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Full text of "Modern Mechanical Engineering Vol-I"

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58                                    PATTERN-MAKING
and they are unjointed. All non-symmetrical fittings, as brackets, feet,
bosses, and flanges, if large, are prepared separately in wood, and attached to
or laid on the loam pattern. A large number of moulds can be taken from it,
since it is hard, and its surface is protected with a coat of hot tar. Among
the articles commonly made thus are cylinders, e.g. for hydraulic machines,
for Corliss engines, for large gas-engines, and for pumps. Allied to this work
is that of strickling patterns of bend pipes in halves. The loam is swept on
grids; and flanges, feet, or other attachments are prepared in wood and fitted.
(rf) Swept Moulds.—Moulds are swept in green sand and in loam
to avoid the expense of complete or of skeleton patterns. The scope of the
first is limited because the fragile character of green sand does not permit
of deep sweeping. That of the second is very extensive, and is practically
the only method available for very large cylindrical moulds. In some details
where contours are irregular and unsymmetrical, loam is laid and worked
against pattern parts of wood embedded in it. But the main moulds are
swept to their symmetrical profiles with boards attached to a central revolving
bar, set concentrically in a step bearing, and their main cores are swept on
the same or a duplicate bar.
From these elements we turn to the consideration of the most suitable
methods of pattern construction.
2. PATTERN CONSTRUCTION
The pattern-maker's craft differs in many ways from that of the carpenter,
joiner, and wood-turner. The first and chief contrast lies in the necessary-
provisions that have to be made for delivery, taper, and the other matters
instanced in the preceding division. In addition, measures have to be taken
to minimize the effects of the severe and destructive treatment to which
patterns are subjected. It is that of insertion in wet sand, of rapping, and
delivery, alternating with storage, and what is as injurious, the alterations
that have to be made in many patterns from time to time. And, in all but
highly standardized work, wood alone is used, yellow pine mostly, soft and
porous, and mahogany to a limited extent for small articles.
Very broadly, pattern construction falls within three great groups: plane
areas, cylindrical articles, and circular work.
Plane Areas.—In dealing with these the aim is to lessen the widths
of individual pieces to relatively narrow strips in order to localize the expan-
sion due to moisture and the shrinkage consequent on storage, which regularly
alternate. The solid glued-up table tops and side-boards of the cabinet-
maker have no analogues in pattern-work. Instead, wide pieces are always
made with " open joints " (fig. 10), that is, a space of about J in. or less is
left between strips, to the extent of which they are free to expand when
moisture is absorbed, so that the out-and-out dimensions of a broad width
are not affected.
Since the edges of the open joints are not united, there is no cohesion
between the pieces as there is when edges are glued, nor can the pieces lie