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Full text of "Modern Mechanical Engineering Vol-I"

122

FOUNDRY  WORK

to within f in. of the joint face in a " plain top ", or to within f in. of the
pattern when the box is made for special work only. In the latter case the
bottom bars are shaped similarly, as in pipe and column work. The bars
retain the sand by their proximity, and the friction of their rough surfaces,
assisted by an application of clay wash before ramming. Middle parts
seldom have any bars, but narrow flanges are cast within top and bottom
edges to assist in sustaining the sand, and to carry rods laid on them and
disposed close to the pattern. Small flasks for brass moulding have internal
flanges, and some have their sides recessed to a very obtuse angle to prevent
risk of the sand falling out.

Fig. 16.—A Corcd-up Mould for a Fusee Barrel

i

Lifters, Rods, and Nails.—When deep portions of moulds or recessed
pockets of sand extend considerably below the lower edges of the bars, these
receive support with " lifters " or " S-hooks " (see fig. 16), suspended from
the tops of the bars, and going down into the sand rammed around them.
They are bent at both ends, one to rest on the bars, the other to assist in
holding the sand. They are wetted with clay water.
When portions of sand extend out horizontally, they would break down
by their own weight, or be washed away by the inflowing metal, unless
supported with rods in the larger sections, or cut nails (sprigs) in the smaller.
In each case the length must be sufficient, when enclosed in the sand, to
counterbalance the portion that overhangs. All weak portions of sand have
to be treated thus, so that a rather large amount of " sprigging " has to be
done in some moulds.
Venting.—With the exception of some small moulds, made with an open
self-venting sand, and most loam moulds, venting done with a rod or wire is
necessary. The vents are driven from the outside of the moulds close to