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Full text of "Modern Mechanical Engineering Vol-I"

CORE-MAKING

133

making the box sides as rigid as possible, and by slightly reducing the interior
ciimensions. These precautions should always be made in standardized
^^vork, and provision made for the
taper of core prints in the boxes,
t:o avoid rubbing the taper on the
oores. Another reason why cores

—A

Fig. 33-—The Ratchet Core in fig. 16

Fig. 34.—End Core without Ratchet in fig. 16
A, Line of joint in core.





should be made slightly below size is that, when dried, they are so hard

a.nd rigid that they retard the

shrinkage of the casting, so that

the  interior  either comes out

too   large,   or  in   some   cases

fracture occurs unless the core

is  loosened while the casting

is cooling.

Details of Core Formation.
—Generally this work is done
by the core makers, a class of
men apart from the moulders.
But this is merely a matter of
economy, a useful division of
tasks, since moulders can pre-
pare their own cores, and do so
frequently in the small shops.
Referring first to those cores
•which are rammed in boxes,
the work is substantially that
of dried sand moulds, with the
difference before noted, the
employment of an interior sup-
porting Skeleton, the "grid",                    Fig.35__ Core Box for Bevel Wheel

in  place of an exterior flask.

The first thing, therefore, which has to be decided is the form and dimen-
sions of the grid.    This both carries the load of sand, and affords the