Modern Mechanical Engineering Vol-I

170

FOUNDRY WORK

en

IfMT tffi

CEE

(XI

in

tZD

ZD

are not only more rapid in action than the severance by hand, but they involve
no risk of tearing out the metal below the surface of the casting. This is
liable to occur when runners are knocked off with the hammer. This may
be prevented by nicking all round with a cold chisel before using the hammer.
In any case the surface has to be smoothed by chipping and filing, which is
avoided when a machine is employed for severance. Fins occur more or
less on all castings, following the mould joints, and the fitting of cores in
their print impressions. These are laboriously removed with hammer and

chisel. The pneumatic chipping
chisels are far more efficient.
Much economy of time results in
this kind of work, and in smooth-
ing lumpy and rough portions,
when emery grinding wheels are
installed. The larger sizes are
mounted on a floor stand, the
smaller on a work bench. To
deal with castings that are too
large to be handled and presented
to the fixed machines, wheels are
mounted on suspended arms to be
swung by the workman into any
required position.

When large quantities of small
castings have to be smoothed, this
is done, after preliminary grinding
for the removal of fins and ex-
crescences, in a tumbling barrel
or nimbler. This is a cylindrical
vessel from 18 in. to 36 in. in
diameter by from 30 in. to 60 in.
in length, rotated round its longi-
tudinal axis about once in a
second. Within this the castings
are tumbled in contact with small

" stars ", and are smoothed and polished by the mutual friction set up.
Iron is tumbled dry; brass, with water. The driving is done with a belt
direct, or through gears, or the drums run on rollers. Its axis is horizontal
or inclined. Polygonal drums are made, with chilled lining plates.

Since large castings cannot be put in tumbling barrels, these, in the more
advanced foundries with a sufficient output, are treated in a sand-blasting
plant, for which an air compressor, giving a low blast pressure ranging
between 5 and 25 Ib. per square inch, is necessary. Having a suitable plant,
castings of any sizes and weights can be cleaned. The castings are placed
in a room, constructed of sheet iron, having a perforated steel floor and a
glass roof, well ventilated. The sand, propelled by the air pressure, is

en

CH

ro

U3

Fig 79.—Multiple Moulds poured in Files through
Ingate A