A Cut Above

While both plasma and oxyfuel cutting systems have their place in the industrial environment, it is plasma that, in many ways, is proving to be "a cut above." Discovered in the 1950s, plasma cutting has become the process of choice in a wide variety of commercial applications.

Some of the more common uses include general fabrication; heating, ventilation and air conditioning (HVAC), where the bulk of the work involves in-house duct cutting; facilities maintenance; truck, trailer and automotive repair; agriculture/farming; steel building construction, shipbuilding and maintenance; manufacturing and metal arts.

The principle behind plasma cutting is as elegant as it is effective. When sufficient electrical energy is applied to a gas, the gas becomes super-heated and ionized, such that it is capable of conducting an electric current. This electrically conductive, ionized gas is called plasma. When a high-velocity jet of plasma is delivered to a metal workpiece, the intense heat melts a thin area of metal and a high-pressure gas blows the molten metal away leaving an edge with good quality that minimizes the need for secondary operations.

One of plasma's key advantages over other technologies is that plasma is able to cut any electrically conductive metal, both ferrous and non-ferrous, even if rusted, painted or dirty. Mild steel, stainless steel, aluminum, galvanized, copper, cast iron, and more are all easily cut with plasma. This is in stark contrast to oxy-fuel cutting, which is ineffective on stainless steel, aluminum and other non-ferrous metals.

Plasma systems are also suitable for a range of metal thicknesses: from thin gauge up to 1¾ in and, in some cases, even thicker. Some plasma systems can operate solely on electricity and compressed air, which reduces gas costs and eliminates cylinder rental, pick up and drop off fees.

The superior speed of plasma over oxyfuel cutting has become even more pronounced over the past decade. On steel up to about 1.5 in thick, plasma is a faster process, an important consideration when project timetables are essential. And because 80 percent of metal cut today is ½ in or less ? thicknesses where plasma enjoys a significant speed advantage over oxyfuel ? the vast majority of metal cutters can experience speed and productivity gains with plasma.

For many cutting applications, piercing is essential, and plasma's piercing capability and speed should not be overlooked. With an oxyfuel system, which requires significant pre-heating of the metal work piece, piercing is a slow process that often yields poor results. The speed and quality of piercing with plasma can greatly increase productivity.

Plasma's cut quality is superior to that of oxyfuel, meaning less finishing work (i.e., grinding and polishing) has to be performed on the cut materials, thus limiting labor costs and driving down operating costs. Further, the smaller size and increased portability of some plasma units allow users to bring them into the field with less effort. And while there are certainly safety considerations with any form of cutting, plasma is considered a safer procedure than oxyfuel, since it does not require any flammable gas (see Figure 1).

Today's portable plasma cutters can cut through a wide variety of metal thicknesses, from sheet metal to metal 1¾ in thick, and sometimes greater. Most portable systems run on a single gas, most commonly compressed air, but sometimes nitrogen for specific applications or when air quality is poor. Consumable life ranges widely from one manufacturer to another, and should be reviewed when making product comparisons. Duty cycle ? a percentage which specifies how many minutes out of every ten the machine may be used without overheating ? also varies. Better products with more advanced thermal cooling technology and internal controls are able to provide higher, "industrial" duty cycles.

If you're now sold on plasma, you might be wondering how to choose the right system. Fortunately, choosing a plasma system can be as easy as using one. Choosing a system with the necessary cut capacity and duty cycle is important, and buyers should also consider factors like versatility, cut quality, reliability, ease of use, and operating cost.

VERSATILITY

Plasma's ability to cut through various metals gives it a distinct advantage over the oxy-fuel cutting process. Some plasma cutters can be used for both handheld and mechanized cutting applications, providing an additional level of versatility. Some of the more advanced models on the market allow the user to switch fluidly between hand- and machine-torches, for use on X-Y tables, track burners, pipe bevellers, and even robotic arms. A few systems include a CNC interface and internal voltage divider, providing even greater options for mechanized applications.

