by Tim Matus, Product Manager,
Miller Electric Co., Appleton, WI
This operator uses his Miller Spectrum®700
in an artistic application. The piece of metal he is cutting will be part of a sculpture.
Over the years, the air plasma arc process has been greatly refined so that plasma provides good quality cutting, gouging and piercing at high speeds and low cost.
Plasma arc cutting is a process in which an open arc, much like TIG welding, can be constricted by passing through a small nozzle, or orifice, from the electrode to the work -- piece. Although the technology behind the plasma arc may seem complicated, the process itself is very easy to learn and perform.
While oxyfuel was the most common method of cutting carbon steel in the past, plasma cutting is gaining favor because of the following:
A properly installed air plasma arc cutting setup is safer than oxyfuel gas cutting because there is a chance of flashbacks and the danger of flammable gases in exposed hoses with oxyfuel torches.
Plasma cutting also is easier for hole piercing due to the plasma jet. The jet is the swirling force of air around the arc, in combination with the arc attachment, which focuses the heat into the metal. The plasma jet also is better suited for cutting stacked materials.
This process can be used to cut any material that is a good electrical conductor. Unlike chemical cutting, it can be used on any metal for applications such as stack cutting, beveling, shape cutting, gouging and piercing.
Plasma cutting can be successfully performed on a variety of material sizes as well. It can cut anything from thin-gage aluminum to stainless and carbon steel up to several inches thick, depending upon the power of the cutting machine.
To setup a plasma arc cutter, simply hook up the compressed air to the plasma cutting unit. Three choices of air are available: bottled, an in-house air supply, or a small air compressor. Most plasma units have a built-in regulator to maintain the proper flow of air for the system.
To set the amperage, or heat, of the cutting unit to the proper level, make a few practice cuts with the amperage set high. Users then can adjust the amperage down according to your travel speed. If the amperage is too high or travel speed is slow, the material being cut may become hot and accumulate dross. Traveling at the right speed and using the right amount of heat will produce a very clean cut with less dross on the bottom of the cut as well as little or no distortion to the metal.
Begin cutting by placing the torch as close as possible to the edge of the base metal. Press the trigger to initiate the preflow air; the pilot arc will light, followed by the cutting arc. Once the cutting arc starts, move the torch slowly across the metal. Adjust your speed so that the cutting sparks emerge from the bottom of the metal. You should be able to see the bottom of the work -- piece and the arc should be directed straight down. If the sparks are not visible at the bottom of the plate, you have not penetrated the metal. This is because your travel speed is too fast, you have insufficient amperage, or the plasma stream is directed at an angle.
At the end of a cut, angle the torch slightly towards the end of the cut or pause briefly to completely finish the cut. The post flow air will continue for a short period of time after the trigger is released to cool the torch and consumable parts. However, cutting can be resumed immediately. (See Fig. 1.).
Plasma arc gouging can be accomplished by placing the torch at approximately a 40-deg. angle to the base metal. Press the trigger for the preflow air and pilot arc. When the cutting arc ignites, form the arc length a short distance from the work -- piece. Further adjust the arc length and travel speed as needed. Do not gouge too deep, as several passes may be needed to accomplish the necessary depth. Again, after releasing the trigger, the post flow air will continue for a short period of time, but gouging can be resumed immediately.
Piercing (creating a hole) can be achieved by placing the torch at a 40-deg. angle to the work -- piece. Press the trigger. When the cutting arc is initiated bring the torch tip to a 90-deg. angle and the arc will pierce the base metal. A good rule to follow is that you can pierce up to 1/2 of the maximum cutting thickness provided by the machine.
Output Power -- The output power needed in a plasma-cutting machine depends upon the thickness and type of material. The metal thickness also will determine the size of the nozzle opening, as well as the type and amount of gas or air required.
After determining the type of metal to be cut, check the cutting capacity of the machine you wish to purchase. For example, the Spectrum 300 CutMate provides 25 amps of cutting output power and cuts metal up to 3/8 in. thick. This type of machine works well for most fabrication, maintenance and automotive repairs.
A more powerful machine may be necessary for operators planning to cut thicker materials. The Spectrum 1250 is a heavy-duty, industrial cutting machine that provides 100 amps of cutting output and can cut stock up to 11/4 in. thick.
