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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 r f d
SCREW THREADS 1727 ISO Metric Thread System.—ISO refers to the International Organization for Standardization, a worldwide federation of national standards bodies (for example, the American National Standards Institute is the ISO national body representing the United States) that develops standards on a very wide variety of subjects. The basic profile of ISO metric threads is specified in ISO 68 and shown in Fig. 2. The basic profile of this thread is very similar to that of the Unified thread, and as previously discussed, 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. Here also, capital letters designate the internal thread dimensions (D, D 2 , D 1 ), and lowercase letters designate the external thread dimensions (d, d 2 , d 1 ). This metric thread is discussed in detail in the section METRIC SCREW THREADS starting on page 1783. Internal threads D, d D2 , d2 D1 , d1 External threads Machinery's Handbook 27th Edition P 8 P P 2 P 3 8 5 H 8 P 4 2 H 4 H 60° 30° H 90° Axis of screw thread H = 3 × P = 0.866025404P 2 0.125H = 0.108253175P 0.250H = 0.216506351P 0.375H = 0.324759526P 0.625H = 0.541265877P Fig. 2. ISO 68 Basic Profile Definitions of Screw Threads The following definitions are based on American National Standard ANSI/ASME B1.7M-1984 (R2001) “Nomenclature, Definitions, and Letter Symbols for Screw Threads,” and refer to both straight and taper threads. Actual Size: An actual size is a measured size. Allowance: An allowance is the prescribed difference between the design (maximum material) size and the basic size. It is numerically equal to the absolute value of the ISO term fundamental deviation. Axis of Thread: Thread axis is coincident with the axis of its pitch cylinder or cone. Basic Profile of Thread: The basic profile of a thread is the cyclical outline, in an axial plane, of the permanently established boundary between the provinces of the external and internal threads. All deviations are with respect to this boundary. Basic Size: The basic size is that size from which the limits of size are derived by the application of allowances and tolerances. Bilateral Tolerance: This is a tolerance in which variation is permitted in both directions from the specified dimension. Black Crest Thread: This is a thread whose crest displays an unfinished cast, rolled, or forged surface. Blunt Start Thread: “Blunt start” designates the removal of the incomplete thread at the starting end of the thread. This is a feature of threaded parts that are repeatedly assembled Copyright 2004, Industrial Press, Inc., New York, NY H 8
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