WIRE DRAWING
Wire drawing is a simple process. In this process, rods made of steel or non ferrous metals and alloys are pulled through conical dies having a hole in the centre. The included angle of the cone is kept between 8 to 24°. As the material is pulled through the cone, it undergoes plastic deformation and it gradually undergoes a reduction in its diameter. At the sametime, the length is increased proportionately. The process is illustrated in Figure 888
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| WIRE DRAWING |
The dies tend to wear out fast due to continuous rubbing of metal being pulled through it.
Hence they are made of very hard material like alloy steel, tungsten carbide or even diamond. In one pass, the reduction in cross-sectional area achieved is about 25–30%.
Hence in a wire drawing plant, the wire has to pass thorugh a number of dies of progressively reducing diameter to achieve the required reduction in diameter.
However as the wire passes through dies and undergoes plastic deformation, it gets strain hardened. Its strength increases and capacity to further undergo plastic deformation decreases.
Therefore during the entire run of the wire, from time to time, it has to be heated (and cooled) to remove the effect of work-hardening.
This process is called “in process annealing”. The aim is to make the material soft and ductile again so that the process of drawing may be smoothly carried out.
The metal rods to be drawn into wires must be absolutely clean. If necessary, they are pickled in an acid bath to dissolve the oxide layer present on the surface. Its front end is then tapered down so that it may pass through the hole in the die which is firmly held in the wire drawing machine.
The wire is drawn by means of a number of power driven spools or rotating drums.
During wire drawing, a great deal of heat is generated due to friction between the wire rod and the die. To reduce friction, dry soap or a synthetic lubricant is used. But despite reducing friction, the dies and drums may have to be water cooled. The preferred material for dies is tungsten carbide but for drawing fine wire, use of ruby or diamond dies is preferred.
The drawing machines can be arranged in tandem so that the wire drawn by the previous die may be collected (in coil form) in sufficient quantity before being fed into the next die for further reduction in diameter.
As the diameter becomes smaller, the linear speed of wire drawing is increased.
The major variable in wire drawing process is (1) Reduction ratio (2) Die angle and (3) Friction. Improper control of these parameters will cause defects in the drawn material. Defects include centre cracking (as in extrusion and for the same reasons) and formation of longitudinal scratches or folds in the material.
TUBE DRAWING
The ‘drawing’ process can also be used for tube drawing. Tube drawing does not mean manufacturing a tube from solid raw material. It means lengthening a tube reducing its diameter. Various arrangements used for tube drawing are shown in Figure.
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TUBE DRAWING |