Process of Tube Drawing | Metallurgy

The process of tube drawing is similar to wire drawing. Generally draw benches are used to obtain straight tubes as shown in Fig. 9.6.

The purpose of tube drawing can be any one of the following:

(i) To regulate the outer diameter only.

(ii) To regulate the outer diameter and to have good surface finish on the inner diameter. The inner diameter may not be regulated.

(iii) To regulate the outer as well as inner diameter.

(iv) To carry out a heavy reduction in thickness of the tube.

In order to realize the above aims, four types of tube drawing processes, are employed. These are illustrated in Fig. 9.17. Besides these processes, the tube drawing may also be carried out through roller dies in which case the die friction is substantially reduced.

1. Tube Sinking:

In this process, tube is simply pulled through the die as shown in Fig. 9.17(a). The outer diameter is regulated by the die diameter but there is no regulation of inner diameter or thickness of tube. The surface finish on inner diameter is also not good. During the drawing operation the thickness of tube generally changes.

The thin tubes in which t/R (ratio of initial thickness to initial outer radius) is less than 0.33, the tubes get thicker on drawing. The tubes having t/R greater than 0.33 get thinner on drawing. With t/R = 0.33, there is negligible change in the thickness of tube.

For threading the front end of tube through the die, many in the industry still follow the old practice of hammering the front end down thus wasting at least 200 mm length of tube. In modern practice the front end is pointed by pointing machines thus minimizing the wastage. Grippers are also designed to minimize the end wastage.

2. Tube Drawing with Floating Mandrel:

The process of tube drawing with a floating mandrel is shown in Fig. 9.17(b). The position of mandrel with respect to the die gets adjusted by the normal and tangential forces exerted by tube material on the mandrel.

The frictional force tends to pull the mandrel into the die while the normal force tries to it push out. If the mandrel moves into the die the tube thickness reduces and hence the normal reaction increases, which also increases the frictional force. The mandrel takes a position where axial components of the forces are balanced.

Since there is no external control on the position of the mandrel, it may change its position if the frictional condition changes, thus resulting in change in tube thickness. The external surface shape of the mandrel may be designed so that the mandrel adjusts its position for desired thickness of tube.

3. Tube Drawing with Fixed Mandrel:

The process is shown in Fig. 9.17(c). The tube is drawn through a die and a mandrel. The position of mandrel may be adjusted by the bar attached to its rear end in order to change the thickness of tube and the internal diameter. The external diameter is determined by the die diameter.

The surface quality of both the surfaces, internal as well as external gets improved. The pull required is certainly more than that in tube sinking because of the additional deformation in the thickness of tube and also due to frictional force between the tube and the mandrel.

4. Tube Drawing with Moving Mandrel:

The process is illustrated in Fig. 9.17(d). The cylindrical mandrel and the tube are pulled together through the die. The process is generally used to reduce the thickness of tube. Since the area of cross section of tube increases towards the entry side its speed decreases while the mandrel being rigid moves with the same speed as the speed of tube at the exit.

Therefore, in the deformation zone the mandrel moves faster than the tube. The frictional force between the tube and the mandrel pulls the tube inside the die while the frictional stress between the tube and die acts in the opposite direction.

The effects of frictional forces acting on contacting surfaces of mandrel and die tend to nullify each other. This results in net decrease of drawing stress. Therefore, the maximum possible reduction in thickness in this process is higher than that in other tube drawing processes.