Tc 9-524 fundamentals of machine tools

Thread Designations
In general. screw thread designations give the screw number
(or diameter) first. then the thread per inch. Next is the thread
series containing the initial letter of the series. NC (National
Coarse). UNF (Unified Fine). NS (National Special). and so
forth. followed by the class of fit. If a thread is left-hand. the
letters LH follow the fit. An example of designations is as
follows:
Two samples and explanations of thread designations are as
follows:
No 12 (0.216) -24 NC-3. This is a number 12 (0.216-inch
diameter) thread. 24 National Coarse threads per inch.
and Class 3 ways of designating the fit between parts.
including tolerance grades. tolerance positions. and
tolerance classes. A simpler fit.
1/4-28 UNF-2A LH. This is a l/4-inch diameter thread.
28 Unified Fine threads per inch, Class 2A fit, and left-
hand thread.
7-46

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TC 9-524
Metric Thread Fit and Tolerance
The older metric screw thread system has over one hundred
different thread sizes and several ways of designating the fit
between parts. including tolerance grades. tolerance
positions. and tolerance classes. A simple system was
devised with the latest ISO Metric thread standard that uses
one internal fit and two external fit designations to designate
the tolerance (class) of fit. The symbol 6H is used to
designate the fit for an internal thread (only the one symbol
is used). The two symbols 6g and 5g6g are used to designate
the fit for an external thread. 6g being used for general
purpose threads and Sg6g used to designate a close fit. A fit
between a pair of threaded parts is indicated by the internal
thread (nut) tolerance fit designation followed by the external
thread (bolt) tolerance fit designation with the two separated
by a stroke. An example is M 5 x 0.8-Sg6g/6H. where the
nominal or major diameter is 5 mm. the pitch is 0.8 mm. and
a close tit is intended for the bolt and nut. Additional
information on ISO metric threads and specific fits can be
found in any updated engineer’s handbook or machinist’s
handbook.