Twist drills have various shank types sized by frac-
tion, number, or letter. Fraction symbols give the actual
size of the drill. Sets that include sizes from 1/16 to 1/2
inch are common in the shop. Number and letter desig-
nators only identify the drill. A drill gauge or reference
chart (table 5-1) gives the actual size of the drill. Note
that the letter sizes are larger and start where the number
sizes stop. The most common type used in HT shops is
the carbon steel drill with a straight shank.
Figure 5-21.--Comparison of a twist drill for plastic and a
A drill should be reground at the first sign of dull-
twist drill for metal.
ness. The increased load that dullness imposes on the
cutting edges may cause a drill to break.
may fall on them. Do not place drills where they will
Twist drills are sharpened differently for boring
rub against each other.
different materials. The two common angles are the
regular point (fig. 5-19) and the flat point (fig. 5-20).
MACHINE SPUR BIT
The regular point has an angle of 118° and is used for
general boring, which includes wood and metal. The flat
The machine spur bit (fig. 5-13, view E) works
point has an angle of 135° and is used to bore hard and
only in a drill press. It is a high-speed, smooth-cutting
bit for boring deep, flat-bottomed holes. It has a
The general-purpose twist drill is made of high-
centering point and a twist to remove waste material
speed steel. Figure 5-21 shows a typical plastic-cutting
and comes in sizes ranging from 1/8 to 1/2 inch in
drill and a typical metal-cutting drill. Notice the smaller
angle on the drill used for drilling plastics.
Before putting a drill away, wipe it clean and give
it a light coating of oil. Do not leave drills in a place
The multispur bit (fig. 5-13, view F) also works
where they may be dropped or where heavy objects
strictly in drill presses. Use it to bore flat-bottomed
holes larger than the machine spur bit and ranges in
size from 1/2 to 4 inches in diameter.
NOTE: As bit sizes increase, drill press speed
The countersink forms a seat for the head of a
flat-headed wood screw and comes in three types (fig.
Figure 5-19--Specifications for grindingaregular point
5-22). Type A is for use with a hand brace. Types B
Figure 5-20.--Specifications for grinding a flat point twist
Figure 5-22.--Types of countersinks.