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Cold drawing is a metal forming process whereby a piece of metal is forced through a single or series of dies, thereby reducing the cross section size of the original part. The cold drawing process yields a dimensionally consistent final product, while improving both yield and tensile strength. The resulting surface is clean and free from scale or other defects. Cold drawing is typically used to produce precision steel, copper and aluminum rods, bars and wire that must meet tight physical and mechanical specifications.
During the cold drawing process, a stationary platform called a drawing bench is used to hold the die in place. The bench has an in-feed and an out-feed side. The out-feed side consists of a long roller platform, used to catch the finished stock after it has been forced through the die. The metal stock, usually in bar or wire form, is loaded into the in-feed side. One end of the metal stock is reduced in size so it fits through the die. A hydraulic cylinder or winch is then used to pull the metal through the die and onto the out-feed rollers.
There are three primary types of cold drawing: tube drawing, bar drawing and wire drawing. Tube drawing reduces both the inside diameter (ID) and the outside diameter (OD), while increasing the length of the tubing. The thickness of the tubing wall remains constant. Bar drawing reduces the cross-section of the original bar stock, while increasing the length. The width or height of the original stock may remain constant, although usually both dimensions are reduced. Wire drawing decreases the OD of the wire stock while increasing the length. The volume of the wire remains constant. Wire drawing usually requires several stages to reduce the wire to the desired size. Dies used in wire drawing are usually manufactured from tungsten carbide or diamond.
In addition to changing the physical dimensions, cold drawing also alters certain mechanical properties of the original stock. Increases of 30% in tensile strength are possible, and gains of 80% in yield strength are often seen in many materials. Changes in mechanical properties are related to the amount of reduction in the cross section of the original stock. A small initial reduction in cross section can lead to large changes in mechanical properties, although the rate of change decreases rapidly as the amount of reduction increases.
Cold drawing is used in the manufacturing of copper electrical cables and household wiring as well as structural steel and industrial piping. The cold drawing process produces many common everyday items, such as paper clips, guitar strings and springs. Other applications include parts for various musical instruments and electrical components as well as hydraulic tubing and piping.
Good article. Can anybody share more information of the cold drawing die design and finish product tolerance?