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Metal fabrication is the building of metal structures by cutting, bending, and assembling processes.
It is a value added process that involves the creation of machines, parts, and structures from various raw materials. A fabrication shop will bid on a job, usually based on the engineering drawings, and if awarded the contract will build the product. Large fab shops employ a multitude of value added processes in one plant or facility including welding, cutting, forming and machining. These large fab shops offer additional value to their customers by limiting the need for purchasing personnel to locate multiple vendors for different services. Metal fabrication jobs usually start with shop drawings including precise measurements, then move to the fabrication stage and finally to the installation of the final project. Fabrication shops are employed by contractors, OEMs and VARs. Typical projects include loose parts, structural frames for buildings and heavy equipment, and stairs and hand railings for buildings.
Fabrication comprises or overlaps with various metalworking specialties:
The raw material has to be cut to size. This is done with a variety of tools.
The most common way to cut material is by shearing.
Special band saws designed for cutting metal have hardened blades and a feed mechanism for even cutting. Abrasive cut-off saws, also known as chop saws, are similar to miter saws but have a steel-cutting abrasive disk. Cutting torches can cut very large sections of steel with little effort.
Burn tables are CNC cutting torches, usually natural gas powered. Plasma and laser cutting tables, and water jet cutters, are also common. Plate steel is loaded on a table and the parts are cut out as programmed. The support table is made of a grid of bars that can be replaced. Some expensive burn tables also include CNC punch capability, with a carousel of different punches and taps. Fabrication of structural steel by plasma and laser cutting introduces robots to move the cutting head in three dimensions around the material to be cut.
Forming is an operation that converts a flat sheet metal work piece into a 3-D part. In this process, a raw material piece is formed by applying force to an object without adding or removing material. The force must be great enough to change the object's initial shape. The process of forming can be controlled with the use of tools such as punches or dies. Machinery can also be used to regulate force magnitude and direction. An example of machine-based forming can also combine forming and welding to produce lengths of fabricated sheeting, most commonly seen in the form of linear grating (used principally for water drainage).
Proper design and use of tools with machinery creates a repeatable form which can be used to create products for many industries, including jewelry, aerospace, automotive, construction, civil and architectural.
Machining is the process of removing unwanted material from the block of metal to get the desired shape. Machining is a trade in and of itself, although fab shops generally entail a limited machining capability including metal lathes, mills, and drills, along with other portable metal working tools.
Most components such as gears, bolts, screws, and nuts are manufactured by a machining process.
Welding is the main focus of steel fabrication. The formed and machined parts will be assembled and tack welded into place then re-checked for accuracy. A fixture may be used to locate parts for welding if multiple weldments have been ordered.
The welder then completes welding as per the engineering drawings if welding is detailed, or as per his/her own judgement if no welding details are provided.
Special precautions may be needed to prevent warping of the weldment due to heat. These may include re-designing the weldment to use less weld, welding in a staggered fashion, using a stout fixture, covering the weldment in sand during cooling, and straightening operations after welding.
Straightening of warped steel weldments is done with an oxy-acetylene torch and is somewhat of an art. Heat is selectively applied to the steel in a slow, linear sweep. The steel will have a net contraction, upon cooling, in the direction of the sweep. A highly skilled welder can remove significant warpage using this technique.
Steel weldments are occasionally annealed in a low-temperature oven to relieve residual stresses. Such weldments, particularly those employed for engine blocks, may be line-bored after heat treatment.