The position at which the half nut should be closed depends upon the pitch of thread.
For odd-numbered threads, close half nuts on any numbered line, such as 3 1/2 threads per inch, the
half-nut must be engaged at any odd-numbered line on the dial.
For quarter-threads, such as 2 1/4 or 3 1/4 threads per inch, the half-nut must be engaged at the same
point on the dial each time a cut is started.
For all numbered threads, close half nuts upon any line on the dial.
For threads involving half threads-any numbered line.
For threads involving quarter threads-return to the original starting point before closing half nuts.
The dial is engaged to the lead screw, as the operator desires, by a feed lever on the apron. This lever
is called a half-nut lever. When the lathe is running but the half-nut lever is not engaged, the dial revolves.
When the half-nut lever is engaged, the carriage moves but the dial remains motionless.
Before you can start the thread-cutting operation, you must decide, from the number of threads required
per inch, at what point on the dial to engage the half-nut. For chasing all even numbers of threads per
inch, such as 4, 6, 8, 10, etc., the half-nut is engaged for the first, and for all successive cuts, at any of
the eight graduation marks on the face of the dial.
For an odd number of threads per inch, engage the half nut at any quarter-turn or numbered line on the
dial (the main graduation marks),
SETTING THE COMPOUND FOR RIGHT AND LEFT-HAND THREAD
To cut right-hand external threads on the lathe the compound rest is turned at a 29-degree angle. This
prevents tearing of the thread and makes it easier to rechase the thread if the tool must be reset. The
carriage is made to travel from right to left or, toward the head stock.
Before each successive cut, the tool is fed in with the compound rest. To cut a left-hand thread, the
compound is turned at a 29-degree angle toward the head stock, and the carriage is made to travel
from left to right.
The threading tool is mounted in the tool post. Adjust the cutter point vertically to the exact center of the
work. Then place a center gage with its back edge in contact with, or parallel to, the work or the tail
stock spindle.
Now adjust the tool horizontally by fitting the cutter point exactly into the 60-degree angle notch in the
front edge of the center gage.