Threads and Threading - Sportpilot.info

Machinery's Handbook 27th Edition
1782 BRITISH UNIFIED THREADS
Limits of Size: Formulas used to determine limits of size are given in Table 4; the limits of
size are given in Table 5. The diagram on page 1781 illustrates the limits of size and Table
6 gives values for the minimum flat at the root of the external thread shown on the diagram.
Classes of Threads: The standard establishes one class of thread with zero allowance on
all diameters. When coatings of a measurable thickness are required, they should be
included within the maximum material limits of the threads since these limits apply to both
coated and uncoated threads.
Hole Sizes for Tapping: Suggested hole sizes are given in the Tapping Section.
British Standard Unified Screw Threads of UNJ Basic Profile
This British Standard B.S. 4084: 1978 arises from a request originating from within the
British aircraft industry and is based upon specifications for Unified screw threads and
American military standard MIL-S-8879.
These UNJ threads, having an enlarged root radius, were introduced for applications
requiring high fatigue strength where working stress levels are high, in order to minimize
size and weight, as in aircraft engines, airframes, missiles, space vehicles and similar
designs where size and weight are critical. To meet these requirements the root radius of
external Unified threads is controlled between appreciably enlarged limits, the minor
diameter of the mating internal threads being appropriately increased to insure the necessary
clearance. The requirement for high strength is further met by restricting the tolerances
for UNJ threads to the highest classes, Classes 3A and 3B, of Unified screw threads.
The standard, not described further here, contains both a coarse and a fine pitch series of
threads.
Copyright 2004, Industrial Press, Inc., New York, NY