LATHE OPERATIONS - OD1645 - LESSON 1/TASK 3
(2) American (National) Standard Thread.
(a) The American (National) Standard thread form is being replaced by
the Unified screw thread form. It is still in use and is interchangeable
with the Unified system.
(b) The American (National) Standard thread form is used in five series
of pitches as follows:
Fine (NF) (formerly the SAE Standard screw thread).
Course (NC) (formerly the USS screw thread).
In the coarse and fine series, the number of threads per inch decreases as
the diameters increase. These series are intended for general use. Eight-
pitch is used bolts, cylinder head studs, high-pressure pipe flanges, and so
on. Twelve-pitch is used in modern machine and boiler construction for thin
nuts, shafts, and sleeves. Sixteen-pitch is intended for adjusting collars,
bearing retaining nuts, or any part requiring a fine thread.
(c) Tables listing general dimensions for the Unified system and the
American (National) system are listed in the appropriate technical manuals.
(3) SAE Extra Fine Threads.
The SAE Extra Fine series has many more
threads per inch for a given diameter than any series of the American
The form of thread is the same as the American
(National) Standard. These small threads are used in thin metal where the
length of thread engagement is small, in cases where close adjustment is
required, and where vibration is great. It is designated EF (Extra Fine).
(4) Acme Screw Thread.
The Acme screw thread form (figure 41 on the
following page) is classified as a power-transmitting type of thread. This
is because the 29 included threaded angle at which its sides are
established reduces the amount of friction when matching parts are under
load. Because the root and crest are wide, this thread form is strong and
capable of carrying a heavy load.
The Acme thread form is especially