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Full text of "Modern Mechanical Engineering Vol-I"

PATTERN-MAKING

have to be machined later.   The question of moulding by bedding-in
or turning-over is settled by the numbers of castings required and the

Fig. 66.—Section through Crank-shaft Bearings of Engine Bed.   Pattern shows boxing-up,
printsjfor cores, and loose pieces

boxes available. The latter method is preferable, except for beds of the
largest dimensions. The choice of self-delivery, or of coring the interior,
depends chiefly on the bed section, and on the
relative proportions of width to depth of interiors.
As there is no objection to giving plenty of taper,
a good slope is always given to the outside, and as
the thickness of metal is equal throughout, the
internal taper favours delivery (fig. 65). Many
deep beds therefore with wide internal spaces
deliver themselves, the interior " green sand core "
being carried on a grid suspended from the stays
of the top box, or, if special boxes are made, the
stays are brought down inside to a distance of about
| in. from the pattern all round. But this method is
not practicable when beds are narrow and deep. In
these cases, the interior is taken out with cores in-
serted in print impressions (fig. 66). This is very
convenient when loose pieces have to be attached to
the outsides, as these can be withdrawn laterally
through the open interior.

There are few patterns which do not carry some
loose pieces, and core prints for the insertion of
small cores (figs. 66 and 67), for bearings, and recessed

portions in varied forms.   In some cases it is convenient to carry all the
outer mould on an encircling plate for the purpose of getting at recessed

y.-shows Loose BOSS
*on end °f Engine