Tech Tips For Better Plasma Cutting
Over the years, the air plasma arc process has been greatly refined so that plasma provides good quality cutting, gouging and piercing at very high speeds for a much lower cost. This cutting process is truly perfect for many industries.
Plasma arc cutting is a process where an open arc can be constricted by passing through a small nozzle, or orifice, from the electrode to the workpiece. Although the technology behind the plasma arc may seem complicated, the process itself is very easy to learn and perform.
Plasma - A Cut Above Oxyfuel
While oxyfuel was the most common method of cutting carbon steel in the past, plasma cutting is gaining favor because it provides numerous advantages. Plasma cuts faster than oxyfuel; a pre-heat cycle is not required; the kerf width (the width of the cut) produced is small; and, it has a smaller heat-affected zone, which prevents the surrounding area from warping or damaging paint.
In addition, plasma cutting can be used on any type of electricity-conducting metal (the oxyfuel process cannot cut stainless steel or aluminum). Moreover, plasma cutting is a cleaner, less expensive and more convenient method of metal cutting because clean, dry air is used for most plasma cutting applications.
A properly installed air plasma arc cutting setup is safer than oxyfuel gas cutting. This is because there is a chance of flashbacks and the danger of flammable gases in exposed hoses with oxyfuel torches.
Plasma cutting is easier for hole piercing due to the plasma jet. The plasma jet is the swirling force of air around the arc, in combination with the arc attachment, which focuses the heat into the metal. In addition, plasma arc piercing does not require the metal to be preheated, which saves production time. The plasma jet also makes for a better choice when cutting stacked materials.
Plasma Cutting Applications
The plasma 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. Plasma can be used to cut anything from thin gauge aluminum to stainless and carbon steel up to several inches, depending upon the power of the cutting machine.
Setting-Up A Plasma Arc Cutter
To set-up 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. You can then adjust the amperage down according to your travel speed. If the amperage is too high or your travel speed is too slow, the material you are cutting 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.
Operating A Plasma Arc For Cutting
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 then 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 workpiece 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.
Operating A Plasma Arc For Gouging
Plasma arc gouging can be accomplished by placing the torch at approximately a 40-degree 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 workpiece. 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 gouge. Again, after releasing the trigger, the postflow air will continue for a short period of time, but gouging can be resumed immediately.
Operating A Plasma Arc For Piercing
Piercing - creating a hole - can be performed by placing the torch at a 40-degree angle to the workpiece. Press the trigger. When the cutting arc is initiated bring the torch tip to a 90-degree 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.
What To Look For In A Plasma Cutter
Output Power. The output power needed in a plasma cutting machine depends primarily on the thickness and type of material to be cut. The metal thickness will also determine the size of the nozzle opening, as well as the type and amount of gas or air required.
Start by determining the type and thickness of metal you'll be cutting and the desired cutting speed. Miller uses three standards: rated, quality and sever cuts. A rated cut is the thickness of mild metal that an operator can manually cut at a rate of 10 inches per minute (IPM). A quality cut is rated at a slower speed but on thicker metal. A sever cut is the maximum thickness a plasma cutter can handle. The travel speed is slower and the cut may require clean up.
For example, Miller Electric's Spectrum® X-TREME 375 has a cut rating of 3/8-in. Rated cutting capacity is the speed at which an operator achieves a smooth, steady cut at 10 inches per minute (IPM). At 6 IPM, the Spectrum 375 X-TREME can cut 1/2-in. mild steel. This type of machine works well for most fabrication, maintenance and HVAC applications, as well as home workshop or hobbyist applications. A more powerful machine may be necessary for operators planning to cut thicker materials.
|An example of cutting capability. The above is for an inverter -based plasma cutter with an output of 25-55A@ 110VDC. See each model's individual specifications for its cutting speed chart.|
Cutting Speed. Also check the cutting speed of the machine (See above). This is usually noted as Inches Per Minute (IPM). A machine that cuts 1/2 in. material may take five minutes to do so, whereas another machine may take one. Cutting speed makes a significant difference in production time.
|The above chart shows a typical relationship between cutting speed and material thickness. (See individual model spec sheets to determine cutting speeds for that model.)|
Input Power. Will you always be using the plasma cutter in the same location, or do you need portability and the ability to use a variety of power sources?
Miller Spectrum plasma cutters offer a wide range of power options. Operating on 115V, the Spectrum 375 and Spectrum 375 X-TREME can deliver a quality cut of 1/2", while the Spectrum 125C can deliver a quality cut of 3/16-in.
The Spectrum 375 and 375 X-TREME also work on 230V. Using Multi-Voltage Plugs (MVPs™), the Spectrum 375 X-TREME makes it especially easy to switch between the two. The operator simply selects the right plug for the receptacle and plugs it onto the power cord.
Both the Spectrum 375 and 375 X-TREME, as well as the Spectrum 625 (230V), feature Line Voltage Compensation (LVC™), allowing the input power to fluctuate by +- 10 percent without affecting performance. This can be very important if you plan to use the plasma cutter in a location subject to power fluctuations or brownouts.
Others can automatically adapt to a wide range of voltages, single or three-phase and compensate for power fluctuations in the supply.
With the benefit of Miller's Auto-Line™ technology, the Miller Spectrum 2050 can accept input voltages ranging 190 through 630 V, single- or three-phase, 50 or 60 Hz. This means you can plug this plasma cutter in anywhere in the world. Even with brownouts or other power fluctuations, as long as the voltage stays anywhere in that range, the cutting quality is unaffected.
These types of units are especially suited for working in the field, using an engine drive's auxiliary power. In the field, units without this type of technology are prone to erratic cutting arcs, frequent breaker trips and blown circuit boards because they can place a load on the line such that voltage levels drop below the plasma cutter's operating range.
Duty Cycle. Duty cycle is an important factor to consider when purchasing a cutting machine. Duty cycle is the amount of time a machine can cut within a ten minute cycle without overheating. For example, if a machine's duty cycle is 60 percent, the machine can run continuously for six out of every ten minutes and then needs to cool for the remaining four minutes. 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 may also have an effect on duty cycle. For example, Miller Electric establishes its duty cycle at 104°F, however this may not be true for all manufacturers. In fact, power sources with an established duty cycle at 75°F will not be able to provide full duty cycle at 85°F.
Torch. The type of torch to purchase depends upon the application for which the plasma cutter will be used. The torch should always be heavy-duty and be able to withstand the necessary work environment, as well as being dropped or thrown. Several options are available for torches. Miller offers an ICE torch with an epoxy cup that 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 to 1/8 in. off of the workpiece. This permits the operator to drag the torch on the workpiece 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 to 1/8 in.
You must also 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 gauge sheet metal applications where a 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.
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. You will want to have consumables available when you need them, 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 especially convenient if the cutting machine has a storage compartment for these consumables to save on downtime.
Weight and Size. Weight and size may be extremely important factors to consider when purchasing a plasma cutter if there is a need for portability. Many hand-portable units are available that weigh less than 45 lbs. The Spectrum 375 X-TREME provides the ultimate in portability, weighing only 18 lbs., allowing the user to carry it comfortable by its shoulder strap.
There are also compatible running gears available for some larger, heavier machines. 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
Safety procedures must be closely followed 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 and 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 stop 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.
When used properly, your plasma arc cutting unit will provide clean, quality cuts at very high speeds. For a demonstration, or for more information on plasma cutting products contact your nearest Miller Electric distributor, or call 1-800-4-A-MILLER (1-800-426-4553).
References: Plasma Arc Metalworking Processes, American Welding Society