What Cold Drawing Is
Cold drawing is a metal-forming process in which a piece of metal is forced through a single die or a series of dies at ambient temperature to reduce the cross-sectional size of the original part. The process produces a dimensionally consistent product with a clean surface, free of the oxide skin of hot processing, and it raises the yield and tensile strength through work hardening. Cold drawing is used to make precision steel, copper, and aluminium bars, wire, and tubes that must meet tight dimensional and mechanical specifications, and it is the finishing process that gives the product its final accuracy.
How the Process Works
The dies are held in a stationary platform called a drawing table, which has an infeed side and an outfeed side. The outfeed side is a long roller platform that captures the finished blank after it passes through the die. The feedstock, usually bar or wire, is loaded on the infeed side, and one end is reduced in size so that it can enter the die. A hydraulic cylinder or winch then pulls the metal through the die and onto the discharge rollers. The reduction per pass is limited by the strength of the pulled section and the die life, so large reductions are made in several passes with intermediate treatment as required.
The Three Types of Cold Drawing
There are three main types of cold drawing. Tube drawing decreases the inside and outside diameters of a tube while increasing its length; in the simplest sinking process the wall thickness stays roughly constant, while drawing over a plug or mandrel can also control the wall. Bar drawing reduces the cross-section of solid bar stock while increasing the length, with both dimensions usually reduced. Wire drawing reduces the wire diameter while increasing the length, with the volume remaining constant, and it normally requires several stages to reach the final size. The dies are made of tungsten carbide or diamond for long life and a consistent surface.
The Effect on Mechanical Properties
Cold drawing changes the mechanical properties of the material through work hardening. In many steels, the tensile strength can increase by up to 30% and the yield strength by up to 80%. The change depends on the amount of reduction in the cross-section: an initial small reduction produces a large change in the properties, but the rate of change decreases as the reduction grows. The same cold work reduces the ductility and toughness, so drawn products that must be formed or loaded in service are often normalized, annealed, or heat treated after drawing to restore the balance of properties.
Applications
Cold drawing is used to manufacture copper cables and home wiring, structural steel and industrial piping, and a wide range of everyday products such as springs, paper clips, and guitar strings. In the pipe industry, cold-drawn seamless tube is the precision product for hydraulic cylinders, bearing races, and instrument tubing, where the tight tolerance, smooth bore, and consistent strength are essential. Drawn bar and wire feed the machining, fastener, and spring industries.
Frequently Asked Questions
Q: What is cold drawing?
A metal-forming process that pulls bar, tube, or wire through a die at ambient temperature to reduce the cross-section, improve dimensions, and raise the strength.
Q: What are the three types of cold drawing?
Tube drawing, which reduces the tube diameter; bar drawing, which reduces the bar cross-section; and wire drawing, which reduces the wire diameter in stages.
Q: How does cold drawing affect strength?
Work hardening can raise the tensile strength by up to 30% and the yield strength by up to 80%, with the effect depending on the reduction and the material.
Q: Does cold drawing reduce ductility?
Yes. The cold work reduces plasticity and toughness, so drawn products are often annealed or heat treated afterwards when ductility is required.
Q: What are the dies made of?
Tungsten carbide for steel drawing and diamond for fine wire, giving long die life and a consistent surface.
Q: What is cold-drawn tube used for?
Hydraulic cylinders, bearing components, and precision tubing, where tight tolerance and a smooth surface are required.
