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Machinery's Handbook 27th Edition 1732 UNIFIED SCREW THREADS UNIFIED SCREW THREADS American Standard for Unified Screw Threads American Standard B1.1-1949 was the first American standard to cover those Unified Thread Series agreed upon by the United Kingdom, Canada, and the United States to obtain screw thread interchangeability among these three nations. These Unified threads are now the basic American standard for fastening types of screw threads. In relation to previous American practice, Unified threads have substantially the same thread form and are mechanically interchangeable with the former American National threads of the same diameter and pitch. The principal differences between the two systems lie in: 1) application of allowances; 2) variation of tolerances with size; 3) difference in amount of pitch diameter tolerance on external and internal threads; and 4) differences in thread designation. In the Unified system an allowance is provided on both the Classes 1A and 2A external threads whereas in the American National system only the Class I external thread has an allowance. Also, in the Unified system, the pitch diameter tolerance of an internal thread is 30 per cent greater than that of the external thread, whereas they are equal in the American National system. Revised Standard.—The revised screw thread standard ANSI/ASME B1.1-1989 (R2001) is much the same as that of ANSI B1.1-1982. The latest symbols in accordance with ANSI/ASME B1.7M-1984 (R2001) Nomenclature, are used. Acceptability criteria are described in ANSI/ASME B1.3M-1992 (R2001), Screw Thread Gaging Systems for Dimensional Acceptability, Inch or Metric Screw Threads (UN, UNR, UNJ, M, and MJ). Where the letters U, A or B do not appear in the thread designations, the threads conform to the outdated American National screw threads. Advantages of Unified Threads.—The Unified standard is designed to correct certain production difficulties resulting from the former standard. Often, under the old system, the tolerances of the product were practically absorbed by the combined tool and gage tolerances, leaving little for a working tolerance in manufacture. Somewhat greater tolerances are now provided for nut threads. As contrasted with the old “classes of fit” 1, 2, and 3, for each of which the pitch diameter tolerance on the external and internal threads were equal, the Classes 1B, 2B, and 3B (internal) threads in the new standard have, respectively, a 30 per cent larger pitch diameter tolerance than the 1A, 2A, and 3A (external) threads. Relatively more tolerance is provided for fine threads than for coarse threads of the same pitch. Where previous tolerances were more liberal than required, they were reduced. Thread Form.—The Design Profiles for Unified screw threads, shown on page 1733, define the maximum material condition for external and internal threads with no allowance and are derived from the Basic Profile, shown on page 1726. UN External Screw Threads: A flat root contour is specified, but it is necessary to provide for some threading tool crest wear, hence a rounded root contour cleared beyond the 0.25P flat width of the Basic Profile is optional. UNR External Screw Threads: To reduce the rate of threading tool crest wear and to improve fatigue strength of a flat root thread, the Design Profile of the UNR thread has a smooth, continuous, non-reversing contour with a radius of curvature not less than 0.108P at any point and blends tangentially into the flanks and any straight segment. At the maximum material condition, the point of tangency is specified to be at a distance not less than 0.625H (where H is the height of a sharp V-thread) below the basic major diameter. UN and UNR External Screw Threads: The Design Profiles of both UN and UNR external screw threads have flat crests. However, in practice, product threads are produced with partially or completely rounded crests. A rounded crest tangent at 0.125P flat is shown as an option on page 1733. Copyright 2004, Industrial Press, Inc., New York, NY
UNIFIED SCREW THREADS 1733 UN Internal Screw Thread: In practice it is necessary to provide for some threading tool crest wear, therefore the root of the Design Profile is rounded and cleared beyond the 0.125P flat width of the Basic Profile.There is no internal UNR screw thread. American National Standard Unified Internal and External Screw Thread Design Profiles (Maximum Material Condition) .— 0.125H 0.625H H 0.125H ;;;;;;;; ;;;;;;;; 90 deg 0.375H 0.25P 0.375H Flanks to be straight beyond 0.25H from sharp apex of root Axis of external thread 0.125P 0.5P Pitch line 0.25H Rounded crest optional 60 deg 30 deg Nominal flat root design minor diameter Rounded root optional ;;;;;;;;; ;;;;;;;;; ;;;;;;;;; ;;;;;;;;; 0.25H 0.625H H 90 deg Machinery's Handbook 27th Edition 0.25P 0.6875H 0.1875H 0.125P 60 deg 0.5P Pitch line r = 0.108P Flanks to be straight beyond 0.25H from sharp apex of root Axis of external thread Thread Series.—Thread series are groups of diameter-pitch combinations distinguished from each other by the numbers of threads per inch applied to a specific diameter. The various diameter-pitch combinations of eleven standard series are shown in Table 2. The limits of size of threads in the eleven standard series together with certain selected combinations of diameter and pitch, as well as the symbols for designating the various threads, are given in Table 3. (Text continues on page 1763) P 0.0625H 0.25H Tangency flank/root rad. UNR design minor diameter specified in dimensional tables ;;;;;; ;;;;;; Min major diameter specified in dimensional tables UN Internal 0.125P Thread (Nut) 0.125H 0.125H 0.25P 0.375H Pitch 60° H line 0.625H 0.25H 0.5P 0.25H 0.25H P 90 deg Axis of external thread (H = height of sharp V-thread = 0.86603 × pitch) P ;;;;;;;; 0.25H Rounded crest optional 30 deg Copyright 2004, Industrial Press, Inc., New York, NY
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