This is the second level of cold working tubes to reduce their size. Here the tube is pulled through a die which is smaller than the tube, so the end of each tube needs to be machined in order to fit. It will be ‘swaged’ or ‘tagged’ before drawing. Following this procedure, the tube end can pass through the die and is clamped to a drawing trolley, also called a pulley, which draws the tube through the die.
The machine used for this process is called a draw bench, and Fine Tubes has them in various sizes. A smaller one is used each time the tube is reduced. They are equipped with different tools so they can perform different types of cold drawing such as:
Sink drawing

This is the simplest drawing procedure. The tube is drawn through a hardened die made of polished tool steel or, for smaller sizes, an industrial diamond. With the correct lubrication, when the tube surface comes into contact with the polished die surface it is worked smooth. The inside bore surface is not constrained so the wall thickness of the tube will always increase when it’s drawn. Large cross sectional area reductions can be performed using this technique.
Rod drawing

Rod drawing is one of the main intermediate drawing stages. The outside diameter and wall thickness are reduced at the same time.
The tube is loaded over a hardened steel mandrel rod and both are then drawn through a die. This squeezes the tube onto the rod and reduces the wall thickness. The die and mandrel determine the size of the drawn tube which is then slightly expanded using pressure rollers so that the rod can be removed. Rod draws are used as intermediate stages to reduce tube sizes prior to the final drawing cycle.
Plug drawing

This is the type of drawing used for the highest quality output. The outside and inside diameters of the tube are worked at the same time. A plug made of high grade tool steel with a polished surface is located precisely in the middle of the outside diameter drawing die. The tube is loaded over the plug attached to a fixed rod. As the tube is drawn through the die the burnishing action of the metal flowing over the stationary plug imparts a high tolerance surface finish inside the tube.
When properly lubricated and prepared the ID will show very few defects and finishes of 0.20microns Ra are achieved. Therefore, plug drawing is chosen to significantly improve the ID surface condition.
We have developed specialist procedures to manufacture tubes with the highest quality ID surface finishes, and these are used specifically on our semiconductor, medical and aerospace products. Improved ID surfaces have several potential advantages from limited contamination and high cleanliness to improved high pressure performance, increasing the fatigue life of the tube.