Section 13. qxd

Drilling machines
are intended for drilling holes, tapping, counterbor-
ing, reaming, and general boring operations. They may be classified
into a large variety of types.
Vertical drilling machines
are usually designated by a dimension which
roughly indicates the diameter of the largest circle that can be drilled at
its center under the machine. This dimensioning, however, does not
hold for all makes of machines. The sizes begin with about 6 and
continue to 50 in. Heavy-duty drill presses of the vertical type, with all-
geared speed and feed drive, are constructed with a box-type column
instead of the older cylindrical column.
The size of a 
radial drill
is designated by the length of the arm. This
represents the radius of a piece which can be drilled in the center.
Twist drills
(Fig. 13.4.13) are the most common tools used in drilling and
are made in many sizes and lengths. For years they have been grouped
according to numbered sizes, 1 to 80, inclusive, corresponding approxi-
mately to Stub’s steel wire gage; some by lettered sizes A to Z, inclusive;
some by fractional inches from 
up, and the group of millimetre sizes.
Straight-shank twist drills
of fractional size and various lengths range
from 
in diam to 1
in by 
in increments; to 1
in by 
in; and
to 2 in by 
in. 
Taper-shank drills
range from 
in diam to 1
in 
by 
increments; to 2
in by 
in; and to 3
in by 
in. Larger
drills are made by various drill manufacturers. Drills are also available
in metric dimensions.
Tolerances have been set on the various features of all drills so that
the products of different manufacturers will be interchangeable in the
user’s plants.
Twist drills are decreased in diameter from point to shank (back
taper) to prevent binding. If the web is increased gradually in thickness
from point to shank to increase the strength, it is customary to reduce
the helix angle as it approaches the shank. The shape of the groove is
important, the one that gives a straight cutting edge and allows a full
curl to the chip being the best. The