Tighten the tool post screw. Be sure not to change the position of the holder. Recheck the tool setting
after tightening the tool post screw.
CUTTING AN EXTERNAL THREADS
After setting the compound rest and positioning the threading tool properly in relation to the work, it is
necessary to select the proper speed and feed. Good thread-cutting practice requires that the back
gears be engaged for this operation.
This reduces the r.p.m. or speed to a minimum, and is necessary if best results are to be obtained. The
correct selection of feed is determined by a gearbox on all modern lathes. Directions are shown on the
gearbox regarding the setting of levers to obtain the correct feed, depending upon the threads per inch
to be cut.
Now the compound feed-screw graduated collar is set to zero and the tool point is brought into contact
with the work by turning the cross feed screw. The tool point should contact the work lightly. Then run the
carriage to the right, using the carriage hand wheel, until the tool clears the end of the work. Notice what
the setting is on the cross-feed collar.
This adjustment must be remembered so, at the end of each cut, the cross-feed is always brought back
to the same number or setting. Feed in on the compound .002" and then start the machine and take the
first trial cut.
If using the thread dial, be sure to engage the half-nut lever at the correct line on the dial, depending
upon the threads per inch you are cutting. This causes the carriage to start in motion.
A check should be made after this first trial cut to see that the correct pitch of thread is being machined.
This is done by using a thread-pitch gage or a rule.
To determine the total number of thousandths of an inch that the threading tool must be fed in by the
compound feed screw, and in order to cut the thread to the desired depth, a simple formula is used.
This formula is: divide the constant .750 by the number of threads per inch.
If it is required that eight threads per inch are to be cut, then divide .750 by 8, which gives a result of
.0937" (.094" can be used).
This is the total number of thousandths of an inch the tool is to be fed into the work to cut eight threads
per inch. However, the compound feed is used until the tool has been fed in '090". Then the cross-feed
is used to remove the final four thousandths of an inch from the thread, making four cuts and feeding the
tool in .001" on each cut. This will help to polish the right side of the thread.