cut. The finishing feeds for cast iron vary
from 1/4 to 3/4 inch on ordinary work. The different tools used on the
vertical mill will be referred to more in detail later.

All medium and large sized vertical boring mills are equipped with two
tool-heads and two tools are frequently used at the same time,
especially on large work. Fig. 9 illustrates the use of two tools
simultaneously. The casting shown is a flywheel, and the tool on the
right side turns the upper side of the rim, while the tool on the left
side turns the outside or cylindrical surface. As a boring mill table
rotates in a counter-clockwise direction, the left-hand tool is reversed
to bring the cutting edge at the rear. By turning two surfaces at once,
the total time for machining the casting is, of course, greatly reduced.
The turning of flywheels is a common vertical boring mill operation, and
this work will be referred to in detail later on.

[Illustration: Fig. 5. Tools for Boring and Reaming Holes]

=Boring Operations.=--There are several methods of machining holes when
using a vertical boring mill. Ordinarily, small holes are cored in
castings and it is simply necessary to finish the rough surface to the
required diameter. Some of the tools used for boring and finishing
comparatively small holes are shown in Fig. 5. Sketch _A_ shows a boring
tool consisting of a cutter _c_ inserted in a shank, which, in turn, is
held in the tool slide, or in a turret attached to the tool slide. With
a tool of this type, a hole is bored by taking one or more cuts down
through it. The tool shown at _B_ is a four-lipped drill which is used
for drilling cored holes preparatory to finishing by a cutter or reamer.
This drill would probably finish a hole to within about 1/32 inch of the
finish diameter, thus leaving a small amount of metal for the reamer to
remove. The tool illustrated at _C_ has a double-ended flat cutter _c_,
which cuts on both sides. These cutters are often made in sets for
boring duplicate parts. Ordinarily, there are two cutters in a set, one
being used for roughing and the other for finishing. The cutter passes
through a rectangular slot in the bar and this particular style is
centrally located by shoulders _s_, and is held by a taper pin _p_. Some
cutter bars have an extension end, or "pilot" as it is called, which
passes through a close-fitting bushing in the table to steady the bar.
Sketch _D_ shows a finishing reamer. This tool takes a very lig