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.

[Illustration: Fig. 13. Geometric Self-opening and Adjustable
Screw-cutting Die Head]

=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.

[Illustration: Fig. 14. Geometric Collapsing Tap]

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 mean