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.
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 0.20 microns Ra are achieved. Therefore, plug drawing is chosen to significantly improve the ID surface condition.