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Thread Series: Thread series are
groups of diameter-pitch combinations distinguished from each other by the
number of threads per inch applied to a series of specific diameters.
There are two general series classifications: standard and special.
Coarse Thread Series Applications
The coarse thread series (UNC/UNRC) is generally used
for the bulk production of screws, bolts, and nuts. It is commonly used in
relatively low strength materials such as cast iron, aluminum, magnesium,
brass, bronze, and plastic because the coarse series threads provide more
resistance to internal thread stripping than the fine or extra-fine
series. Coarse series threads are advantageous where rapid assembly or
disassembly is required, or if corrosion or damage from nicks due to
handling or use is likely.
Fine Thread Series Applications
The fine thread series (UNF/UNRF) is commonly used for
bolts and nuts in high strength applications. This series has less thread
depth and a larger minor diameter than coarse series threads.
Consequently, thinner walls are permitted for internal threads and more
strength is available to external threads than for coarse series threads
of the same nominal size.
8-Thread Series
The 8-thread series (8UN) is a uniform-pitch series for
large diameters or as a compromise between coarse and fine thread series.
Although originally intended for high-pressure-joint bolts and nuts, it is
now widely used as a substitute for the coarse thread series for diameters
larger than 1 inch.
12-Thread Series
The 12-thread series (12UN) is a uniform-pitch series
for large diameters requiring threads of medium-fine pitch. Although
originally intended for boiler practice, it is now used as a continuation
of the fine thread series for diameters larger than 1-1/2 inch.
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