Fabrication of structural steel elements has historically been performed by sequential operations involving sawing, drilling and high temperature flame cutting to remove material, each operations being performed on special purpose machinery.
Developments in plasma cutting of metals, combined with computer motion control, accomplish the sequential operations on a single machine. This has the advantage of minimizing the non-productive loading/unloading/transport time between machines.
Background: Structural Steel Elements
Structural steel – the “skeleton” of multi-story construction – provides a framework on which floor, wall and exterior cladding systems are affixed. Individual structural steel elements are produced in steel mills or foundries, conforming to chemical composition and geometric/dimensional specifications established by regulatory agencies and industry associations.
The most common structural steel elements are beams (I-beams, H-beams or girders), channels, HSS (hollow structural shapes), angles, columns and plate. These elements are cut to required lengths and joined together, either by welding or bolting in the manner prescribed to achieve objectives for supporting both static and dynamic loads.
Traditional Fabrication Methods
Fabrication of structural steel elements has always been performed using “metal against metal” techniques, and these remain the most widespread methods today. The emergence of CNC (computer numerical control) technology brought automation and greater accuracy to these techniques, resulting in special purpose machines dedicated to performing individual fabrication tasks.
The most common such machine is a bandsaw. A bandsaw employs a continuously rotating band of toothed metal to saw through the structural steel and is generally used to cut through the entire cross section of the element.
A beam drill line has long been considered an indispensable way to drill holes and mill slots. CNC beam drill lines are typically equipped with feed conveyors and position sensors to move the element into position for drilling, plus probing capability to determine the precise location where the hole or slot is to be cut.
For cutting irregular openings or non-uniform ends on dimensional (non-plate) elements, a cutting torch is typically used. Oxy-fuel is the most common type and range from simple hand-held torches to automated CNC ‘coping machines’ that move the torch head around the structural element in accordance with cutting instructions programmed into the machine.
Plasma Technology Applied to Industrial Metal Cutting
Plasma Cutting emerged as a very productive way to cut sheet metal and plate in the 1980s. It had the advantages over traditional “metal against metal” cutting of producing no metal chips and giving accurate cuts, and produced a cleaner edge than oxy-fuel cutting.
As with other machine tools, CNC technology was applied to plasma cutting machines in the late 1980s into the 1990’s, giving plasma cutting machines greater flexibility to cut diverse shapes “on demand” based on a set of instructions programmed into the machine’s control. These CNC plasma cutting machines were, however, generally limited to cutting patterns and parts in flat sheets of steel, using only two axes of motion.
Multi Axis Plasma Cutting of Structural Steel Sections
Starting in the late 1990s, programmable industrial robots were integrated with plasma cutting to accomplish more generalized cutting of non-flat shapes. These “3D Systems” use the robot to move the plasma cutting head around the element to be cut, so the cutting path may encompass the entire outer surface of the element. Many systems also grip the element to be cut in a “chuck” so the element itself can be rotated or indexed forward or backward with the cutting head.
Robotic plasma cutting is widely used for cutting of pipe, including HSS, used as structural steel elements. The task of robotic plasma cutting of more diverse shapes, such as beams and channels, has proven to be more challenging. The large sizes and variety of shapes involved make the technique of gripping the structural steel element in a chuck impractical. This places the entire burden of cutting motion back on the robot. In order to have the cuts and features placed where they are intended on the element, the robot must be given some instruction as to the location, size and shape of the element.
Burlington Automation developed software capable of reading CAD drawings of the structural element, and combining this information with motion control and sensor feedback to arrive at a 3D plasma cutting system that in effect “sees” the structural steel element it is to cut. There are no vision systems involved, rather the robotic arm that carries the plasma torch head gently touches (probes) the element to be cut in multiple locations and combines this information along with the CAD drawing data to determine the exact contours of the element in three dimensions. With this information, the robotic plasma cutting system, which goes by the trade name PythonX is able to cut a variety of features (bolt holes, copes, notches) or marks into exact locations along the structural elements. This extends 3D plasma cutting capability to the complete range of structural steel elements, thus allowing the PythonX system to replace beam drill lines, coping machines, bandsaws and plate burning centers.
More info on how PythonX replaces beam coping machines visit http://www.pythonx.com