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Full text of "Modern Mechanical Engineering Vol-I"

THE  ELEMENTS

55

typical example (fig. 5) the top flange is fast to the pattern, but the rib
below and the middle strip must not be (apart from the employment of a
drawback plate). Loose pieces are located during ramming, either with
skewers or with dovetails, the first-named being removed during ramming,
previous to the withdrawal of the pattern.

Obviously, before a loose piece can be drawn inwards, there must be an
open space large enough to receive it. If, in fig. 5, the pieces have to be drawn
into the narrow space left on the withdrawal of the main rib, they must be
thinner than the space thickness, as is the case in A. But the bottom strip
in c is thicker, hence it must be divided into two or three thicknesses, one to
follow the other. Since the pricker has to be inserted diagonally (B), getting
the pieces out of so deep a
space is troublesome, and                              Q

no mending-up or cleaning
can be done if the sand
breaks down. But the
conditions are altered if
the interior has to be taken
out with cores, or if,
though rammed wholly in
green sand, the ramming
is done on a grid that
permits of the removal of
the interior mould. Ample
space is then left, into
which strips of greater
width than those shown
can be withdrawn. But
even then there are limi-
tations to the widths that can be dealt with in this manner, where fracture
of the sand and convenience of cleaning and blackening have to be con-
sidered.

Drawbacks or false cores.—These avoid this awkward method of with-
drawal, but they have a vastly wider scope. They are either grids or plates
on which outer portions of moulds are rammed, to be lifted bodily away from
the pattern, to be replaced and reset accurately by some form of joint between
the plate, or its sand, and the sand in the main body of the mould. This is
capable of very extensive applications, since there is no limit to the width of
the encircling portions that can be carried thus.

Internal portions, Cores.—The conditions that control the delivery of
internal parts differ from those of external. Thus, it will be obvious that
depths and diameters are related. A shallow hole will deliver satisfactorily,
though it has but a slight amount of taper. A deep hole of the same diameter
will not, and therefore it must be taken out with an independent core. Frames
of large dimensions may be regarded as patterns having large holes, relatively
shallow. They deliver freely as well within as without, and they are tapered

B
Fig, 5.—Loose Pieces