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Full text of "Modern Mechanical Engineering Vol-I"

THE   ESSENTIALS  OF  ECONOMICAL  MACHINING    201
The drain line flushing arrangement is shown at c, fig. 34, the object
of which is to prevent sediment from accumulating and impeding the free
movement of the liquid. The oil is sterilized in the vessel D before being
filtered. The machines are lettered as follows: fig. 33, E indicates flat urret
lathes of various sizes; F, lathes; G is a centring-machine; H, one for testing;
J, one for cutting off stock; K, a Jones & Lamson lathe; L, various automatics;
M, is a stock rack.
DIVISION II
Speeds and Feeds.—The speed of a cutting-tool, relatively to that
of the piece of work on which it operates, irrespective of whether the tool or
the work moves, is expressed by the number of linear or peripheral feet
passed through per minute by the tool or the work. The feed in turning
and planing is the lateral distance traversed between each cut; in drills and
face-milling cutters it is the depth of penetration estimated in some minute
fractional part of an inch per revolution of the tool; in edge-milling cutters
it is stated usually as the linear distance travelled by the work under the
cutter per minute; in grinding wheels it is the depth of cut given by each
setting-in of the wheel.
There are standard speeds memorized in the shops, just as there are
standard tool angles for different materials. But they are more honoured
in the breach than in the observance, and are exceeded in favourable con-
ditions. There are no commonly recognized feeds. But, with the increasing
stiffness of machine-tools and with improved lubrication and suitable tool
angles, feeds are generally very much coarser than of old, notably in high-
speed turning, in drilling, and in milling.
Relations of Speeds and Feeds.—There is no hard-and-fast rule as
to whether high speeds and fine feeds, or low speeds and coarse feeds are
preferable. In drills, for instance, it is more economical to increase speed
than feed. In edge-milling cutters the best results are secured by low
speed with coarse feed. In turning and planing, high speeds and coarse
feeds may go on simultaneously. The old speeds for carbon tools were:
cast iron, from 15 to 20 ft. per minute; steels, from 15 to 30 ft.; wrought
iron, from 25 to 40 ft.; and brass from 50 to 100 ft. These are now
generally exceeded, except in the harder qualities, and tools of high-speed
steel will cut at double these rates. But any general statements can only be
approximate, since results are controlled by many variables, as tool angles,
depths of cut, rate of feed, grade of material, the rigidity of the machine, and
the volume of lubrication—often the largest factor of all. Because of these
facts, no ratios of speeds and feeds could be tabulated that would be of any
general value.
Depth of Cut.—This may range from o-ooi in. in grinding wheels to
i in. in cutting took. An increase in depth of cut involves a reduction in
cutting speed and feed, because the capacity of a tool is measured by the
area of cut plus the feed. Heavy cuts at slow speeds are more economical
than light cuts at high speeds. But the horse-power required is greater,