Cold drawing machine is the process of reducing the diameter of metal bars, rods and tubes by passing them through a series of dies of decreasing size. The result is a tube with a smoother finish and tighter dimensional accuracy than can be achieved through hot rolling or extrusion alone. The process involves both tension and compression, resulting in work hardening. The tensile strength of the finished product may be higher than that of the starting material, depending on its ductility. This process is mainly used on copper, aluminum and other nonferrous metals.
The process of drawing is usually carried out without heating the metal, and as a result the tensile and yield strengths increase while plastic properties decrease. The dependences on which these indicators change during cold working differ for each alloy. Therefore, the construction of the hardening curves for each of them is a laborious task. It is necessary to know the equations that describe changes in tensile and yield strength during drawing, in order to construct an accurate energy-power mathematical model of this process.
In the process of bar drawing, the beginning stock is a coiled coil of hot-rolled wire of a suitable size. The wire is drawn (pulled) through a series of dies that reduce the diameter and shape it into the desired cross sectional area. The last die in the drawing cycle has a final tooth geometry that corresponds to the desired end use of the bar or rod.
A mandrel may be inserted into the center of the die to assist in maintaining the correct position during the drawing process. The drawing dies are usually made of tungsten carbide or diamond. To achieve the desired dimensions, multiple passes through a progressively smaller set of dies may be required, along with intermediate anneals to prevent the formation of cracks in the wall.
Tube drawing is similar to bar drawing, except the beginning stock is a tubular piece of metal rather than a coiled coil. In this case, the tube is expanded to regain its original shape by applying pressure while passing it through cross rolls. The radial stresses cause the tube to thicken, which is useful in eliminating internal flaws and achieving high dimensional accuracy. The optimum die angle varies with the diameter-to-thickness ratio; high-angle dies tend to thin the tube walls, while low-angle dies cause them to thicken. The dies are also usually made of tungsten carbide or a hard material to provide good wear resistance. The drawing process can be controlled with the help of a computer and can be monitored via strain gauges. This allows the operator to control the bending of the dies to compensate for any changes in the size of the tube being drawn. The finished products are then coiled for storage or further processing. This type of drawbench is available in several different models and can be used to produce a wide variety of sizes and shapes of metal tube, pipe or rod.