The following points highlight the four main types of machines used for rolling threads in the industries. The types are: 1. Reciprocating Thread Rollers 2. Rotary Planetary Machines 3. Cylindrical Die Machines 4. Screw Machines.
Type # 1. Reciprocating Thread Rollers:
It use two flat dies, one stationary and other moving. The dies are arranged with their rolling faces located opposite each other and blank placed in between. (Refer Fig. 6.25). A thread is rolled on one blank at a time during the forward stroke of the machine. During rolling, no axial movement of the blank takes place.
The diameter of the finished thread is controlled by the diameter of the blank and the distance between the faces of the dies at the finishing end of the stroke. These machines are made in a number of sizes, each for a limited diameter range and with a specified length of die.
This machine is generally used for rolling threads on blanks upto 25 mm diameter. The number of strokes depends on the die length, and the rate of penetration is determined by the shape of the die.
Type # 2. Rotary Planetary Machines:
This machine comprise one central rotary die on a fixed axis and one or more stationary concave segment dies located outside the rotary die (Fig. 6.26). The starting end of the segment die is set closer to the rotary die so that is will first accommodate the blank, and the finishing end of the segment die is set closer to the axis of the rotary die so that a thread is fully formed when the blank rolls pass it.
It is also possible to roll more than one blank depending on the set up. In this machine also there is no axial movement of the blank during rolling. These machines are made in several small sizes, with limited diameter ranges. Speed of operation depends on the composition and hardness of the metal.
Type # 3. Cylindrical Die Machines:
They have a wide range of speeds and feeds and these are widely used for in feed rolling with no axial movement of the blank during rolling. These machines use two or three circular dies, which are mounted at equiangular around the periphery of the blank. In the case of two dies arrangement the blank is supported on a work rest between the dies.
ADVERTISEMENTS:
The axis of one die remains stationary and that of other moves towards the axis of the stationary die to roll the thread on the blank. In the three die arrangement, the work floats between the three dies which are mounted on parallel axes and rotate in the same direction.
All the dies move towards the centre of the blank to roll the threads. The rate of penetration is controlled by the amount of feed applied to the dies for each revolution of the blank; it is less for harder materials and hollow parts.
Type # 4. Screw Machines:
Rotary threads on screw machines and turret lathes are most commonly made with attachments using one or two thread rolls. In case of single thread roll pressure is applied radially towards the work and it is fed directly towards the centre of the work.
In the case of two rolls, they straddle the blank, and the final size is obtained when the rolls are on the centre of the work (Fig. 6.27). The size of the finished thread is controlled by the size of the blank and fixed setting of the rolls in the attachment.
ADVERTISEMENTS:
The cross-slide travel is used only as a means of supplying movement to the rolls. In case of forming of threads between two rolls as they straddle the blank, the direct pressure on the spindle and blank is reduced to approximately 10% of the pressure exerted by one roll. As a result, in this case there is less spring of the blank and reduced thread wear on the spindles.
