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}                                                          THE  TOOLS                                     185
A;,                 and the work revolves.    A cup is screwed into the shank to receive the
connection from the oil supply, this connection being usually a flexible pipe.
I                        Twisted Twist-drills.—These are being used in increasing numbers
j                  in preference to those in which the flutes are cut by milling.   The demands
I                  of high-speed work are partly responsible for this design, which is a return
to the primitive twist-drills made by twisting a flat bar of steel.
|                        Drill Shanks.—These are standardized both for tapered and parallel
shanks. The tapered shank is used with drill sockets, and the second
when the drill is held in a chuck or in a turret. There are seven sizes of
Morse tapers. One size can only be used for a small range of drill diameters
differing by a few eighths of an inch. Adapter sleeves or sockets are then
employed, the first for shanks larger than a machine-spindle takes, the second
for those of smaller sizes.
Boring-tools.—Boring is distinguished from drilling not precisely
because bored holes are usually larger than those that are drilled, but
the term signifies the enlargement of a hole which has been already
" drilled ". Though drilling may be done up to 5 or 6 in., and boring
so small as 2 or 3 in. diameter, yet the latter operation is mostly associated
with holes that range, say, from about 3 in. to 20 or 30 ft.
Boring-cutters.—The single-edged lathe boring-tool is the type on
which all boring-cutters are designed. The single cutter is retained in many
cases for roughing. The lathe tool itself has but a limited use in the boring
i                   practice of to-day.   The solid shank of the tool is a cantilever that chatters
if it overhangs much, or if the pressure of the single cut is unbalanced.
1                   For long holes two or more cutters in balance are used, either inserted in
*                   slotted bars or carried in heads, which are either fixed or are fed along
their bars. Different shapes and cutting angles may be used for roughing
and for finishing, but frequently no difference is made. In minor details
the tools follow the usual practice in tool design which has already been
described. The boring-tool is a tool point of an expensive but hard
steel, which is gripped in a bar or holder of common material.
Cutters in Bars.—Only the smaller holes are bored with cutters that
fit in slots in bars. They are single or double, and are differently secured.
A wedge is common (fig. 8) but is liable to shift, so is a round tapered pin,
flattened on the side next the cutter (fig. 9). Neither alone would provide
for setting the cutter to exact radius, which must be done by gently tapping.
j     .             Many single- and double-ended cutters are therefore "self-centred" with
a notch fitting over the diameter of the bar (figs. 8, 9, 10), and then they
'                   cannot shift.   Single cutters are adjusted radially with light hammer-taps,
1                  and are then tightened with set-screws (figs, n and 14).   They may be set
with a grub-screw at the rear, and clamped with a set-screw (fig. 12).    A
very common method is that in fig. 13, where the head of a cheese-head screw
/                  entering a notch in the shank of the cutter adjusts it finely.   Another method
applied to double cutters is shown in fig. 15.   A grub-screw with a conical