by Kip hanson
Industry experts help to define the term “thick plate” and offer recommendations on how to cut it most effectively
When it comes to cutting big hunks of metal into smaller ones, fabricators have plenty of options, each with its unique pros and cons. Lasers, the undisputed champions for cutting thin gauge materials, are both fast and accurate, yet expensive. Waterjets can also be quite accurate, and care naught for hardness or thickness, but are generally much slower than competing processes. For everything else, shops have two choices—plasma cutters or oxy-fuel.
Rapid rusting
Need a primer? As its name implies, oxy-fuel uses oxygen together with a fuel gas such as acetylene, MAPP gas (methylacetylene-propadiene), propane, LPG (liquid petroleum gas), and even hydrogen to create an exothermic reaction that oxidizes the workpiece material. You can think of it as extremely fast rusting. It is limited to mild steels (up to 0.3 per cent carbon) and cannot be used to cut aluminum, stainless and other non-ferrous materials.
That said, oxy-fuel rules the world of thick steel plate, which most agree starts at 50 mm (2 in.) and up. Need to cut a battleship hull? How about an extreme pressure vessel or the deck plate for a deep-sea oil platform? Chances are you’ll cut it with oxy-fuel. You’ll need to preheat the metal to a bright cherry red first (roughly 1000° C or 1800° F), which is one of the arguments against this popular cutting technology, and a reasonably skilled operator (another argument). Properly applied, however, oxy-fuel provides a clean cut in practically any thickness of mild steel imaginable.
The fourth state
If you paid attention in science class, you know that matter comes in three forms: liquids, solids and gases. Apply enough energy, however, and the fourth state of matter emerges. It’s called plasma. If you want to see some, step outside and look upwards. That big yellow thing up there—our sun—is mostly plasma, defined as a high energy, electrically-conductive, blindingly hot soup of ionized atoms and free electrons.
Aside from making life on Planet Earth possible, plasma performs a more mundane though still important task—it’s excellent at cutting metal. Several types of plasma technology exist. Air plasma systems use compressed shop air and up to 20,000 amps of electrical power per square inch to provide general purpose cutting capabilities using either a hand torch or a machine torch mounted on a CNC table (note: the actual power needed to drive the system is far less than this, and well within the electrical service capabilities of most industrial buildings).
A second type of plasma is conventional multi-gas plasma which uses air, oxygen or nitrogen as the plasma gas and air or others as shield gases. And then there are so-called “high-def” and X-Definition plasma systems that apply two or more times as much energy—up to 70,000 amps per square inch—together with oxygen, nitrogen, argon or a combination of hydrogen, nitrogen and argon to generate a plasma stream. In all cases, temperatures of 22,000° C (40,000° F) or higher are possible, relatively cold in solar terms, but hot enough to blast through 50 mm or so (around 2 in.) of mild steel and even thicker stainless steel and aluminum. Superalloys such as titanium and Inconel and more, can also be cut quickly and accurately.
Choices, choices
So which one’s better? The answer depends to some degree on whom you ask, but most system integrators and equipment manufacturers agree that choosing between oxy-fuel or plasma comes down to several factors, including what you’re cutting, its thickness, your production and accuracy requirements and available budget.
Steve Liebold, plasma process engineering leader at Hypertherm Inc., suggests that plasma, though slightly more expensive upfront, is the preferred method for cutting plate stock (whatever the alloy) between 10 mm (0.38 in.) and 50 mm (2.0 in.) thick. He says plasma is actually an option for cutting or severing mild steel up to 80 mm (3 in.) in thickness. At this thickness, plasma cut speeds slow to the same speed as oxy-fuel, although edge quality tends to degrade with heavy dross formation.
“Similar to oxy-fuel, oxygen creates an exothermic reaction that facilitates thicker cutting than would be possible with nitrogen, for example, or argon-hydrogen,” he says. “The downside is that you’ll see more dross, the speeds fall off at that thickness, and piercing becomes problematic, so you’re forced to either edge start or pre-drill a starter hole. Still, for shops that want to steer clear of oxy-fuel, it’s a viable alternative.”
The case for oxy
The irony here is that, even though plasma’s piercing capabilities begin to fall off rather quickly on these thicker materials, you’d best be wearing your patience pants with oxy-fuel. “At two inches, plasma’s going to pierce in five seconds or less,” says Dirk Ott, vice-president of Global Mechanized Plasma Systems at Thermal Dynamics, an ESAB integrator. “Oxy-fuel will require 20 seconds, maybe more.”
Slower piercing or not, oxy-fuel is less expensive to operate from a consumables perspective, and the equipment itself is much less costly—where a basic oxy-fuel torch might run a few thousand dollars, a plasma system is going to cost fifteen times that or more—possibly a good deal more. Yet Ott is quick to point out that additional investment cost is easily offset by plasma’s far greater flexibility.
For a job shop that cuts 12.7 mm (1/2-in.) thick 316 stainless steel one day and 50 mm (2 in.) thick 6061-T6 aluminum the next, plasma is a no-brainer. But what happens when a production order of 80 mm (3 in.) thick low carbon bridge girders comes along? This is why most experts agree that the most effective solution—at least for those who have the part volume and material variety to justify it—might be a CNC plasma cutter that is also equipped with multiple oxy-fuel torches.
Best of both
“We’re interested in providing maximum productivity to the customer, no matter what they’re cutting, which is why we often suggest dual technology systems,” says Brice Turner, president of Hornet Cutting Systems, a manufacturer of plasma, oxy-fuel and waterjet machines. “This provides the flexibility to cut a higher variation of parts with reduced pierce time in a range of alloys with high definition plasma such as the Hypertherm XPR 300, as well as the ability to tackle large quantities with high repeatability in thick plate steel with multiple oxy-fuel torches. It’s a best of both worlds solution.”
An important part of that solution is a high quality motion control system, notes Turner, a qualified equipment supplier for aerospace and metal fabricators with stringent accuracy requirements. And for shops that are cutting large quantities of heavy gauge material—say 6.35 mm (1/4-in.) up to 12.7 mm (1/2 in.)—on their fiber or CO2 lasers, Hornet (and others) can provide higher throughput and equivalent part quality for “a significantly lower acquisition cost.”
“Plasma and oxy-fuel have both improved greatly over recent years, as have the CNC platforms used to drive them,” Turner says. “Today’s systems are safer, more accurate and considering the newer power supplies, automatic height control and preheat cycles, variable gas pressure control, water mist and other advanced features, they’re often far more cost-effective than lasers, especially for shops that routinely cut thicker materials.” SMT
