January 17, 2014
Metal fabricators with plasma cutting equipment that is 10 years or older may be surprised to learn just how far plasma cutting has come. The technology has evolved to the point that outside edges are smoother than ever, and precision holes can be cut to accommodate fasteners.
Plasma cutting has come a long way since it was first developed in the late 1950s by engineers at Union Carbide Corp. Today it is one of the most widely used metal plate cutting processes for a large variety of industries.
Early plasma cutting systems (see Figure 1) were used primarily for cutting stainless steel and aluminum plate from 0.5 to more than 6 in. thick. These systems, primitive by today’s design standards, were the most practical method for cutting heavy nonferrous plate. Most were mounted on XY cutting pantograph-style machines that used either photo-cell tracers to duplicate large black line engineering drawings of the parts to be cut, or a magnetic tracer to follow the path of a steel template.
Engineers continuously worked on the process throughout the 1960s with the goal of improving cut quality and the life of the consumable nozzles and electrodes in the cutting torch. Plasma began gaining momentum during this period as the process improved and as users became aware of its ability to cut complex shapes in nonferrous materials at very high speeds.
In 1968 radial water injection was introduced. This patented nozzle technology used pure water injected radially around the plasma jet to constrict the arc, increasing its energy density while improving nozzle cooling and thus allowing faster cut speeds, higher-quality cuts, and the ability to cut carbon steels at speeds four to six times faster than an oxyfuel cutting process.
At about that same time, XY coordinate drive cutting machine technology was being improved. Microprocessor control technology started to become the brains of the XY motion control machines, allowing for better accuracy, higher cutting speeds (necessary for the new-technology plasma systems), and higher levels of automation and productivity on the shop floor.
Through the 1970s plasma cutting technology replaced many oxyfuel-based steel cutting applications from 0.25 to 1 in. thick, while still maintaining its stronghold on the stainless and aluminum markets. While plasma could cut steel thicker than 1 in., the oxyfuel process still was a lower-cost alternative for heavier steel plate.
With the baseline of plasma’s early history established, let’s take a look at some of the major engineering breakthroughs with this technology:
1957 The plasma cutting process was developed and patented by Union Carbide as an extension of the gas tungsten arc welding (GTAW) process.
1962-1967 Several new developments were completed in consumable design, and the dual flow torch was designed to help improve consumable life and cut quality on nonferrous materials.
1968 The water injection process was commercialized. This process allowed for cutting with clean, square-cut edges and faster speeds, as well as cutting of carbon steels with acceptable cut quality.
1970-1979 The water table and water muffler, designed to provide fume and smoke control, debuted. Automated arc voltage-based height controls for more consistent cut quality and longer consumable parts life emerged.
1980-1984 Oxygen-based plasma cutting systems that helped improve edge squareness and edge metallurgy (softer, weldable edge) and allowed for cutting carbon steels at lower power levels and higher cut speeds (see Figure 2) were introduced.
1984-1990 Many developments in the air plasma cutting process allowed for better portability and lower power levels for hand cutting and mechanized thin-sheet cutting.
1990 Better power supply designs using pulse width-modulated, current-controlled outputs were developed. Some systems started to use lighter-weight, smaller inverter technology power supplies suitable for portable, hand-held plasma systems.
1992 Long-life oxygen process technology was introduced. This was essentially a microprocessor-controlled method of controlling plasma gas ramping pressures as well as power supply output amperage. It helped increase typical oxygen plasma consumable parts life by four to six times; improved parts consistency; and helped lower the cost of plasma cutting.
1993 High-definition plasma, a technique that required the previous long-life oxygen technology to implement, was developed. This process allowed for a new nozzle design that increased the energy density of an oxygen plasma arc by as much as four times, allowing for squarer, cleaner cuts in all material thicknesses.
1996 Automated gas flow control systems emerged. They interfaced digitally with the machines’ CNCs. These gas flow controls eliminated some of the potential for machine operator-related errors in setting parameters for the cutting process.
1996-2006 Many developments occurred relating to improving cut quality and productivity and automating the many process cut parameters. These included integrated plasma, a system that closely coupled the CNC, the plasma power supply, the gas flow control, the CAM software, and the height control system to automate the process. With this expertise built into the system, the machine operator’s job became much simpler, and the process relied less on operator expertise.
In the last seven years, developments in plasma cutting technology have come at a fast pace. The latest revision on high-definition machines is their full integration with the CNC machines they are coupled with. New CNCs have touchscreen accessibility, minimizing the number of buttons involved in operating a plasma cutting machine and making operation as simple as almost any Windows®-based software. Operator training has been simplified on even the largest, most complex CNC plasma cutting machines.
The operator’s job also has been made easier with improvements in auto-calibrating height control functionality. The operator does not need to make adjustments as the consumable parts in the torch wear out.
Hole cutting has been improved with a large database of information in the CAM software that automatically recognizes CAD features and implements the best possible cut path and plasma cutting parameters, including on-the-fly shield gas changes that nearly eliminate the normal taper found in plasma-cut holes on steel (see Figure 3). This process is transparent to the machine operator and system programmer, eliminating the need for them to be experts.
Improvements in cut-to-cut cycle times have been incorporated into CAM software. The software automatically recognizes areas of a full cutting nest (multiple parts) and modifies the traverse time, torch retract time, and gas preflow time to decrease production times and improve product throughput.
Nesting software now applies the lead-in points in the most effective way to avoid traversing over areas prone to collisions with previously cut parts.
Improved plate beveling software has simplified the integration and operation of a bevel head with XY CNC cutting machines. This advancement, again associated with the system’s CAM software, saves much of the programmer/operator trial-and-error testing that has always been necessary to hold the best tolerances on plate edge beveling applications, such as weld prep.
Very new vented nozzle and gas mixing technology has helped improve stainless steel edge quality. Edges are squarer, shiny, and weldable.
Air plasma cutting systems from the major manufacturers also improved dramatically in terms of cut quality, consumable life, and duty cycles. These systems, primarily designed for portable and in-shop hand-held cutting applications, now are available with quick-change mechanized torches and interface easily to a variety of lower-cost CNC machines. Systems are available from a 30-amp, toaster-sized unit that operates on 120-V household current to sever materials up to 0.5 in. thick, to a 125-amp, 100 percent duty cycle industrial unit that can sever 2.25-in. materials. Both portable systems can be used with a hand torch or can be mechanized for a variety of automated cutting applications.
Industrial mechanized systems typically are 100 percent duty cycle, available with machine torches, and designed to use a variety of compressed gases to fine-tune the cut quality for different materials. These systems are available in various sizes and capacities from 130 to 800 amps.
Many other advances have been made to improve reliability, performance, consumable life, cut quality, and ease of use since the first plasma system was created. The process shares the cutting market with laser cutting, abrasive waterjet, and oxyfuel cutting, all of which deliver accuracy, productivity, and long-term cost-effectiveness when used for the appropriate applications.
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