Plasma Arc Cutting PAC

Objectives Define plasma arc cutting (PAC). Explain how a PAC cutter operates. Identify the parts of a PAC cutter. Explain advantages and disadvantages of the PAC system. Identify materials that can be cut with the PAC. Explain safety associated with using the plasma arc cutter.

History The plasma-arc process had its origin almost 50 years ago, during the height of World War II. Plasma cutting was accidentally discovered by an inventor who was trying to develop a better welding process. In an effort to improve the joining of aircraft materials, a method of welding was developed that used a protective barrier of inert gas around an electric arc to protect the weld from oxidation.

History It was discovered that by restricting the opening through which the inert gas passed, the heat produced by the process was greatly increased. At the same time, the smaller opening caused the flow of gas to speed up dramatically, ultimately blowing out a channel in the work. The plasma-arc cutting process started seeing commercial use in the first few years of the sixties. It was an extremely expensive process to undertake, and most cutting was performed by large burning services.

4th State of Matter

Plasma Plasma has two meanings. The fluid portion of blood. A state of matter that is found in the region of an electrical discharge (arc). Plasma created by an arc is an ionized gas that has both electrons and positive ions whose charges are nearly equal to each other. Plasma is present in any electrical discharge.

Plasma Plasma consists of charged particles that conduct the electrons across the gap. Both the glow of a neon tube and the bright fluorescent light bulb are examples of low-temperature plasmas. Plasma results when a gas is heated to a high enough temperature to convert into positive and negative ions, neutral atoms, and negative electrons. The temperature of an unrestricted arc is about 11,000°F The temperature created when the arc is concentrated to from a plasma is about 23,000°F.

Machines Most, if not all, of the light portable plasma cutters are 110 volt machines. Suited primarily for cutting sheetmetal and other light work. The next level up are the 220 volt machines with 50 to 80 amp output current. These are portable from the standpoint that one person can put it on a truck and take it to the job.

How PAC works Plasma cutters work by sending an electric arc through a gas that is passing through a constricted opening. The gas can be shop air, nitrogen, argon, oxygen. etc. This elevates the temperature of the gas to the point that it enters a 4th state of matter. Scientists call this additional state plasma. As the metal being cut is part of the circuit, the electrical conductivity of the plasma causes the arc to transfer to the work. The restricted opening (nozzle) the gas passes through causes it to squeeze by at a high speed. This high speed gas cuts through the molten metal. The gas is also directed around the perimeter of the cutting area to shield the cut.

How a Plasma Cutter works A complete plasma cutter consists of a power supply, a ground clamp, and a hand torch. The main function of the power supply is to convert the AC line voltage into a user-adjustable regulated (continuous) DC current. The hand torch contains a trigger for controlling the cutting, and a nozzle through which the compressed air blows. An electrode is also mounted inside the hand torch, behind the nozzle.

PAC System

Operation Initially, the electrode is in contact with (touches) the nozzle. When the trigger is squeezed, DC current flows through this contact. Next, compressed air starts trying to force its way through the joint and out the nozzle.

Operation Air moves the electrode back and establishes a fixed gap between it and the tip. The power supply automatically increases the voltage in order to maintain a constant current through the joint - a current that is now going through the air gap and turning the air into plasma. Finally, the regulated DC current is switched so that it no longer flows through the nozzle but instead flows between the electrode and the work piece. This current and airflow continues until cutting is halted.

Starting the Arc In many of today's better plasma cutters, a pilot arc between the electrode and nozzle is used to ionize the gas and initially generate the plasma prior to the arc transfer. Other methods that have been used are touching the torch tip to the work to create a spark, and the use of a high-frequency starting circuit.

PAC versus Oxy-Fuel In general, fabricators consider oxy-fuel to be superior to plasma for cutting steel when thicknesses exceed about 1/2 inch. This is because of the slight bevel (4 to 6 degrees) in the cut face that plasma produces. It is not noticeable in thinner materials, but becomes more so as thicknesses increase. Also, at thicknesses above 1/2 inch, plasma has no cutting speed advantage over oxy-fuel.

PAC versus Oxy-Fuel If you are planning to cut non-ferrous metals such as stainless or aluminum, which cannot be cut by oxy-fuel, consider a 50 to 80 amp, 220 volt plasma cutter. Plasma cutting is by far the simplest and most economical way to cut a variety of metal shapes accurately. Plasma cutters can cut much finer, faster, and more automatically than oxy-acetylene torches.

PAC Cutting Examples

Plasma Torch A device depending on its design, which allows the creation and control of the plasma for welding and cutting processes. Plasma torch supplies electrical energy to a gas to change it into the high energy state of a plasma

Torch Body Made of a special plastic that is resistant to high temperatures, ultraviolet light, and impact. Provides a good grip area and protects the cable and hose connections to the head. Torch body is available in a variety of lengths and sizes.

