Once you have determined plasma cutting is the right process for you, look at the following factors when making a buying decision.
1. Determine the thickness of the metal that you will most frequently cut
One of the first factors you need to determine is the thickness of metal most frequently cut. Most plasma cutting power sources are rated on their cutting ability and amperage. Therefore, if you most often cut ¼" thick material, you should consider a lower amperage plasma cutter. If you most frequently cut metal that is ½" in thickness look for a higher amperage machine. Even though a smaller machine may be able to cut through a given thickness of metal, it may not produce a quality cut. Instead, you may get a sever cut which barely makes it through the plate and leaves behind dross or slag. Every unit has an optimal range of thickness - make sure it matches up with what you need. In general, a ¼" machine has approximately 25 amps of output, a 1/2" machine has a 50-60 amp output while a ¾" - 1" machine has 80 amps output.
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? When buying a plasma cutter, the manufacturer should provide cutting speeds for all thickness of metal measured in IPM (inches per minute). If the metal you cut most frequently is ¼", a machine that offers higher amperages will be able to cut through the metal much faster than one rated at a lower amperage, 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 ¼" steel, choose a 1/2" class (60 amp) machine.
If you are performing long, time-consuming cuts or are cutting in an automated set-up, be sure to check into the machine's duty cycle. Duty cycle is simply the time you can continuously cut before the machine or torch will overheat and require cooling. Duty cycle is rated as a percentage of a ten-minute period. For example, a 60 percent duty cycle at 50 amps means you can cut with 50 amps output power continuously for six minutes out 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 cutters have a pilot arc that utilizes high frequency to conduct electricity through the air. However, high frequency can interfere with computers or office equipment that may be in use in the area. Thus, starting methods that eliminate the potential problems associated with high frequency starting circuits may be advantageous.
The lift arc method features a DC+ nozzle with a DC- electrode inside. Initially, the nozzle and the electrode physically touch. When the trigger is pulled, current flows between the electrode and the nozzle. Next, the electrode pulls away from the nozzle and a pilot arc is established. The transfer from pilot to cutting arc occurs when the pilot arc is brought close to the work piece. This transfer is caused by the electric potential from nozzle to work.
4. Compare consumable cost versus consumable life
Plasma cutting torches have a variety of wear items that require replacement, commonly called consumables. Look for a manufacturer that offers a machine with the fewest number of consumable parts. A smaller number of consumables mean less to replace and more cost savings.
Look in the manufacturer's specifications for how long a consumable will last - but be sure when comparing one machine against another that you are comparing the same data. Some manufacturers will rate consumables by number of cuts, while others will use the number of starts as the measurement standard.
5. Test the machine and examine cut quality
Make test cuts on a number of machines, traveling at the same rate of speed on the same thickness of material to see which machine offers the best quality. As you compare cuts, examine the plate for dross on the bottom side and see if the kerf (the gap left by cut) angle is perpendicular or angular.
Look for a plasma cutter that offers a tight, focused arc. Lincoln Electric consumables are specially designed to concentrate the plasma swirl, offering a tighter arc and concentrating more cutting power on the work piece.
Another test to perform is to lift the plasma torch up from the plate while cutting. See how far you can move the torch away from the work piece and still maintain an arc. A longer arc means more volts and the ability to cut through thicker plate.
6. Pilot to cut and cut to pilot transfers
The transfer from pilot arc to cutting arc occurs when the pilot arc is brought close to the work piece. A voltage potential from nozzle to work is mechanism for this transfer. Traditionally, a large resistor in the pilot arc current path created this voltage potential. This voltage potential directly affects the height at which the arc can transfer. After the pilot arc transfers to work a switch (relay or transistor) is used to open the current path.
Look for a machine that provides a quick, positive transfer from pilot to cutting at a large transfer height. These machines will be more forgiving to the operator and will better support gouging. A good way to test transfer characteristics is by cutting expanded metal or gratings. In these instances, the machine will be required to quickly transfer from pilot to cut and back to pilot very quickly. To get around this, they may recommend you cut expanded metal using only the pilot current.
7. Check the machine's working visibility
As you are working on an application, you want to be able to see what you are cutting, especially when tracing a pattern. Visibility is facilitated by the geometry of the torch - a smaller, less bulky torch will enable you to better see where you are cutting, as will an extended nozzle.
8. Look for the portability factor
Many consumers use their plasma cutter for a variety of cutting applications and need to move the machine around a plant, job site or even from site to site. Having a lightweight, portable unit and a means of transportation for that unit - such as a valet style undercarriage or shoulder strap - make all the difference. Additionally, if floor space in a work area is limited, having a machine with a small footprint is valuable.
Also, you want a machine that offers storage for the work cable, torch and consumables. Built-in storage drastically improves portability since these items will not drag on the ground or get lost during machine transport.
9. Determine the ruggedness of the machine
For today's hard job site environments, look for a machine that offers durability and has protected controls. For example, fittings and torch connections that are protected will wear better than those that aren't. Some machines offer a protective cage around the air filter and other integral parts of the machine. These filters are an important feature since they ensure oil is removed from the compressed air. Oil can cause arcing and reducing cutting performance. Protection of these filters is important as they ensure oil and water, which reduces cutting performance, is removed from the compressed air.
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 that is user-friendly. Such a panel allows someone who does not normally use a plasma cutter to be able to pick it up and use it. In addition, a machine with procedural information clearly printed on the unit will help with set-up and troubleshooting.
How does the torch feel in your hand? You want something that has good ergonomics and feels comfortable.
BONUS
Look for safety features
Look for 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 in place. Some safety systems can be fooled into thinking the nozzle is in place (i.e. shield cup sensing), even when it is not. If the output is turned on, the operator will be exposed to 300 VDC, a very unsafe condition. This cannot happen with the Lincoln Nozzle-in-Place safety sensor.
Look for a machine that provides a pre-flow sequence. This feature provides an advanced warning to the use before the arc initiates. In addition, look for a machine which provides a three-second pre-flow safety which gives users advanced warning to make sure all body parts are clear of the nozzle before the arc initiates.