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Manufacturing Processes > Forming Processes > Drawing

 

Drawing

 

Process description

A number of processes where long lengths of rod, tube or wire are pulled through dies to progressively reduce the original cross-section through plastic deformation. The process is performed cold.

Materials

Any ductile metal at ambient temperatures.

Drawing

Process variations

  • Rod or bar drawing: reduction of the diameter of rod or bar through a single die or progressive reduction through a number of dies.
  • Wire drawing: performed on multiple wire drawing machine where the wire is wrapped around blocks before being pulled through the next die to successively reduce diameter. Wire diameters that cannot be wrapped around blocks are drawn out on long benches at low speeds, but give lower production rates.
  • Tube drawing: reduction of either the diameter of a tube or simultaneous reduction of diameter and thickness using mandrel.
  • Can use rollers in place of dies for plastic deformation.
  • Sizing is a low deformation operation sometimes used to finish the drawn section giving closer dimensional accuracy and surface roughness.

Economic considerations

  • Production rates from 10 (rod, tube) to 2000 m/min (wire).
  • Lead time typically days.
  • Material utilization excellent. Some scrap may be generated when cutting to length.
  • High degree of automation possible.
  • Economical for high production runs (1000m+).
  • Tooling costs low.
  • Equipment costs moderate.
  • Direct labor costs low to moderate.
  • Finishing costs very low. Cutting long lengths of rod, bar and tube to length only.

Typical applications

  • Drawing is an important process for producing the stock material for many other processes, e.g. machining and cold heading.
  • Rod, bar, wire, tubes
  • Fabrication and machine construction
  • Spring wire, musical instrument wire

Design aspects

  • Simple shapes with rotational symmetry only.
  • No draft angles required.
  • Rod drawing:
  • Minimum diameter =Ø0.1 mm.
  • Maximum diameter =Ø50 mm.
  • Wire drawing:
  • Minimum diameter =Ø0.1 mm.
  • Maximum diameter =Ø20 mm.
  • Tube drawing:
  • Minimum diameter =Ø6 mm.
  • Maximum diameter =Ø600 mm.
  • Minimum section =0.1 mm.
  • Maximum section =25 mm.

Quality issues

  • Strain hardening occurs in material during cold working, giving high strength.
  • High directionality (anisotropy) due to nature of plastic deformation and grain orientation in direction of drawing.
  • High friction between work and die causes high temperatures which must be reduced through external cooling.
  • Surface detail excellent.
  • Surface roughness ranging 0.2–6.3 µm Ra.
  • Finer surface roughness values obtained with finer grit grades.
  • Process capability charts showing the achievable dimensional tolerances for cold drawing various materials are provided.

costing