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Full text of "Modern Mechanical Engineering Vol-I"

6o

PATTERN-MAKING

pipes and columns, and work of which these are typical. Only when these
are of small dimensions, say of 6 in. diameter and under, are they made in
solid stuff, usually jointed along the central plane and dowelled. When of
over this size the patterns are built up with narrow strips, glued on cross-
pieces, located at short intervals—" lagging ". This method is adopted up
to the largest diameters for which entire patterns of wood are constructed.

As diameters increase, the num-
ber of lags is multiplied. In all
cases they are very narrow to
localize shrinkage, ranging with-
in small limits, say from 2 in.
in small bodies to not more than
5 in. in the largest. It is neces-
sary to secure rigidity as well as
freedom from changes of form.
Strips must not be too thin,
nor may the cross-bars be
spaced very far apart. Propor-

M2.—Ring built with Segments                   tions .are governed by diameter

and length. Subsequent draw-
ings (figs. 34 and 35) indicate suitable proportions. Good close joints
must be made between adjacent edges, and be united with glue. If the
work is done carefully and well, and seasoned stuff used, the patterns will
retain their accuracy for an indefinite period. There are details in the
methods of construction that are dealt with in later sections, where
examples of work are illustrated.

Circular Work.—This is built up with sectors of circles—" segmental

work ". Obviously, if rings
were cut from solid material,
they would shrink into ellip-
tical forms, and fracture
along the short grain. Built
with sector pieces overlap-
ping, " breaking joint ", they
mutually reinforce each other,
shrinkage is minimized, and
the circular shape is main-
tained. (See Chapter II,
Section 3.) To secure this result perfectly, it is necessary to limit the
length and the thickness of the individual pieces. Those too long would
shrink in width, and those too thick would shrink and lack something of
reinforcement by other pieces. The maintenance of a judicious relationship
between these proportions is necessary to secure permanence of form. This
method of building-up is suitable alike for rings that are shallow or deep.
The shallower the work, the thinner the sectors are. A thin flange of large
diameter should not be built in less than three or four courses. Rings of

Fig. 13.—Semi-spherical Pattern built up