Plasma cutting boosts maintenance productivity
Research into and experience with air plasma cutting has led many companies to the conclusion that it offers a reliable, efficient, high-quality, low-cost method of metal cutting.
Research into and experience with air plasma cutting has led many companies to the conclusion that it offers a reliable, efficient, high-quality, low-cost method of metal cutting. The use of plasma cutting for production and maintenance has expanded greatly in recent years in both the number of industries and variety of applications where it is used. This trend has been driven in large part by the advances in solid-state electronics technology that have affected the plant in areas ranging from lighting to motor speed control.
The origin of plasma arc cutting dates back to World War II. As part of the war effort, the U.S. government searched for ways to improve the welding process. This fundamental research resulted in a new process that used electric current surrounded by an inert gas shield to produce a superior weld.
This new process, called tungsten inert gas (TIG), gave rise to a whole new industry sector and became a mainstay in industrial welding. A few years later, scientists within the new industry began to tinker with the process, hoping for improvements that would mean greater performance.
Their optimism was well founded. They learned that constricting the orifice through which the gas flows causes the arc heat to increase dramatically. In addition, the increased velocity of the gas actually removes molten metal. To their surprise, the result was not an improved method of joining metal, but rather a highly effective way of removing it. Thus, the plasma arc cutting process was born.
Science educators around the world teach students about the three familiar states of matter: solid, liquid, and gas. In each state, beginning with solid, energy can be added to create a new state of matter. The most common example is water. We know that water in its solid form is ice.
When heat energy is added, the solid is transformed into a liquid. The addition of even more heat energy transforms the liquid into a gas (water vapor or steam).
The plasma state takes this natural progression one step further and is generally thought of as a fourth state of matter. With the introduction of still greater amounts of energy to the gaseous state, gas molecules break into atoms and many of the atoms lose an electron, a process called ionization. This hot, ionized gas, called a plasma, is electrically conductive.
Older methods vs air plasma
Oxyfuel cutting is often referred to in the same breath as plasma cutting. Oxyfuel cutting employs a chemical reaction, heating steel to kindling temperature with a fuel like acetylene and then injecting a stream of pure oxygen to oxidize the steel — an exothermic reaction that both melts and literally burns the steel. Flame temperatures are typically in the 5000-6000 F range. However, this reaction occurs only in ferrous metals, limiting the process primarily to steel.
The prime attraction is low initial investment in simple equipment. Because the oxyfuel process doesn’t lend itself well to cutting sheet metal, mechanical means such as shears or nibblers are often employed.
Plasma cutting, which is an electrical/thermal process, utilizes a highly constricted arc surrounded by a swirled column of ionized gas. This combination results in high arc current density, as well as increased voltage and heat. Arc temperatures often reach well into the 30,000-F range. At these temperatures, there isn’t a need for a chemical reaction as the arc melts the metal away at very high speeds.
Plasma cutting systems are available in mechanized, handheld, or table-mounted systems. While handheld systems have the edge in portability, the tabletop or mechanized systems are useful for volume operations where cut consistency and productivity are important.
Plasma’s most obvious advantage is its ability to cut all electrically conductive metals, including stainless steel and aluminum. But, the primary reason for plasma cutting’s emerging preeminence in steel cutting is its speed and productivity.
The preheat delay is eliminated in plasma cutting, and cutting speeds can be anywhere from 100%-700% faster than oxyfuel, depending on the size of the machine. Typical handheld plasma cut speeds on machines recommended for cutting 1/2-in. steel range from 20-30 in./min.
Users of plasma are also quick to point out that the absence of a preheat cycle results in less heat transfer to the workpiece, which can reduce warping and twisting of the metal. Plasma cutters, as opposed to their oxyfuel counterparts, also leave a smaller heat affected zone. This translates into stronger welds at the edge of the cut.
When plasma cutting systems were first introduced, the primary objection to their use was that they produced only crude, low-quality cuts. Because the technology has rapidly advanced, plasma now produces cut quality comparable and sometimes superior to oxyfuel.
Because of these significant speed advantages, input costs per foot of metal cut are also 40-90% lower. And, for most hand-operated systems, all that’s required is electric power and compressed air. The only items consumed in air plasma cutting are the torch electrode and nozzle, or “tip.”
The sheer versatility of handheld plasma is a tremendous advantage. Most handheld plasma torches can be set directly on the workpiece and dragged, virtually eliminating the need to hold a difficult “stand-off.” This feature increases cutting precision, as templates are easily utilized. It also allows for easy setup and repeat, production-type jobs.
Another important feature of handheld plasma systems is their ability to remove metal at the surface of the workpiece, or “gouge.” Within minutes, the plasma torch can be configured for gouging by simply replacing the standard nozzle with a wider, lower-velocity type. Gouging, in most industrial applications, is primarily used to remove old or broken welds. The gouging process is far more efficient than other tedious and often labor-intensive manual methods.
Portability is an essential aspect of a system’s overall value. Handheld units generally weigh 30-90 lb and can be easily equipped with an optional wheel kit. Typically, these units also come with either a 25 or 50-ft torch lead. This feature, combined with the power supply mobility, allows for plasma cutting just about anywhere power and air are available.
Finally, users will be hard pressed to find another technology that is as easy to learn as plasma cutting. Hunter Herman, who teaches welding and cutting processes at community centers and universities, says “At all levels, I’ve found that students can produce good cuts within just a few minutes of being introduced to the process. Satisfaction comes quickly, increasing confidence, use, and proficiency. There’s nowhere near the complexity that’s involved in welding training.”
Whether you are cutting HVAC ductwork, steel plate, or pipe, the handheld plasma unit is a “go anywhere” system capable of cutting metal from thin gauge to 2-in. thick.
Metal cutting methods have changed.
Plasma arc cutting has replaced oxyfuel and mechanical methods in numerous situations.
Compact, portable plasma equipment is ideal for many maintenance tasks
What to look for in a plasma system
Plasma systems are often rated according to their output current, with typical hand cutting systems operating in the 20-200 A range. Manufacturers generally provide a “maximum thickness capacity” rating for their systems, but these numbers reflect the top end of the machine’s capability. To avoid frustration with slow, poor-quality cuts, potential users should determine the primary thickness to be cut and then purchase a machine with rated maximum capacity of approximately twice that thickness.
Here are three questions plasma buyers should ask before buying a system.
1. What will be the primary types and frequency of use? The thickness range to be cut is the primary determinant of machine capacity needed. However, information on frequency of use is also important, because it determines desirable cutting speed. A high number of arc hours per day usually means higher cutting speeds can save money.
2. What are the comparative operating costs of the machine? Labor and input energy costs are about the same for machines of comparable ratings. The other cost area is consumables. Most plasma systems “consume” electrodes and nozzles through the course of cutting. The life spans of these consumables often differ significantly from machine to machine, as do their purchase costs.
3. Which machine features are “must have,” and which are “nice to have?” Features that come as standard on one machine may be options on another and may not even be available on a third. Above all, take the time to “test drive” each of the different alternatives, especially since there are no rating standards governing the industry.
The author will answer technical questions concerning this article. Mr. Kanda is available at 800-643-0030, ext. 536; e-mail: steve.kanda@ hypertherm.com. The company web site is www.hypertherm.com.