Steel shops are commonly organized into departments such as receiving, detail material, main material cut-and-preparation, assembly and shipping. Many shops also have paint departments.
Material is received on trucks or by rail, off loaded, compared to order requirements, and stored by project or by size and grade. Material is received from the mill or warehouse marked with the size, specification, grade, and heat number. The specification and grade marks are maintained on the material that is returned to stock from production.
Material handling is a major consideration in a structural shop and organized storage is a key to reducing handling.
Flame cutting steel with an oxygen-fed torch is one of the most useful methods in steel fabrication. The torch is used extensively to cut material to proper size, including stripping flange plates from a wider plate, or cutting beams to required lengths. The torch is also used to cut complex curves or forms, such as those encountered in finger-type expansion devices for bridge decks. In addition, two torches are sometimes used simultaneously to cut a member to size and bevel its edge in preparation for welding. Also, torches may be gang-mounted for simultaneous multiple cutting.
Flame-cutting torches may be manually held or mechanically guided. Mechanical guides may take the form of a track on which is mounted a small self-propelled unit that carries the torch. This type is used principally for making long cuts, such as those for stripping flange plates or trimming girder web plates to size. Another type of mechanically guided torch is used for cutting intricately detailed pieces. This machine has an arm that supports and moves the torch. The arm may be controlled by a device following the contour of a template or may be computer-controlled.
In the flame-cutting process, the torch burns a mixture of oxygen and gas to bring the steel at the point where the cut is to be started to preheat temperature of about 1600F. At this temperature, the steel has a great affinity for oxygen. The torch then releases pure oxygen under pressure through the cutting tip. This oxygen combines immediately with the steel.
As the torch moves along the cut line, the oxidation, coupled with the erosive force of the oxygen stream, produces a cut about 1â„8 in wide. Once cutting begins, the heat of oxidation helps to heat the material.
Structural steel of certain grades and thicknesses may require additional preheat. In those cases, flame is played on the metal ahead of the cut.
In such operations as stripping plate-girder flange plates, it is desirable to flame-cut both edges of the plate simultaneously. This limits distortion by imposing shrinkage stresses of approximately equal magnitude in both edges of the plate. For this reason, plates to be supplied by a mill for multiple cutting are ordered with sufficient width to allow a flame cut adjacent to the mill edges. It is not uncommon to strip three flange plates at one time using 4 torches.
Plasma-arc cutting is an alternative process for steel fabrication. A tungsten electrode may be used, but hafnium is preferred because it eliminates the need for expensive inert shielding gases. Advantages of this method include faster cutting, easy removal of dross, and lower operating cost. Disadvantages include higher equipment cost, limitation of thickness of cut to 11â„2 in, slightly beveled edges, and a wider kerf. Plasma is advantageous for stainless steels that cannot be cut with oxyfuel torches.
Shearing is used in the fabricating shop to cut certain classes of plain material to size.
Several types of shears are available. Guillotine-type shears are used to cut plates of moderate thickness. Some plate shears, called rotary-plate shears, have a rotatable cutting head that allows cutting on a bevel. Angle shears are used to cut both legs of an angle with one stroke. Rotary-angle shears can produce beveled cuts.
Sawing with a high-speed friction saw is often employed in the shop on light beams and channels ordered to multiple lengths. Sawing is also used for relatively light columns, because the cut produced is suitable for bearing and sawing is faster and less expensive than milling. Some fabricators utilize cold sawing as a means of cutting beams to nearly exact length when accuracy is demanded by the type of end connection being used. Sawing may be done with cold saws, band saws, or in some cases, with hack saws or friction saws. The choice of saws depends on the section size being cut and effects the speed and accuracy of the cut. Some saws provide a cut adequate for use in column splices. The adequacy of sawing is dependent on the maintenance of blades and on how the saw and work piece is set up.
