Threads and Threading - Sportpilot.info

1776 UNIFIED SCREW THREADS
10–32 UNF-2A (22) or 0.190–32 UNF-2A (22)
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.
Example:
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 followed by the words BEFORE COATING.
Example:
Machinery's Handbook 27th Edition
3 ⁄4 –10 UNC-2A (21)
a Major dia 0.7500 max
PD 0.6850 max
b Major dia 0.7482–0.7353
} AFTER COATING
}
BEFORE COATING
PD 0.6832–0.6773 }
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 maximum
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 National
Standard ASME/ANSI B1.1-1989 (R2001).
Designating UNS Threads.—UNS screw threads that have special combinations of
diameter 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
modified limits of size or with special lengths of engagement, reference should be made to
American National Standard ASME/ANSI B1.1-1989 (R2001).
Hole Sizes for Tapping.—Hole size limits for tapping Classes 1B, 2B, and 3B threads of
various lengths of engagement are given in Table 2 on page 1925.
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 11 ⁄ 2 diameters.
However, some thread applications have lengths of engagement which are greater
than 11 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.
⁄ 2 diameters or less than the nominal diameter. For such applications it may be
advantageous to increase or decrease the tolerance, respectively, as explained in the Tapping
Section.
Copyright 2004, Industrial Press, Inc., New York, NY