Four distinct cutting technologies--oxyfuel, plasma, waterjet and laser--offer value-adding versatility to service centers and processors By Kevin Woodward, ASSOCIATE EDITOR - METAL CENTER NEWS
Cutting metal is a difficult and time-consuming task, especially when the job calls for a tight tolerance or an unusual shape that can't be cut with a saw. In those cases, metal processors can choose from four cutting systems-oxyfuel, plasma, waterjet or laser-depending on which one can make a cut of the needed quality in a particular type of metal at the fastest speed.
Plasma arc cutting Quickly becoming the preferred shape-cutting technology among service centers and toll processors, plasma arc cutting uses a high-velocity jet of electrically charged gas to cut metal at up to 50,000 degrees Fahrenheit.
"Plasma arc cutting is a lightning bolt generator," says Chris Forman of Centricut, a plasma consumables supplier in Lebanon, N.H. It can be used to cut plate up to six inches thick. The basic components of the system include a torch, a DC power supply, a gas supply, a control system, a coolant system (air or water) for the torch and torch parts, and leads that connect the torch, power, gas and coolant.
Used solely on mechanized systems, water-shielded PAC tends to have a lower operating cost because water is cheaper than gas. Water shielding also reduces top-edge rounding and the amount of smoke and fumes.
Manufacturers report that service centers are casting a keen eye on high-tolerance PAC, sometimes known as high-definition or fine-plasma cutting. High-tolerance PAC is used for cutting metals from .20 gauge to 3/8-inch thick. This system uses a nozzle with a smaller orifice diameter so the flow rate of the spinning plasma gas is much higher. The cut quality is nearly as good as laser, but at a lower cost, Forman says.
Plasma advantages: Plasma is faster cutting than oxyfuel and can cut thicker materials. It is also less expensive than waterjet and laser. Nitrogen-based systems are well suited for high-performance materials such as stainless steel, aluminum and nickel. Oxygen-based systems are better for carbon products and leave no nitride deposits, which complicate further processing.
Plasma disadvantages: Plasma consumables can be short-lived depending on gas selection, operator proficiency and water selection. Heat-affected zones appear in the area surrounding the cut. Dross (resolidified metal that attaches to the bottom of the cut) can occur. Under water cutting helps reduce the size of the heat-affected zone.
Oxyfuel cutting At one time the only choice for cutting carbon steel, oxyfuel technology is still the predominant choice for cutting plate. Using gases, acetylene and oxygen, to produce a controlled flame, this technology cheaply burns through carbon steel and most alloys, producing near-net shapes with relative ease. But Oxyfuel cutting creates a heat-affected zone around the cut that must be removed by additional machining or through a post-cutting annealing process.
Oxyfuel advantages: Oxyfuel systems' multiple-torch capability is good for high production runs. Oxyfuel is well suited for cutting material that is to be machined. Oxyfuel disadvantages: Oxyfuel is slower than other cutting systems and leaves a large heat-affected zone. Cut tolerances typically are not as tight as with plasma, waterjet or laser.
Laser Cutting A laser beam is a high-intensity beam of light that can be tightly focused onto a spot only 0.005 inches in diameter. Laser light is produced by passing electrical energy through a lasing medium. In gas lasers, CO2 is mixed with helium and nitrogen to make the lasing medium. In solid-state lasers, yttrium-aluminum-garnet (YAG) crystals containing neodymium ions are used as the medium. Laser processing systems are made of five basic components: the laser, beam-focusing optics, material-handling system, heat exchanger and the control computer. The heat exchanger cools the laser and optical components.
Laser advantages: Lasers produce the most precise cut with the smallest heat-affected zone of the thermal cutting technologies. They can cut up to one-inch thick carbon products and half-inch thick specialty metals. Consumables usage is less than with plasma or oxyfuel. Laser disadvantages: Laser systems typically cost more than plasma, oxyfuel and waterjet systems. They can handle material of limited thickness and may have difficulty cutting reflective material.
Abrasive waterjet cutting A waterjet machine pressurizes a stream of water to between 36,000 and 60,000 pounds per square inch, powerful enough to cut through many metals. Abrasive waterjet systems inject an abrasive, usually garnet, into the stream as it leaves the nozzle. This combination can cut a wider variety of materials than plain water. The water-jet head is often submerged in a water table to dissipate the noise. Waterjet advantages: Waterjet systems are usually less costly than laser machines and create virtually no heat-affected zone. They are well suited for high-performance metals.