Cutting Speed -- Also check the cutting speed of the machine. This is usually noted as inches per minute (IPM). A machine that cuts 0.500 in. material may take 5 min. to do so, whereas another machine may take 1 min. Cutting speed makes a significant difference in production time.
Input Power -- Check the primary voltage and amperage the power source requires. Also, determine if you need a machine that can operate on multiple voltages and amperages. Some machines can use either 115V, 230V or 460V, single-phase or three-phase primary power. Spectrum 300 CutMate can use 115V or 230V single-phase power and Spectrum 650 CutForce can use 230V or 460V single-or three-phase power.
Duty Cycle -- Duty cycle is the amount of time a machine can cut within a 10-min. cycle without overheating. For example, if machine's duty cycle is 60 percent, the machine can run continuously for six out of every 10 min. and then needs to cool for the remaining 4 min. A larger duty cycle is important when making long cuts, in high productivity applications or when using the machine in a hot environment. The duty cycle is usually given for the maximum output of the unit. If the machine is used at a lower output, the duty cycle will increase correspondingly. The temperature of the surrounding air also may have an effect on duty cycle. For example, Miller establishes its duty cycle at 104 deg. F, however, this may not be true for all manufacturers.
Torch -- Torch selection depends upon the application for which the cutter will be used. It always should be heavy-duty and be able to withstand the work environment, as well as being dropped or thrown. Several options are available.
An ICE torch with an epoxy cup is more rugged than ceramic and virtually unbreakable. This torch also provides a stand-off guide, or drag shield, that attaches to the cup and holds the tip 1/16 in. to 1/8 in. off the work -- piece. This permits the operator to drag the torch on the work -- piece while cutting at full output, which increases operator comfort and makes template cutting easier.
Torch stand-off is the distance the outer face of the nozzle is to the base metal surface. The torch stand-off is determined by the thickness of the material and the amperage required. Low amperage cutting may allow dragging the tip or nozzle on the metal. High amperage cutting, above 40 amps, requires a stand-off distance of 1/16 in. to 1/8 in. (See Fig. 2.)
The user also must choose between a single-flow torch and a dual-flow torch. A single-flow torch has only a flow of air for cutting. This is because its use is limited to low amperage, thin gage sheet metal applications. Shielding gas flow used to cool the torch is not necessary. A dual-flow torch has a flow of air for cutting, as well as a shielding gas flow for cooling the torch. Dual-flow torches are used for cutting thicker materials that require higher amperages.
Plasma cutting can be especially
useful in a fabrication setting. Here operators are using Miller's Spectrum®
1000 (left) and Spectrum® 300 CutMate (bottom).
Consumables -- Besides compressed air or nitrogen, there can be as few as two consumables needed for plasma arc cutting. These are the tip and the cutting electrode. If either the tip or the electrode become worn or damaged, the quality of the cut will be affected. The consumables will wear with each cut, but factors like moisture in the air supply, cutting excessively thick materials or poor operator technique will increase the deterioration of the consumables. The user will want to have consumables available when needed so the ability to order and receive them in a timely fashion is important. Best practice is to replace the tip and the electrode together for optimal quality cuts. It is convenient if the cutting machine has a storage compartment for these consumables to save on downtime.
Weight and Size -- These are extremely important factors to consider when purchasing a plasma cutter if there is a need for portability. There are many hand-portable units available that weigh less then 45 lbs. In addition, there are inverter-based plasma cutters available that provide high cutting output power, yet weigh much less than regular cutting machines that offer the same cutting capacity.
Proper Safety Procedures -- Must be followed closely in any application of the plasma cutting machine. Be aware of potential hazards involved with the process including, high voltages and temperatures, fumes, ultraviolet radiation and molten metal. Proper welding clothing should be worn, as well as welding helmets with dark lenses, as specified by the manufacturer.
Before cutting, inspect the shield cup, tip and electrode. Do not operate the unit without the tip or electrode in place. Hitting the torch on a hard surface to remove spatter can damage the torch and prevent proper operation. In addition, avoid constant starting and restarting of the plasma arc to lengthen consumable life. As with all industrial products, read the owner's manual for proper safety procedures. MF