Plasma Cutting: Then and now

It wasn't that long ago that plasma cutting was the domain of seasoned metal fabrication veterans who knew just how to tweak gas settings and adjust torch height to get the best cut on a plasma cutting table. Today many of the highly trained technicians have left the shop floor.

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What is left is the plasma cutting system, which in many cases still provides a quality cut but without the expertise needed to guide the machine. For many fabricators, this type of plasma cutting is the only type of plasma cutting they use.

That doesn't have to be the case, however. Technological advances have helped to automate programming for plasma cutting systems, and highly precise plasma cutting is now a reality.

To get a better idea of just how far plasma cutting has come, let's take a look at where it started and where it's headed.

That Was Then

Plasma cutting was invented in the mid-s. The patent holder learned that by sending a high-velocity jet of superheated gas through a constricted opening, ionized gas, or plasma, is created that can melt metal.

The predominant method of thermal cutting at that time was oxyacetylene cutting. An operator was required to control the torch head by hand, following a template, or the torch head could be mounted to a gantry-style trace-eye machine or a linear tracking rail.

By the mid-s NC gantry machines started becoming popular. The marriage of oxyacetylene to the NC gantry machines revolutionized the flame cutting process.

Plasma torches also were available on the NC gantry machines. Paper punch machines were used to write G-codes to the controller; however, only a limited number of people were capable of programming the machines.

Manufacturers and fabricators did not generally accept plasma cutting until the mid-s in the U.S. The reason for the delay in the popularity of plasma cutting can be attributed to the:

• system costs, especially the price of the gases required.
• Technology's inability to match the thickness levels that oxyacetylene could cut.
• Lack of public knowledge about plasma cutting capabilities.
• Lack of programming expertise.

In the s plasma cutting grew in popularity as more low-amperage systems began to be manufactured. Thinner metals now could be cut with plasma. Electronic torch height control also was introduced. Electronically controlling the torch-to-workpiece distance allowed the torch head to pierce the material at a greater distance from the workpiece, which minimized consumable wear and contributed to a more precise cut.

The PC also gained in popularity during this period. Equipment programmers used the PC to generate machine G-code or ASCII text files that were used to direct the machine controller. Many machines continue to use this technology today.

CAD programs that could generate machine code became available in the late s. The CAD programs combined with the PC provided a simple solution to programming the machine. The knowledge of G-code programming was no longer the only source to program the machine control. CNCs that allowed programming at the machine also became popular.

Also in the s, Apple introduced the icon-style user interface to the computer, and Microsoft soon after released the first Windows® operating system to compete. The navigation of the PC via text alone became outdated.

The s were a time that encouraged some pioneering fabricators to run machine tools directly from the PC. Many who operated their machines in this way found the user interface to be much more user-friendly when compared to rudimentary commercial control software offerings.

High-precision plasma cutting became available in the s. Many cutting applications required the edge quality a laser machine could produce&#;no dross and smooth edges&#;but not the precise accuracy. These high-precision-plasma cutting machines became an affordable option for these types of applications.

This Is Now

Today fabricators benefit from the combination of the innovations that emerged in the previous decades. The refinement of high-precision plasma technology, such as the HyDefinition® torches from Hypertherm and the FineLine® torches from InnerLogic Inc.; the advancement of electronic torch height control systems; and the evolution of computer control technology have resulted in a popular contour cutting solution.Machine motion has improved because of advancements made in linear guiding, servomotors, and gearing:

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• Linear guiding technology now features higher load capacities, allowing for a stronger guide system to be mounted in a smaller place.
• Sinusoidal AC servomotors are tuned digitally. The result is smoother motion and higher speeds.
• Antibacklash gear heads provide a more precise rotary-to-linear motion to achieve higher accuracy and torque.

Plasma cutting systems that feature these technologies have machine motion positioning accuracy of 0.004 inch. Systems of the past without sinusoidal AC servomotors, antibacklash gear heads, and precision linear guides normally would achieve only 0.015-in. positioning accuracy.

The tighter tolerance as related to positioning accuracy minimizes corner overshoot and maintains straighter lines during diagonal moves. This higher accuracy in machine motion directly corresponds to higher part accuracy.

