LATHE OPERATIONS - OD1645 - LESSON 1/TASK 3
i.
Thread Cut tin Operations.
(1) Before cutting threads on the workpiece, turn down the the major
diameter of the thread to be cut. The workpiece may be set up in a chuck or
between centers. If a long thread is to be cut, it is advisable to use a
follow rest because thread cutting can place a great strain on the
workpiece.
(2) The usual practice in cutting threads is to take a very light cut and
then check to see that the lathe has been geared correctly for the right
number of threads per inch. If it is correctly geared, continue taking cuts
until the thread reaches the depth wanted; in the case of Unified and
American (National) Standard threads, this is determined by measuring the
pitch diameter of the thread, and in the case of Sharpe V-threads, when the
thread becomes pointed.
(3) After each pass of the cutter bit, the operator must move the bit out
of engagement with the thread being cut and traverse the carriage and bit
back to the beginning of the thread. At the end of each cut, the half-nuts
are usually disconnected and the carriage returned to the position of the
next cut by hand. Some device must be provided, therefore, to engage the
half-nuts for the following cut at a point on the lead screw which will
cause the cutter bit to follow the previous cut. If such a device is not
available, it is necessary to leave the half-nuts engaged at the end of the
cut and return the cutter bit by reversing the lathe.
(4) The usual device for accomplishing correct alignment of the cutter bit
after the half-nuts have been disengaged is the thread chasing dial (figure
46 on the following page). This device is supplied as standard or optional
equipment on all screw cutting lathes. It consists of a worm wheel which
meshes with the lead screw, a dial, and a short shaft connecting the worm
wheel to the dial.
It is usually mounted to the right side of the carriage apron. The dial is
calibrated with four numbered lines and four unnumbered lines between them.
To use the thread chasing dial, engage the half-nuts when the dial is lined
up for the particular number of threads per inch being cut.
If the number of threads per inch is an even number, the half-nuts can be
reengaged for the following cuts when any line on the dial is
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