er-head is bored to fit the bar closely and it is prevented from
turning by a key against which a set-screw is tightened. By referring to
the end view, it will be seen that each cutter is offset with relation
to the center of the bar, in order to locate the front of the tool on a
radial line. The number of cutters used in a cutter-head varies. By
having several cutters, the work of removing a given amount of metal in
boring is distributed, and holes can be bored more quickly with a
multiple cutter-head, although more power is required to drive the
boring-bar. The boring-bar is also steadied by a multiple cutter-head,
because the tendency of any one cutter to deflect the bar is
counteracted by the cutters on the opposite side.

A disk-shaped head having four cutters is illustrated in Fig. 6. The
cutters are inserted in slots or grooves in the face of the disk and
they are held by slotted clamping posts. The shape of these posts is
shown by the sectional view. The tool passes through an elongated slot
and it is tightly clamped against the disk by tightening nut _n_. This
head is also driven by a key which engages a keyway in the boring-bar.

[Illustration: Fig. 6. Cutter-head with Four Boring Tools]

Two other designs of cutter-heads are shown in Fig. 7. The one
illustrated at _A_ has three equally spaced cutters which are held in an
inclined position. The cutters are clamped by screws _c_ and they can be
adjusted within certain limits by screws _s_. The cutters are placed at
an angle so that they will extend beyond the front of the head, thus
permitting the latter to be moved up close to a shoulder. The
cutter-heads shown in Figs. 5 and 6 can also be moved up close to a
shoulder if bent cutters are used as shown in the right-hand view, Fig.
5. The idea in bending the cutters is to bring the cutting edges in
advance of the clamping posts so that they will reach a shoulder before
the binding posts strike it. The arrangement of cutter-head _B_ (Fig. 7)
is clearly shown by the illustration.

Cutter-heads are often provided with two sets of cutters, one set being
used for roughing and the other for finishing. It is a good plan to make
these cutters so that the ends _e_ (Fig. 6) will rest against the bar or
bottom of the slot, when the cutting edge is set to the required radius.
The cutters can then be easily set for boring duplicate work. One method
of making cutters in sets is to clamp the annealed stock in the
cutter-hea