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Machinery's Handbook 27th Edition 1856 ANSI BUTTRESS THREADS External Thread: Max. Major Diameter: D − G = 1.9926 (see Table 5) Min. Major Diameter: D − G − Major Diameter Tolerance = 1.9814 (see Tables 4 and 5) Max. Pitch Diameter: D − h − G = 1.8426 (see Tables 2 and 5) Min. Pitch Diameter: D − h − G − PD Tolerance = 1.8314 (see Table 4) Max. Minor Diameter: D − G − 2hs = 1.6612 (see Tables 2 and 5) Buttress Thread Designations.—When only the designation, BUTT is used, the thread is “pull” type buttress (external thread pulls) with the clearance flank leading and the 7degree pressure flank following. When the designation, PUSH-BUTT is used, the thread is a push type buttress (external thread pushes) with the 7-degree load flank leading and the 45-degree clearance flank following. Whenever possible this description should be confirmed by a simplified view showing thread angles on the drawing of the product that has the buttress thread. Standard Buttress Threads: A buttress thread is considered to be standard when: 1) opposite flank angles are 7-degrees and 45-degrees; 2) basic thread height is 0.6p; 3) tolerances and allowances are as shown in Tables 4 and 5; and 4) length of engagement is 10p or less. Thread Designation Abbreviations: In thread designations on drawings, tools, gages, and in specifications, the following abbreviations and letters are to be used: BUTT for buttress thread, pull type PUSH- BUTT for buttress thread, push type LH for left-hand thread (Absence of LH indicates that the thread is a right-hand thread.) P for pitch L for lead A B for external thread for internal thread Note: Absence of A or B after thread class indicates that designation covers both the external and internal threads. Le for length of thread engagement SPL for special FL for flat root thread E for pitch diameter TPI for threads per inch THD for thread Designation Sequence for Buttress Inch Screw Threads.—When designating singlestart standard buttress threads the nominal size is given first, the threads per inch next, then PUSH if the internal member is to push, but nothing if it is to pull, then the class of thread (2 or 3), then whether external (A) or internal (B), then LH if left-hand, but nothing if righthand, and finally FL if a flat root thread, but nothing if a radiused root thread; thus, 2.5-8 BUTT-2A indicates a 2.5 inch, 8 threads per inch buttress thread, Class 2 external, righthand, internal member to pull, with radiused root of thread. The designation 2.5-8 PUSH- BUTT-2A-LH-FL signifies a 2.5 inch size, 8 threads per inch buttress thread with internal member to push, Class 2 external, left-hand, and flat root. A multiple-start standard buttress thread is similarly designated but the pitch is given instead of the threads per inch, followed by the lead and the number of starts is indicated in parentheses after the class of thread. Thus, 10-0.25P–0.5L – BUTT-3B (2 start) indicates a 10-inch thread with 4 threads per inch, 0.5 inch lead, buttress form with internal member to pull, Class 3 internal, 2 starts, with radiused root of thread. Copyright 2004, Industrial Press, Inc., New York, NY
Machinery's Handbook 27th Edition WHITWORTH THREADS 1857 WHITWORTH THREADS British Standard Whitworth (BSW) and British Standard Fine (BSF) Threads The BSW is the Coarse Thread series and the BSF is the Fine Thread series of British Standard 84:1956—Parallel Screw Threads of Whitworth Form. The dimensions given in the tables on the following pages for the major, effective, and minor diameters are, respectively, the maximum limits of these diameters for bolts and the minimum limits for nuts. Formulas for the tolerances on these diameters are given in the table below. Whitworth Standard Thread Form.—This thread form is used for the British Standard Whitworth (BSW) and British Standard Fine (BSF) screw threads. More recently, both threads have been known as parallel screw threads of Whitworth form. With standardization of the Unified thread, the Whitworth thread form is expected to be used only for replacements or spare parts. Tables of British Standard Parallel Screw Threads of Whitworth Form will be found on the following pages; tolerance formulas are given in the table below. The form of the thread is shown by the diagram. If p = pitch, d = depth of thread, r = radius at crest and root, and n = number of threads per inch, then d 1 ⁄3 p 27 ° cot 30 ′ = × = 0.640327p = 0.640327 ÷ n r = 0.137329p = 0.137329 ÷ n It is recommended that stainless steel bolts of nominal size 3 ⁄ 4 inch and below should not be made to Close Class limits but rather to Medium or Free Class limits. Nominal sizes above 3 ⁄ 4 inch should have maximum and minimum limits 0.001 inch smaller than the values obtained from the table. Tolerance Classes : Close Class bolts. Applies to screw threads requiring a fine snug fit, and should be used only for special work where refined accuracy of pitch and thread form are particularly required. Medium Class bolts and nuts. Applies to the better class of ordinary interchangeable screw threads. Free Class bolts. Applies to the majority of bolts of ordinary commercial quality. Normal Class nuts. Applies to ordinary commercial quality nuts; this class is intended for use with Medium or Free Class bolts. Table 1. Tolerance Formulas for BSW and BSF Threads Class or Fit Tolerance in inchesa (+ for nuts, − for bolts) Major Dia. Effective Dia. Minor Dia. Close 2 ⁄3T Bolts Medium T Free 3 ⁄2T Close … 2 ⁄3T 0.2p + 0.004 Nuts }{ b Medium … T 0.2p + 0.005c Normal … 3 ⁄2T 0.2p + 0.007d p d r 55˚ r 2 ⁄3T + 0.01 p 2 ⁄3 T + 0.013 p T + 0.01 p T + 0.02 p 3 ⁄2T + 0.01 p 3 ⁄2 T + 0.02 p a The symbol T 0.002 D , where D = major diameter of thread in inches; L = length of engagement in inches; p = pitch in inches. The symbol p signifies pitch. b For 26 threads per inch and finer. c For 24 and 22 threads per inch. d For 20 threads per inch and coarser. 3 = + 0.003 L + 0.005 p Copyright 2004, Industrial Press, Inc., New York, NY
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