Wire or rod stock is drawn through one or more tapered wire drawing die in succession. As the wire is pulled through the die(s), its volume remains the same, so the change in diameter is inversely proportional to the change in length, i.e. as the diameter decreases, the length increases, and vice versa. Reductions of the diameter of the wire can range up to 45 percent per pass—smaller wires being reduced 15-25 percent, and larger sizes at 20-45 percent. Drawing is usually performed at room temperature, but it may be performed at elevated temperatures for large wires or rods or hollow sections in order to reduce forces (Kalpakjian).
Drawing very fine wires wire by wire is possible but costly. Rather than drawing one wire at a time, wires can be drawn in bundles through a die. In a bundle, wires are separated by a metal with similar properties as the wires being drawn, but with lower chemical resistance so that it can be removed after drawing. Wires and rods can also be drawn into different shapes by using differently shaped dies. This is a very difficult process though, and takes much experience to perform correctly.
Drawing dies are typically made of tool steels and carbides. For hot drawing, cast-steel dies are used, and for drawing fine wires, diamond dies may be used. For steel wire drawing, a tungsten carbide inner "nib" is inserted inside a steel casting. The hard carbide provides a wear resistant surface for the reduction of the steel wire. Die angles usually range from 6-15 degrees, and each die has at least 2 different angles—the entering angle and approach angle.
Lubrication in the drawing process is essential for maintaining good surface finish and long die life. The following are different methods of lubrication:
• Wet drawing: the dies and wire or rod are completely immersed in lubricant
• Dry drawing: the wire or rod passes through a container of lubricant which coats the surface of the wire or rod
• Metal coating: the wire or rod is coated with a soft metal which acts as a solid lubricant
• Ultrasonic vibration: the dies and mandrels are vibrated, which helps to reduce forces and allow larger reductions per pass (Kalpakjian)
Kalpakjian, Serope and Steven R. Schmid. Manufacturing Engineering and Technology. New Jersey: Pearson Prentice Hall, 2006: p.415-419, 5th Ed.