LATHE OPERATIONS - OD1645 - LESSON 1/TASK 3
(8) Thread Angle.
The angle between the sides of the adjacent threads,
measured in an axial plane. For most V-threads, the angle is fixed at 600.
(9) Depth. The depth of the thread is the distance between the crest and
the root of a thread, measured perpendicular to the axis.
(10) Major Diameter (D).
The major diameter is the largest diameter of a
screw thread.
(11) Minor Diameter (K).
The minor diameter is the smallest diameter of a
screw thread.
(12) Pitch Diameter.
The pitch diameter is the diameter of an imaginary
cylinder formed where the width of the groove is equal to one-half of the
pitch. This is the critical dimension of threading as the fit of the thread
is determined by the pitch diameter.
(13) Number of Threads Per Inch.
The number of threads per inch may be
counted by placing a rule against the threaded parts and counting the number
of pitches in 1 inch. A second method is to use the screw pitch gage. This
method is especially suitable for checking the finer pitches of the screw
threads.
c. Screw Thread Forms. The most commonly used thread forms are discussed
below.
Of these threads, the Unified screw thread and the American
(National) Standard thread are the most widely required for locking devices.
The Acme, square, and 290 worm threads are the most common used for devices
which function to transmit motion.
(1) Unified Screw Thread.
The Unified screw thread (figure 40 on the
following page) is the newest of standard thread forms, being acceptable for
interchangeable parts by the United States, Great Britain, and Canada. The
Unified thread is a variation of the American (National) Standard thread
form, having an included thread angle of 60. On external threads of the
unified form, the root is rounded and the crest is optionally rounded or
left flat; in the United States, the flat crest is preferred. The internal
thread of the Unified form is like the American (National) thread form but
is not cut as deep, leaving a crest of one-fourth the pitch instead of one-
eight the pitch.
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