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cutting.    In fig. 9 shouldered cutters are seen, one for roughing the other
for finishing, each being secured with a tapered pin.   In fig. 10 two cutters
.                      are secured with circular nuts that bear against their

HI                    faces.    In fig. n provision is made for three cutters

/Tl                   in a bar, two slots being occupied; in fig. 14 a similar

provision is made. Counter-bores are tools that pro-
duce shouldered recesses in holes already bored.
They are centred and steadied with pilots, often
with provision for changing pilots.

Cutters in Heads.—These are either flat- or
round-nose tools (fig. 16) set out with a conical screw
and clamped. Figs. 17 and 18 show two of the best
methods. The tools being set diagonally cut sweetly.
A large range of adjustment is provided for, and the
set-screws clamp the cutters securely. The heads fit
easily on the bars over half the bore only, and are
held securely with set-screws.


Reamers.—A reamer is used to finish accurately
a hole previously drilled, since no drill leaves a hole
correct to fine limits or perfectly straight. Though
the reamer removes an exceedingly minute amount,
two passages with tools of different sizes are often
necessary for the finest tolerances. The reamer has
many cutting-blades which counterbalance each other.
The old D-bit and the rose reamer (fig. 19) cut by
their leading edges; present-day reamers cut with the
whole length of their blades. The ends are slightly
tapered to enable them to enter easily. The blades
are often spaced irregularly in order to prevent chatter
and risk of " cornering ", due to the fact that if blades
are pitched equally they come round to exactly the
same place in each revolution, so that any initial in-
accuracy will be perpetuated. But the evil is lessened
by imparting a small amount of clearance. Blades are
straight, or spiral; in the latter the spiral should run
contrary to the cutting edge in order to avoid the
tendency of the reamer to " draw " into the hole.
Reamers are either solid with shanks, or are shells.
They are parallel, or tapered. They are made with
blades solid, or adjustable (fig. 19).

Clearances.—Though a reamer is a scraping tool—the faces of the
teeth being disposed radially — it will not operate well unless suitable
clearances are provided. A very slight longitudinal clearance is necessary