Even though many job shops would like to have a laser for metal sheet and plate cutting, Jim Colt, strategic accounts manager at Hypertherm says, "Lasers have a high capital equipment cost. In many cases, they generally can do the type of cutting the customer wants, but plasma is superior in cut quality on certain materials and thicknesses. Also, in many situations plasma fits better because the capital equipment cost or the operating costs are better."
Tim Walsh, president of Plasma Automation, says, "The first thing we ask when someone is looking for a plasma system is what is the size of the maximum material sheet or plate he'll be cutting, such as a 5-ft.-by-10-ft. sheet or an 8-ft.-by-20-ft. plate. Also, the material thickness and types of materials to be cut are very important."
Colt adds, "If a company is going to cut 3-in. or 4-in. thick carbon steel, that's oxy-fuel territory. If it is going to cut 20-gauge stainless steel all the time, while plasma can do it, we have to ask a few more questions. But maybe the company needs to go with a waterjet or laser system. So we need to know the range of the material and what the company spends most of its time cutting. We have to narrow the cutting down to one or two thicknesses that represent 75 percent or 80 percent of the company's work."
Walsh says, "Plate thickness would also have a lot to do with a purchase. Depending on its needs, we'd point a company in the direction of either conventional plasma or a high precision system. Of course, the equipment budget is very important too. What can the company spend for a system?"
Dave Coleman, senior sales engineer at Koike Aronson, says, "If a company is cutting material only up to 1/4 in. thick, it has a lot of things to consider. First, is the percentage of cutting done at the 1/4 in. thickness. Is it 50 percent of the time, 75 percent, or is it less than that? Is the company cutting it only 10 percent of the time with the other 90 percent being used for lighter gauge materials? So there are a lot of variables that will come into play when you select the type of plasma that's right for you."
Part accuracy, edge condition
Final part accuracy depends on a lot of variables, says Walsh. Is a company working within a 1/32 tolerance or does it need a tighter one? If it is willing to live within plus or minus 1/32, then standard plasma would normally be the correct system. If it wants to cut more accurately than that, it should go with the high performance or precision plasma. But cut edge quality can also be important. If a company needs a superior edge quality, then a precision plasma system could be the right choice."
Walsh adds, "We then ask if a company is looking to do any plasma marking with the plasma torch. Also, many machines now offer an optional drill attachment. That would be for producing small, precise holes. So if a company can't live with the accuracy of a plasma-cut hole, we can equip it with up to a 3/8-in. drill.
"Another question is what kind of tolerances are needed? The job shop guy is going to say, 'I want perfect parts. I want 0.001 in. plus or minus tolerances,'" says Colt. "To get that you'll have to spend a lot of money for those parts and do secondary operations--even a laser or waterjet can't do that. You have to get to the bottom line: What are your real tolerance needs? The tolerance capability is also a steering factor for all of us in the metal cutting industry and that's with plasma. It can determine if a company fits into a plasma cutting system and productivity range. Tolerances are what might steer it toward a high-definition or high-performance type plasma system, or a conventional one. There's a difference in price between those two systems, but tolerances are what make the difference."
System types
CNC plasma systems are also offered in different types primarily based on size, cut performance and cut accuracy needed. Walsh says that he uses three levels of equipment that give different cut accuracies. All three use a dual rack-and-pinion drive system that produces more accurate parts than single-end drive machines. High accuracy machines have high-performance axis and plasma head guides and high-speed digital servo motors. Usually these also offer a third axis to allow the plasma head to be moved up and down automatically for different material thicknesses. Construction is a one-piece design, while some plasma systems are rail-style machines.
Colt says, "In order to improve the cut quality and to produce consistent cuts, a person needs to look at CNC cutting machines that have superior acceleration capability. Today they're using A/C brushless drive motors that have superior acceleration capabilities over older machines. They're also using linear bearings on the machine. Instead of just having a drive wheel that rides on a track, today they're using linear bearings with pre-circulating ball bearings similar to what's used on machine tools as the moving parts of the machine to make the machine movement very fluid and smooth.
"Some plasma cutting machines use a ball screw on the cross axis, which is the shortest axis, and generally use a rack and pinion on the long axis. However, rack and pinion drives have improved dramatically over the years using helical racks, zero backlash gear boxes and superior drive motors."
Operating costs
Another important consideration for a system is ongoing operating costs. Colt says, "Our systems are getting more complicated, and they cost more than they did 20 years ago, but we have greatly succeeded in lowering the operating costs--the cost per foot of a cut. It's far less expensive to cut with plasma today than it was 20 years ago because the consumables last longer, we use less gas and the cut quality is better for reduced secondary operations.
