Unified Screw Threads Machinery's Handbook, 31st Edition
1991
7 ∕ 16 -20 UNRF-2A (23) or 0.4375-20 UNRF-2A (23) 2-12 UN-2A (21) or 2.000-12 UN-2A (21) 1 ∕ 4 -20 UNC-3A-LH (21) or 0.250-20 UNC-3A-LH (21) For uncoated standard series threads these designations may optionally be supplemented by the addition of the pitch diameter limits of size. Examples: 1 ∕ 4 -20 UNC-2A (21) PD 0.2164-0.2127 (Optional for uncoated threads) Designating Coated Threads.— For coated (or plated) Class 2A external threads, the basic (max) major and basic (max) pitch diameters are given followed by the words AFTER COATING. The major and pitch diameter limits of size before coating are also given fol - lowed by the words BEFORE COATING. Example: 3 ∕ 4 -10 UNC-2A (21) a Major dia 0.7500 max } AFTER COATING PD 0.6850 max b Major dia 0.7482-0.7353 } BEFORE COATING PD 0.6832-0.6773 } a Major and PD values are equal to basic and correspond to those in Table 3 for Class 3A. b Major and PD limits are those in Table 3 for Class 2A. Certain applications require an allowance for rapid assembly, to permit application of a proper lubricant, or for residual growth due to high-temperature expansion. In such applications where the thread is to be coated and the 2A allowance is not permitted to be consumed by such coating, the thread class symbol is qualified by the addition of the letter G (symbol for allowance) following the class symbol, and the maximum major and maxi mum pitch diameters are reduced below basic size by the amount of the 2A allowance and followed by the words AFTER COATING. This arrangement ensures that the allowance is maintained. The major and pitch diameter limits of size before coating are also given followed by SPL and BEFORE COATING. For information concerning the designating of this and other special coating conditions reference should be made to American Na - tional Standard ANSI/ASME B1.1-2003 (R2018). Designating UNS Threads.— UNS screw threads that have special combinations of diam - eter and pitch with tolerance to Unified formulation have the basic form designation set out first followed always by the limits of size. Designating Multiple Start Threads.— If a screw thread is of multiple start, it is designated by specifying in sequence the nominal size, pitch (in decimals or threads per inch) and lead (in decimals or fractions). Other Special Designations.— For other special designations including threads with mod- ified limits of size or with special lengths of engagement, reference should be made to American National Standard ANSI/ASME B1.1-2003 (R2018). Hole Sizes for Tapping.— Hole size limits for tapping Classes 1B, 2B, and 3B threads of various lengths of engagement are given in Table 3 on page 2160. Internal Thread Minor Diameter Tolerances.— Internal thread minor diameter tolerances in Table 3 are based on a length of engagement equal to the nominal diameter. For general applications these tolerances are suitable for lengths of engagement up to 1 1 ∕ 2 diameters. However, some thread applications have lengths of engagement which are greater than 1 1 ∕ 2 diameters or less than the nominal diameter. For such applications it may be advan- tageous to increase or decrease the tolerance, respectively, as explained in the Tapping Section.
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online