Cutting Screw Threads
Screw threads often are most conveniently and
quickly cut on the lathe using taps for internal
threads and dies for external threads. For best
control and safety, these operations should be
done by engaging a shop-made handle into the
left end of the spindle bore and turning the
spindle manually using the handle. Detailed
procedures for using taps and dies are beyond
the scope of this manual.
Alternatively, both metric and inch threads may
be cut using the threading feature of the lathe.
This operation is known as single-point thread
cutting because the tool used has a single
point in the shape of one turn of the thread,
rather than multiple cutting surfaces as in a tap
or die. Threading is an advanced operation, so
it is best to master the operations previously
described in this manual before attempting
The shape, or form, of a thread cut by single-
point threading is determined by the profile of
the cutting tool. Both metric and inch threads
use a very similar thread form, so the same
type of tool may be used for cutting both types
of threads. The type of thread, metric or inch,
is determined by the pitch (the spacing
between adjacent threads). Each system of
units has standard pitches.
In general, larger diameter screws have fewer
threads per inch (a larger pitch) than smaller
diameter is manufactured in several pitches for
fine and coarse threading applications. For
example, a ¼" diameter bolt may have either
20 threads per inch (TPI) for coarse threads or
28 TPI for fine threads. These are designated
as National Coarse (NC) and National Fine
A TPI designation that is NC for one screw size,
for example 6-32, may be NF for a larger
screw size, such as 8-32.
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A thorough discussion of screw threads and
threading is beyond the scope of this manual.
Consult the Internet or technical reference
manuals for detailed information on these
Setting Up a Gear Train
To cut a screw thread with a given number of
threads per inch or per mm, a sequence of
gears, called a gear train, is set up on the left
end of the lathe. The train of gears is driven by
a gear permanently attached to the spindle and
drives the leadscrew in a fixed number of
rotations for each rotation of the spindle.
For example, to cut 20 TPI, the lead screw
must advance exactly one inch during 20
revolutions of the spindle. The specific gears
required for each TPI setting are designated on
charts attached to the lathe.