Machinery Repairman

Figure 6-96.—Finishing the end of a threaded piece.
collar on the compound rest feed screw to the depth of
cut previously taken.
NOTE: Be sure that the thread mechanism is
engaged and the tool is set square with the work
before adjusting the position of the tool along the axis
of the workpiece.
If it is inconvenient to use the compound rest for
readjusting the threading tool, loosen the lathe dog (if
used); turn the work so that the threading tool will
match the groove, and tighten the lathe dog. If
possible, however, avoid doing this.
Another method, which is sometimes used, is to
disengage the reverse gears or the change gears; turn
the headstock spindle until the point of the threading
tool enters the groove in the work, and then reengage
the gears.
The end of a thread may be finished by any one of
several methods. The 45° chamfer on the end of a
thread, as shown in view A of figure 6-96, is commonly
used for bolts and capscrews. For machined parts and
special screws, the end is often finished by rounding it
with a forming tool, as shown in view B of figure 6-96.
LEFT-HAND SCREW THREADS
A left-hand screw (fig. 6-97) turns counterclockwise
when advancing (looking at the head of the
screw), or just the opposite to a right-hand screw.
Figure 6-97.—A left-hand screw thread.
6-58
Figure 6-98.—Setup for left-hand external threads.
Left-hand threads are used for the cross-feed screws
of lathes, the left-hand end of axles, one end of a
turnbuckle, or wherever an opposite thread is desired.
The directions for cutting a left-hand thread on a
lathe are the same as those for cutting a right-hand
thread, except that you swivel the compound rest to
the left instead of to the right. Figure 6-98 shows the
correct position for the compound rest. The direction
of travel for the tool differs from a right-hand thread
in that it moves toward the tailstock as the thread is
being cut.
Before starting to cut a left-hand thread, it is good
practice, if feasible, to cut a neck or groove into the
workpiece. (See fig. 6-97). Such a groove enables
you to run the tool in for each pass, as you do for a
right-hand thread.
Make the final check for both diameter and pitch
of the thread, whether right-hand or left-hand, with
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
any point.
MULTIPLE SCREW THREADS
A multiple thread, as shown in figure 6-99, is a
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