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Full text of "Modern Mechanical Engineering Vol-I"

ESSENTIAL  MACHINES  AND APPLIANCES         159
CHAPTER  X
Essential Machines and Appliances
The more advanced foundries of the present day employ labour-saving
methods to an extent that would have been deemed impracticable a few
years ago. Yet in too many shops wasteful ways, which are a financial
handicap in competitive efforts, are retained. It seems desirable, therefore,
to give attention to this particular aspect of foundry work, dealing with
the preparation of the sands, with machine moulding, with fettling, and
the lifting and transport systems.
The Preparation of Sands.—Sand when new from the quarry is not
suitable for moulds without preliminary treatment. This is performed in
isolated machines, or in one large plant, which is only installed in the big
foundries. New sands are wet and lumpy, often having pebbles intermixed.
Drying is necessary. In small shops this is done in the core stoves, the sand
being spread on iron plates. In the bigger foundries, drying cylinders,
which measure about 6 ft. in diameter by several feet in length, are em-
ployed. They are either disposed with the axis horizontally, or at a slight
angle. The sand, fed through a hopper, is carried along the interior of the
revolving cylinder with spiral plates, and thrown against baffle plates, which
bring it into intimate contact with the hot gases from a furnace that traverse
the cylinder. The rotation is slow, being about i r.p.m. These machines,
in different capacities, will dry from 10 cwt. to 3 tons of sand per hour.
After drying it is necessary to crush, pulverize, and grade the sand.
The machines used for these processes are edge runners, disintegrators,
riddles, and sieves. Crushing is only necessary with the coarser, harder,
clayey sands, and is not adopted with the finer qualities, but instead the
lumps are triturated. In small foundries they are broken with a punner,
and the product with the ordinary mass is put through a riddle. The
machines that crush (fig. 63) are also used for mixing wet loam, hence termed
" loam mills ". They are similar to mortar mills. The lumpy sand is
ground between revolving runners and the bottom of the pan, which is
commonly fitted with removable chilled plates. The runners are frequently
chilled, or they are steel-tyred. Scrapers are fixed at an angle to heap up the
sand in front of the runners. These revolve on their shafts, and are at the
same time rotated around the pan on a central vertical shaft. In some cases
the pan revolves under the runners. Driving is done through belt pulleys
and bevel gears, and the pulverized sand is discharged through a shoot at
the bottom of the pan. Many pans used for mixing loam have their rollers
deeply indented like huge cogs. These throw up the loam, and amalgamate
it very thoroughly. One of these is often used with a smooth roller on the
opposite shaft. Some runners again are deeply grooved, in annular fashion.
The next process is the trituration of the sand to bring it into a fine,