Machinery's Handbook, 31st Edition
2072 Centralizing Acme Screw Threads Example, 1.750-6-ACME-4C-LH: Indicates the same thread left-hand.
Example, 2.875-0.4P-0.8L-ACME-3C (Two Start): Indicates a Centralizing Class 3C Acme thread with 2.875-inch major diameter, 0.4-inch pitch, 0.8-inch lead, double thread, right-hand. Example, 2.500-0.3333P-0.6667L-ACME-4C (Two Start): Indicates a Centralizing Class 4C Acme thread with 2.500-inch nominal major diameter (basic major diameter 2.500 inches), 0.3333-inch pitch, 0.6667-inch lead, double thread, right-hand. The same thread left-hand would have LH at the end of the designation. Acme Centralizing Threads—Alternative Series with Minor Diameter Centralizing Control.— When Acme centralizing threads are produced in single units or in very small quantities (and principally in sizes larger than the range of commercial taps and dies) where the manufacturing process employs cutting tools (such as lathe cutting), it may be economically advantageous and therefore desirable to have the centralizing control of the mating threads located at the minor diameters. Particularly under the above-mentioned type of manufacturing, the two advantages cited for minor diameter centralizing control over centralizing control at the major diam - eters of the mating threads are: 1) Greater ease and faster checking of machined thread dimensions. It is much easier to measure the minor diameter (root) of the external thread and the mating minor diameter (crest or bore) of the internal thread than it is to determine the major diameter (root) of the internal thread and the major diameter (crest or turn) of the external thread; and 2) better manufacturing control of the machined size due to greater ease of checking. In the event that minor diameter centralizing is necessary, recalculate all thread dimen sions, reversing major and minor diameter allowances, tolerances, radii, and chamfer. American National Standard Stub Acme Threads.— This American National Standard ANSI/ASME B1.8-1988 (R2016) provides a Stub Acme screw thread for those unusual applications where, due to mechanical or metallurgical considerations, a coarse-pitch thread of shallow depth is required. The fit of Stub Acme threads corresponds to the Class 2G General Purpose Acme thread in American National Standard ANSI/ASME B1.5- 1997 (R2014). For a fit having less backlash, the tolerances and allowances for Classes 3G or 4G General Purpose Acme threads may be used. Thread Form: The thread form and basic formulas for Stub Acme threads are given on page 2054 and the basic dimensions in Table 13. Allowances and Tolerances: The major and minor diameter allowances for Stub Acme threads are the same as those given for General Purpose Acme threads on page 2054. Pitch diameter allowances for Stub Acme threads are the same as for Class 2G General Purpose Acme threads and are given in Table 4 . Pitch diameter tolerances for Stub Acme threads are the same as for Class 2G General Purpose Acme threads given in Table 5. Limiting Dimensions: Limiting dimensions of American Standard Stub Acme threads may be determined by using the formulas given in Table 14a , or directly from Table 14b. The diagram below shows the limits of size for Stub Acme threads. Thread Series: A preferred series of diameters and pitches for General Purpose Acme threads (Table 15 ) is recommended for Stub Acme threads. Stub Acme Thread Designations.— The method of designation for Standard Stub Acme threads is illustrated in the following examples: 0.500-20 Stub Acme indicates a 1 ∕ 2 -inch major diameter, 20 threads per inch, right hand, single thread, Standard Stub Acme thread. The designation 0.500-20 Stub Acme-LH indicates the same thread except that it is left hand.
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