Plasma Cutting: Then and now
To get a better idea of just how far plasma cutting has coe, let's take a look at where it started and where it's headed.
|Plasma cutting has come a long way since paper punch machines were used to write G-codes for the cutting controller. For example, 10-gauge cold-rolled steel sheet that's cut with a high-precision plasma torch yields a part with an accuracy of ±0.012 in.|
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.
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-1950s. 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-1960s 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-1970s 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.
|User-friendly controls, at the cutting machine or on a nearby desktop, make setup on today's plasma cutting machines much simpler and quicker.|
In the 1980s 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 1980s. 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 1980s, 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 1980s 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 1990s. 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:
- 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.
|Automatic nesting software, common with many new plasma cutting machines, has made it easier to maximize material usage.|
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.
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.
Interested in learning how to clean-cut stainless steel with plasma technology? Click here for the answer.
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
The FABRICATOR is North America's leading magazine for the metal forming and fabricating industry. The magazine delivers the news, technical articles, and case histories that enable fabricators to do their jobs more efficiently. The FABRICATOR has served the industry since 1971.