Wire Drawing © Stuller

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 a. Too great of a reduction for the feedstock being drawn; b. Insufficient lubrication, which causes increased friction; c. Defects in wire stock such as voids or delamination; d. Wire being drawn in the wrong direction through die; e. Reductions between dies are not matched to the draw speed of the wire drawing machine; f. Wire stock is tangled on feed side of draw die; g. Wire die is at an angle in relation to the wire being drawn; h. Dies used are not designed for nonferrous materials. 2. Poor quality finish on wire a. Insufficient lubrication, which results in, increased friction.; b. Trash at entrance of die, which feeds into the die with the feed stock; c. Wire being drawn the wrong direction through die; d. Poor finish on die either due to excessive die wear or poorly polished die from manufacturer; e. Feedstock is of poor quality possibly from grain growth as a result of over annealing, blistering on surface of stock resulting from excessive copper oxide in stock and annealing in hydrogen atmospheres or surface defects on stock; f. Lubricant is contaminated with abrasive particles. 3. Wire size drawn is not accurate a. Die is worn to a larger size; b. Wire stock is too soft for final draw and stretches (necks) at output side of die. 4. Wire has grooves on the surface  a. Die has trash caught at the entrance and is fed in with the wire stock; b. Square stock not drawn to round cross section with one die; c. Original feedstock was defective. Stock may have become finned while square rolling. 5. Wire has wavy patterns (chatter marks) a. Distance between die and drawing source is too large, which causes wire to vibrate as it feeds through the die; b. End of wire stock on the feed side of die is not held in a fixed position and vibrates. Special Tip 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 Back to ShopTalk Back to Home Page