Activate Javascript or update your browser for the full Digital Library experience.
Previous Page
–
Next Page
OCR
_ MACHINE TOOLS
It is easily seen that power is an important
factor in modern civilizaton. - If we go back a
century or so, to the time when mechanical
power was not nearly so common as at the
present time, we see a great amount of man-
ual labor, and not very much _ production.
Work, in those days, required a great deal of
time, and, consequently, the higher things of
life suffered, and living was not as comforta-
ble as it now is. If we were suddenly put back
a hundred years, life would be almost unbear-
able to us.
Consider therefore, the difficulties which
James Watt had in building his steam engine.
The boring machine for the cylinder consisted
of a boring bar supported at both ends, and
thus gave a fairly accurate cylinder. When we
think of the great amount of time necessary
for. making just the cylinders, we can com-.
pare the work with that of modern times,
when cylinders are made in a few minutes, and
with an accuracy far superior to that of the
olden days. .The cylinders in Watt’s day were
marvelled at if they were true to within one
thirty-second of an inch. ae :
The modern machine tool supplies the means
for working metals accurately. and economical-
ly. There are three primary:elements in a
machine tool: structural or. frame elements,
whose function it is to support the.-work and
tool; transmission elements, which give the
work or tcol, or. both, movements for shaping
the work to be produced; and control elements,
for adjusting the structural elements relative-
ly to each other, and for controlling the func-
tion of the transmissions for moving either the
tool or the work, or both. ek
It is a known fact that once a machine tool
has been invented, the frame elements under-
go very little or no functional change at. all.
If one compares the machine of about a cen-
tury ago with those used today, he finds very
little change in the principle of most of these
machines. They are, however, built heavier
and more rigid, and strengthening elements
have been added. | oe
Going back a century. or so,: we find that the
biggest step in the development of modern ma-
chine tools was from the hand-driven tool to the
power-driven tool... It was a slow process, and
all know that. even toward the end of the last
century a number.of machine-tools were still in
use with hand- or foot-power drive. At the
' present time, however, it-is safe. to say that
these machines have completely disappeared
from the industrial field. ~~ __
Back in the beginning of the nineteenth cen-
‘tury there were driving cones on various ma-
chine tools driven from. countershafts of line-
shafts.: These cone pulleys for driving or feed-
ing the work or tool at different rates were
the controlling variable speed transmission ele-
ments then used. One element was lacking,
however; they did not have enough power. In
order to meet this requirement they sacrificed
speed, and back gears were invented. This
gave a great variety of speeds, however, and
starting from this, we now have our highly or-
‘ganized and well-developed variable-speed and .
feed-change transmission. Nevertheless, chang---”
ing from one rate of speed to another required»
a certain amount of skill on the part of the
operator, or else required stopping, the machine’
for the shifting of belts. There was also a pos-
sibility of the belt slipping, which, for certain
operations, was destructive to the work of the
tool. In the following years, the productivity
of the machine became one of the most impor-
tant factors in the industrial development. To
increase ‘this productivity, it was necessary to
increase the cutting capacity of the machine.
Harder and stronger steel was produced, which
revolutionized the speed of production, as it
wes possible to run the machines at a much
higher speed, and the tool could stand this
enormous force against it. Cutting tools of
earlier days. could-not withstand. this. high
speed, for they. were. not strong and -hard
enough. They would give under the. pressure
of the work and'after a few cuts at high speed,
would prove valueless.. In order to get the full
benefit of this harder steel, more powerful and
faster machines had to be made. This result-
ed in the so-called single-pulley machine which
adopted the positive-gear change mechanism of
the gear box, and used it for the drive of the
machine. : a
The advent of the single-pulley machine made
it possible to attach or couple an electric mo-
tor directly to the machine, thus making the
machine, so arranged, a self-contained operat-
ing unit. The variable’ speed motor, very
popular at the beginning of this century, was
quite frequently used as a speed variator, re-
placing some of the transmission. gearing, and
providing at the same time a greater number ~
of. speeds with very close increments. This ar-
rangement also did away with a great amount
of friction due to back gears, etc. The varia-
ble speed motor, however, had the: disadvan-
tage of being inefficient at low speeds.» It was
larger than the constant speed motor, and cost
considerably more. Then too, at that time
alternating current was fast replacing direct
current, and the variable speed motor disap-
peared very quickly as a driving member for
machine tools, It is. still: used, however,. to
drive planers and grinder machines, and its
advantages are so great in these two machines
that very frequently a motor-generator set. is
used to convert the ‘alternating current: into
direct current... Later on,’ machines were
equipped with a number of electric motors, each
doing a certain part of-the work. It can be
easily seen that machines of the future will
make ‘use of this type of transmission drive.
The electrical connections are so arranged that
if one motor stalls, the whole system is cut off,
and thus no damage can be.done. © oo