Torch head Torch head is attached to the torch body where the cables and hoses attach to the electrode tip, nozzle tip, and nozzle. Torch and head may be connected at any angle such as 90°, 75°, 180° (straight), or it can be flexible. Because of the heat in the head produced by the arc, some provisions for cooling the head and its internal parts must be made. Cooling for low power torches may be either by air or water. High power torches must be liquid cooled. It is possible to replace just the torch head on most torches if it becomes worn or damaged.

Power switch Manual power switch used to start and stop the power source, gas, and cooling water. Thumb switch on the torch body most often used. Foot control can be used.

Common Torch Parts Parts of the torch Electrode Tip nozzle insulator Nozzle tip Nozzle guide Nozzles and the metal parts are usually made out of copper, and they may be plated. The plating of copper parts will help stay spatter-free longer.

PAC

Electrode tip Electrode tip is often made of copper electrode with a tungsten tip attached. Use of copper/tungsten tip has improved the quality of work they can produce. By using copper, the heat generated at the tip can be conducted away faster. Keeping the tip as cool as possible lengthens the life of the tip and allows for better quality cuts for a longer time. Old torches you must grind the tungsten electrode

Nozzle Insulator Located between the electrode tip and the nozzle tip Provides the critical gap spacing and electrical separation of the parts. The spacing between the electrode tip and nozzle tip called the electrode setback is critical to the proper operation.

Nozzle tip Has a small, cone-shaped, constricting orifice in the center. The electrode setback space, between the electrode tip and nozzle tip is where the electric current forms the plasma. The preset close-fitting parts provide the restriction of the gas in the presence of the electric current so the plasma can be generated. The diameter of the constricting orifice and electrode setback are major factors in the operation of the torch. As the diameter of the orifice changes, the plasma jet action will be affected. When the setback distance is changed, the arc voltage and current flow will change.

Nozzle Sometimes call the cup. Made of ceramic or any other high-temperature resistant substance. Helps prevent the internal electrical parts from accidental shorting and provides control of the shielding gas or water injection if they are used.

Water shroud Water shroud nozzle may be attached to some torches. Water surrounding nozzle tip is used to control the potential hazards of light, fumes, noise, or other pollutants produced.

Power Requirements Requires a drooping arc voltage or constant current, direct current, high-voltage, power supply. Drooping arc voltage allows for a rapid start of the plasma arc at the high open circuit voltage and more controlled plasma arc aas the voltage rapidly droops. Ranges from 50-200 volts closed circuit Ranges from 150-400 volts open circuit.

Amperages High voltage Amperage range from 10-200 amps. Some automated machines may have 1,000 ampere capacities. Higher the amperage capacity the faster and thicker they will cut.

Cutting speeds High cutting speeds are possible Up to 300 inches per minute 25 feet a minute ¼ mile an hour

Metals to be cut Any material that is conductive can be cut using the PAC process. In a few applications nonconductive materials can be coated with conductive material so that they can be cut. Most popular materials cut Carbon steel up to 1” Stainless steel up to 4” Aluminum up to 6” Other metals commonly cut Copper Nickel-alloys High-strength, low alloy steels Clad materials Expanded metal

Starting Methods Two methods are used to establish a current path through the gas High frequency alternating current Momentary shorting

High frequency alternating current Most common Uses a high frequency alternating current carried through the conductor, the electrode and back from the nozzle tip. High frequency current will ionize the gas and allow it to carry the initial current to establish a pilot arc. After the pilot arc has been started, the high frequency starting circuit can be stopped. When the torch is brought close enough to the work, the primary arc will follow the pilot arc across the gap, and the main plasma is started. Once the main plasma is started, the pilot arc power can be shut off.

Momentary shorting Requires the electrode tip and nozzle tip to be momentarily shorted together. This is accomplished by automatically moving them together and immediately separating them again.

Safety Electrical shock Moisture Because the open circuit voltage is much higher for this process than for any other, extra caution must be taken. The chance that a fatal shock could be received from this equipment is much higher than from any other welding equipment. Moisture Often water is used with PAC torches to cool the torch, improve the cutting characteristics, or as a part of a water table. Any time water is used it’s very important that there be no leaks or splashes. The chance of electrical shock is greatly increased if there is no moisture on the floors, cables, or equipment.

Safety Noise Light Fumes Because the plasma stream is passing through the nozzle orifice at a high speed, a loud sound is produced. The sound level increases as the power level increases. High levels of sound can have a cumulative effect on one’s hearing. Light PAC produces light radiation in all three spectrums. Large quantity of visible light, if the eyes are unprotected, will cause night blindness. Most dangerous of the lights is ultraviolet. This light can cause burns to the skin and eyes. Infrared can be felt as heat, and it is not as much a hazard. Fumes PAC produces a large quantity of fumes that are potentially hazardous. A specific means for removing them from the work space should be in place.

Safety Gases Sparks Some of the plasma gas mixtures include hydrogen. Hydrogen is a flammable gas. Make sure that the system is leak-proof. Sparks Danger of accidental fire is present. Use a fire watch person if excessive sparks are present.