Releasing Die and Tap Holders.—Threads are cut in the turret lathe by means of dies for external threading, and taps for internal threading, the die or tap being held in a holder attached to the turret. A simple form of releasing die holder is shown at B, [Fig. 11]. This holder was designed for the spring-screw type of threading die shown to the left. The die is clamped in the holder a by the set-screw shown, and the shank b of the holder is inserted in the turret hole. Holder a has an extension c which passes through the hollow shank. When the die is pressed against the end of the work, holder a and its extension moves back until lug d on the holder engages lug e on the shank. The die and holder are then prevented from rotating with the work and the die begins to cut a thread. It continues to screw itself onto the work with the turret following, until the thread has been cut to the required length; the turret is then stopped and as the die and holder a are drawn forward, lugs d and e disengage so that the die simply rotates with the work without continuing to advance. The lathe spindle is then reversed and as the turret is moved back by hand, pin f comes around and enters notch g, thus holding the die stationary; the die then backs off from the threaded end. Some tap holders are also constructed the same as this die holder, so far as the releasing mechanism is concerned. There are also many other designs in use, some of which operate on this same principle.
Self-opening Die Heads.—The type of die holder shown at B in [Fig. 11] is objectionable because of the time required for backing the die off the threaded end; hence, self-opening dies are extensively used in turret lathe work. As the name implies, this type of die, instead of being solid, has several chasers which are opened automatically when the thread has been cut to the required length. The turret can then be returned without reversing the lathe spindle. The dies are opened by simply stopping the travel of the turret slide, the stop-rod for the feed of the turret being adjusted to give the proper amount of travel.
Fig. 13. Geometric Self-opening and Adjustable Screw-cutting Die Head
A well-known die head of the self-opening type is shown in [Fig. 13]. The dies open automatically as soon as the travel of the head is retarded, or they can be opened at any point by simply holding back on the turnstile or lever by which the turret slide is moved. The die is closed again by means of the small handle seen projecting at right-angles from the side of the head. The closing may be done by hand or automatically by screwing a pin into a threaded hole opposite the handle and attaching a small piece of flat steel to the back edge of the turret slide. The latter will then engage the pin as the turret revolves, thus closing the die head. This die head has a roughing and finishing attachment which is operated by handle A. When this handle is moved forward, the dies are adjusted outward 0.01 inch for the roughing cut, whereas returning the handle closes and locks the dies for the finishing cut. The die head has a micrometer scale which is used when making slight adjustments to compensate for the wear of the chasers or to make either a tight-or a loose-fitting thread.
