Machinery's Handbook, 31st Edition
1992
Miniature Screw Threads 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. Addi - tionally, 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 standard. 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 0.52 p , instead of 0.54127 p , 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 to simplify calcula - tions and achieve more precise agreement between 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 0.54127 p . 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 0.52 p 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 in - ternal 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) used in formulas Thread Element Symbol Formula Angle of thread 2 α 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 0.86603 p Addendum of basic thread h ab 0.32476 p Height of basic thread h b 0.52 p
0.32475953 P (0.375 H )
0.108253318 P (0.125 H )
P
0.86602540 P ( H )
P 2 0.125 P
60°
Pitch Line
30°
0.27777223 P (0.321 H ) 0.48000000 P (0.554 H )
90°
0.321 P
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