Fine-Thread Series: This series, UNF/UNRF, is suitable for the production of bolts, screws, and nuts and for other applications where the Coarse series is not applicable. Externalthreads of this series have greater tensile stress area than comparable sizes of the
Coarse series. The Fine series is suitable when the resistance to stripping of both external and mating internal threads equals or exceeds the tensile load carrying capacity of the externally threaded member (see page 1443). It is also used where the length of engagement
is short, where a smaller lead angle is desired, where the wall thickness demands a fine pitch, or where finer adjustment is needed.
Extra-Fine-Thread Series: This series, UNEF/UNREF, is applicable where even finer
pitches of threads are desirable, as for short lengths of engagement and for thin-walled
tubes, nuts, ferrules, or couplings. It is also generally applicable under the conditions stated
above for the fine threads. See Table 4c.
Fine Threads for Thin-Wall Tubing: Dimensions for a 27-thread series, ranging from 1⁄4-
to 1-inch nominal size, also are included in Table 3. These threads are recommended for
general use on thin-wall tubing. The minimum length of complete thread is one-third of the
basic major diameter plus 5 threads (+ 0.185 in.).
Selected Combinations: Thread data are tabulated in Table 3 for certain additional
selected special combinations of diameter and pitch, with pitch diameter tolerances based
on a length of thread engagement of 9 times the pitch. The pitch diameter limits are applicable
to a length of engagement of from 5 to 15 times the pitch. (This provision should not
be confused with the lengths of thread on mating parts, as they may exceed the length of
engagement by a considerable amount.) Thread symbols are UNS and UNRS.