Fabrication of structural steel by plasma and laser cutting

Fabrication of dimensional (non-flat) structural steel elements has historically been performed by sequential operations involving sawing, drilling and high temperature flame cutting to remove material. Each of these operations is performed on special purpose machinery; hence the time involved in loading/unloading and transporting the structural steel elements between machines can add considerable time to the total fabrication process.

In recent years, developments in plasma cutting and laser cutting of metals have been combined with computer motion control to accomplish the sequential operations on a single machine. This has the advantage of minimizing the non-productive loading/unloading/transport time, and can also improve dimensional accuracy of the fabricated element, due to the use of position sensors and highly accurate servo motor drives to postion the cutting head or "torch" of the machine.

Background: Structural Steel Elements

Structural steel is often thought of as the "skeleton" of multi-story construction, in that it provides the framework upon which floor, wall and exterior cladding systems are affixed. Individual pieces of structural steel (interchangeably called elements, sections or members) are produced in steel mills or foundries, conforming to chemical composition and geometric/dimensional specifications established by regulatory agencies and industry associations, such as the American Institute of Steel Construction.

The most common structural steel elements are beams (also known as I-beams, H-beams or girders), channels, HSS (for hollow structural shapes), angles, columns and plate. These elements are cut to required lengths and joined together, either by welding or mechanical fastening (bolting) in the manner prescribed to achieve the objectives for supporting both static and dynamic loads.

Traditional Fabrication Methods

Fabrication (cutting and drilling features) of structural steel elements has always been performed using "metal against metal" techniques, and these remain the most widespread fabrication methods today. The emergence of CNC (computer numerical control) technology brought automation and greater accuracy to these techniques, resulting in families of special purpose machines dedicated to performing individual fabrication tasks.

Perhaps the most common such machine is the 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 to achieve the prescribed length.

A beam drill line (drill line) has long been considered an indispensible way to drill holes and mill slots into beams, channels and HSS elements. 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 torches are the most common technology 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.

Fabricating flat plate is performed on a plate processing center where the plate is laid flat on a stationary 'table' and different cutting heads traverse the plate from a gantry-style arm or "bridge." The cuttting heads can include a punch, drill or torch.

Plasma and Laser Technologies Applied To Industrial Metal Cutting

[http://home.howstuffworks.com/plasma-cutter1.htm/ Plasma Cutting] - a technology that grew out of plasma welding in the 1960s - 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. Early plasma cutters were large, somewhat slow and expensive and, therefore, tended to be dedicated to repeating cutting patterns in a "mass production" mode.

As with other machine tools, CNC (computer numerical control) 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 that were programmed into the machine's numerical 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 (referred to as X Y cutting).

Industrial [http://science.howstuffworks.com/laser.htm/ Laser technology] followed a commercialization path for industrial use similar to that of plasma, but roughly a decade later. Industrial laser cutting technology for metals has the advantages over plasma cutting of being more precise and using less energy when cutting sheet metal, however, most industrial lasers cannot cut through the greater metal thickness that plasma can. Newer lasers machines operating at higher power (6000 watts, as contrasted with early laser cutting machines' 1500 watt ratings) are approaching plasma machines in their ability to cut through thick materials, but the capital cost of such machines is much higher than that of plasma cutting machines capable of cutting thick materials like steel plate.

The majority of industrial laser cutting machines are also used to cut flat materials, using two axes of motion for the cutting head.

Multi-Axis Plasma and Laser Cutting of Structural Sections

Starting in the late 1990s, programmable industrial robots were integrated with plasma and laser cutting to allow these metal cutting technologies to be applied to more generalized cutting of non-flat shapes. These "3D Systems" use the industrial robot to move the laser or plasma cutting head around the element to be cut, so that 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 that the element itself can be rotated or indexed forward or backward in concert with the movement of the cutting head. This serves to decrease overall cutting time and increase accuracy by optimizing the motion of the element with the motion of the cutting head.

Robotic 3D laser cutting systems frequently make use of this technique of moving the element to be cut, because laser systems work well with smaller thin-wall elements such as tubes. As OD and wall thickness of the pipe/tube increases, 3D laser cutting becomes less attractive due to the increased cutting time and higher capital cost of laser cutting technology.

Robotic plasma cutting is more widely used for 3D cutting of pipe, including HSS, used as structural steel elements. [http://www.vernontool.com/pagesnew/pipecutting.htm/ Vernon Tool Company] was an early innovator in developing 3D plasma cutting machinery for oil/gas field and structural tube/pipe. Similar systems introduced by Quick-Pen, Watts Specialties and Bickle Manufacturing are capable of cutting pipe diameters up to 32 inches and making straight, angled and saddle cuts, including beveled-edge cuts needed for joining together different pipes.

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.

[http://www.burlingtonautomation.com/ 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 [http://www.pythonx.com. PythonX] is able to cut a variety of features (bolt holes, copes, notches) or marks into exact locations along the structural elements. This extends the automated 3D plasma cutting capability pioneered by Vernon Tool and others 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.

If the past is prologue, it might be expected that robotic 3D laser cutting technology will soon be commonly applied to the fabrication of structural steel elements, as has already been done with plasma cutting. The steel thickness limitation of laser cutting has been overcome by the evolution of more powerful laser systems. However, as a general rule, tolerances on structural steel elements are less exacting than for other manufactured steel goods (such as auto components), therefore the extra precision that laser cutting offers is typically not required for structural steel. Areas of exception may be structural elements for ships and large, highly customized fabrications for power plants. For the time being, the lower capital cost and higher cutting speeds of robotic 3D plasma cutting make it the technology of choice for generalized fabrication of structural steel elements.

Citations

* [http://www.modernsteel.com/Uploads/Issues/August_2007/30768_nucor_web.pdf "How Structural Steel Is Made" by Bradford McKee, Modern Steel Construction, August 2007.]
* [http://www.thefabricator.com/PlasmaCutting/PlasmaCutting_Article.cfm?ID=1781 "The Life and Times of Plasma Cutting - How The Technology Got Where It Is Today" by Thierry Renault and Nakhleh Hussary, The Fabricator, November 2007.]
* [http://www.thefabricator.com/PlasmaCutting/PlasmaCutting_Article.cfm?ID=675 "Making Plasma Cutting Easier - Using CNC Automation Technology" by Brad Thompson and Kris Hanchette, The Fabricator, August 2003.]
* [http://www.thefabricator.com/Sawing/Sawing_Article.cfm?ID=623 "Making Hands-Free Straight, Saddle and Miter Cuts" by Eric Lundin, Tube & Pipe Journal, June 2003.]
* [http://www.thefabricator.com/LaserCutting/LaserCutting_Article.cfm?ID=144 "Science Nonfiction" by Kevin Cole, The Fabricator, August 2002.]
* [http://www.thefabricator.com/LaserCutting/LaserCutting_Article.cfm?ID=1182 "Tube, Profile Cutting With Lightning Speed - Laser Cutting Tube With A Rotary Axis" by Dr. A. Pieter Schwarzenbach, The Fabricator, October 2005.]
* [http://www.thefabricator.com/TubePipeFabrication/TubePipeFabrication_Article.cfm?ID=137 "Focusing On Tube Cutting Lasers" by Eric Lundin, Tube & Pipe Journal, November 2002.]
* [http://www.pythonx.com/library/ABB_casestudy_burlington_3.pdf "Burlington Automation - Breaking The Robot Barrier" Case Study from ABB Robotics.]