Plasma is superheated, compressed air that ionizes to form a conductive gas. This gas conducts electricity from the torch of the plasma cutter to the work piece, through a electrode and copper nozzle on the machine. The nozzle constricts the high velocity gas, causing a tornado effect, and the arc melts and blows away the metal.
Compared to oxyacetylene, which works by burning through the metal, plasma can cut through nonferrous materials, has a lower skill level and offers faster travel speeds. However, plasma cutters are more expensive that oxyacetylene units and require access to electrical power and compressed air, making them less convenient in some circumstances.
If you decide that plasma cutting is the right process for your shop, here are some factors to consider before making your purchase:
1. Determine the thickness of the metal you will most frequently cut.
Most plasma cutting power sources are rated on their cutting ability and amperage. So if you most often cut 1/4-in. thick material, you should consider a lower amperage plasma cutter. If you most frequently cut metal that is 3/4-in. thick, look for a higher amperage machine. Even though a smaller machine may be able to cut through the thicker metal, it may not produce a quality cut. Instead, you might get a sever cut that barely makes it through the plate and leaves behind dross or slag.
2. Select your optimal cutting speed.
Do you perform most of your cutting in a production environment or in an atmosphere where cutting speed isn't as critical? The manufacturer should provide cutting speeds for all thickness' of metal measured in IPM (inches per minute). If the material you cut most frequently is 1/2-in., a machine that offers higher amperages will be able to cut through the metal much faster than one rated lower, although both will do the job. For production cutting, a good rule of thumb is to choose a machine which can handle approximately twice your normal cutting thickness. For example, to perform long, fast, quality production cuts on 1/4-in. steel, choose a 1/2-in. class (60 amp) machine.
If you're doing long, time-consuming cuts or cutting in an automated set up, be sure to check into the plasma cutter's duty cycle. Duty cycle is the time you can continuously cut before the machine or torch will overheat and require cooling. Duty cycle is typically rated as a percentage of a 10-minute period. For example, a 60% duty cycle at 60 amps means you can cut with 60 amps output power continuously for six minutes of a 10 minute period. The higher the duty cycle, the longer you can cut without taking a break.
3. Can the machine offer an alternative to high frequency starting?
Most plasma cutter have a pilot arc that utilizes high frequencies to conduct electricity through the air. But this may not be the best way to initiate the cutting arc, since high frequencies may interfere with computers or office equipment that might be in use in the area (or it can reduce the life of their internal electronics).
The alternative, the lift arc method, features a DC+ nozzle with a DC- electrode inside and touching it. When the trigger is pressed, current flows between the electrode ands the nozzle. The electrode pulls away from the nozzle and a pilot arc is established.
4. Compare consumable cost vs. the life of the consumable.
Look for a manufacturer that offers a machine with the least number of consumable parts. These parts are considered the wear items of the unit and have to be replaced as they pit and degrade. A smaller number of consumables means less to replace and more cost savings.
Look in the manufacturer's specifications for how long a consumable is expected to last. Be sure you are comparing the same data. Some manufacturers rate consumables by the number of cuts while others use the number of starts as the measurement standard.
5. Test the machine and examine cut quality.
Demo several plasma cutters traveling at the same speed on the same thickness of material to see which one offers the best quality. As you compare cuts, examine the plate for dross on the bottom side and see if the kerf (gap left by the cut) angle is perpendicular or angular. The plasma cutter should offer a tight, focused arc.
6. Pilot to cut and cut to pilot transfers.
Seek a machine that provides a quick, positive transfer from pilot to cutting at a large transfer height. Such machines will be more forgiving to the operator and will better support gouging. A good way to test this is by cutting expanded metal or gratings. The machine will be required to quickly transfer from pilot to cut and back to pilot very quickly.
7. Check the plasma cutter's working visibility.
As you are working, you'll want to see what you're cutting, especially when tracing a pattern. A smaller, less bulky torch and an extended nozzle will allow you to better see what you're doing.
8. The portability factor.
Many consumers use their plasma cutter for a variety of applications and need to move the machine around a plant or job site. Having a lightweight, portable unit and a means of transportation for it, such as a valet-style undercarriage or shoulder strap can make all the difference. If floor space in a work area is limited, having a machine with a small footprint is valuable.
9. Determine the ruggedness of the machine.
For today's hard job site environments, look for a plasma cutter that offers durability and has protected controls. Fittings and torch connections that are protected will work better than those that are not. Some machines offer a protective cage around the air filter and other integral parts of the machine. These filters ensure oil is removed from the compressed air. Oil can cause arcing and reduce cutting performance.
10. Find out if the machine is easy to operate and feels comfortable.
Look for a plasma cutter that has a big, easy to read control panel. A well-designed panel allows someone who isn't too familiar with plasma cutters to use them. A machine with step by step procedures clearly printed on the unit will also help with set up and troubleshooting.
How does the torch feel in your hand? You want something that has good ergonomics. Try it out the same way you would test the grip of a golf club.
11. Consider safety features.
Seek out a machine that offers a true "Nozzle in Place" safety sensor. With such a feature, the plasma cutter will not start an arc unless the nozzle is there. Other safety systems can be fooled into thinking the nozzle is in place. Additional good safety features include a pre-flow sequence that warns before the arc initiates.
Editor's note: The following article was supplied by Lincoln Electric Co.
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