Modern Mechanical Engineering Vol-I

EXAMPLES OF WORK

pattern can be constructed in wood, divided
longitudinally, but this is not a very satisfactory
method. If a cast worm is used, the pattern
should be cut in metal and moulded vertically,
screwing it out of the mould through a stripping
plate, and relieving its weight with a counter-
balance suspended from a rope passing over a
pulley. But it is better to cut worms in the
machine-shop, in which case the pattern-maker
can employ the actual worm as a perfect guide
by which to shape the teeth of the wheel.

Pattern wheels must be jointed along the
middle plane, either through the central plate,
or leaving this in one piece, undivided, the half
depth of rim is registered to it, as in the case of
sheave wheels. Segments are built up, overlap-
ping, and the concavity for the tooth blocks is
turned with a templet, the interior of the rim
being similarly dealt with. Blocks for the teeth
are fitted and glued to each half-rim, and the
abutting ends that come in the joint face are
turned at separate chuckings of each half-pattern,

and at the same time the curves are imparted Fig. 56—Rim for Bevel wheel
to the points with the aid of a templet working t
from the joint face. The outer ends of the teeth are then finished.

The teeth are pitched and their thicknesses and shapes marked in the

Fig. 57.—Worm-wheel Pattern mounted for Cutting the Teeth