has a standard taper hole into which the shank of the drill is inserted.
The adjustable type of holder _M_ is extensively used, especially on
small and medium sized machines when several sizes of drills are
necessary. This holder is simply a drill chuck fitted with a special
shank. For large drills the plain style of holder _K_ is recommended,
and if only a few sizes of drills are required, it is more satisfactory
and economical than the adjustable type.

The various types of small turret lathe tools referred to in the
foregoing for turning, threading, tapping, knurling, etc., are a few of
the many different designs of tools used in turret lathe practice.
Naturally, the tool equipment for each particular job must be changed
somewhat to suit the conditions governing each case. The tools referred
to, however, represent in a general way, the principal types used in
ordinary practice. Some of the more special tools are shown in
connection with examples of turret lathe work, which are referred to in
the following.

=Turning Gasoline Engine Pistons in Turret Lathe.=--The making of
pistons for gas engines, especially in automobile factories, is done on
such a large scale that rapid methods of machining them are necessary.
The plan view _A_, Fig. 16, shows the turret lathe tools used in one
shop for doing this work. As is often advisable with work done in large
quantities, the rough castings are made with extra projections so
arranged as to assist in holding them. These projections are, of course,
removed when the piece is completed. In this case the piston casting _a_
has a ring about 1-1/4 inch long and a little less in diameter than the
piston, at the chucking end. The piston is held in suitable chuck jaws
_b_ which are tightened against the inside of this ring. The set-screws
in these special jaws are then tightened, thus clamping the casting
between the points of the screws and the jaws. This method of holding
permits the whole exterior of the piston to be turned, since it projects
beyond the chuck jaws. This is the object in providing the piston with
the projecting ring by which it is held.

[Illustration: Fig. 16. (A) Method of Boring and Turning Pistons in
Gisholt Lathe. (B) Special Chuck and Tools for Turning, Boring and
Cutting Off Eccentric Piston Rings]

The first operation consists in rough-boring the front end of the
piston. The double-ended cutter _n_ is held in boring-bar _m_, which is,
in turn, supported by