Machinery's Handbook, 31st Edition
2114 LAMP AND ELECTRICAL FIXTURE THREADS Base Screw Shell Gage Tolerances: Threaded ring gages—“Go,” Max. thread size to minus 0.0003 inch; “Not Go,” Min. thread size to plus 0.0003 inch. Plain ring gages— “Go,” Max. thread O.D. to minus 0.0002 inch; “Not Go,” Min. thread O.D. to plus 0.0002 inch. Socket Screw Shell Gages: Threaded plug gages—“Go,” Min. thread size to plus 0.0003 inch; “Not Go,” Max. thread size to minus 0.0003 inch. Plain plug gages—“Go,” Min. minor dia. to plus 0.0002 inch; “Not Go,” Max. minor dia. to minus 0.0002 inch. Check Gages for Base Screw Shell Gages: Threaded plugs for checking threaded ring gages—“Go,” Max. thread size to minus 0.0003 inch; “Not Go,” Min. thread size to plus 0.0003 inch. Electric Fixture Thread.— The special straight electric fixture thread consists of a straight thread of the same pitches as the American standard pipe thread, and having the regular American or US standard form; it is used for caps, etc. The male thread is smaller, and the female thread larger than those of the special straight-fixture pipe threads. The male thread assembles with a standard taper female thread, while the female thread assembles with a standard taper male thread. This thread is used when it is desired to have the joint “make up” on a shoulder. The gages used are straight-threaded limit gages. Instrument and Microscope Threads British Association Standard Thread (BA).— This form of thread is similar to the Whitworth thread in that the root and crest are rounded (see illustration). The angle, how- ever, is only 47 degrees 30 minutes and the radius of the root and crest are proportion - ately larger. This thread is used in Great Britain and, to some extent, in other European countries for very small screws. Its use in the United States is practically confined to the manufacture of tools for export. This thread system was originated in Switzerland as a standard for watch and clock screws, and it is sometimes referred to as the “Swiss small screw thread standard.” See also Swiss Screw Thread . This screw thread system is recommended by the British Standards Institution for use in preference to the BSW and BSF systems for all screws smaller than 1 ∕ 4 inch except that the use of the “0” BA thread be discontinued in favor of the 1 ∕ 4 -in. BSF. It is further recom mended that in the selection of sizes, preference be given to even numbered BA sizes. The thread form is shown by the diagram.
s
47
1
2
r
. 113634 060000 018083 026817 . . .
H
p p p p
= = = =
# # # #
H h
r
h r s
23 3
4
s
p
British Association Thread
It is a symmetrical V-thread, of 47 1 ∕ 2 degree included angle, having its crests and roots rounded with equal radii, such that the basic depth of the thread is 0.6000 of the pitch. Where p = pitch of thread, H = depth of V-thread, h = depth of BA thread, r = radius at root and crest of thread, and s = root and crest truncation.
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