times the pitch, and so on.
Most screw threads are single. The single screw thread has a single ridge and groove.
The thread angle is the angle included between the sides of the thread. The thread angle of the Unified
National Form is 60 degrees.
Screw threads are so widely used to connect parts that they are of prime importance when produced in
the machine shop.
Great amounts of study and research have been devoted to the standardization of screw thread forms.
The basic form of the screw and nut, with a 60-degree angle of thread 55. The crest of the thread may
be flat or rounded. The flat crest is preferred in American practice, and the rounded crest is given
preference in British practice.
Over many years, different screw thread forms and standards have been adopted in the United States.
These different forms originated chiefly because of special requirements or because they were
considered superior to other forms. In addition to the American Standard thread, they are the Acme,
Square, and the 29-Degree Worm threads.
The American Standard thread form is used as the standard locking thread form in the United States.
This thread form is used on practically all-mating parts in modern machine construction. The Unified
thread form is essentially identical to the former standard, the American National thread form. The two
forms are interchangeable for most diameter-pitch combinations. A 1-64 National Coarse thread is
interchangeable with a 1-64 Unified National Coarse thread.
Unified National Coarse (UNC)
Unified National Fine (UNF)
Unified National Extra-Fine (UNEF)
Unified National 8-Pitch (8 UN)
Unified National 12-Pitch (12 UN):
Unified National 16-Pitch (16 UN)
In the coarse, fine, and extra-fine series, the number of threads per inch increases as the diameters
decrease. The coarse and fine threads are widely employed for general use.
In order to produce an American Standard thread on the engine lathe, the ratio of tool feed to work