Tube drawing is a metalworking process used to create a tube with a smaller diameter by pulling, or drawing, a larger diameter tube through a die. There are five methods of tube drawing that are commonly used. These methods are fixed plug drawing, floating plug drawing, tethered plug drawing, rod drawing and tube sinking.
This process is a cold-working process, meaning that the metal tubing is not heated prior to being shaped in the tube drawing process. This gives the finished product added strength because the metal tubing is not affected by thermal expansion during the process. In addition, this process produces tubing with more precise measurements than other methods of production.
Fixed plug drawing is the oldest form of tube drawing. Using a mandrel that is locked in a fixed position near the die, the process of fixed plug drawing produces the best interior surface finish of any tube drawing method. Fixed plug drawing is also the slowest method in use and is extremely limited in the amount of diameter reduction possible.
Floating mandrel, or floating plug, drawing incorporates a free-floating mandrel placed inside the tube stock. The plug is forced to the throat of the die by friction and pressure, called axial force. The floating plug method is capable of producing very thin tubing diameters. This method of tube drawing is noted for producing tubing with high-quality inner and outer surface finishes.
Tethered plug drawing, or semi-floating mandrel drawing, combines the features of fixed plug drawing and floating plug drawing. Although the mandrel floats within the tube stock, it is tethered to an anchor point. Used in the process of making straight tubes, this method of tube drawing delivers a better finish on the inside surface of the tube.
Rod drawing, also known as mandrel drawing, uses a mandrel to ensure the inside diameter of the drawn tubing remains constant. The mandrel is a solid rod as long as 100 feet (about 30 m) that is inserted in the tube stock and pulled along with the tubing through the die. The inside surface of the finished tubing takes on the characteristics of the rods' exterior as the metal tubing is compressed around the rod as it passes through the die. The rod is then extracted from the finished tubing in a process called reeling.
Tube sinking is one of the primary methods of tube drawing for products requiring a quick, cost-effective setup without a great deal of concern over the inner surface finish of the tubing, such as the tubing used to build lawn furniture. It differs from the other forms of tube drawing because there is no mandrel used inside the tubing. Although this is the simplest form of tube drawing, it results in an inferior surface finish quality when compared with the other four methods.