Choosing the right plasma cutting machine
1. Specify the thickness of the metal most frequently cut
Most plasma cutting power sources are rated on their cutting ability and amperage. 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 unit.
Plasma cutters operating at the limit of their current capacity may make poor quality cuts. 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. Establish the optimal desired cutting speed
Production plants and job shops often have widely different needs in cutting speed. 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 most frequently cut is ¼”, a machine that offers higher amperages will be able to cut through the metal much faster than a low current unit, 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.
Like welding equipment, the duty cycle is important for automated, or continuous cutting. 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 downtime.
3. How does the plasma cutter start?
Air is a pretty good insulator, and plasma cutting requires that air to ionize in order to establish the current flow needed to form a plasma. Most plasma cutters have a pilot arc that utilizes high frequency to conduct electricity through the air. This makes starting easy, but there is a penalty: high frequency can interfere with computers or office equipment. If the production environment involves sensitive PLC or PC controlled equipment, it’s important to choose alternate starting methods that eliminate that potential problem.
One answer is the lift arc method, which uses a DC positive nozzle with a DC negative electrode inside. Initially, the nozzle and the electrode physically touch. When the trigger is pulled, current flows between the electrode and the nozzle and as the electrode pulls away from the nozzle, 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. Watch out for the hidden cost: consumable cost versus consumable life
Plasma cutting torches have a variety of consumable items that require routine replacement. For hand-held torches, the retaining cap, shield, nozzle, electrode and swirl ring are easily replaced and should be swapped out as cutting performance deteriorates, rather than at the point of failure. It’s a hidden cost. 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. Production equipment that runs continuously, for example, will have a different wear profile than a similar unit that starts and stops frequently, as in a custom fab shop or repair environment.
5. Try before you buy
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. A well designed unit offers a tight, focused arc.
Another useful test 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 a higher potential (voltage) 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. 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, essential for proper fit-up in heavy plate welding. 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, a lower performance mode for busy job shops.
7. Portability is important, even on the plant floor
Many users operate 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 can make all the difference. Additionally, if floor space in a work area is limited, having a machine with a small footprint is valuable. If portability is essential, consider units which offer storage for the work cable, torch and consumables.
8. Durability is essential for most industrial applications
Protected controls are a must for many hard-use environments. Some machines offer a protective cage around the air filter and other integral parts of the machine. Filters are important because they ensure oil is removed from the compressed air. Oil can cause arcing and reducing cutting performance.
10. Can your workforce understand the controls on the plasma cutter?
It’s more important than you think. Look for a plasma cutter that has a big, easy-to-read control panel that is user-friendly. A well designed panel allows a user with limited experience to pick up a plasma torch and be productive quickly. A machine with settings and procedures clearly printed on the unit will help with set-up and troubleshooting. Ergonomics are important for hand-held units. How does the torch feel in the hand? Comfort reduces operator fatigue and promotes cleaner, faster cuts.
11. Safety first
Plasma cutting requires considerable voltage to establish the arc, in the neighbourhood of 300VDC. The nozzle protects the operator from this hazard, but if the machine can be inadvertently started without it in place, a serious accident can result. Machines are available with a nozzle-in-place safety sensor, preventing the unit from starting 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. Another safety advantage is a machine with 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.
The Original Posted by Lincoln