The distinction between a rod and a wire is somewhat arbitrary because the word wire refers to small diameter products under 5 mm which may be drawn rapidly on multiple-die machines.
Wire drawing is the operation of stretching a metal rod into a small diameter wire by passing or extruding the rod through a die. The die has a typical tapering hole which helps in reducing the rod size gradually into a reduced diameter wire. When the rod diameter is sufficiently small to permit coiling, block drawing on multistep bull block is used as it allows making of long lengths in a much smaller flow area than required for a draw bench. According to the metal to be drawn and reduction required, wires are drawn by pulling the metal rod through one die or a series of dies (of decreasing hole diameter) arranged in tandem one after the other, to bring down the reduction in size gradually to obtain finally a thin wire. Because the area reduction per drawing pass is rarely greater than 30-35%, many reductions are needed to achieve overall reduction.
Multi-block machines with one die and one draw block for each reduction are common. The raw material for wire drawing is available in the form of hot rolled rods in a coiled form. The metals used for wire drawing include carbon steels, copper, aluminium, stainless steel, titanium and high temperature alloys.
Schematic representation of a wire drawing operation. The loosely coiled treated rod shown is placed at reel (A), whereas the finished drawn wire is coiled at reel (B). Note the gripping jaws (C) at reel (B) which grip the pointed end of treated rod (after it is passed through the die) for starting the drawing operation.
Stepped cone multiple pass wire drawing. Since the wire diameter will decrease after each pass, the velocity and length of the wire will increase proportionately. Thus the peripheral speed of each draw block must increase (by increasing diameter).
Before the actual operation of wire drawing, the raw material is given cleaning treatment by placing it in hot dilute sulphuric acid to remove rust and oxide from the rod, followed by washing in water and after drying, coating with lubricant to avoid corrosion and to reduce friction during drawing. With the high strength materials, a soft surface coating of copper or tin may be used as lubricant. Conversion coatings such as phosphates or chromates may also be used for lubrication.
These are used in conjunction with a lubricant, soap, in dry drawing. Soap solutions are used as lubricant which is picked up by the rod stock while passing through the soap containers before entering the die. Phosphoting of steel wires is sometimes done for lubrication purposes. The dies used for wire drawing are made of chilled cast iron, hardened alloy steel or tungsten carbide. The die may have several number of holes of varying diameters.
When wire is to be drawn to considerably smaller diameter, it is required to pass through more than one hole of the die during intermediate operations of drawing the wire to the correct size. Since considerable heat is generated in wire drawing operation, the dies are cooled by water circulating around them.
Multistage wire drawing machines (using multistep bull block or capston driven by single motor) are available which are capable of handling very long rods and wires. These are cheaper and more effective.
See More: Roll Design
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See More: Hot Rolling Process