Tuesday, 01 May 2007 16:29

Plasma cutting in a nutshell: a step-by-step guide to the process and the materials needed

Written by Toby Chess
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I asked the supervisor if the car that he was inquiring about was a 2004 or newer Volvo XC90, to which he responded,

"Yes!” He was curious how I knew which car he was asking about. I explained that two Volvo models were using advanced steel allied with boron on the rear body panels and rear bumper reinforcements. (A related article is available online at www.i-car.com. Click on technical information for I-CAR Advantage article dated April 22, 2005.)
    “What is boron steel?” he asked. I told him about advanced steel with boron and that the most effective way of removing it from a damaged vehicle was to use a plasma arc cutter or a 3-fluted drill bit designed for boron steel, available at Dent Fix Corp. This drew yet another question. “What is a plasma arc cutter?” he asked. At this point I realized I had another idea for an article. So here is everything in a nutshell about plasma arc cutting beginning with the plasma process.
The four states of matter
    Physical matter can exist in four states, and I am not talking about states as in the United States of America. I am referring to physical property states of solid, liquid, gas and plasma. To change from one state of matter to another, energy is added or subtracted from the matter.
    To get a better understanding of matter, let’s look at water. Water freezes at 32 degrees Fahrenheit. This is its solid state. Energy in the form of heat is added to the frozen water and as the temperature rises water changes from its solid state to a liquid state. If additional heat is applied and the temperature is elevated to 212 degrees Fahrenheit, the liquid water changes to steam or a gas.
    The fourth state of matter is plasma, but what the heck is it?  It is defined as “an assemblage of charged particles called electrons and ions that react collectively to forces exerted by electric and magnetic fields.” If you can understand that definition, maybe you should get out of the collision repair business and enroll at MIT.
Electrons separate from nucleus
    In layman’s terms, plasma occurs when gas reaches an extremely hot temperature. 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,” according to www.howstuffworks.com/plasma-cutter3.htm.
How a plasma arc cutter works in simple terms
    The difference between plasma and gas is that plasma can conduct electricity and this physical property is the basis of plasma arc cutting. Pressurized gas, compressed air or nitrogen, which is the best gas for cutting stainless steel, flows into the upper chamber of the torch. The electrode is located in the center of the torch. Note that the electrode is the negative part of the electrical circuit that will be created. When the tip of the gun touches the metal that is to be cut, an electrical circuit is established. A powerful spark is produced in the chamber and this spark heats up the compressed air to an extremely high temperature. This generates the plasma gas.
    The plasma expands in volume and pressure - up to 30,000 degrees Fahrenheit and 20,000 ft/sec - and is directed in a controlled beam to the metal that is to be cut. The metal heats up to a molten state and the pressurized gas forces open a hole that is the start of the cut line. The gun is moved in a path away from the hole and the metal is severed or cut. The speed and depth at which the plasma arc cutter works depends on a few variables.
Speed and thickness variables
    Some of the variables that affect cutting speed and the depth of cut depend upon the voltage, amps (110 volts with produce up 27 amps), air pressure, nozzle size, and the metal thickness. Most plasma arc cutting manufacturers have both 110 volt models as well as 220 volt models. Miller Electric’s 375X-TREME has multi-plugs that can be switched from 120 to 220-volts in a matter of moments, depending on the type of receptacle. I have outlined all the major manufacturer’s machines in a comparison chart at the end of this article.
    The next generation of plasma cutters is now using inverter technology. To help reduce the number of variables, I will look at machines that are rated to cut 3/8-inch thick metal at 10 inches per minute. First and foremost by using the 3/8-inch thickness, I cannot think of any part on today’s cars or light trucks (one ton or less) where a tech would encounter a piece of metal that would be that thick. Nearly all of the 120-volt machines will cut a 1/8-inch thickness, but you may have an occasion to cut a thicker piece of metal and it would be nice to have those capabilities at your fingertip. One more item when selecting the right machine is to stay away from any high frequency or capacitor discharge units. They will play havoc on your company phone and computer systems as well as the nearby vehicle’s computers. {mospagebreak}
Torch tips

