IOO

PATTERN-MAKING

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since having a level bed swept, the block can be set and reset on a circle
struck round with a trammel. It is held securely during ramming with a
weight. The interior is formed with cores. The two methods just de-
scribed are in common use for crane beds and centres which are of fairly
large dimensions, but which are seldom ordered in considerable numbers.

With the exception of open moulds, only used in making foundry ap-
pliances and the roughest castings, a top box-part is necessary. When a
sectional mould is made, the top cannot be rammed in its place on it, as is
done over a complete pattern. Then it is either swept with a strickle and
turned over on the mould, or it is rammed on a hard levelled bed of sand
away from the mould, transferred to the latter, and set on it by measurement.

Fig. 82. — A Sweeped
Pattern Segment, from
which a ring is moulded

Fig. 83.—A Sweeped Pattern Segment set for Ramming an
External Mould

The second method possesses this advantage over the first, that supple-
mentary pieces, as facings, bosses, brackets, Sec., can be laid on the prepared
bed in their correct positions, and the top box be rammed on them. This
is rather better than cutting away the sand in a strickled top and bedding
them in. .
Skeleton-like Patterns.—These differ from those just described in
the fact that they include the correct outlines, the complete contours, and
cardinal dimensions, but that the timber construction is not continuous.
The outlines are represented by a series of ribs, which leave open spaces to
be filled with sand. A large quantity of timber is saved, and labour is
economized, with no disadvantages to set-off. The method is employed for
large pipe-bends, large cylinders, condensers, and the casings of steam
turbines. It is used also in making alterations to some patterns. Enlarge-
ments of portions of patterns and reductions in diameters of core boxes are