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Full text of "Modern Mechanical Engineering Vol-I"

EXAMPLES  OF  WORK

89

wheels, to correspond with the tooth points. These vary in details with the
shapes of wheels. A plane top is general for spurs, but the top for a bevel
wheel is curved to follow the edges of the vertical arms (fig. 63). The edges
for sweeping the bottom and top moulds are usually cut on the same board
which is reversed on the bar. The latter is of a standard size (fig. 62), so that
all boards are shorter than the real radius of the wheel by the radius of the
bar, to which they are attached with
an iron strap. The radius of the
tooth block, though set by the bed
swept, is checked, and, if neces-
sary, finely corrected with a strip
that gives the exact distance from
the central bar to the point or the
root of a tooth.

Core Boxes. — Arms of all
shapes can be made with cores,
but the most convenient are those
of H-section, and these therefore

are mOSt  COmmon (fig.  64).     Bevel         Fig. 64.—Core Box for Wheel Arms of H-section

wheels   have  arms   of  T-section.

Cores are rammed, dried, blackened, and set in the mould on the swept
bed without aid from prints. The spaces between the cores corresponding
with thicknesses of metal are set with wooden gauges. Their own weight
and the pressure of the cope when the mould is closed prevents them from
shifting. Central bosses and prints are swept, or bedded-in.

5. BEDS AND ALLIED FORMS

These being the bases for engines, pumps, machine-tools, cranes, &c.,
occur in an immense variety of outlines and dimensions. Only broad
principles can be stated here.

If

Fig. 65.—An Engine Bed suitable for Self-delivery—the top face being lowermost in the mould
Method of Moulding.—This is the first thing to be determined.
Usually the top face of the bedplate pattern goes to the bottom of the mould.
This ensures that sound metal shall be present in those surfaces which