Waterjet disadvantages: Waterjet technology has a higher entry cost than plasma or oxyfuel machines and can be slower cutting. Deciding Which System Makes the CutMaximizing the productivity of cutting systems is a challenge because so many variables come into play: How many different types of metal must be cut? How quickly must the cuts be made? How tight are the tolerances on the cuts? Must the edges be clean or will the scale and dross of a heat-affected zone be acceptable for the application? Is the speed of a plasma cutter or the precision of a laser cost-justifiable compared to the utility of an oxyfuel system, and so on?In some cases, the best system for an operation is one that combines cutting technologies. Nivert Metal Supply, Throop, Pa., recently added an MG-brand cutting machine with four oxyfuel and two plasma torches, which can cut material up to 8 feet wide and 20 feet long. It supplements the 8-head machine (7 oxyfuel torches and 1 plasma torch) already in service. The new machine will handle more precise cutting while the older machine will handle simple burning. "The MG picked up our cutting time and increased the range of materials we can cut," says Gus Griffiths, a salesman at Nivert. "The old way was to burn it with oxyfuel and then saw cut it. Plasma does that in one step. We can cut quicker with two plasma torches than with seven oxyfuel torches, up to one inch thick. It's a lot cheaper and faster." Nivert's MG system cost about $500,000, owing to the multiple heads and other options.Nivert's decision to add a machine with both types of torches follows the advice of Gavan Farley, president of Farley Cutting Systems, Willowbrook, Ill. "Work out which technology is most appropriate for you," he says. "No one system fits every situation. You can buy combination machines."Plasma arc cutting is quickly becoming the preferred choice for many service centers.
"We want to use plasma machines to cut bigger pieces faster and increase our productivity," says John Klabacha, vice president of operations at Central Steel & Wire, Chicago. Klabacha's staff is evaluating plasma machines to supplement the company's six oxyfuel machines.Marty Ceccardi, operations manager at Heflin Steel, Phoenix, which runs four FARLEY plasma machines and two oxyfuel burners, suggests buying a machine with a slightly larger cutting capacity than necessary. "If you typically cut 1-inch thick material, you should buy a machine that cuts 1.5 inches, so it will cut the 1-inch material with ease," he says.Ceccardi also looks for systems that are easy to learn and to operate. Be wary of manufacturers' claims of cutting speed, he says, which often don't take into account the other tasks associated with a cutting operation. In Heflin Steel's experience, a good plasma operator might get three arc hours in an eight-hour shift, even with someone else assisting with material handling.Don't expect the machine to cut 100 percent of each shift, Farley agrees. "Of the time the machine is staffed, only about 25 to 30 percent of it is spent converting parts."
Management of the machine, including scheduling, setup, material handling and maintenance account for the remainder of the time. Waterjet vs. laserThe cost of a waterjet system is largely attributable to its power and the accuracy o f its control system. The more powerful and precise, the more expensive it is. Therefore, service centers should analyze their needs first to be sure they don't spend money buying a system with more power or more accuracy than they really need."Cutting simple shapes out of two-, three- or four-inch steel will lead you to consider the power of the machine," says Laird Parry, international sales manager at OMAX Corp., an Auburn, Wash., waterjet manufacturer. "On the other hand, if you're cutting one-inch thick or less and making intricate parts, then table precision and the control system become more important."For thicker material, Parry recommends an abrasive waterjet system, which costs more than a conventional torch table, but about half that of a comparable laser machine. (One waterjet manufacturer says his machines cost between $100,000 and $250,000, but can go up to as much as $3 million, depending on the size and components.) About 60 percent of the operating cost of an abrasive waterjet is the garnet that is injected into the water stream, says Michael Ruppenthal, marketing manager at Flow International, Kent, Wash.
This fall, his company plans to introduce machines with a garnet recycler that recovers about half the abrasive for reuse.The waterjet machine installed recently at TW Metals in San Francisco has been a big hit with customers, says Barry Ronsheimer, area general manager. "We got it because our customers asked us to supply process plate closer to the finished size. In a lot of cases we're being asked to supply the finished parts," Ronsheimer says. The price per piece is high, he adds, but it's worth it for the customer willing to consider the total cost of the part.Ronsheimer advises service centers to experiment with the machine's capabilities. Examine what happens when cutting speed, orifice and mixing tube sizes are adjusted and different grits of garnet are used. He also suggests making the training investment. 's not as easy as programming the machine, pushing a button and it's done. You need someone with a good working mechanical knowledge."Easily the most precise of the four cutting methods, laser cutting is not for every service center, no matter how attractive the high-tech equipment may appear.Because of the high initial cost, laser-cutting machines must be evaluated on the total finished cost to make a part.
If a part is cut with plasma and requires machining to get it at the customer's tolerance and finish requirements, it may be less costly to cut with a laser machine and get the needed edge in one step. Sustainable tolerances and small heat-affected zones are key advantages to a laser-cutting machine, says Jeff Defalco, laser product manager for ESAB Welding & Cutting, Florence, S.C. "With laser cutting, you have a more consistent process. You can practically turn out the light and walk away while the machine works." In addition, consumable cost is lower with laser, compared to plasma, he says. Another operating cost issue associated with laser machines is the amount of gas and power they consume. The higher the wattage of the laser, the thicker the material it can punch through. But as the power increases, the cut quality may decline slightly because the beam tends to change shape.
