These screws are available in countersunk head, round
head, and brazier head types. See figure 5-5.
The commonly used machine screws are the round
head, flat head, fillister head, pan head, truss head, and
socket head types.
A self-tapping screw is one that cuts its own
internal threads as it is turned into the hole.
Self-tapping screws may be used only in comparatively
soft metals and materials. Self-tapping screws may be
further divided into two classes or groupsmachine
self-tapping screws and sheet metal self-tapping
Machine self-tapping screws are usually used for
attaching removable parts, such as nameplates, to
castings. The threads of the screw cut mating threads in
the casting after a hole has been predrilled undersize.
Sheet metal self-tapping screws are used for such
purposes as temporarily attaching sheet metal in place
for riveting. Sheet metal self-tapping screws may be
used to permanently assemble nonstructural units
where it is necessary to insert screws in difficult to get
Self-tapping screws should never be used to
replace standard screws, nuts, or rivets originally
used in the structure.
Setscrews are used to position and hold com-
ponents in place, such as gears on a shaft. Setscrews are
available with many different point styles. They are
classified as hexagon-socket and fluted-socket headless
Aircraft nuts may be divided into two general
Nonself-locking nuts are those that must be safetied by
external locking devices, such as cotter pins, safety
wire, or locknuts. The locking feature is an integral part
of self-locking nuts.
The most common of the nonself-locking nuts are
the castle nut, the plain hex nut, the castellated shear
nut, and the wing nut. Figure 5-6 shows these
Figure 5-5.Structural screws.
PLAIN HEX NUT
Figure 5-6.Nonself-locking nuts.