Machinery Repairman

placing the levers as indicated on the index plate for
the thread desired.
When you have the lathe set up to control the
carriage movement for cutting the desired thread
pitch, your next consideration is shaping the thread.
Grind the cutting tool to the shape required for the
form of the thread to be cut, that is-V-form, Acme,
square, and so on.
Mounting Work in the Lathe
When you mount work between lathe centers for
cutting screw threads, be sure the lathe dog is securely
attached before you start to cut the thread. If the dog
should slip, the thread will be ruined. Do not remove
the lathe dog from the work until you have completed
the thread. If you must remove the work from the
lathe before the thread is completed, be sure to replace
the lathe dog in the same slot of the driving plate.
When you thread work in the lathe chuck, be sure
the chuck jaws are tight and the work is well
supported. Never remove the work from the chuck
until the thread is finished.
When you thread long slender shafts, use a
follower rest. You must use the center rest to support
one end of long work that is to be threaded on the
inside.
Figure 6-91.—Compound rest set at 29 1/2°.
6-55
Position of Compound Rest for Cutting Screw
Threads
Ordinarily on threads of fine lead, you feed the
tool straight into the work in successive cuts. For
coarse threads, it is better to set the compound rest at
one-half of the included angle of the thread and feed
in along the side of the thread. For the last few
finishing cuts, you should feed the tool straight in
with the cross-feed of the lathe to make a smooth,
even finish on both sides of the thread.
In cutting V-form threads and when maximum
production is desired, it is customary to place the
compound rest of the lathe at an angle of 29 1/2°, as
shown in part A of figure 6-91. When you set the
compound rest in this position and use the compound
rest screw to adjust the depth of cut, you remove most
of the metal by using the left side of the threading tool
(B of fig. 6-91). This permits the chip to curl out of
the way better than if you feed the tool straight in, and
keeps the thread from tearing. Since the angle on the
side of the threading tool is 30°, the right side of the
tool will shave the thread smooth and produce a better
finish; although it does not remove enough metal to
interfere with the main chip, which is taken by the left
side of the tool.