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Full text of "Modern Mechanical Engineering Vol-I"

THE   ELEMENTS

53

Fig. i.—Pattern of Sheave Wheel built up

(a) Complete Patterns.—In these, the first question that arises is
that of the direction or directions of jointing the mould, with the usual
though not necessary concomitant, that of jointing the pattern similarly.
This very often admits of alternative solutions. In a fair number of
instances, only one is practicable, though others may be possible if the
cost of moulding is overlooked. The best way to approach the subject
is to consider the simple elementary geometrical forms which are constantly
recurring.

Jointing.—All moulds, except the
relatively very small number which are
" open", comprise bottom and top
portions, included in bottom and top
box-parts (" drag " and " cope "). An
exception occurs in bedded-in moulds,
for which the bottom box is not re-
quired. The jointing between top and

bottom is determined by the facility afforded for delivery of the pattern,
with the least risk of damage to the mould, and bearing in mind too the
extent of subsequent details of finishing, coring, and of pouring, and the
disposition of upper and lower faces. This latter consideration is most
important when tooling enters into the case, since machined portions must
be free from specks and blowholes. In general, if one portion is of greater
depth than another, the deeper section goes in the bottom. The reason is
that it is much better to withdraw a pattern from a bottom mould than to
lift the top sand off the pattern. This is not always necessary, because when
a top box-part is turned
over, and the pattern
parts along the joint, the
upper portion can be
left loose from that
below, to come up with
the top sand, and be
withdrawn after turning

Over.                                                                     Fig. 2.—Pattern of Trolley Wheel built up

Elementary   sections

that deliver well are illustrated in many subsequent diagrams. The
patterns may or may not be jointed along the same planes. Very often
they are not; seldom in those of small dimensions used by brass moulders,
because dowels work loose with usage, and the edges of the pattern parts
overlap. When unjointed, the moulder makes the joint face, guided by
the eye alone, or, in repetitive work, some form of joint-board, odd-side,
or plate is provided.

In many instances, the pattern joint cannot coincide with that of the mould
(fig. i and fig. 2 are typical examples). The patterns must have divisions
to permit of withdrawing them from the moulds, but the joints of the latter
do not coincide with those of their patterns. In the examples (figs, i and 2)