ained to show, in a
general way, how a vertical boring machine is operated. It should be
understood, however, that the arrangement differs considerably on
machines of other makes. The construction also varies considerably on
machines of the same make but of different size.

[Illustration: Fig. 2. Small Boring and Turning Mill with Single
Turret-head]

All modern vertical boring mills of medium and large sizes are equipped
with two tool-heads, as shown in Fig. 1, because a great deal of work
done on a machine of this type can have two surfaces machined
simultaneously. On the other hand, small mills of the type illustrated
in Fig. 2 have a single head. The toolslide of this machine, instead of
having a single tool-block, carries a five-sided turret _T_ in which
different tools can be mounted. These tools are shifted to the working
position as they are needed, by loosening binder lever _L_ and turning
or "indexing" the turret. The turret is located and locked in any of its
five positions by lever _I_, which controls a plunger that engages
notches at the rear. Frequently, all the tools for machining a part can
be held in the turret, so that little time is required for changing from
one tool to the next. Some large machines having two tool-heads are also
equipped with a turret on one head.

=Boring and Turning in a Vertical Boring Mill.=--The vertical boring
mill is, in many respects, like a lathe placed in a vertical position,
the table of the mill corresponding to the faceplate or chuck of the
lathe and the tool-head to the lathe carriage. Much of the work done by
a vertical mill could also be machined in a lathe, but the former is
much more efficient for work within its range. To begin with, it is more
convenient to clamp work to a horizontal table than to the vertical
surface of a lathe faceplate, or, as someone has aptly said, "It is
easier to lay a piece down than to hang it up." This is especially true
of the heavy parts for which the boring mill is principally used. Very
deep roughing cuts can also be taken with a vertical mill. This type of
machine mill is designed for turning and boring work which, generally
speaking, is quite large in diameter in proportion to the width or
height. The work varies greatly, especially in regard to its diameter,
so that boring mills are built in a large range of sizes. The small and
medium sizes will swing work varying from about 30 inches to 6 or 7 feet
in diameter, whereas large