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 strength 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 for the finish desired.
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 where tool failure is the
limiting factor in the size of the roughing cut, you may
be able to reduce the speed slightly and increase the feed
to remove more metal. This will prolong tool life.
Consider an example where the depth of cut is 1/4 inch,
the feed 0.020 inch per revolution, and the speed 80 fpm.
If the tool will not permit additional feed at this speed,
you can drop the speed to 60 fpm and increase the feed
to about 0.040 inch per revolution without having tool
trouble. The speed is therefore reduced 25 percent, but
the feed is increased 100 percent. Thus the actual time
required to complete the work is less with the second
setup.
For the FINISH TURNING OPERATION, take a
very light cut, since you removed most of the stock
during the roughing cut. Use a fine feed to run at a high
surface speed. Try a 50 percent increase in speed over
the roughing speed. In some cases, the finishing speed
may be twice the roughing speed. In any event, run the
work as fast as the tool will withstand to obtain the
maximum speed during this operation. Be sure to use a
sharp tool when you are finish turning.
COOLANTS
A cutting lubricant serves two main purposes: (1) It
cools the tool by absorbing a portion of the heat and
reducing the friction between the tool and the metal
being cut. (2) It also keeps the cutting edge of the tool
flushed clean.
The best lubricants to use for cutting metal must
often be determined by experiment. Water-soluble oil
is acceptable for most common metals. Special
cutting compounds containing such ingredients as
tallow, graphite, and lard, marketed under various
names, are also used. But these are expensive and used
mainly in manufacturing where high cutting speeds
are the rule.
Some common materials and their cutting
lubricants are as follows:
Metal
Cast iron
Lubricant
Usually worked dry
Mild steel
Oil or soluble oil
Hard steel
Mineral lard oil
Monel metal
Bronze
Dry (or soluble oil)
Dry (or soluble oil)
Brass
Dry (or soluble oil)
Copper
Dry (or soluble oil)
Babbitt
Dry (or soluble oil)
Aluminum
Dry (or soluble oil)
A lubricant is more important for threading than for
straight turning. Mineral lard oil is recommended for
threading the majority of metals that are used by the
Navy.
CHATTER
Chatter is vibration in either the tool or the work
The finished work surface appears to have a grooved or
lined finish instead of a smooth surface. The vibration
is set up by a weakness in the work, work support, tool,
or tool support and is probably 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 by a center rest or follower rest.
The fault may be in the machine adjustments. Gibs
may be too loose; hearings may, after a long period of
heavy service, be worn; the tool may be sharpened
improperly, and so on. If the machine is in excellent
condition, the fault may be in the tool or tool setup.
Grind the tool with a point or as near a point as the finish
specified will permit; avoid a wide, round leading edge
on the tool. Reduce the overhang of the tool as much as
possible. Be sure all the gib and bearing adjustments are
properly made. See that the work receives proper
support for the cut and, above all, do not try to turn at a
surface speed that is too high. Excessive speed is
probably the greatest cause of chatter. The first thing you
should do when chatter occurs is reduce the speed.
Direction of Feed
Regardless of how the work is held in the lathe, the
tool should feed toward the headstock. This causes most
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