Threads and Threading - Sportpilot.info

Machinery's Handbook 27th Edition
STUB ACME SCREW THREADS 1843
Designation of Centralizing Acme Threads.—The following examples are given to
show how these Acme threads are designated on drawings, in specifications, and on tools
and gages:
Example, 1.750-6-ACME-4C:Indicates a Centralizing Class 4C Acme thread of 1.750inch
major diameter, 0.1667-inch pitch, single thread, right-hand.
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 diameters 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 dimensions,
reversing major and minor diameter allowances, tolerances, radii, and chamfer.
American National Standard Stub Acme Threads.—This American National Standard
ASME/ANSI B1.8-1988 (R2001) provides a Stub Acme screw thread for those
unusual applications where, due to mechanical or metallurgical considerations, a coarsepitch
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 B1.5-1988.
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 1826 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 1825.
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.
Copyright 2004, Industrial Press, Inc., New York, NY