Wire drawing is a metalworking process that reduces wire diameter by pulling it through a die. It was originally done by hand and later with the help of a steam engine. The Bessemer process allowed for consistently high-quality wire production. The wire is tapered and dragged through the die to reduce its diameter. The wire is used for various applications, such as electrical networks, paper clips, springs, and musical instruments. Additional processing, such as annealing or galvanizing, may be done depending on the intended use.
Wiredrawing is a metalworking process that reduces the diameter of a wire by pulling it through a die designed for that purpose. Usually performed at room temperature, wire drawing differs from extrusion in that the wire is pulled through the die, rather than pushed. While the most commonly known application for drawn wire is wiring used for electrical and communications networks, there are also countless other uses: paper clips, springs, tire spokes, and musical wire (the wires used in violins, cellos, and other tools) are all made with drawn wire.
Wire was originally made by hammering metal, such as gold and silver, into very thin sheets and then cutting very thin slices from the sheets. These thin slices would be hammered back into shape until they were fine enough to be used for jewelry or to be woven into clothing. Archaeological evidence suggests that around 400 BC, metal workers were experimenting with wire drawing, shaping rough molds and drawing wire through them by hand.
Until the mid-19th century, the wire drawing process became more sophisticated, as craftsmen developed different techniques, including the use of the steam engine to power the actual wire drawing process. They learned to lubricate the wire as it was drawn, which reduced the amount of energy required to draw the wire and marginally improved its quality. However, the quality of drawn wire has always been limited by the quality of the metal it was made from. Metals of inconsistent purity and malleability break regularly when drawn into wire. The broken wire would have to be spliced together, a time-consuming process leading to a loss of quality, which was a critical problem for applications such as telegraph communication. The poor quality of the drawn wire increased the time required for production and made the wire very expensive.
It was not until the invention of the Bessemer process in the late 1850s, which produced consistently workable metal, that wire drawing was able to produce wire of a consistently high quality. The metal cast from the converters into molds called billets is cooled only slightly and then begins the wire forming process in a hot rolling mill, using the residual heat from the Bessemer process. In this process, large coils of thick wire, called wire rod, weighing 150 to 300 pounds (68 to 136 kilograms) are made.
Once the wire rod has been cleaned of surface impurities, the end is tapered enough to pass through the die, which in turn is tapered with the opening on one side wide enough to accommodate the wire rod, narrowing up to 40 percent on its length. The tip of the tapered rod is gripped tightly and dragged, reducing its diameter. Narrow wire is usually wrapped around a core, although it can sometimes be passed through a smaller die to continue the shrinking process. A thick wire can be reduced in diameter by up to 40 percent in one pass; thinner wire can be reduced by 15 to 25 percent.
To produce the very fine wires used in telephone wires and stranded electrical cables, the wire is drawn through successively narrower dies. Once drawn, the wire is sometimes subjected to additional processing, depending on its intended use. For example, a process called annealing, or heating the finished product to a certain temperature for a set amount of time, is done if the wire is to be flexible and springy. The thicker wire that will be cut into the nails is not annealed, but will often be galvanized or zinc coated, to prevent rusting. The wire used in fencing, such as barbed wire, is usually both annealed and galvanized.
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