Threads and Threading - Sportpilot.info

Machinery's Handbook 27th Edition
MINIATURE SCREW THREADS 1777
American Standard for Unified Miniature Screw Threads
This American Standard (B1.10-1958, R1988) introduces a new series to be known as
Unified Miniature Screw Threads and intended for general purpose fastening screws and
similar uses in watches, instruments, and miniature mechanisms. Use of this series is recommended
on all new products in place of the many improvised and unsystematized sizes
now in existence which have never achieved broad acceptance nor recognition by standardization
bodies. The series covers a diameter range from 0.30 to 1.40 millimeters
(0.0118 to 0.0551 inch) and thus supplements the Unified and American thread series
which begins at 0.060 inch (number 0 of the machine screw series). It comprises a total of
fourteen sizes which, together with their respective pitches, are those endorsed by the
American-British-Canadian Conference of April 1955 as the basis for a Unified standard
among the inch-using countries, and coincide with the corresponding range of sizes in ISO
(International Organization for Standardization) Recommendation No. 68. Additionally,
it utilizes thread forms which are compatible in all significant respects with both the Unified
and ISO basic thread profiles. Thus, threads in this series are interchangeable with the
corresponding sizes in both the American-British-Canadian and ISO standardization programs.
Basic Form of Thread.—The basic profile by which the design forms of the threads covered
by this standard are governed is shown in Table 1. The thread angle is 60 degrees and
except for basic height and depth of engagement which are 0.52p, instead of 0.54127p, the
basic profile for this thread standard is identical with the Unified and American basic
thread form. The selection of 0.52 as the exact value of the coefficient for the height of this
basic form is based on practical manufacturing considerations and a plan evolved to simplify
calculations and achieve more precise agreement between the metric and inch dimensional
tables.
Products made to this standard will be interchangeable with products made to other standards
which allow a maximum depth of engagement (or combined addendum height) of
0.54127p. The resulting difference is negligible (only 0.00025 inch for the coarsest pitch)
and is completely offset by practical considerations in tapping, since internal thread
heights exceeding 0.52p are avoided in these (Unified Miniature) small thread sizes in
order to reduce excessive tap breakage.
Design Forms of Threads.—The design (maximum material) forms of the external and
internal threads are shown in Table 2. These forms are derived from the basic profile
shown in Table 1 by the application of clearances for the crests of the addenda at the roots
of the mating dedendum forms. Basic and design form dimensions are given in Table 3.
Nominal Sizes: The thread sizes comprising this series and their respective pitches are
shown in the first two columns of Table 5. The fourteen sizes shown in Table 5 have been
systematically distributed to provide a uniformly proportioned selection over the entire
range. They are separated alternately into two categories: The sizes shown in bold type are
selections made in the interest of simplification and are those to which it is recommended
that usage be confined wherever the circumstances of design permit. Where these sizes do
not meet requirements the intermediate sizes shown in light type are available.
Table 1. Unified Miniature Screw Threads — Basic Thread Form
Formulas for Basic Thread Form
Metric units (millimeters) are used in all formulas
Thread Element Symbol Formula
Angle of thread 2α 60°
Half angle of thread α 30°
Pitch of thread p
No. of threads per inch n 25.4/p
Height of sharp V thread H 0.86603p
Addendum of basic thread hab 0.32476p
Height of basic thread hb 0.52p
Copyright 2004, Industrial Press, Inc., New York, NY