TC 9-524
Threads per inch is the number of threads per inch may be
counted by placing a rule against the threaded parts and
counting the number of pitches in 1 inch. A second
method is to use the screw pitch gage. This method is
especially suitable for checking the finer pitches of screw
threads.
A single thread is a thread made by cutting one single
groove around a rod or inside a hole. Most hardware
made, such as nuts and bolts, has single threads. Double
threads have two grooves cut around the cylinder. There
can be two, three, or four threads cut around the outside
or inside of a cylinder. These types of special threads are
sometimes called multiple threads.
A right-hand thread is a thread in which the bolt or nut
must be turned to the right (clockwise) to tighten.
A left hand thread is a thread in which the bolt or nut
must turn to the left (counterclockwise) to tighten.
Thread fit is the way a bolt and nut fit together as to being
too loose or too tight.
Metric threads are threads that are measured in metric
measurement instead of inch measurement.
Screw Thread Forms
The most commonly used screw thread forms are detailed in
the following paragraphs. One of the major problems in
industry is the lack of a standard form for fastening devices.
The screw thread forms that follow attempt to solve this
problem; however, there is still more than one standard form
being used in each industrial nation. The International
Organization for Standardization (IS0) met in 1975 and drew
up a standard metric measurement for screw threads, the new
IS0 Metric thread Standard (previously known as the
Optimum Metric Fastener System). Other thread forms are
still in general use today, including the American (National)
screw thread form, the square thread, the Acme thread, the
Brown and Sharpe 29° worm screw thread, the British
Standard Whitworth thread, the Unified thread, and different
pipe threads. All of these threads can be cut by using the lathe.
The IS0 Metric thread standard is a simple thread system
that has threaded sizes ranging in diameter from 1.6 mm
to 100 mm (see Table 7-8 in Appendix A). These metric
threads are identified by the capital M, the nominal
diameter, and the pitch. For example, a metric
thread with an outside diameter of 5 mm and a pitch of 0.8
mm would be given as M 5 x 0.8. The IS0 metric thread
standard simplifies thread design, provides for good strong
threads, and requires a smaller inventory of screw fasteners
than used by other thread forms. This IS0 Metric thread has a
60° included angle and a crest that is 1.25 times the pitch
(which is similar to the National thread form). The depth of
thread is 0.6134 times the pitch, and the flat on the root of the
thread is wider than the crest. The root of the ISO Metric
thread is 0.250 times the pitch (Table 7-9).
The American (National) screw thread form is divided
into four series, the National Coarse (NC), National Fine
(NF), National Special (NS), and National Pipe threads
(NPT), 11 series of this thread form have the same shape
and proportions. This thread has a 60° included angle.
The root and crest are 0.125 times the pitch. This thread
form is widely used in industrial applications for
fabrication and easy assembly and construction of
machine parts. Table 7-9 in Appendix A gives the
different values for this thread form.
The British Standard Whitworth thread form thread has a
55° thread form in the V-shape. It has rounded crests and
roots.
The Unified thread form is now used instead of the
American (National) thread form. It was designed for
interchangeability between manufacturing units in the
United States, Canada, and Great Britain. This thread is a
combination of the American (National) screw thread
form and the British Whitworth screw thread forms. The
thread has a 60° angle with a rounded root, while the crest
can be rounded or flat. (In the United States, a flat crest is
preferred.) The internal thread of the unified form is like
the American (National) thread form but is not cut as
deep, leaving a crest of one-fourth the pitch instead of
one-eighth the pitch. The coarse thread series of the
unified system is designated UNC, while the fine thread
series is designated UNF. (See Tble 7-9 in Appendix A
for thread form and values.
The American National 29° Acme was designed to
replace the standard square thread, which is difficult to
machine using normal taps and machine dies. This thread
is a power transmitting type of thread for use in jacks,
vises, and feed screws. Table 7-9 lists the values for
Acme threads.
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