March 11, 2008
Plasma cutting is a widely used process in the welding industry that provides both speed and precision, but it doesn't happen by accident. Being knowledgeable about your cutting applications, and familiarizing yourself with cutting techniques, maintenance, and safety features, will allow for a productive plasma experience.
More than 50 years of effort have made plasma cutting an economically competitive choice for today's metal fabricators. Becoming knowledgeable about what you need from the process, as well as familiarizing yourself with cutting techniques and basic preventive practices, will help you get the most from your plasma cutting machine.
Once a gas is heated to an extremely high temperature and ionized, it becomes electrically conductive and is considered to be plasma. The plasma arc cutting and gouging processes use plasma to transfer an electrical arc to the workpiece. The metal to be cut or removed is melted by the heat of the arc and then blown away.
Plasma cutting fits most cutting applications but is particularly well-suited for applications in which speed and cut quality are important. The intense heat of the plasma arc—about 40,000 degrees F—results in a cut that is clean with little or no dross. The process usually requires very little rework or cleanup. The heat also allows the arc to burn through most surface coatings such as paint and rust, so less preparation work is needed. Plasma can cut ferrous and nonferrous metals with little or no heat-affected zone (HAZ), especially on thin metals. Difficult shapes, such as ventilation ductwork in HVAC, tanks, and vessels, can be cut with plasma.
By answering a few basic questions, you will be directed toward a plasma cutting system that best fits your needs. Ask yourself: How thick is the material to be cut? What level of cut quality do I require? What type of material am I cutting? What is my primary input power? Knowing the answers to these questions will help your local distributor or welding supply house guide you in the right direction.
As you consider the advantages of plasma cutting and the applications for which you expect to use it, you should also take some time to understand terminology and execution of common applications.
Drag Tip Cutting. Drag tip cutting is keeping the torch tip in direct contact with the work (see Figure 1). This is the preferred method for cutting light-gauge metal up to inch thick and produces the best cut quality, a narrow kerf width, and a fast cutting speed. While this method is user-friendly, it lacks arc visibility, and slag can build up on consumable parts, especially if you're using a high amperage to cut thick material.
Standoff Cutting. The preferred method for cutting metal thicker than in. at current levels higher than 60 amps, standoff cutting requires you to keep the torch tip 1/8 to in. from the workpiece (see Figure 2). This provides maximum arc visibility and extends parts life by keeping them away from excessive heat and slag. At 120 amps, standoff cutting can be used to cut material up to 2 in. thick.
Drag Shield Cutting. In drag shield cutting, shown in Figure 3, a shield cap or spacer is used to maintain an optimum spacing between the work and the torch tip. This is an operator-friendly method of cutting between 70 and 120 amps while maintaining a constant standoff distance. As with drag tip cutting, drag shield cutting can be used with nonconductive templates to trace or cut out a pattern.
Gouging. A simple method of metal removal, gouging is accomplished by angling the torch to a lead angle of 35 to 45 degrees and using a gouging tip (see Figure 4). Commonly used to remove unwanted welds, this technique is faster than grinding.
Plasma machines require little routine maintenance. However, supplying clean, dry air and keeping the consumables in good condition will help to ensure optimal performance day in and day out.
Because of the demanding environment they work in, most plasma cutting units are fairly rugged. However, units that have protection against the inevitable fall off a workbench or tailgate, for example, can lead to a longer productive life than those that do not. The addition of a roll bar is a simple way to protect all the gauges and connections from being damaged in a fall.
Some of the more advanced models aid in troubleshooting with onboard diagnostics that indicate what type of setup or error condition is occurring. For example, if incoming air pressure drops below an acceptable level for optimal performance, the unit will shut down and an LED will flash a low-pressure-condition message on the front panel. Or, if the consumable components are not installed correctly, another LED will show a parts-in-place error. Units have become more user-friendly in recent years with the focus being on uptime and ease of use.
Plasma cutting produces an intense light from the arc that could cause eye damage without proper protection. The Occupational Safety and Health Administration (OSHA) recommends eye protection with a minimum protective shade of 8. Leather gloves should be worn to protect hands from heat and sparks.
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