Threads and Threading - Sportpilot.info

Machinery's Handbook 27th Edition
THREADS AND THREADING 1725
SCREW THREAD SYSTEMS
Screw Thread Forms
Of the various screw thread forms which have been developed, the most used are those
having symmetrical sides inclined at equal angles with a vertical center line through the
thread apex. Present-day examples of such threads would include the Unified, the Whitworth
and the Acme forms. One of the early forms was the Sharp V which is now used only
occasionally. Symmetrical threads are relatively easy to manufacture and inspect and
hence are widely used on mass-produced general-purpose threaded fasteners of all types.
In addition to general-purpose fastener applications, certain threads are used to repeatedly
move or translate machine parts against heavy loads. For these so-called translation
threads a stronger form is required. The most widely used translation thread forms are the
square, the Acme, and the buttress. Of these, the square thread is the most efficient, but it is
also the most difficult to cut owing to its parallel sides and it cannot be adjusted to compensate
for wear. Although less efficient, the Acme form of thread has none of the disadvantages
of the square form and has the advantage of being somewhat stronger. The buttress
form is used for translation of loads in one direction only because of its non-symmetrical
form and combines the high efficiency and strength of the square thread with the ease of
cutting and adjustment of the Acme thread.
V-Thread, Sharp V-thread.—The sides of the thread form an angle of 60 degrees with
each other. The top and bottom or root of this thread form are theoretically sharp, but in
actual practice the thread is made with a slight flat, owing to the difficulty of producing a
perfectly sharp edge and because of the tendency of such an edge to wear away or become
battered. This flat is usually equal to about one twenty-fifth of the pitch, although there is
no generally recognized standard.
Owing to the difficulties connected with the V-thread, the tap manufacturers
agreed in 1909 to discontinue the making of sharp Vthread
taps, except when ordered. One advantage of the V-thread is
that the same cutting tool may be used for all pitches, whereas,
with the American Standard form, the width of the point or the flat
varies according to the pitch.
The V-thread is regarded as a good form where a steam-tight joint
is necessary, and many of the taps used on locomotive work have
this form of thread. Some modified V-threads, for locomotive boiler taps particularly, have
a depth of 0.8 × pitch.
The American Standard screw thread is used largely in preference to the sharp V-thread
because it has several advantages; see American Standard for Unified Screw Threads. If p
= pitch of thread, and d depth of thread, then
d = p× cos30
deg. = 0.866 × p = -------------------------------------------------------
0.866
No. of threads per inch
United States Standard Screw Thread.—William Sellers of Philadelphia, in a paper
read before the Franklin Institute in 1864, originally proposed the screw thread system that
later became known as the U. S. Standard system for screw threads. A report was made to
the United States Navy in May, 1868, in which the Sellers system was recommended as a
standard for the Navy Department, which accounts for the name of U. S. Standard. The
American Standard Screw Thread system is a further development of the United States
Standard. The thread form which is known as the American (National) form is the same as
the United States Standard form. See American Standard for Unified Screw Threads.
American National and Unified Screw Thread Forms.—The American National form
(formerly known as the United States Standard) was used for many years for most screws,
bolts, and miscellaneous threaded products produced in the United States. The American
Copyright 2004, Industrial Press, Inc., New York, NY