Figure 6-96.--Finishing the end of a threaded piece.
collar on the compound rest feed screw to the depth of
Figure 6-98.--Setup for left-hand external threads.
cut previously taken.
NOTE: Be sure that the thread mechanism is
engaged and the tool is set square with the work
Left-hand threads are used for the cross-feed screws
before adjusting the position of the tool along the axis
of lathes, the left-hand end of axles, one end of a
of the workpiece.
turnbuckle, or wherever an opposite thread is desired.
If it is inconvenient to use the compound rest for
The directions for cutting a left-hand thread on a
readjusting the threading tool, loosen the lathe dog (if
lathe are the same as those for cutting a right-hand
used); turn the work so that the threading tool will
thread, except that you swivel the compound rest to
match the groove, and tighten the lathe dog. If
the left instead of to the right. Figure 6-98 shows the
possible, however, avoid doing this.
correct position for the compound rest. The direction
of travel for the tool differs from a right-hand thread
Another method, which is sometimes used, is to
in that it moves toward the tailstock as the thread is
disengage the reverse gears or the change gears; turn
the headstock spindle until the point of the threading
Before starting to cut a left-hand thread, it is good
tool enters the groove in the work, and then reengage
practice, if feasible, to cut a neck or groove into the
The end of a thread may be finished by any one of
you to run the tool in for each pass, as you do for a
several methods. The 45° chamfer on the end of a
Make the final check for both diameter and pitch
used for bolts and capscrews. For machined parts and
of the thread, whether right-hand or left-hand, with
special screws, the end is often finished by rounding it
the nut that is to be used, or with a ring thread gauge
if one is available. The nut should fit snugly without
play or shake, but it should not bind on the thread at
LEFT-HAND SCREW THREADS
A left-hand screw (fig. 6-97) turns counter-
MULTIPLE SCREW THREADS
clockwise when advancing (looking at the head of the
screw), or just the opposite to a right-hand screw.
combination of two or more threads, parallel to each
other, progressing around the surface into which they
are cut. If a single thread is thought of as taking the
form of a helix, that is of a string or cord wrapped
around a cylinder, a multiple thread may be thought of
as several cords lying side by side and wrapped
around a cylinder. There may be any number of
threads, and they start at equally spaced intervals
around the cylinder. Multiple threads are used when