How fast and thick a plasma arc cutter works depends upon a few variables, some of which affect cutting speed and depth. Learn how these machines work to operate them properly and get the most out of them.
A supervisor from an insurance company called me recently about replacing a rear body panel on a vehicle. He wondered why it took the body shop 22 drill bits and three days to remove the panel. I asked if the car was a 2004 or newer Volvo XC90. He responded, "Yes" and asked me how I knew what 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. (See I-CAR's Advantage article April 22, 2005—go to www.i-car.com and click on technical information). When he replied, "What is boron steel?" I proceeded to tell him about advanced steel with boron and explained that the most effective way to remove it from a damaged vehicle was to use a plasma arc cutter or a three-fluted drill bit designed for boron steel. He then asked, "What's a plasma arc cutter?" At this point I realized some detailed explanation was necessary. So here, in a nutshell, is everything I can share about "Plasma Arc Cutting" starting with the plasma process.What's the matter?
Physical matter can exist in four states, and I am not talking about states as in the United States of America. I'm talking about physical property states: 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. At 32 degrees Fahrenheit water freezes. 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 a 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.
In layman's terms, plasma occurs when gas reaches a super hot temperature. According to the Web site science.howstuffworks.com/plasma-cutter3.htm, 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."
The difference between plasma and gas is that plasma can conduct electricity and this physical property is the basis of a plasma arc cutting. Let's see how a plasma arc cutter works in simple terms that we can all understand.
Pressurized gas, compressed air or nitrogen (best gas for cutting stainless steel) flows into the upper chamber of the torch. In the center of the torch is located the electrode (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, which 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 opens with force 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.
How fast and thick the plasma arc cutter works depends upon a few variables. Some of the variables that affect cutting speed and depth depend upon the voltage, amps, air pressure, nozzle size and metal thickness. Most plasma arc cutting manufacturers have both 110-volt models as well as 220-volt models.The next generation of plasma cutters is now using inverter technology. To help reduce the number of variables, I recently looked at machines that were rated to cut ⅜-inch thick metal at 10 inches per minute. I looked at these machines because I cannot think of any part on today's cars or light trucks (one ton or less) that a tech would encounter that would be that thick. Nearly all of the 120-volt machines will cut a ⅛-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 fingertips.
One more opinion when selecting a machine – I'd stay away from any high frequency or capacitor discharge units. They will play havoc on your company's phone and computer systems, as well as the nearby vehicle's computers.
Speaking of tips
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 increases the velocity of the plasma arc stream. The torch tips are made from copper and are considered a consumable item. In other words, they wear out with use. 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. First, the gas needed 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 that criteria for plasma arc cutting are Nitrogen, bottled compressed air or just good old shop air.
If you're 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, suggests that to get the most life out of the torch, install a 5-micron filter. The filter comes with all the necessary brackets and fittings to work on any plasma arc cutting machine on the market.
Some machines have internal compressors, but I would suggest 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.
Cutter operation
Now that you have a thorough understanding of the physics and components of plasma arc cutting, let's move on to operating such a machine.
The first step in starting the plasma arc cutting process is to hook up the unit to power and air supplies. Step two is to place the work clamp as close as possible to the cut area. 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 (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 researching this article. Dross is the oxidized metal that melts during the cutting operation and 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 warpage and small amount of dross.
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 had a drag shield. If you are not using a drag shield, then you will have to use a deflector shield and the stand off (distance from the tip to the work space) is 11/416th of an inch. Pull the trigger and an arc is struck. 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 your gun angle (the cutting stream needs to be at 90 degrees to the surface of the metal).
If the plasma arc cutter is working properly and you are now approaching the end of the cut, there are a couple ways to end the process. You can briefly pause at the end of the cut to make sure the metal is completely severed, or you can angle the arc stream (move 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. You are probably wondering if you can start a cut in the center of base metal to create a hole — and yes, you can; it's called piercing.
Piercing is the process of producing a hole in the base metal. If you were replacing the rear bumper reinforcement on a Volvo XC90 and didn't have an STRS welder, you'd need to have 8mm holes in the part for MIG welding plugholes. The problem is you cannot punch it or drill it (remember, it's made of advanced steel with boron). You can use the plasma arc cutting machine to cut the 8mm holes in the panel. Let's see how this is done.
First, hold the gun at about a 45-degree angle (remember when we cut we were at a 90-degree angle). Depress the trigger and strike an arc. At the beginning, the molten steel will "blow out" the side instead of back into the gun. Next, 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 you have perfected the technique, the process goes quickly.
Finally, we need to consider the safety aspects when cutting with a plasma arc cutting machine. 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 you should wear a welding jacket as the best protection along with a good pair of MIG welding gloves).
A full-face shield with a number five or six 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.
It's also important to remember that as with all welding, proper ventilation and a welding respiration unit is a must.
Finally, read the instruction manual supplied with your machine.
I hope that after reading this article you now have enough information about how these machines work to know what you are looking for when shopping around and how to safely and correctly operate them.