    Speaking of tips, let’s look at the torch tip. The torch tip is designed to focus and constrict the plasma arc beam. By constricting the beam, the plasma arc increases the amount of energy, which translates into more heat and it also increases the velocity of the plasma arc stream. The torch tips are made from copper and they are considered a consumable item. In other words, they wear out with use. Finally, when replacing a worn out tip, make sure that it is the correct size for the amperage of the machine. The last variable that we will look at is the gas.
Gas should be nitrogen, bottled compressed air or shop air
    First, the gas need for plasma arc cutting has to have the ability for “high ionization” energy, high thermal conductivity and a high atomic weight to push or blow out the molten metal from the cut site. The most common gases that fit the criteria for plasma arc cutting are Nitrogen, bottled compressed air or just good old shop air. If you are going to use shop air, it should be clean and dry. Moisture in the air will cause a premature wearing of the consumables (electrode, rings and nozzle) in the torch.
    My good friend and welding guru, Jody Valentin, owner of Welding Machine & Torch Repair in San Antonio, Texas, suggested that to get the most life out of the torch is to install a 5-micron filter. Motor Guard USA makes a filter that works extremely well. It (Model M26) comes with all the necessary brackets and fittings to work on any plasma arc-cutting machine on the market.
Internal compressor not recommended
    Some machines have an internal compressor, but I would suggest that you stay away from them. Their primary use is portable cutting, in other words, cutting out in the back 40 (away from any air supply.) These units are more expensive than their non-internal compressor counterparts and a lot heavier too. Again, my chart will only deal with units that do not have internal compressors.
Machine operation

    Now that you have a thorough understanding on the physics and components of plasma arc cutting, let’s move on how to operate it. The first step in starting the plasma arc cutting process is to hook up the unit to power and air supplies. The next step is to place the work clamp as close as possible to the cut area. Then you should make some test cuts to determine the proper level of the amps or volts needed for the cutting process.
    During the test phase of cutting, the voltage should be set as high as possible to make the cut followed by reduction in the voltage to accommodate your travel speed. More power is better, except when you need a precision cut or narrow cut, also known as a kerf.
    If you use amperage that is too high or your travel speed is too slow, the base metal will get way too hot, which will lead to a buildup of dross, this is called low-speed dross. I will bet you have never heard of dross. Well to tell you the truth, I never heard of the word dross either until I began to research this article. Dross is the oxidized metal that melts during cutting operation that sticks to the bottom of the cut metal. If you set the machine at the best possible amps and at the right travel speed, you will get a very clean cut with little metal warp and a small amount of dross.
The initial cut
    The next step is the initial cut. On a straight cut, you should start the cut as close as possible to the edge of the base metal. You can place the gun on the metal if your gun has a drag shield (see photo). If you are not using a drag shield, then you will have to use a deflector shield (see photo) and the stand off, which is the distance from the tip to the workspace, or 1/16 of an inch. Pull the trigger and an arc is stuck.
    Begin by moving the gun and you should see sparks emerging from the bottom of the metal. If, by chance you can’t see the sparks, you probably have not penetrated the metal. You may have to increase the amps, reduce the travel speed, check the ground on the work clamp or check you gun angle. The cutting stream needs to be 90-degrees from the surface of the metal.
End of the cut
    If the plasma arc cutter is properly working and you are now approaching the end of the cut, there are a couple of ways to end the process. You can briefly pause at the end of the cut to make sure that the metal is completely severed or you can angle the arc stream by moving the stream away from 90-degrees to “blow out” the edge of the base metal. One note—there will be air continuing to flow out of the gun after you release the trigger at the end of the cut—this airflow is for cooling down the gun parts.
Producing a hole
    You are probably wondering if you can start a cut in the center of the base metal to create a hole and the answer is yes. It is called piercing. Piercing is the process of producing a hole in the base metal. If you were replacing rear bumper reinforcement on a Volvo XC90 and did not have STRS Welder, you would need to have 8-mm holes in the part for MIG welding plugholes. The problem is that you cannot punch it or drill since it is made of advanced steel with boron. You can use the plasma arc cutting machine to cut the 8-mm holes in the panel. Let’s see how this is done.
    First, hold the gun at about a 45-degree angle, remember that when we cut we are at a 90 degree angle. Depress the trigger and strike an arc. In the beginning, the molten steel will “blow out” the side instead of back into the gun. Next, you will move your gun angle to 90-degrees and move the gun in a circular motion to create a hole in the metal. It will take a little practice, but once perfected, it goes quickly.
Safety aspects
    Remember that you are using a high-voltage machine, so make sure that standard precautions are observed. Check for loose or broken wires and cracked torches. Proper welding protection garments need to be worn. Long sleeve shirts are a minimum, but for the best protection you should wear a welding jacket along with a good pair of MIG welding gloves. A full-face shield with a number 5 or 6 shade rating is recommended. If you are cutting on a vehicle with a plasma cutter, you will have to remove a lot of parts for access as well as for fire protection.
    A safety note—sparks fly great distances with plasma arc cutting. I got nailed by some hot molten sparks 15-feet from the tech who was using the plasma arc cutting machine on the Volvo in the above photograph. Moreover, remember that as with any welding, proper ventilation and a welding respiration is a must. Finally, read the instruction manual supplied with your machine.
    I hope that after reading this article that now have enough info on purchasing a new machine as well as on its safe and correction operation. Happy cutting.

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