The improvements to contouring technology, electronic torch height control, and high-precision torch systems enabled plasma machines to produce parts similar to laser-cut parts, but with slightly less accuracy. For example, 10-gauge cold-rolled steel sheet that's laser-cut normally yields a part with an accuracy of ±0.005 in., while the same material cut with a high-precision plasma torch yields a part within ±0.012 in. Having said that, plasma cutting systems can cut thicker materials faster than lasers and produce quality parts at the same time.

Based on operating costs and periodic machine maintenance, it is safe to say that plasma cutting is one of the most affordable contour-cutting machine choices to purchase and to operate. For instance, consider the cost of consumables, which are changed on a pay-as-you-go basis. Consumables are the electrodes, nozzles, and shield caps that total approximately $15 to $35 per change, depending on the torch system and material being cut. Laser and waterjet cutting machines also use consumables with similar costs, but those technologies require additional maintenance. Lasers periodically need mirrors and lenses replaced or aligned. Waterjets need water purifiers, seals replaced, and abrasive media systems. Both plasma cutters and lasers need cutting gases, but some fabricators are able to run their plasma systems on just compressed air.

What's Next?

When considering a contour-cutting machine, fabricators should keep the following in mind: edge quality and part accuracy needed, maintenance and operating costs, and simplicity of use.

A fabricator interested in newer plasma cutting technology also should focus on the machine's programming methods. A new plasma cutting system should have the ability to program parts with complex geometries, import DXF files from other CAD programs, or accept G-code programs. The system's software package also should provide automatic nesting of the part geometry and have the ability to change the nested part geometry by allowing the operator to drag the part around the sheet displayed on the computer screen. Finally, the software system should keep track of jobs&#;what has and has not been cut at the machine.

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Laser cutting grabs a lot of the headlines in the trade press, but plasma technology has remained a viable cutting technology alternative even in today's world of precision fabricating. Check out "Plasma pleases plenty"for more details.

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Keeping the guesswork out of the operator's hands, the control also should guide cutting parameters for different material types. A big point to consider is whether this type of programming will take place in an office, at the machine, or in both places.

Programming and the user interface are two of the most important differences among plasma cutting systems manufactured today as opposed to those manufactured even a decade ago. These two technologies make it possible to capture labor savings that are unattainable with older equipment and controls

Matt Walsh is vice president, Plasma Automation Inc., Arctic Ave., Bohemia, NY , 631-563-, fax 631-563-, info@plasma-automation. com, www.plasma-automation.com.

What is plasma cutting? It is a process that uses a plasma torch to cut through steel and other kinds of metal or materials of various thicknesses. An inert gas (in a few units, compressed air is used instead of a gas) comes out of a nozzle at high speed while an electrical arc forms through the gas to the surface that is to be cut. The arc turns some of the gas into plasma, which is hot enough to melt the metal that is to be cut and is also fast enough to blow the molten metal away from the cut as it is produced.

Plasma cutting began in the s. It came about as a part of plasma welding and was a very productive way of cutting plate and sheet metal in the s. There were several advantages to plasma cutting. For one thing, no metal chips were produced and cuts were more accurate. Another advantage was that plasma cutting produces a cleaner edge than oxy-fuel cutting. In the beginning of plasma cutting, the cutters were rather large, expensive, and slow. This led to plasma cutting being used in mass production areas. Take a look at the image below of an old plasma cutter.

Plasma torches used to be found only in well-stocked shops and private garages as well as in professional welding shops. Over time, the plasma torches became much cheaper and even hobbyists are now able to afford them. Advances in technology have led to newer plasma cutters that have inverter technology. This allows the units to work just as well, and in some cases better, than the original plasma cutters, but they are much lighter and easier to work with.

In just the last decade, advances have been made that allow plasma cutters to have smaller nozzles, thinner plasma arcs, and precision that is almost as exact as a laser. Some models of plasma cutters have become so small that they are easily held by hand and can be used for precision work anywhere a hand can reach. This has opened the door to many new uses for plasma cutting. Originally, plasma cutting machines were dedicated to simply cutting patterns in factories for the mass production of products. Even when the plasma cutting machines came to be controlled by computers &#; CNC plasma cutting machines &#; they were still too big and expensive. They were still limited to cutting parts and patterns out of flat steel sheets using X Y cutting.

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