Self Opening Die Heads

Self Opening Die Heads

Self Opening Die Heads - A major disadvantage of solid type threading dies is that they must be unscrewed from the workpiece in order to remove them. They thus are not suitable for use on high speed, production type machines. Therefore, self opening die heads are used on turret lathes, screw machines and special threading machines for cutting external threads.

There are three types of self opening die heads, all having four sets of adjustable, multiple point cutters or chasers that can be removed for sharpening or for interchanging for different thread sizes. This permits one head to be used for a range of thread sizes. In the type, the cutters are positioned radially.

In the type shown in, the cutters are positioned tangentially so that the contact area between the cutters and the workpiece is reduced, resulting in less friction. This arrangement also provides for somewhat easier sharpening of the cutters. In the third type, the cutters are circular, with an interruption in the circular form to provide an easily sharpened cutting face. The cutters must be mounted on the holder at an angle equal to the helix angle of the thread.

The mechanism for retracting the cutters varies in different makes of die heads, but in each case the opening occurs when the thread has been cut to the desired length. On die heads that are used on turret lathes, the operator usually must set the cutters in the close position, prior to cutting the thread, by means of a hand lever.

Self opening die heads for use on screw machines and automatic threading machines are provided with a tripping mechanism that closes the cutters automatically on each cycle.

Self opening die heads are made in stationary and rotary types. The stationary type is used on turret lathes and screw machines, where the workpiece rotates, whereas the rotary type is used on threading machines where the workpiece remains stationary.

Opening mechanisms are used, time is saved because it is not necessary to backtrack the tool off the work. Time is also saved and the tool life is lengthened appreciably, because the individual sets of chasers (Cutters) are readily adjustable for pitch diameter and may be reground or replaced when dulled or worn. The quality of thread is improved since limits may be more closely mainained. Since the necessity for backtracking is eliminated, there is less danger of damage to threads in stainless steel or copper and other soft materials. Cutting threads by means of self opening die heads frequently is called Thread chasing.

Die heads with radial chasers are somewhat simpler to manufacture but the chasers cannot allow many regrinds for sharpening. These heads are being superseded at the present time by other designs. Die heads with tangential chasers are of more advanced construction and their chasers, owing to their great length, can be sharpened many more times.

A still better construction is the die head with circular chasers. These chasers designed on the lines of a circular form tool, allow the greatest number of regrinds.

Self opening die head having radial cutters	Self opening die hand using tangential cutters

Radial	Tangential	Circular