Tubes can be produced from 2 to 6 inches outside diameter by the hot finishing process. When necessary to produce (1) tubes of less than 2 inches diameter, or (2) larger tubes requiring the smoothest surfaces, or (3) tubes requiring greater accuracy in diameter and thickness of wall than can be produced by the hot process, this process must be followed by a series of cold drawing operations.

Fig. 72. - The Operation of Cold-Drawing Tubes.

These operations consist of drawing the hot-rolled tube through dies and over mandrels to reduce its diameters and thickness of wall.

The various steps in the cold drawing operations may be described as follows: The hot-rolled tube, after having cooled, is heated at one end and forged down to a rough point or tag, as shown at H, Fig. 72. After pointing, the tube is cleaned of dirt and scale by pickling in hot dilute sulphuric acid. It is washed in an alkali water to remove all traces of acid, and is then immersed in a lubricating vat. The lubricant consists of flour, water and tallow. After lubricating, the tube is taken to the cold-draw bench shown in the lower sketch of Fig. 72.*

* Fig. 71 shows three views of a cold-draw bench in the shop.

This machine consists of a frame arranged with a support for a die D, and is provided with a traveling chain or other means of moving a gripper or plier G away from the die. The other end of the machine forms a support for the end of the mandrel rod B. A die shown in cross section at DD of the upper view is placed in the die support. The pointed end of the tube is then pushed through the opening in the die and a cylindrical mandrel M on the end of the rod B is entered into the tube. A head C formed on the rod B engages with the frame of the bench, so that the mandrel and die are held rigidly in proper relation to each other as shown in the figure.

The opening in the die is smaller by from 1/8 inch to 1/2 inch than the diameter of the tube operated upon. The mandrel M is also smaller than the inside diameter of the tube, but to a slightly less degree than the die, so that the difference between the die and the mandrel size is less than the difference between the outside and inside diameters of the tube operated upon. The result is that in passing the tube through the die and over the mandrel, the diameter and wall thickness are simultaneously reduced.

The point of the tube having been placed through the die with the mandrel M in position, is gripped by the gripper G, which in turn is engaged by a hook with a moving chain J. The tube is pulled through the die and over the mandrel at the speed of the gripper. By this means, the sectional area of the tube is reduced about 15% for small tubes of light wall and up to 25% or to the strength of the bench for larger sizes. The drawing operation is repeated from 2 to 10 times until the desired diameter and wall thickness are obtained.

After each cold-drawing operation it is necessary to anneal the tube in order to soften it for the succeeding cold drawing. After annealing, the tube is pickled and lubricated as before. For many purposes the cold-drawn tube is used without annealing; but for purposes which require ductility in the material the tube is annealed at various degrees corresponding to the physical properties required. A temperature of 1200° F. removes all traces of the effect of cold drawing.

After receiving final operation, the tube is transferred to the finishing department where it is straightened and cut to length, inspected, gaged and tested and is then ready for shipment.