Threads and Threading - Sportpilot.info

Machinery's Handbook 27th Edition
UNIFIED SCREW THREADS 1775
ance provides adequate undercut for such coatings. There may be variations in thickness
and symmetry of coating resulting from commercial processes but after plating the threads
should be accepted by a basic Class 3A size GO gage and a Class 2A gage as a NOT-GO
gage. Class 1A provides an allowance which is maintained for both coated and uncoated
product, i.e., it is not available for coating.
Class 3A does not include an allowance so it is suggested that the limits of size before
plating be reduced by the amount of the 2A allowance whenever that allowance is adequate.
No provision is made for overcutting internal threads as coatings on such threads are not
generally required. Further, it is very difficult to deposit a significant thickness of coating
on the flanks of internal threads. Where a specific thickness of coating is required on an
internal thread, it is suggested that the thread be overcut so that the thread as coated will be
accepted by a GO thread plug gage of basic size.
This Standard ASME/ANSI B1.1-1989 (R2001) specifies limits of size that pertain
whether threads are coated or uncoated. Only in Class 2A threads is an allowance available
to accommodate coatings. Thus, in all classes of internal threads and in all Class 1A, 2AG,
and 3A external threads, limits of size must be adjusted to provide suitable provision for
the desired coating.
For further information concerning dimensional accommodation of coating or plating
for 60-degree threads, see Section 7, ASME/ANSI B1.1-1989 (R2001).
Screw Thread Selection — Combination of Classes.—Whenever possible, selection
should be made from Table 2, Standard Series Unified Screw Threads, preference being
given to the Coarse- and Fine- thread Series. If threads in the standard series do not meet
the requirements of design, reference should be made to the selected combinations in
Table 3. The third expedient is to compute the limits of size from the tolerance tables or
tolerance increment tables given in the Standard. The fourth and last resort is calculation
by the formulas given in the Standard.
The requirements for screw thread fits for specific applications depend on end use and
can be met by specifying the proper combinations of thread classes for the components.
For example, a Class 2A external thread may be used with a Class 1B, 2B, or 3B internal
thread.
Pitch Diameter Tolerances, All Classes.—The pitch diameter tolerances in Table 3 for
all classes of the UNC, UNF, 4-UN, 6-UN, and 8-UN series are based on a length of
engagement equal to the basic major (nominal) diameter and are applicable for lengths of
engagement up to 1 1 ⁄ 2 diameters.
The pitch diameter tolerances used in Table 3 for all classes of the UNEF, 12-UN, 16-
UN, 20-UN, 28-UN, and 32-UN series and the UNS series, are based on a length of
engagement of 9 pitches and are applicable for lengths of engagement of from 5 to 15
pitches.
Screw Thread Designation.—The basic method of designating a screw thread is used
where the standard tolerances or limits of size based on the standard length of engagement
are applicable. The designation specifies in sequence the nominal size, number of threads
per inch, thread series symbol, thread class symbol, and the gaging system number per
ASME/ANSI B1.3M. The nominal size is the basic major diameter and is specified as the
fractional diameter, screw number, or their decimal equivalent. Where decimal equivalents
are used for size callout, they shall be interpreted as being nominal size designations
only and shall have no dimensional significance beyond the fractional size or number designation.
The symbol LH is placed after the thread class symbol to indicate a left-hand
thread:
Examples:
1 ⁄4–20 UNC-2A (21) or 0.250–20 UNC-2A (21)
Copyright 2004, Industrial Press, Inc., New York, NY