In addition to cutting, many plasma models may also be used for gouging, and a number of manufacturers have developed consumables to optimize this process.

The portability of plasma adds to its versatility. There are lightweight plasma tools weighing anywhere from 20 lb to 35 lb, making them easily portable to a variety of locations ? including hard to reach locations ? whether in the shop, the field, indoors and out. A few utilize a single-handle design and include a shoulder strap for even greater mobility.

The portability factor should not be underestimated. Imagine a piece of farm, mining or construction equipment that requires in-the-field repair. A lightweight plasma device can be quickly loaded into any vehicle or carried through rough terrain to reach the site in a timely fashion. Even people who work in large shops can benefit from having a portable system since their cutting jobs are often spread throughout the facility.

A higher-quality plasma cutting tool provides multiple power-source options, as well as strong power efficiency.  A good system will deliver consistent performance even on low-line conditions. Further, if AC power is unavailable, a quality system can run off a generator ? a few of the smaller tools can even run on generators with a power output as low as 6 kW. This is a critical factor in industries such as agriculture, where many farmers have already invested in smaller generators in the 6 kW range for general use. 

CUT QUALITY

All of the above means very little without paying attention to the tool's cut quality. Improved cut quality saves time in later stages of production.

For example, if a work piece cut by oxy-fuel needs to be welded, significant time grinding edges is typically necessary to prepare them. The clean, smooth edge normally produced by today's plasma systems mean edges may be ready for welding directly after cutting, or at least with minimal grinding. Additionally, the finer cuts cause only a minimal heat-affected zone, further enhancing cut quality.

What's more, the better systems create a much narrower kerf (cut width), which means more precise cuts and less wasted metal. Dross, the metal residue that remains adhered to the work piece after the cut, is another factor to consider. With many plasma cutters, any dross that is generated is typically much easier to remove than dross left by oxy-fuel cutting. The amount of dross created varies by product and can be observed during a product demonstration.

RELIABILITY

Reliability is another important consideration for many people. The most reliable systems are engineered with fewer parts, use software instead of hardware where possible, are carefully manufactured to exacting ISO standards, and thoroughly tested.

One example of how design aids reliability can be found in the innovative cooling system utilized by certain systems. In many plasma systems, the fan draws in air from the outside into one end of the system, sends it along the heat sink, and out the other end of the system. As the air travels through the system and cools down internal components, it begins to heat up; as a result, a cooler end and a warmer end are created, resulting in non-uniform, sub-optimal cooling.

Newer systems, on the other hand, use a centralized fan configuration. In this design, the tool brings cool air in through the center of the system, where the most thermally sensitive components are located, and is subsequently moved along the heat sink in both directions, resulting in more efficient and consistent cooling. In addition to superior cooling and higher reliability, this design has the added benefit of enabling a higher, more industrial duty cycle.

EASE OF USE

Simple operation is a requirement in a plasma cutting tool. Obviously, for an experienced operator, ease of use allows a job to be done faster and more efficiently, but it also means that people with little or no experience can get good results. Regardless of experience level, operators using plasma can get jobs completed quickly and with good quality, keeping time-sensitive projects on schedule.  

OPERATING COST

Consumable life, especially of the nozzles and electrodes that are utilized in both cutting and gouging, varies from one brand of plasma system to another and should be considered during the buying process. Longer consumable life is important as it reduces downtime for change-outs and lowers costs. The best brands utilize patented technology to extend consumable life, while also delivering high-quality cuts.
The most-recent design features a dual-angle nozzle that angles the shield gas into the plasma arc resulting in both better cut quality and longer nozzle life ? two significant advantages for the operator.

WHICH TO CHOOSE?

Few plasma cutting tools will meet all of these requirements. Discuss your cutting needs with your local authorized plasma dealer and use the criteria listed here to help determine the best cutting tool for the job you want done.

Hypertherm, Inc., Etna Road, P.O. Box 5010, Hanover, NH 03755, 603-643-3441, Fax: 603-643-5352, www.hypertherm.com.