Skip to main content

Full text of "Modern Mechanical Engineering Vol-I"




tions in all castings made. If the pattern-maker does not put a radius, the
" hollow " or " fillet " in a keen angle, the moulder rubs one, as at D, thus
altering the weak crystallization of B to that shown. Additional strength is
afforded by the bracket at D, common in flanged structures, and which steel
makers often insert when not done in the pattern, to prevent cracking of the
casting. It is better to fit a bracket as shown at intervals, than to make the
fillet very large, because the result might be a " draw " (a cavity in the casting)
due to internal shrinkage, such as is seen at E, where three ribs meet with
large fillets. This would be prevented, and the casting be stronger if the
radii vvere smaller, which, while favouring suitable crystallization, would
reduce the mass of metal in the corner.

Some Common Precautions.—Castings, apparently sound, not infrequently
fracture during machining or subsequently. This is because they are in a
condition of internal tensile
stress, dangerously equal to
that of the ultimate strength
of the metal. Inspectors test
roughly for this condition with
hammer blows. Hard sand
cores and portions of dried
moulds interfere with shrink-
age, and a careful moulder will
break these up as soon as the
metal has congealed. At the

best the shrinkage is only lessened, but this in large castings may be
sufficient to counteract the allowance for tooling. Bars in flasks adjacent
to flanges (fig. 53) will check shrinkage, requiring the breaking away of the
intervening sand. Pulley arms are commonly curved, because they will
accommodate themselves to the pull of a shrinking boss instead of fractur-
ing. Pulleys with wrought-iron arms must have the boss cast after the rim
has become nearly cold. Large runners and risers will interfere with shrink-
age, and the moulder often knocks these off so soon as the mould is full.

Fig-S3-—Illustrates Shrinkage of Flanges

Right hand, Flange and weak.    Left hand, Flange reinforced
with brackets.

The  Furnaces

The furnaces include several types with many variations: for melting
iron, steel, the brasses and bronzes, and malleable cast iron. A large amount
of plant and machinery is associated with the operation of each, on which
greatly depend not only the economies of working, but the soundness and
strength of the castings produced.
The Cupolas.—With many differences in details, the essentials of a