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Lesson 3.4DeformChapter 3 · Bulk forming

Wire drawing

Pull a rod through ever-smaller dies

Wire drawing pulls a metal rod through a series of ever-smaller holes until it becomes wire. Electric cables, paper clips, springs, fences and the steel cords inside car tires all start this way. It is also the first step in making a screw: the wire you see here feeds the machines of the next two lessons.

Wire drawing: a real photo
Photo: European Coal and Steel Community, CC BY-SA 4.0, via Wikimedia Commons
A capstan pulls the rod through a cone-shaped die, so it leaves thinner and longerEach die removes only part of the size, so fine wire passes through many dies in a row.One die, cut in half(side view)lubricant boxrodthinner wiredrawing diesteel casehard nibconebearingcapstanpulling force1234powered drum pulls the wireA drawing machine(overview, not to scale)rod coildie, capstan, die, capstan...annealerspool56Key:metal being shapeddies and machine partsheatlubricant
Wire drawing: one die up close, then a multi-die machineTop: what happens inside one die (steps 1 to 4). Bottom: a drawing machine that repeats it many times (steps 5 and 6). Circled numbers match the steps below.

How it works

  1. Prepare the rod. The starting material is hot-rolled rod, typically about 5.5–8 mm thick, delivered in coils. It is cleaned of scale, and its tip is pointed (made thinner) so it can be threaded through the die.

  2. Lubricate. The rod passes through a box of soap powder or a bath of oil. The lubricant cuts friction and heat inside the die.

  3. Squeeze in the die. The die is a very hard nib (tungsten carbide, or diamond for fine wire) with a cone-shaped hole. The cone squeezes the metal, and a short straight section called the bearing sets the final diameter.

  4. Pull with a capstan. A capstan (a powered drum) pulls the wire through the die. Pulling, not pushing, is what makes this work: the wire comes out thinner, longer and faster than it went in.

  5. Repeat through many dies. One die can reduce the cross-section only by about 15–30%, or the wire would snap under the pull. So a drawing machine chains many dies and capstans, each die slightly smaller than the last.

  6. Anneal and spool. Drawing makes the metal harder and stiffer (work hardening). Copper wire is usually annealed (heated to soften it again), then wound onto spools; steel wire for screws is coiled for the next step.

Key facts

Materials
Copper, aluminum, low- and high-carbon steels, stainless steel, tungsten, gold and other precious metals
Typical parts
Electrical wire, wire for screws and nails, spring wire, fencing, tire cord, welding wire, guitar strings
Part size
From about 25 mm rod or bar down to wires thinner than a hair (around 0.01 mm)
Precision
Diameter typically within about ±0.01–0.05 mm; very fine wire much tighter
Surface finish
Smooth and bright; shiny on copper, often coated with leftover lubricant on steel
Batch size it suits
Very high volume; lines run for hours on one coil weighing a tonne or more
Tooling and machine cost
Dies: tens to hundreds of dollars each (diamond dies more). Multi-die machines: hundreds of thousands of dollars or more
Speed
Steel wire often leaves the last die at several meters per second; fine copper wire at tens of meters per second

Strengths and limits

Strengths

  • Very precise, round and smooth wire at low cost per kilogram
  • Drawing cold makes the metal much stronger (useful for springs and tire cord)
  • Fast, continuous and highly automated
  • Cheap, long-lasting dies, easy to swap for a new size

Limits

  • Only long, thin products with a constant section
  • Each die removes only a little, so many passes are needed
  • The metal hardens and often needs annealing between stages
  • Dirt, poor lubrication or a flaw in the rod can snap the wire

Where it's used

  • ElectricalCopper and aluminum conductors for cables, motor windings, power lines
  • FastenersSteel wire for screws, bolts, rivets and nails
  • AutomotiveTire cord, springs, seat frames, brake and clutch cables
  • ConstructionPrestressing wire for concrete, fencing, mesh, binding wire
  • Consumer goodsPaper clips, coat hangers, guitar and piano strings, jewelry chain

How to recognize it

  • A perfectly round, smooth wire with a constant diameter along its whole length.
  • Fine, straight lines along the wire under a magnifier, left by the die.
  • A slightly dull or greasy film on steel wire, from the drawing lubricant.
  • Bright, springy metal: drawn wire bends back more than soft, annealed wire.

Compare with

Extrusion3.3extrusion pushes metal through a die, so it can make thick, complex shapes; drawing pulls and suits thin, simple sections.Rolling3.2rolling makes the rod that drawing starts from; drawing then gives the precise final diameter.Cold heading3.5the next step in the screw story: drawn wire is cut and its head is formed.

Watch it

Mecholic - For Mechanical Engineering · The whole process, explained step by step

Fundamentals of manufacturing processes · Die shape and how much each pass can reduce

International Wire Group · A real wire plant: drawing, annealing and spooling

Bekaert · Industrial drawing of steel wire

See real photos

Remember

  • Wire drawing pulls a rod through cone-shaped dies; each die makes it a little thinner and longer.
  • Many dies in a row turn a thick rod into fine wire, with annealing to soften it when needed.
  • Drawn steel wire is the raw material for screws, the subject of the next two lessons.

More in Bulk forming

All 100 processes
3.1ShortForging (open-die and closed-die)Squeeze hot metal between flat or shaped diesOpen the lesson3.2RollingPull metal through the gap between turning rollsOpen the lesson3.3Extrusion (aluminum profiles and plastic pipes)Push metal or plastic through a shaped dieOpen the lesson3.5Cold headingCut wire into blanks and punch a head, coldOpen the lesson3.6Thread rolling and tappingPress a thread onto a blank, or cut or press one in a holeOpen the lesson

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