e bar by a taper pin which
bears against a circular recess in the side of the cutter. This form
has the advantage of only requiring a hole through the boring-bar,
whereas it is necessary to cut a rectangular slot for the flat cutter.

[Illustration: Fig. 4. Boring with a Flat Double-ended Cutter]

Fig. 4 shows how a hole is bored by cutters of the type referred to. The
bar rotates as indicated by the arrow _a_ and at the same time feeds
longitudinally as shown by arrow _b_. The speed of rotation depends upon
the diameter of the hole and the kind of material being bored, and the
feed per revolution must also be varied to suit conditions. No definite
rule can be given for speed or feed. On some classes of work a long
boring-bar is used, which passes through the hole to be bored and is
steadied at its outer end by the back-rest _B_, Figs, 1 and 2. On other
work, a short bar is inserted in the spindle having a cutter at the
outer end. An inexpensive method of holding a cutter at the end of a bar
is shown at _D_, Fig. 3. The cutter passes through a slot and is clamped
by a bolt as shown. When it is necessary to bore holes that are "blind"
or closed at the bottom, a long boring-bar which passes through the work
cannot, of course, be used.

Sometimes it is necessary to have a cutter mounted at the extreme end of
a bar in order to bore close to a shoulder or the bottom of a hole. One
method of holding a cutter so that it projects beyond the end of a bar
is indicated at _E_. A screw similar to the one shown at _B_ is used,
and the conical end bears in a conical hole in the cutter. This hole
should be slightly offset so that the cutter will be forced back
against its seat. The tool shown at _F_ has adjustable cutters. The
inner end of each cutter is tapering and bears against a conical-headed
screw _b_ which gives the required outward adjustment. The cutters are
held against the central bolt by fillister-head screws _f_ and they are
clamped by the screws _c_. Boring tools are made in many different
designs and the number and form of the cutters is varied somewhat for
different kinds of work.

[Illustration: Fig. 5. Cutter-heads for Boring Large Holes]

=Cutter-heads for Boring Large Holes.=--When large holes are to be
bored, the cutters are usually held in a cast-iron head which is mounted
on the boring-bar. One type of cutter-head is shown in Fig. 5. This
particular head is double-ended and carries two cutters _c_. The
cutt