Plasma cutting for tube, pipe, profiles

Plasma’s versatility enables it to cut a variety of pipe diameters and thicknesses. In this system, a single power supply is connected to three torches cutting three pipe sizes.

From its infancy in the 1960s, plasma cutting has progressed to become widely used in both machine-based and hand-held applications. Over the years technology improvements have led to better edge quality, faster cutting speed, longer consumable parts life, and lower long-term operating cost. These improvements have been applied primarily to CNC machines that cut flat sheet and plate into precision components for a variety of industries.

Technology developments also have opened the door to 3-D cutting. Virtually all of the technology applied to 2-D cutting can be applied to tube and pipe for industries such as construction and energy. While cut quality and overall plasma system performance are just as important for tube and pipe as they are for sheet and plate, the intricacies of 3-D cutting require newer and better motion control technology. These motion control capabilities were developed for the steel construction industry and come in many forms—robots, conventional CNC tables equipped with rotary axes, portable pipe cutting and beveling machines for field use, and CNC machines designed specifically for tube and pipe.

Plasma Improvements Condensed

Plasma cutting comes in two types: air plasma, which is the basic option, and high-definition, which uses gas mixtures to assist the cutting process.

Air Plasma Systems. In use since the mid-1980s, air plasma systems were developed primarily for hand-held cutting applications. Over time the major manufacturers have improved these portable cutting systems to the point that, equipped with a machine-mountable torch, they are highly productive cutting tools for many shop-based and field-based cutting.

Inverter technology has reduced power supply size, and at the same time power supplies have improved in reliability, duty cycle, operating cost, cut quality, and cut speed. A typical 85-amp air plasma system can pierce ¾-in. plate in 1.4 seconds, has a duty cycle of at least 60 percent, and operates on a variety of input voltages (200 to 600 V, single- or three-phase). A typical weight is 75 pounds, so they are both robust enough for day-in, day-out shop work and portable enough for field work. Many have quick-change torches, so they can be changed from machine-mounted to manually operated in a matter of seconds. The drawback is that air plasma cutting systems can create some edge hardening on many steels because of the nitrogen content in ambient air.

Typical uses of air plasma systems in tube and pipe fabrication are:

• Portable pipe cutoff machines that, in addition to cutting round tube and pipe, can do weld prep bevels in one or two passes.
• Robotic workcells for nearly any 3-D cutting application. The latest systems have a two- to six-wire CNC interface for easy integration and do not use high-frequency or high-voltage starting technology, which initially plagued robotic systems. Specifically designed robotic torches further improve capability and performance on 6-axis robotic applications.
• Purpose-built pipe and square tube cutting machines that cut to length, bevel, and cope.
• Small, low-cost, entry-level CNC machines with an optional rotary axis that can cut to length, cope, and make other tube- and pipe-related cuts, such as saddles. These usually are programmed as flat, 2-D cuts that the software adapts to 3-D cuts.
• Manual torches can be used for trimming or cutting, usually aided with guides or templates. They also can be used for low-powered plasma arc gouging to remove welds for rework.

High-Definition Plasma Systems. These are designed for shop use in mechanized applications only. These systems are meant for 100 percent duty cycle in shops that need high productivity and accurate, metallurgically pure cuts. Because high-definition plasma systems use oxygen as the plasma gas with compressed air as the shield gas, the cut edge has minimal chemical- or heat-affected zones, maintaining the structural integrity of the base material. Cut quality on low-carbon steels is similar to that on high-strength steels (HSS), and can be fine-tuned by adjusting the power to match the material type and thickness; for example, 30 amps for thin materials, 400 amps for piercing and cutting 2-in.-thick steel, and 800 amps to cut 6.25-in.-thick stainless steel or aluminum.

High-definition plasma systems have been in widespread use for flat plate cutting applications for nearly 20 years, yet recent developments have resulted in better plasma-to-CNC machine integration. This improved communication between the motion control device (whether robot, CNC cutting machine, or specialized tube cutting machine) simplifies the operator’s task, using PC-based control and CAM software to set gas flows, torch height, cut speeds, piercing technique, and other parameters. Further, hole quality with many of these integrated systems has improved to the point that bolt-quality holes can be made that meet many construction industry specifications for roundness and smoothness. In many cases, the taper is minimal, eliminating the need for separate drilling or punching operations.

Typical shop uses for high-definition plasma systems for tube and pipe fabrication include:

• Cutting to length, beveling, and coping on a rotary axis, an option that can be added to many X-Y cutting tables. Some of the more complex machines can bevel up to 45 degrees automatically on any shape of tube or pipe.
• Purpose-built machines use robotic technology and software that take the difficulty out of making complex cuts on hollow structural sections (HSS) and wide-flange sections. This conventionally has been a time-consuming process that required skilled layout and cutting personnel.
• Custom robotic cells often are developed by system integrators and industrial robotic manufacturers for precise, high-volume tube and pipe cutting. These applications are common, but usually designed for particular high-volume jobs.