Machinery Repairman

FEED is the amount the tool advances in each
revolution of the work. It is usually expressed in
thousandths of an inch per revolution of the spindle.
The index plate on the quick-change gear box
indicates the setup for obtaining the feed desired. The
amount of feed to use is best determined from
experience.
Cutting speeds and tool feeds are determined by
various considerations: the hardness and toughness of
the metal being cut; the quality, shape, and sharpness
of the cutting tool; the depth of the cut; the tendency
of the work to spring away from the tool; and the
rigidity and power of the lathe. Since conditions vary,
it is good practice to find out what the tool and work
will stand, and then select the most practical and
efficient speed and feed consistent with the finish
desired.
If the cutting speed is too slow, the job takes
longer than necessary and the work produced is often
unsatisfactory because of a poor finish. On the other
hand, if the speed is too fast the tool edge will dull
quickly and will require frequent regrinding. The
cutting speeds possible are greatly affected by the use
of a suitable cutting coolant. For example, steel that
can be rough turned dry at 60 rpm can be turned at
about 80 rpm when flooded coolant.
When ROUGHING parts down to size, use the
greatest depth of cut and feed per revolution that the
work, the machine, and the tool will stand at the
highest practical speed. On many pieces, when tool
failure is the limiting factor in the size of the roughing
cut, it is usually possible to reduce the speed slightly
and increase the feed to a point that the metal removed
is much greater. This will prolong tool life. Consider
an example of when the depth of cut is 1/4 inch, the
feed is 20 thousandths of an inch per revolution, and
the speed is 80 fpm. If the tool will not permit
additional feed at this speed, you can usually drop the
speed to 60 fpm and increase the feed to about 40
thousandths of an inch per revolution without having
tool trouble. The speed is therefore reduced 25
percent, but the feed is increased 100 percent. The
actual time required to complete the work is less with
the second setup.
On the FINISH TURNING OPERATION, a
very light cut is taken since most of the stock has been
removed on the roughing cut. A fine feed can usually
be used, making it possible to run a high surface
speed. A 50 percent increase in speed over the
roughing speed is commonly used. In particular
cases, the finishing speed may be twice the roughing
6-34
speed. In any event, run the work as fast as the tool
will withstand to obtain the maximum speed in this
operation. Use a sharp tool to finish turning.
COOLANTS
A coolant serves two main purposes: (1) It cools
the tool by absorbing a portion of the heat and reduces
the friction between the tool and the metal being cut.
(2) It keeps the cutting edge of the tool flushed clean.
A coolant generally allows you to use a higher cutting
speed, heavier feeds, and depths of cut than if you
performed the machining operation dry. The life of
the cutting tool is also prolonged by coolants. The
most common coolants used are soluble oil and
synthetic coolants. Refer to the manufacturers’
recommendations for proper mixing rates.
The various operations used and materials
machined on a lathe may cause problems in the
selection of the proper coolant. A possible solution is
to select a coolant that is suitable for the majority of
the materials you plan to work with.
CHATTER
A symptom of improper lathe operation is known
as “chatter.” Chatter is vibration in either the tool or
the work. The finished work surface will appear to
have a grooved or lined finish instead of the smooth
surface that is expected. The vibration is set up by a
weakness in the work, work support, tool, or tool
support and is perhaps the most elusive thing you will
find in the entire field of machine work. As a general
rule, strengthening the various parts of the tool
support train will help. It is also advisable to support
the work with a center rest or follower rest.
Begin your search for the cause of the chatter by
making sure that the surface speed is not excessive.
Since excessive speed is probably the most frequent
cause of chatter, reduce the speed and see if the
chatter stops. You may also increase the feed,
particularly if you are taking a rough cut and the
finish is not important. Another adjustment you can
try is to reduce the lead angle of the tool (the angle
formed between the surface of the work and the side
cutting edge of the tool). You may do this by
positioning the tool closer and perpendicular to the
work.
If none of these actions work, examine the lathe
and its adjustments. Gibs may be loose or bearings
may be worn after a long period of heavy service. If
the machine is in perfect condition, the fault may be in