Wire Drawing

Introduction

In drawing, the cross section of a long rod or wire is reduced or changed by pulling (hencethe term drawing) it through a die called a draw die (Fig. 7.1). Thus, the difference betweendrawing and extrusion is that in extrusion the material is pushed through a die, whereas indrawing it is pulled through it. Although the presence of tensile stresses is obvious indrawing, compression also plays a significant role because the metal is squeezed down asit passes through the die opening. For this reason, the deformation that occurs in drawing issometimes referred to as indirect compression. Drawing is a term also used in sheetmetalworking. The term wire and bar drawing is used to distinguish the drawing processdiscussed here from the sheet metal process of the same name. Rod and wire productscover a very wide range of applications, including shafts for power transmission, machineand structural components, blanks for bolts and rivets, electrical wiring, cables,..Etc.

    Fig (7.1) Process variables in wire drawing. The die angle, the reduction

in cross sectional area per pass, the speed of drawing,the temperature and the lubrication all affect the drawing force, F.

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 interface, and drawing speed. The die angle influences the drawing force and the quality of the drawn product.

The basic difference between bar drawing and wire drawing is the stock size that is processed. Bar drawing is the term used for large diameter bar and rod stock, while wire drawing applies to small diameter stock. Wire sizes down to 0.03 mm (0.001 in) are possible in wire drawing.

Bar drawing is generally accomplished as a single-draft operation—the stock is pulled through one die opening. Because the beginning stock has a large diameter, it is in the form of a straight cylindrical piece rather than coiled. This limits the length of the work that can be drawn. By contrast, wire is drawn from coils consisting of several hundred (or even several thousand) feet of wire and is passed through a series of draw dies. The number of dies varies typically between 4 and 12.

In a drawing operation, the change in size of the work is usually given by the area reduction, defined as follows:

Where r=area reduction in drawing; Ao=original area of work, mm square (in2); and Af=final area, mm square (in2). Area reduction is often expressed as a percentage.

In bar drawing, rod drawing, and in drawing of large diameter wire for upsetting and heading operations, the term draft is used to denote the before and after difference in size of the processed work. The draft is simply the difference between original and final stock diameters: