Drawing is a metalworking process in which material is pulled through a die to reduce and control its cross-section. Wire drawing is used to produce wire with controlled diameter and surface finish, while related drawing processes are used to reduce and finish seamless tubes.
Wire and tube drawing are normally cold-working processes and may require several successive reductions to achieve the required dimensions. Die material, lubrication, reduction per pass and the properties of the workpiece all influence the drawing process and the quality of the finished product.
Wire drawing involves pulling rod or wire through a shaped die to reduce its diameter. The starting material may be hot-rolled rod which is cleaned or descaled before drawing.
As the material passes through the die its cross-sectional area is reduced and its length increases. The finished wire is normally collected on a motor-driven block or coiling system which also provides the pulling force.
Where a substantial reduction in diameter is required, it is generally impractical to obtain the complete reduction in a single pass. The wire is therefore progressively drawn through a series of dies, with each die reducing the diameter by a controlled amount.
Industrial wire-drawing machines can incorporate a number of dies and drawing blocks so that successive reductions are carried out continuously. The amount of reduction possible at each stage depends on the material, its condition, lubrication, die geometry and the required finished properties.
Drawing dies must withstand high contact pressure, sliding friction and wear while maintaining the required wire dimensions. Tungsten carbide is widely used as a die material, while diamond dies are used particularly for fine wire and applications requiring high wear resistance and close dimensional control.
A drawing die normally provides a controlled approach to the reducing region followed by a bearing or sizing region which assists in controlling the final wire diameter.
Lubrication is important in wire drawing because it reduces friction between the wire and die, limits die wear and helps control the surface condition of the finished wire.
Dry drawing processes commonly use drawing soaps or other solid lubricants, while different lubrication systems may be used according to the material, wire size, drawing speed and production process.
Modern continuous wire-drawing equipment can operate at very high speeds, particularly for small-diameter wire. The practical drawing speed depends on the wire material and diameter, reduction sequence, lubrication, cooling and drawing-machine design.
Seamless steel tube can be produced by first forming a hollow tube from a heated solid billet and subsequently reducing and finishing the tube using rolling and drawing operations.
Several manufacturing routes are available. A historically important method uses rotary piercing to create the initial hollow section, followed by further rolling operations to control the diameter and wall thickness. Cold drawing can subsequently be used where closer dimensional control, smaller dimensions or improved surface finish are required.
In rotary piercing, a heated cylindrical billet is fed between inclined rotating rolls. The arrangement causes the billet to rotate while also moving axially through the mill.
The deformation produced by the angled rolls encourages formation of a central cavity within the billet. A suitably profiled piercing plug or mandrel controls and enlarges the internal opening as the billet passes through the mill.
This principle is associated with the Mannesmann rotary piercing process and became an important method of producing hollow starting material for seamless tubes.
The process subjects the material to substantial deformation at elevated temperature. Material quality, temperature control and process conditions are therefore important in preventing internal and surface defects.
Following piercing, the rough tube may require further reduction in diameter and wall thickness. Pilger rolling is one process used to progressively reduce tube dimensions around an internal mandrel.
In the process illustrated, profiled rolls act on the tube while an internal mandrel controls the bore. The tube is progressively fed and worked as the rolls move through their operating cycle.
Repeated deformation reduces the outside diameter and wall thickness towards the required tube dimensions.
Cold drawing may be used as a finishing and sizing process where seamless tube requires smaller dimensions, thinner walls, improved dimensional control or a smoother surface finish.
Before cold drawing, surface scale and contamination produced by previous hot-working operations must be removed using a suitable cleaning process. Historically, pickling has commonly been used as part of the preparation of steel tube for subsequent cold working.
A draw bench pulls the tube through a reducing die. One end of the tube is first reduced sufficiently to pass through the die and provide a section which can be gripped by the drawing carriage.
The carriage then moves away from the die and pulls the tube through it. The pulling system may be mechanically or hydraulically operated depending on the equipment. Industrial draw benches may process more than one tube where the machine and product arrangement permit.
The dimensions controlled during tube drawing depend on whether the tube is drawn through an external die alone or whether internal tooling is also used.
In tube sinking the tube is drawn through a reducing die without internal tooling. The die directly controls the outside diameter, while the resulting wall thickness and internal diameter depend on material flow during the drawing operation.
The process is relatively simple but provides less direct control of the internal tube dimensions than methods using a plug or mandrel.
A plug positioned within the tube can be used together with the external die to provide greater control of the internal diameter and wall thickness. The tube is drawn through the annular space formed between the die and the plug.
A mandrel can provide internal support and dimensional control during drawing. Depending on the particular process, fixed, floating or moving internal tooling may be used.
Internal tooling increases the contact area and friction involved in the process, but it can provide substantially better control of tube geometry than simple tube sinking.
Effective lubrication is essential during tube drawing. Friction occurs between the tube and the external die and, where internal tooling is used, between the tube and the plug or mandrel.
Lubrication reduces drawing force, limits tool wear, assists surface finish and helps prevent undesirable transfer or adhesion of workpiece material to the tooling.
Drawing tools require high hardness and wear resistance. Tool steels and tungsten carbide are used depending on the application, while specialised tool materials such as diamond may be used for particular wire-drawing applications.
Tool selection depends on factors including the workpiece material, drawing load, production quantity, dimensional requirements, lubrication and required tool life.
The processes described above are particularly important for steels and other ferrous products, but drawing is also widely applied to non-ferrous metals and alloys.
Copper and aluminium alloys can be manufactured into tube by extrusion and subsequently subjected to drawing or other finishing operations where smaller dimensions, closer tolerances or particular mechanical properties are required.
Because wire and tube drawing are commonly cold-working operations, the process can alter the mechanical properties of the material as well as its dimensions.
Plastic deformation produces strain hardening in many engineering metals, generally increasing strength and hardness while reducing the remaining ductility. Where a large total reduction is required, intermediate heat treatment may therefore be required depending on the material and process.
The drawing sequence should be selected as part of the complete manufacturing route rather than considering only the required finished diameter.
Important factors include:
Although the processes share the term drawing, they should not be confused.
Deep drawing and related sheet-forming processes are covered separately in the RoyMech sheet metal forming section.
The following external links provide additional information on wire drawing, cold drawing and seamless tube manufacture.