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Full text of "Modern Mechanical Engineering Vol-I"

130

FOUNDRY  WORK

and removed in sections, the pattern is unscrewed and taken away, leaving
the core to be removed. An advantage of this method over the making of
a separate pattern and core box is that the correct thicknesses are ensured.
But the real reason for the adoption of the method is economy of timber and
pattern-maker's time. It is reserved therefore for the larger castings.

Loam Patterns.—These, swept in loam, are used instead of those made
of wood, to be rammed in moulds of green or dry sand. This is a rather
large and important section of foundry work, the object being, as in loam
moulding, to save the prohibitive cost of complete patterns of wood. It
includes symmetrical work, revolved against the profiled edge of a board
fixed on the core trestles, and non-symmetrical articles, formed as half pat-
terns with strickles, the longitudinal movements of which are controlled by
guide irons, or by the edges of contour plates on which the pattern halves

Fig. 27.—Pipe made in Loam
A, Guide iron.    B, Core grids,    c, Core.    D, Strickle.    E, Mould with core and its chaplets.
are swept. The longitudinal shapes are determined by the character of the
castings required. They may have regular or irregular curves, or curves
combined with straight portions. Instead of using loam patterns, it is often
cheaper to make a rough skeleton pattern of wood, with outline ribs, fill
the spaces with sand, and ram it in the mould. Fig. 21 shows a skeleton
pattern for a pipe bend as sent from the pattern-shop, and fig. 22 one
for an S-pipe, having the spaces filled with sand.
Figs. 23 to 26 illustrate the making of a pattern, and core for a loam
bend. A is the guide iron, set with weights, B is a slender body of loam
which forms the vent channel of the core, E, a part of which, D, is seen
roughly daubed on the grid c in fig. 24, with its vents, and which is com-
pleted in fig. 25. In fig. 26 the pattern "thickness" F, corresponding
with the thickness of metal in the casting, has been laid on, and the
standard iron pattern socket G and spigot H set, completing the half
pattern. After the pattern has been moulded, the thickness is stripped
off, leaving the core ready, when blackened, for insertion. In fig. 27 the
core strickle, controlled by the guide iron, is seen bridging the core, and
the mould is shown to the right, with the core inserted.