Drawing Process
Drawing involves pulling the hollow tube through a series of hardened steel dies of gradually decreasing diameter. Before each step of the drawing process , the tube is pointed at one end to fit through the next smaller die. It is then gripped by automatic jaws attached to a rotating 7 feet diameter drawing machine called a bull block. Before drawing a tapered plug mandrel is placed inside the tube ( floating plugs are used with bull blocks, stationary mandrels are used for relatively short length of tube that are drawn linear draw benches). As the tube is drawn onto the spinning bull block, the mandrel and die act together to reduce both the tubes outside diameter and its wall thickness.
The mandrel also makes the tubes inside surface smoother. Tube that is passed through a series of straightening rolls for straighting. Annealing softens the tube so that it can be bent to shape during installation. Finally the tube is cleaned to remove any traces of drawing lubricants or other contaminants. Samples of the finished tube are analyzed at regular intervals to ensure that it meets of all requirements of size, wall thickness and quality as required under applicable standard.
