output can typically be plugged into a 110V outlet and will have a limited cut rate in the 1/8" to 1/2" range. Machines for cutting a thicker range of metals will run at higher amperages through a 220V outlet which will have to be installed if your home or shop does not have one.
Running a Plasma Cutter off a Generator: It is possible to run some plasma cutters off a generator if you don't have the right power set up. However, make sure that your cutter is able to handle the dirty power that generators produce, which means that the voltage fluctuates. If your plasma cutter is not equipped to handle these fluctuations, there's a chance your machine could shut down in the middle of a job or even break. Your plasma cutter will need Line Voltage Compensation (LVC) in order to operate properly.
Plasma Cutting Tips
Torch Position: The torch should remain at a 90-degree angle and depending on the machine you can keep a standoff or drag either the tip or the drag shield on the metal.
Settings: There are only two settings on a plasma cutter: the on/off switch and the amperage control. The amperage is adjusted depending on the thickness of the metal being cut, though some operators recommend simply leaving the amperage switch at the highest level.
Compressed Air for Plasma Cutting: Each plasma cutter will specify a particular air pressure in order to cut at certain levels. The manual will specify what each machine needs. The only other gas commonly used is nitrogen for particularly clean cuts on stainless steel.
Before Cutting: The shield cup, cutting tip, and electrode should be inspected prior to cutting. Also make sure all of the lines to the plasma cutter are in excellent shape--always replace a damaged plasma cutter cord. Avoid hitting the torch on a hard surface in order to remove spatter. In addition, the consumable will last longer if the plasma cutter is not restarted frequently while in use.
Operating A Plasma Arc For Cutting: Start cutting metal by placing the plasma cutter torch as close to the edge of the metal workpiece as possible. by pressing the trigger, the preflow air will begin, which will be followed by the lighting of the pilot arc, and then the cutting arc. While cutting, move the torch at a slow, steady rate and make sure that the sparks fall from the bottom of the metal.
If you don't see sparks falling through the metal, this means that the metal is not being cut all of the way through. Make sure there is enough amperage, that your torch is angled properly, and that the torch is not traveling too fast.
In order to finish a cut, angle the torch slightly toward its end or pause briefly at the end of the metal to create a clean cut. Don't forget that some air will continue to pass through the torch for a short period of time after releasing the trigger to the torch.
Operating A Plasma Arc For Gouging: A plasma cutter can be used to gouge metal by holding the torch at a 40 degree angle relative to the base metal. Begin by pressing the trigger for the pre-flow air and pilot arc. The arc should be formed slightly away from the workpiece in order to avoid gouging too deep into the metal, though the arc length and travel speed will have to be adjusted as needed.
Operating A Plasma Arc For Piercing: In order to pierce a hole with a plasma cutter, hold the torch once again at a 40 degree angle to the workpiece. Press the trigger on the torch and then bring to the torch to a 90 degree angle when the arc forms in order to pierce the work piece.
The Disadvantages of Plasma Cutting
Besides the risk of a fatal shock, there is a significant upfront investment for a plasma cutter. Plasma cutters range anywhere from $1100 to $2000, with plenty of more expensive models as well. However, once you've made that purchase, you need to only replace your tips and consumable electrodes for $11 per pair.
For those who do a lot of cutting, plasma cutting's steep up front costs may pay off when compared to an oxy-fuel setup that costs around $500 for getting started and then regular replacement of tips ($5) and gas tank refills ($30). In addition, the versatility and speed of plasma cutting may make a plasma cutter a worthwhile investment.
Plasma Cutting Resources
- Learn About Plasma Cutters at Baker's Gas and Welding Supplies
- Basic Welding Equipment and Gear
- Cutting with Alternative Fuels
- How to Choose a Plasma Cutter
- Plasma Cutting Tips
- Plasma Cutting for Home Repair and Hobbies
- Plasma Cutting Safety
- Plasma Cutting Compared to Oxy-Fuel
Written Exclusively for Baker's Gas and Welding by Ed Cyzewski