Machinery's Handbook, 31st Edition
Screw Thread Systems 1941 of the former standard as is explained below and on page 1947 . 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 1942. P 0.125 P 0.125 H
Basic Major Diameter D bsc, d bsc
60°
0.625 H
H
30°
Basic Pitch Diameter D 2 bsc, d 2 bsc
0.5 p
0.25 H
0.25 P Axis of Screw Thread
Minor Diameter D 1 bsc, d 1 bsc
90°
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 B1.7, “Screw Threads: Nomenclature, Definitions, and Letter Symbols.” 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 Congress for the standardization of screw threads held in Zurich in 1898. The thread form is similar to the American standard (formerly US 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 Euro - pean countries, Japan, and many other countries, including metric thread standards of the United States.
P
f
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.
Nut 60 °
d
Tap drill dia = major dia. − pitch
Screw
r
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 .
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