Drawing operations involve pulling metal through a die by means of a tensile force applied to the exit side of the die. Most of the plastic flow and hence cold work is caused by compressive radial forces, which arise from the reaction of the metal with the die. The reduction in diameter of a solid bar or rod by successive drawing is known as bar, rod or wire drawing, depending on the diameter of the final product.
Wire drawing is usually carried out at room temperature. However, because large deformations are usually involved, there is considerable temperature rise during the drawing operation. The main reason that the wire drawing works is because of the work hardening of the soft annealed wire as it gets drawn through the die.
The following points should be noted before any wire drawing operation:
1. If the starting stock is annealed square wire, make sure it is the same size as the first wire die. If multiple dies are required to change the cross section from square to round, seams can be formed down the length of the wire. Example: If the first die is .080" round, start with a .080" square stock.
2. Lubricant is very important during wire drawing. A common lubricant for drawing short lengths of wire through drawplates is bee's wax. Wire drawing machines with multiple die capacity require special water-soluble lubricants. Companies that specialize in lubricants should be consulted.
3. Individual wire dies are available with "nibs" made from either natural diamond or synthetic diamond or inserts made of tungsten carbide. Dies with diamond nibs are often the choice for the final wire die. Dies with tungsten-carbide inserts are often used for intermediate dies.
4. Draw dies eventually wear out with use. When the die loses the polished finish, particles from the wire being drawn will build at the entrance of the die. Another more obvious indication of die wear is the size of the wire becomes larger than the indicated die size.
5. Excessive die wear can be caused by: a. Abrasive particles (trash) attached to the outside of the wire stock. Clean the wire stock prior to drawing. b. Abrasive particles in the lubricant used for drawing. c. Improper lubrication (not enough lubricant or the wrong lubricant).
6. Reduction between sequential dies must be matched to the multiple, die-wire drawing machine being used. Drawing drums on wire mills have fixed rates of speed that vary with each pass that is done during a multi-die sequence. Failing to match the reduction per draw can result in breaking the wire being drawn.
7. Wire that is annealed will sometimes "neck" (stretch to a smaller cross section) when being drawn, resulting in a wire size that is smaller than the die size. To ensure accurate final wire size, avoid trying to finish wire with one single die after annealing.
8. Minimize the distance between the die and the drawing source. Movement of the wire stock from side to side during drawing will result in wave patterns on the drawn wire.
9. Failing to keep wire dies clean and free of residues can result in a diminished surface quality on the drawn wire.
10. Platinum alloys used for jewelry often leave particles of the platinum alloy bonded to the surface of the die and are difficult to remove. Failing to remove these particles will negatively impact the surface quality of wire stock being drawn afterwards.
Wire Drawing Tips
1. Wire stock breaks during drawing
2. Poor quality finish on wire
3. Wire size drawn is not accurate
4. Wire has grooves on the surface
5. Wire has wavy patterns (chatter marks)
To straighten wire: first, anneal and quench to soften; then, fasten one end and pull on the opposite end with tongs or draw bench until you feel a slight movement. Release pressure, and the piece should be straight.
Exerpted from the Stuller Metals Book