How to Select a Plasma Cutting Machine for your Shop
James Anderton posted on March 02, 2015 | 17904 views

With the cost of plasma machines on the decline and with smaller-sized, portable machines flooding the market, it may be time to take a serious look at plasma for your cutting applications. The benefits of plasma cutting include ease of use, higher quality cuts and faster travel speeds.

Structural and plate steel are the bread and butter of this sector.  That can also be said for multiple other techniques for ferrous cutting such as abrasive water jet, laser, saw, abrasive wheel and oxy/fuel cutting.  However, plasma cutting has distinct advantages in many applications. This post will explain what the advantages are, and when to exploit them for best part making efficiency.

What is plasma cutting technology?

In simplest terms, plasma cutting is a process that uses a high velocity jet of ionized gas that is delivered from a constricting orifice. The high velocity ionized gas, that is, the plasma, conducts electricity from the torch of the plasma cutter to the work piece. The plasma heats the work piece, melting the material. The high velocity stream of ionized gas mechanically blows the molten metal away, severing the material and making the cut.

How does plasma cutting compare to oxyfuel cutting?

Plasma cutting can be performed on any type of conductive metal - mild steel, aluminum and stainless are some examples. With mild steel, operators will experience faster, thicker cuts than with alloys.

Oxyfuel cuts by burning, or oxidizing, the base metal. It’s limited to steel and other ferrous metals that support the oxidizing process. Metals like aluminum and stainless steel form an oxide that inhibits further oxidization, making conventional oxyfuel cutting impossible. Plasma cutting, however, does not rely on oxidation to work, and thus it can cut aluminum, stainless and any other conductive material.

While different gasses can be used for plasma cutting, most people today use compressed air for the plasma gas. In most operations, compressed air is readily available, and thus plasma does not require fuel gas and compressed oxygen for operation. Some portable units also supply air from an on-board compressor.

Plasma cutting is typically easier for the novice to master, and on thinner materials, plasma cutting is much faster than oxyfuel cutting. However, for heavy sections of steel (1 inch and greater), oxyfuel is still preferred since oxyfuel is typically faster and requires lower capacity power supplies than plasma.

What can I use a plasma cutter for?

Plasma cutting is ideal for cutting steel, and non-ferrous material less than 1 inch thick. Oxyfuel cutting requires that the operator carefully control the cutting speed so as to maintain the oxidizing process. Move too quickly and the cutting stops. Plasma is more forgiving. Plasma cutting shines in many niche applications, such as cutting expanded metal, which is awkward and slow with oxyfuel. Also, compared to mechanical cutting, plasma is usually much faster, and can easily make non-linear cuts.

What are the limitations to plasma cutting? Where is oxyfuel preferred?

Oxyfuel may still be the preferred process for some applications. Plasma cutting machines are more expensive than oxyacetylene. Torch cutting also operates independent of electrical power or compressed air which may make it a more convenient method for some users. Oxyfuel can also cut thicker sections (>1 inch) of steel more quickly than plasma. In-plant users, however, can benefit from plasma technology using pre-existing shop services at little additional cost, while eliminating the consumable cost of oxygen and fuel gas, as well as the safety hazard associated with compressed gases.

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.

Thanks to Lincoln Electric for information used in this article. www.lincolnelectric.com

Lincoln Tomahawk 625 Plasma Cutter

Lincoln’s Tomahawk 625 plasma cutter features continuous output control, focussing the arc for different material thickness. A touch start system provides reliable plasma arc initiation without high frequency and rapid arc restrike allows fast cutting through gaps, including expanded metal. The Tomahawk 625 also features a front panel purge control, making it easy to set the air flow rate without initiating the plasma arc. A new electrode and nozzle design reduces consumable use to reduce operating cost. The unit cuts mild steel, stainless steel, aluminum, brass or copper.

www.lincolnelectric.com

Hypertherm Powermax 30XP

Hypertherm’s Powermax 30XP is an easy-to-use, two-in-one design with high-power cutting on thick metal, plus FineCut consumables for detailed, thin metal cutting. The Powermax 30XP plugs into any 120 or 240V outlet using with Auto-Voltage technology and included plug adapters. Compared to the Powermax 30, the XP allows a 50% increase in cutting power for fast cut speeds with less edge preparation – patented consumables are designed to provide superior cut quality. The XP also has twice the consumable life and 70% more efficiency on average for lower cost of cutting. A new Duramax LT torch is designed to withstand impact and heat. The Powermax 30 XP includes a durable carrying case protects the system and gear.

www.hypertherm.com

Miller Spectrum 625 X-TREME

Miller’s Spectrum 625 X-TREME is a 40-amp portable plasma unit capable of cutting 5/8-inch mild steel. The 625 features a quick connect torch cable and a flexible, quick-connect ground cable with a smaller, heavy-duty clamp. It is available with factory-ready long- or short-body machine torches and upgrade kits are available to convert hand-torch machines for automated cutting.

www.millerwelds.com

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