The Drawing Process

The Drawing Process

In drawing, the cross-section of a long rod or wire typically is reduced or changed by pulling (hence the term drawing) it through a die called a draw die (Fig). Thus, the difference between drawing and extrusion is that in extrusion the material is pushed through a die, whereas in drawing it is pulled through it. Rod and wire products cover a very wide range of applications, including rods for shafts for power and motion transmissions, machine and structural components, and as blanks for bolts and rivets, electrical wiring, cables, tension-loaded structural members, welding electrodes, springs, paper clips, spokes for bicycle wheels, and stringed musical instruments.
The major processing variables in drawing are similar to those in extrusion-that is, reduction in cross-sectional area, die angle, friction along the die-workpiece interfaces, and drawing speed. The die angle influences the drawing force and the quality of the drawn product.

As can be seen from these equations, the drawing force increases as reduction increases. However, there has to be a limit to the magnitude of the force, because when the tensile stress reaches the yield stress of the metal being drawn, the work-piece will simply yield and, eventually, break. It can be shown that, ideally and without friction, the maximum reduction in cross-sectional area per pass is 63%. Thus, for example, a 10-mm diameter rod can be reduced (at most) to a diameter of 6.1 mm in one pass without failure.
It can be shown that, for a certain reduction in diameter and a certain friction-al condition, there is an optimum die angle at which the drawing force is a minimum. This does not mean that the process should be carried out at this optimum angle, be¬cause there are other product quality considerations.

Drawing of other shapes. Various solid cross-sections can be produced by draw¬ing through dies with different profiles. Proper die design and the proper selection of reduction sequence per pass require considerable experience to ensure proper material flow in the die, reduce internal or external defects, and improve surface quality.

The wall thickness, diameter, or shape of tubes that have been produced by extrusion or by other processes described in this book can be reduced further by tube-drawing processes (Fig). Tubes as large as 0.3 m (12 in.) in diameter can be drawn by these techniques. Mandrels of various profiles are available for these operations.

Wedge-shaped dies are used for the drawing of flat strips and are used only in specific applications. However, the principle of this process is the fundamental de¬formation mechanism in ironing, used extensively in making aluminum beverage cans.