Cold Drawing is a metal forming process where a piece of metal is forced through a single or series of dies to reduce the cross-sectional size of the original part. The cold drawing process produces a dimensionally consistent end product while increasing yield and tensile strength. The resulting surface is clean and free of oxidized skin or other defects. Cold drawing is commonly used to produce precision steel, copper and aluminum bars and wire that must meet stringent physical and mechanical specifications.
During the cold drawing process, dies are held in place using a stationary platform called a drawing table. The table has an infeed side and an outfeed side. The outfeed side consists of a long roller platform used to capture the finished blank after it has been forced through the die. Metal feedstock, usually in the form of bar or wire, is loaded onto the infeed side. One end of the metal billet is reduced in size to allow it to pass through the mold. A hydraulic cylinder or winch is then used to pull the metal through the die and onto the discharge rollers.
There are three main types of cold drawing: tube drawing, bar drawing and wire drawing. Tube drawing decreases the inside diameter (ID) and outside diameter (OD) while increasing the length of the tube. The thickness of the tube 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 can remain constant, although usually both dimensions are reduced. Drawing reduces the outer diameter of the wire while increasing the length. The volume of the wire remains constant. Drawing usually requires several stages to reduce the wire to the desired size. Drawing dies are usually made of tungsten carbide or diamond.
In addition to changing the physical dimensions, cold drawing changes certain mechanical properties of the original blank. In many materials, the tensile strength can be increased by up to 30% and the yield strength by up to 80%. The change in mechanical properties is related to the amount of reduction in the cross-section of the virgin billet. An initial small reduction in cross-section results in a large change in mechanical properties, although the rate of change decreases rapidly as the reduction increases.
Cold drawing is used in the manufacture of copper cables and home 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 a variety of musical instruments and electrical components as well as hydraulic tubing and piping.
