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Full text of "Modern Mechanical Engineering Vol-I"

CASTINGS   MADE   IN   METALLIC  MOULDS        141
and so tend to burst them. The precaution, therefore, is often taken of
bonding chills with a wrought-iron ring, shrunk on.
Shrinkages.—When a chill mould is poured, two shrinkages occur, that
of the casting inwards, and that of the mould in the contrary direction, so
that a space is quickly left between the two of J in. or more. Attempts have
been made to control and minimize this result, but the practice of dead-
melting the metal is usually adopted, that is, allowing it to cool slightly                            *
before pouring. Metal thus treated will lie better to the chill than that ]
which is in ebullition.                                                                                                            'l
An effect of the large amount of shrinkage that is consequent on chilling                           I
is that the portion cast in sand is weakened if not suitably proportioned. A
wheel rim having light arms must almost certainly snap in cooling. Hence
these are either made with massive arms, curved lengthwise, or the centres
are solid, having a " dished " or corrugated section. Chills do not have a
very long life. Though they may not fracture, the surfaces against which
the metal makes contact become roughened by the formation of minute
cracks, the result of repeated expansions and shrinkages. The metal too
deteriorates, approaching the condition of " burnt iron ". A new chill
must be cast, and finished by boring. Plumbago is used for facing at the
time of casting.
Die-casting.—This is a development, less than a dozen years old, of
the linotype castings. Originating with the white metal alloys, those having
a basis of lead, tin, or zinc, it now includes those with an aluminium base,
and efforts are being made to deal with those of copper. The phenomenal
demand for, and the immense supply of these castings is in response to the
call for those smaller mechanisms of universal use. These include type-
writers, telephones, gas meters, electrical instruments, speedometers, as well
as parts of engineers' mechanisms, lubricators, oil cups, bushes, small gear
wheels, &c.
Die-castings are made in metal moulds of steel, the liquid alloy being
subject to a pressure of 100 Ib. per square inch or more, which is maintained
until it has set. The result is that the castings do not require machining,
being correct to size within a thousandth of an inch, so that they will fit other
parts tightly or with sliding allowances and external and internal screw
threads will match perfectly. The teeth of gear wheels will mesh. Letters
and figures will come out sharply as though engraved. If finely threaded
screws or hard contact pieces are required, these can be cast accurately into
the softer alloys. Die-casting in steel moulds is used for-many small intricate
castings which can neither be made economically in sand, nor drop-forged,
and many for which the cost of machining would be prohibitive. Though
these dies are always expensive, their cost increasing with complexity and
the limits of accuracy insisted on, the outlay is relative. A rather com-
plicated die may cost from £50 upwards, but it will endure 50,000 casts of
a white-metal alloy, and a slightly smaller number for an aluminium-base
alloy. And the advantages and economies just stated are secured by its use,
at the cost of a fraction of a penny per casting.