Chapter 8 Power Tools 189
Copyright Goodheart-Willcox Co., Inc.
Using carbide bits in handheld drills should be avoided because they are
likely to break if excessive side stress is applied to the bit.
Drill bit surfaces are fi nished differently to improve performance in
a variety of materials. Most common drill bits have a polished surface to
reduce friction and speed the fl ow of chips or swarf. As the quality and price
of drill bits increase, the bits are coated with black oxide, titanium nitride
(gold color), or titanium carbonitride (bluish-gray color) to further reduce
friction and to extend the useful life of the tool.
A sharp drill bit cuts continuously and creates less friction than a dull
drill bit, which tends to rub into the material. Allowing a drill bit to turn
against the material without cutting generates excess friction, which creates
heat buildup. The excess heat dulls the drill bit faster and may damage the
work material.
Drill Bit Sizes
The size of a drill bit determines the diameter of the hole that it can
create. There are three standard measuring systems for drill bits. Drill bit
size is usually expressed in inches and fractions of an inch. Size may also be
indicated in millimeters and decimal fractions of a millimeter. Machinists,
professionals who fabricate and repair machinery and machine parts, often
use a system of sizing drill bits that uses a series of letters, A to Z, and whole
numbers, 1 to 80. A drill bit chart is essential when drilling precision holes.
Drill bits may be purchased separately, but usually they are sold in sets
that include a drill index. A drill index is a storage box with holes or pockets
that arrange each drill bit by diameter from smallest to largest. Drill bits
should always be stored in the drill index to preserve their sharpness and to
make fi nding the correct size easier.
Portable Drills
Portable power drills are handheld tools intended
primarily for boring holes in wood, metals, and other
common construction materials. See Figure 8-10. They
work by applying torque to a drill bit to cut a circular
hole through the material. Many drills have a variable
speed setting that allows the operator to match drilling
speed to the type of work material. Power drills do
not follow the general rule of relying on the weight
of the tool to apply pressure to the work. Instead, it is
necessary to manually apply enough force to a drill to
keep the bit cutting at all times.
Portable drills are usually powered by electricity,
either 120-volt AC or rechargeable batteries. Pneumatic
portable drills are used in environments where fl ammable
gases are present and a fl ash fi re hazard exists. Powerful
drills powered by an internal combustion engine
are used for tasks such as digging fence postholes or
boring blast holes in quarries.
Naruedom Yaempongsa/Shutterstock.com
Figure 8-10. A typical cordless electric power drill.