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THE  WORK  OF THE  MACHINES

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CHAPTER  IV

The Work of the
Machines

DIVISION I

The Lathes. —These in-
clude some forty to fifty groups,
each having well-defined spheres
of operation. They range in size
from very small to mammoth
dimensions, while extreme ma-
chines have little in common
except the fact that the work
revolves between centres or in
chucks. The prototype of most
of these is the standard, " self-
acting, sliding, surfacing, and
screw-cutting lathe "—the all-
round machine-tool, the econ-
omic value of which gets less
and less as specialized manufac-
ture increases.

Short screws, and those of
which large quantities are re-
quired, are now manufactured
on turret lathes, screwing
machines, and brass - finishers'
lathes. The longer screws and
stays are still made in screw-
cutting lathes.

The later lathes, fig. 47
being an example, nearly all
differ from the earlier in the
provisions made for speed and
feed changes. Stepped belt cones
are now almost entirely super-
seded by all-geared heads. When
cones are retained, with back
gears, speeds are arranged in
carefully chosen ratios instead of
in a haphazard way. All-geared
heads, being driven from a single

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