Threads and Threading - Sportpilot.info

Machinery's Handbook 27th Edition
1726 SCREW THREAD SYSTEMS
National Standard for Unified Screw Threads now in use includes certain modifications of
the former standard as is explained below and on page 1732. The basic profile is shown in
Fig. 1 and is identical for both UN and UNR screw threads. In this figure H is the height of
a sharp V-thread, P is the pitch, D and d are the basic major diameters, D 2 and d 2 are the
basic pitch diameters, and D 1 and d 1 are the basic minor diameters. Capital letters are used
to designate the internal thread dimensions (D, D 2 , D 1 ), 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 1727.
Fig. 1. Basic Profile of UN and UNF Screw Threads
In the past, other symbols were used for some of the thread dimensions illustrated 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 termonology 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 Congress for the standardization of screw threads held in Zurich
in 1898. The thread form is similar to the American standard (formerly U.S. Standard),
excepting the depth which is greater. There is a clearance between the root and mating 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.
Depth d =0.7035 P max.; 0.6855 P min.
Flat f =0.125 P
Radius r = 0.0633 P max.; 0.054 P min.
Tap drill dia = major dia.− pitch
Screw
International Metric Fine Thread: The International Metric Fine Thread form of thread
is the same as the International system but the pitch for a given diameter is smaller.
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.
P
60°
Nut
Copyright 2004, Industrial Press, Inc., New York, NY
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