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 ofthe early forms wasthe 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 isthe most efficient, but it is also the most difficult to cut owing to its parallelsides 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 tapmanufacturers agreed in 1909 to discontinue the making of sharp Vthread taps, except when ordered. One advantage ofthe 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 thisform ofthread. SomemodifiedV-threads,forlocomotive boilertaps 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 hasseveral advantages;see American Standard for Unified Screw Threads. If p = pitch of thread, and d depth of thread, then 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 asthe American (National) form isthe 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 asthe United States Standard) was used for many yearsfor mostscrews, bolts, and miscellaneous threaded products produced in the United States. The American 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 asthe American (National) form isthe 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 asthe United States Standard) was used for many yearsfor mostscrews, bolts, and miscellaneous threaded products produced in the United States. The American

National Standard for Unified Screw Threads now in use includes certain modifications of the formerstandard as is explained below and on page 1719. The basic profile is shown in Fig. 1 and isidentical for both UN and UNR screw threads. In thisfigure H isthe height of a sharp V-thread, P is the pitch, D and d are the basic major diameters, D2 and d2 are the basic pitch diameters, and D1 and d 1 are the basic minor diameters. Capital letters are used to designate the internal thread dimensions (D, D2, D1), and lowercase letters to designate the external thread dimensions (d, d 2, d 1). Definitions of Basic Size and Basic Profile of Thread are given on page 1714.

In the past, othersymbols were used forsome of the thread dimensionsillustrated above. These symbols were changed to conform with current practice in nomenclature as defined in ANSI/ASME B1.7M, “Nomenclature, Definitions, and Letter Symbols for Screw Threads.” The symbols used above are also in accordance with terminology and symbols used for threads of the ISO metric thread system. International Metric Thread System.—The Système Internationale (S.I.) Thread was adopted at the International Congressfor the standardization ofscrew threads held in Zurich in 1898. The thread form issimilar to the American standard (formerly U.S. Standard), excepting the depth which is greater.There is a clearance between the root andmating crest fixed at a maximum of 1 ⁄16 the height of the fundamental triangle or 0.054 × pitch. A rounded root profile is recommended. The angle in the plane of the axis is 60 degrees and the crest has a flat like the American standard equal to 0.125 × pitch. This system formed the basis of the normal metric series(ISO threads) of many European countries,Japan, and many other countries, including metric thread standards of the United States. International Metric Fine Thread: The International Metric Fine Thread form of thread isthe same asthe Internationalsystem but the pitch for a given diameter issmaller. German Metric Thread Form: The German metric thread form is like the International Standard but the thread depth = 0.6945 P. The root radius is the same as the maximum for the International Standard or 0.0633 P.