Principle of Rod, Wire, and Tube Drawing

Definition

Drawing is a metal forming process used to reduce the cross-section of a workpiece by pulling it through a die. In rod and wire drawing, the material is pulled through a conical die to decrease its diameter. In tube drawing, the process reduces the diameter and/or wall thickness of a hollow cylindrical workpiece, often using an internal support (mandrel) to control the inner dimensions.

Main Content

1. Rod and Wire Drawing Principles

• The process involves a tensile force applied to the exit end of the rod or wire, causing it to be pulled through a converging die.
• The cross-sectional area reduction occurs because the material undergoes plastic deformation as it passes through the die opening.

2. Tube Drawing Techniques

• Tube drawing is classified based on whether a support is used for the inner surface: Sinking (no support), Stationary Mandrel (plug), or Moving Mandrel.
• Sinking is the simplest method, whereas using a mandrel provides better control over the wall thickness and the internal surface finish of the tube.

3. The Role of Die Geometry

• The die angle is critical; a smaller angle reduces friction and force but increases the length of contact.
• Lubrication is essential in both rod and tube drawing to minimize friction between the workpiece and the die, preventing surface defects.

[Schematic of Rod Drawing Process]
      Tensile Force (Pull)
        |
        V
    |-------|  <-- Die
   /         \
--/           \------> Reduced Diameter Rod
  \           /
   \         /
    |-------|

Working / Process

1. Material Preparation

• The starting material (rod, wire, or tube) must be cleaned of scale or rust through "pickling" to prevent damage to the die surface.
• The end of the workpiece is pointed or "swaged" so it can pass through the die opening to be gripped by the pulling mechanism (draw bench or bull block).

2. Lubrication and Feeding

• A lubricant (oil, soap, or polymer) is applied to the surface of the workpiece to reduce the force required and prevent the metal from "welding" to the die.
• The workpiece is fed into the entrance of the die, which has a tapered opening designed to gradually compress the material.

3. Drawing Operation

• The pulling mechanism applies a continuous tensile force, forcing the material to conform to the smaller cross-section of the die.
• For tubes, if a mandrel is used, the tube is drawn over it, ensuring the inner diameter remains uniform and accurate throughout the length of the draw.

Advantages / Applications

• High Dimensional Accuracy: Produces parts with very tight tolerances and smooth surface finishes.
• Mechanical Property Enhancement: The cold working process increases the yield strength and hardness of the material due to strain hardening.
• Versatility: Used extensively in the production of electrical wiring, structural rods, copper plumbing tubes, and hypodermic needles.

Summary

Drawing is a metal forming operation where a workpiece is pulled through a die to reduce its cross-section. Wire drawing focuses on creating long, thin strands, while tube drawing emphasizes controlling wall thickness and internal diameters using mandrels. This process improves surface quality and mechanical strength through cold working. Important terms to remember include: Die Angle, Reduction in Area, Tensile Force, Mandrel, and Strain Hardening.