August 11, 2009
What is plasma cutting, and when is it your best metal cutting option? What information do you need to choose the right plasma system? This article answers these questions and more about plasma cutting.
The basic technology for plasma cutting has been around for decades. Researchers and engineers remain focused on increasing cut speed, improving cut quality, and extending consumable life, while making systems smaller and more powerful.
Once a gas is heated to an extremely high temperature and ionized, it becomes electrically conductive and is considered to be plasma. 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 arc's heat and then blown away.
The plasma arc's heat—around 40,000 degrees F (22,000 degrees C)—produces a clean, dross-free cut with minimal heat input. The process usually requires very little rework or cleanup.
Plasma cutting is an excellent choice for most cutting applications, as it cuts both ferrous and nonferrous materials, but it is particularly well-suited for those in which speed and cut quality are important.
Plasma cuts with low or no heat-affected zone, especially on thin metals. However, plasma cutting has its limitations. It remains more cost-effective to cut thicker (more than 2 in.) ferrous materials with oxyfuel.
A plasma arc is hot enough to burn through most surface coatings, such as paint and rust, as long as the ground clamp makes a good connection with the workpiece. Therefore, less preparation work is required. Difficult shapes, such as ventilation ductwork (HVAC), tanks, and vessels can be easily cut with plasma.
Selecting a plasma cutting system simply requires you to answer some basic questions. The answers will direct you toward the system that best fits your needs.
1. How thick is the metal I want to cut? The thicker the material, the higher the required amperage.
The industry is flooded with terms such as severance, recommended, maximum, rated, pierce, edge-start, and productionthat can confuse plasma machine selection. Look for a machine that can cut the desired thickness all day, every day, but has the extra punch to cut thicker material when the need arises.
2. What cut quality do I require? A higher-amperage machine with variable power will give you the flexibility to adjust the output to produce the desired cut quality on multiple thicknesses.
3. What type of metal am I cutting? Nonferrous materials are more difficult to cut and require slightly more power than mild steel of the same thickness. If you will be cutting nonferrous material, make sure you read the specifications closely, as most published specifications are based on cutting mild steel.
4. What is my primary input power? Small plasma systems have the ability to run on 110-V and 220-V power. However, most plasma systems require more amperage and need at least 220-V power in single- or three-phase. Systems in the 60- to 120-amp range can use various input powers; increasing voltage lowers the amperage draw of the plasma system.
If you are prepared with your answers, a plasma system distributor or welding supply house can suggest an appropriate machine.
As you consider plasma cutting for your application, it is helpful to understand process terminology: drag tip cutting, standoff cutting, drag shield cutting, and gouging.
Drag tip cutting—The preferred method of cutting light-gauge metal up to ¼ in., drag tip cutting produces the best cut quality, narrowest kerf width, and fastest cutting speeds. This method allows the tip to be in contact with the work. The distance between the work and plasma arc remains constant, and the cut quality stays consistent. Using nonconductive material, you can use a template to trace a pattern.
Standoff cutting—Preferrable for thicker metal and at current levels above 60 amps, standoff cutting separates the tip from the workpiece and requires you to keep the distance between the tip and workpiece constant. Keeping the tip away from the molten metal prevents the buildup of slag on the tip and can improve consumable life. Standoff cutting guides can be used to maintain a constant distance.
Drag shield cutting—An operator-friendly method of cutting between 70 and 120 amps while maintaining a constant standoff distance, drag shield cutting allows the torch to rest on the work during the cutting process, but uses a special shield to prevent the buildup of slag and spatter on the tip. Nonconductive templates can be used to cut straight lines or a pattern.
Gouging—Using a gouging tip and angling the torch to the lead angle of 35 to 45 degrees is a simple way to remove metal. A typical application is the removal of an existing weld during repair work. Gouging tips allow the plasma arc to diffuse and widen depending on the size of the weld and how deep you want to remove the metal.
Plasma units work in demanding environments. Because they lack moving parts, they require little routine maintenance. However, supplying clean, dry air and keeping the consumables in good condition ensure optimum performance day in and day out.