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Full text of "Modern Mechanical Engineering Vol-I"

The Machine-shop

CHAPTER   I
The Work of the Machine-shop
Changed Aspects.—This department of the engineers' factory shows
changes more extensive than those which have occurred in any other, even
though the foundry and the smithy have been considerably remodelled. To
an old craftsman the changes are remarkable. A few years ago, the cost of
machining was so high that it was avoided as far as possible, and the work
of the foundry and the smithy was arranged to this end. Those were the
days of cored holes and " black fits ", when the pattern-maker and
smith were brought to book if undue allowances were left for machining. It
was the period, too, of weak machines, of single-cutting tools, mostly made
of carbon steel, with only a meagre proportion of " Mushet " tools. To-day
opposite conditions rule. Coring, black fits, and scanty allowances for
machining are discouraged. Holes are drilled in the solid; pieces of fairly
large dimensions are turned, bored, and cut off from solid parallel bars; and,
instead of being made from expensive forgings, articles are shaped from
blocks of metal first severed in the ubiquitous hack-sawing machine, then
chucked and reduced rapidly.
The Causes of Changed Practice.—These reforms are due to several
causes. There is an intelligent distrust of some of the old long-standardized
methods of dealing with certain articles on certain machines, and new methods
of solving problems of machining are justified by results. Another improve-
ment is the increased strength of machine-tools, which is accompanied by
more rational design and a general speeding-up. There are also improve-
ments in the forms and in the materials of the cutting-tools used, and of the
very numerous appliances, such as jigs, fixtures, and devices for multiple
and continuous machining. A rigid system of gauging is now in vogue.
These and other influences have brought the machine-shops of to-day into
strong contrast with their forerunners.
The manufacture of small arms, the smaller machines, motor vehicles,
aero-engines, and so on, in large numbers, has called for the production of
thousands of similar parts with fine tolerances. This work has had to
VOL. I.                                                           177                                                               12