USING THE PORTABLE ELECTRIC DRILL - CONTINUED
Twist drills are the most common tools used in drilling
metal and are made in many different sizes and lengths.
These drills are made of carbon steel or high speed alloy
steel. Carbon steel drills are used for general drilling
while the alloy steel drills are used for drilling hard
metals such as stainless steel.
A twist drill has three main parts: point, body and shank.
The point of the drill is the cone shaped end
(normally 31 degrees) which does the actual cutting.
The body is the center section of the drill between
the point and shank. The cut-out portions of the
body are called flutes. The flutes serve a definite
function In that they cause the metal chip to curl
tightly within itself occupying a minimum amount of
flow easily down to the cutting edge.
The shank of the drill is the end that fits into the drill
The actual cutting is done by the cutting lips or edges
which are formed by the intersection of the flutes and the
cone-shaped point. The dead center of the drill is the
edge at he extreme tip of the point. The dead center
should always be in the exact center of the drill axis.
When drills are reground, it is possible to have the dead
center point off center, resulting in one cutting lip doing
most of the cutting and placing excessive strain on the
drill. The narrow strip at the edge of each blade is called
the margin. This strip, which extends the entire length of
the flutes, is part of a cylinder interrupted by the flutes.
The actual drill diameter is measured from margin to
The twist drills available at the sites are designated in
two different ways:
Fractional sizes - These drills come in sizes from
1/32 inch to 1/2 inch. The difference between one
drill size and the next larger is always 1/64 inch.
Numbered sizes - These drill sizes vary from #1
(0.2280 inches) to =80 (0.0135 inches). The
smallest numbered size drill stocked at the sites is
=60 (0.0400 inches).
If the size number, which is etched on the drill shank has
worn off, the drill size can be obtained by using a
micrometer. Measure the drill from the margin to margin
on the drill body near the shank end. This shank
diameter is usually a few ten thousandths of an inch
smaller than the point diameter.
USING THE DRILL
Prior to performing the job the proper size drill bit and
drill motor must be selected. Tables 3 and 4 in the
Appendix list the various drill bits available. There are
two drill motors available at the site:
1. A high-speed motor, 1/4 inch capacity drill motor is
used for general drilling of light metals
2. A 1/2 inch capacity used for drilling large holes. The
motor of the 1/2 inch capacity drill is geared down to
prevent overheating the drill bit.
The object to be drilled should be held in a vise when
possible. Never attempt to hold the work with your
hands. The drill may catch or jam and start the stock
spinning. When this occurs the stock may fly loose and
injure personnel In the immediate area. When
52-5.0 Change 3