"The first day that you buy a CNC plasma cutting machine, you might have spent $100,000 in capital equipment costs. Then as you start using it, you have to add in the operating costs: electrodes, nozzles, torch parts, maintenance and gas. A used system might be cheaper than a new one. But you must look at your total costs. After six months a new system will be less expensive because of its newer technology that saves consumable costs and lowers operating expenses," Colt says.
He adds, "most of the major advances in plasma cut quality and consumable life have happened in the last five years. Many progressive companies do look at that, but many don't, and we try to bring a company up to speed before it buys a system. We'll try to sell up from a low duty cycle, short consumable life machine to a high duty cycle, long consumable life machine and show the company the long-term benefits and cost savings. It's beneficial to the customers, and they appreciate it after they've had the machine."
Duty cycle
A plasma system's duty cycle is another key factor to consider when purchasing equipment. A company that does high-volume cutting needs a 100 percent duty cycle system, but this also adds to the cost.
Colt says, "We make a line of manual systems. They are generally lower cost air plasma systems that can be mounted on a CNC cutting machine. But these torches are designed for handheld cutting. Generally you don't run a hand torch 100 percent of the time in an eight-hour shift. But you could approach that on a CNC mechanized cutting machine where you would need a 50 percent to 80 percent duty cycle. So anything that's going to be mounted in a high-duty cycle application should be rated for 100 percent duty cycle for the thickness ranges that it's being used for." (See sidebar, below, for more on duty cycles.)
Software and controller
Software and the plasma system's controller can play a major role in getting the most productivity. Colt says, "There's low-cost software and high-cost software. It all does essentially the same thing. It produces a shape and turns that into motion on the machine that cuts the part. But the real advantage of higher-end software is it can do a lot of other things such as intricate automated nesting. It can save your plate, do remnant tracking, keep track of plates you don't use, and put them back into inventory. It can also tie into the company's inventory system."
Before the advent of automated plasma systems and fully automated parameter setups, an operator would have to look up the thickness of the material he wanted to cut. There could be anywhere from five to 10 gas-arc-voltage current parameters that had to be set to make it operate correctly. With PC-based controls (usually Windows based) on the cutting machine that interface serially to the plasma system, the operator only has to choose the type of material and thickness and the software sets all the cutting parameters.
"On some of our machines we offer a true multi-tasking CNC, where we use two separate processors to manage the cutting jobs," says Coleman. "One we use for actually cutting the plate, and the other processor is used for managing the part. While the operator is cutting a part, he can import a new program into the controller and change the lead-in. It's a true multitasking CNC."
"At the machine's PC control, if the operator needs to quickly change the cutting speeds, we have the ability to program the different mixed gases that are required to do it," says Walsh. "The software is able to set these parameters for the gases automatically. Also with our own software we accept any dxf files from other CAD programs. A buyer needs to know how he is going to get the geometry into the control. That's done with the programming software with our equipment.
"What's more, if a company has a scanner and it scans a part print or drawing in, it can be imported right into our software. The software can then size the part at 100 percent or to any size. Then the software automatically sets the cutting parameters."
Another important software feature is automatic nesting. If you have a different part or a number of dissimilar parts, it will automatically configure the area on the sheet for each part and optimize the material so there is less scrap.
"Nesting a plate is the most efficient way to run a business," adds Coleman. "With the high-performance or high-definition plasmas you can get a much tighter nest because your lead-ins are usually shorter and your lead-outs are virtually nonexistent."
Two other important considerations when buying a plasma system are electronic torch height control and a collision protection device for the torch. Height control will allow the torch to be automatically set by the controller for the type of material being cut, saving the operator from manually adjusting it. Collision protection guards the torch from damage in the event of material collision. FFJ
Understanding duty cycle ratings
A duty cycle rating on a plasma machine means it's rated for cutting a certain amount during a 10-minute cycle, usually six minutes (60 percent). For instance, a 60 percent duty cycle means you could run any plasma system wide open for about the first 10 minutes because the internal components of the plasma system haven't reached room temperature. But once you've reached a steady state of cutting, it gets a little more complicated. A 60 percent duty cycle machine is rated at 60 percent duty cycle at a certain power output under certain ambient temperature conditions. So if the air temperature in the room is 100 degrees, it's going to have less duty cycle than if the air temperature is 60 degrees. At higher room temperatures, a 60 percent duty cycle allows you to cut for six minutes out of ten.
Why use a duty cycle?
Why don't they just make all machines 100 percent? To make a
100 percent duty cycle plasma system for all of its cutting capability requires larger components in the power supply and better cooling capability. So the transformer has to be enhanced, the switching power supply has to be bigger and cooled better and the torch has to be larger and cooled more effectively. If you can take some duty cycle out without taking away performance when the torch is running, you can lower the cost of the machine dramatically, but in many situations it won't deliver the needed performance.
