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Full text of "Modern Mechanical Engineering Vol-I"

Pattern-Making

Introductory

THE  CRAFTSMAN

The work of a competent pattern-maker is both exacting and com-
prehensive. He must be skilled in woodwork, an accomplishment which
he shares with the carpenter, joiner, and turner, who, however, may not
understand how to construct patterns that will deliver from the sand, how
to economize in material and avoid the employment of complete patterns by
the substitution of skeleton-like structures, how and when to use sweeping
boards, sectional pieces, or cores.
He has to know the best methods of countering the effects of the damp
sand on porous timber, by the judicious employment of open joints, of
segmental pieces, and of framed structures of many kinds.
He has to be fully conversant with the different systems of moulding
—green and dry sand, and loam—and core-making in all their branches, and
with the handling of light and heavy work. It is necessary to be familiar
with the evils that result from shrinkages in unequally proportioned castings.
He is primarily responsible for the methods of moulding (since he
has to determine how patterns shall be constructed for the mould joints),
for ramming and delivery, and for the determination of upper and lower
faces for pouring.
An intimate acquaintance with the operations of the machine-shop is
necessary, as machining allowances vary considerably in different classes of
castings, while the variations that occur in similar pieces are often large,
due to the presence of hard cores, the straining of top-boxes, the absence of
risers, and the differences between the results that are associated with the
practice of hand-rapping and delivery and machine-moulding.
Elementary knowledge of arithmetic and geometry are required for the
estimation of weights and the laying out of work. In all shops some men
have to specialize in toothed gears, or in motor-work, or marine-castings, in
plating metal patterns, in odd-side work, and so on. In truth, the craft of
the pattern-maker is a many-sided one.
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