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Full text of "Modern Mechanical Engineering Vol-I"

138

FOUNDRY  WORK

I     »'

liquid pressure, as large copes and the bottoms of deep moulds, should be
rammed with stronger mixtures than the sides. But venting must be more
thorough, or the casting will be " scabbed ".
Green, Dry, and Core Sands.—The feature which these have in common
is that they are consolidated with the rammer while in a moistened condition.
They are never wet, but sufficiently damped to retain a shape imposed when
squeezed in the hand. The retention of the form produced during ramming
depends partly on the coherence of the sand, but largely on the means by
which it is sustained in flasks, and on grids. Green sands cannot be dried,
except slightly on the surface, without losing their coherence. Sand, to be
dried, must be of a strong clayey character, and be mixed with horse manure,
which, by its carbonization during drying, counteracts the close texture of
the mould, favouring venting. But the vent wire must be used freely too.
Coal dust is also used. Core sand is mixed with clay wash, peasemeal, or
beer grounds; and generally, dry sand mixtures are suitable for cores.
Loam Mixtures.—These are made with strong sands, vented with horse
manure, with which generally a large proportion of old loam is mixed, the
whole being ground in a mill with water, and swept thus while in a pasty,
plastic condition, to be dried subsequently. It is used in coarse and fine
grades, the first for embedding the bricks in, and for the rough coats, the
second, finely sieved, for the final coats.
Chemical and Mechanical Analysis.—During recent years, attempts have
been made to grade moulding sands by chemical analysis, supplemented
with microscopical examination of the grains. These are helpful when new
sands are concerned, but their value is discounted when, as is usually the
case, large proportions of old sands are mixed with new. It is important
that the percentages of silica and of alumina should be known, and also the
quantities of iron oxide, lime, magnesia, and alkalies, which tend to lower
the fusing point of a sand, and flux it. Silica, the refractory element, must
be present in more than 80 per cent, alumina in from 7 to 10 per cent, the
proportions varying for weak and strong mixtures. But it is held that the
texture of a sand when passed through sieves of different meshes is of more
importance when deciding its suitability for a certain class of work than
chemical analysis, and that mechanical testing affords an approximate index
of the cohesive character of a sand. Weak sands have fine grains, and least
alumina. The strong sands possess coarse grains, and a large proportion
oi alumina. Castings with smooth skins can be obtained with the use of
coarsely grained sands. Fine grains are suitable for dry mixtures, cores,
and loam, with a large proportion of alumina.