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Full text of "Modern Mechanical Engineering Vol-I"

56                                   PATTERN-MAKING

by the same amount. The question of coring scarcely arises here, but it does
in all cases where interiors will not deliver.

The plain interior of a casting, made by ramming sand within a pattern
which is the exact replica of its casting, may be termed a core. But the
accepted meaning of the term is, a body of sand, generally dried, rammed in
a box distinct from its pattern, and inserted, and located in the mould by the
impression of a core print attached to the pattern.

The alternative of coring an interior to making the pattern like its casting
arises. In the majority of instances no doubt exists. No intricate shapes
can be delivered. These must be rammed in a separate box or boxes, and
inserted in the mould. In some instances it is more convenient to make
cores than to self-deliver. In a fair number of cases a core is preferred,
because, using prints, a stronger pattern can be constructed than if the timber



pig, 6.—Skeleton Frame united with Halvings, with Interior Strickled
were cut away to allow the interior to deliver. Lastly, projecting portions
are frequently cored over in preference to making a joint in the pattern, or
" down-jointing ", or to employing loose pieces or drawbacks.
(£) Skeleton Patterns.—Patterns of large dimensions, and those of
moderate sizes, when they are of shapes that would require considerable
quantities of timber and much tedious cutting, are not made of solid, con-
tinuous stuff, but are of more or less open construction. The numbers of
moulds required count in this scheme, so that while a skeleton pattern might
be used for a few castings, a complete pattern would be more economical for
large numbers.
The open frame (fig. 6) is the simplest example of the skeleton pattern.
Narrow strips jointed at the corners provide the outside dimensions. The
interior is strickled, or it is occupied with loose removable strips (fig. 7) for
repetitive mouldings, the latter being better to ram the cope on than sand is.
The same method is employed for plated portions that are curved in outlines
(fig* 8), but with increased economy, because more timber and labour are
required for working these than for plane frames. Here the strickle may be
used, or strips be fitted at intervals to form a discontinuous guide, the spaces
between the strips being filled with sand, to be rammed on. This method