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Full text of "Modern Mechanical Engineering Vol-I"

86

PATTERN-MAKING

central joint plane of the pattern precisely as for an involute spur wheel.

The tooth sections change constantly from the centre to the outer ends.

The larger the angle or
slope of the teeth, as in
worms of small diameters
and those with multiple
threads, the more marked
are the changes in section.
Here the advantage of em-
ploying the worm as a
templet guide for cutting
the wheel teeth is apparent.
The worm is set between
lathe-centres, and the wheel
is mounted on a stem in
the T-rest. The wheel
and the worm are moved
into contact (fig. 57). The
application of chalk or of
red lead to the worm
indicates, by its trans-
ference to the wheel
teeth, the high parts from
which material must be
removed.

If the wheel contains
a large number of teeth,
the work of cutting may
be hastened by shaping,
say, half a dozen correctly
from worm contact, and
then marking the shapes of
the other ends so obtained
on the remaining teeth.
These can then be roughed
out rapidly with gouge and
chisel, leaving the finish to
be imparted by the assist-
ance of worm contact.
A massive spur pinion

Fig. 58.—RoUing-mill Pinion, 15 teeth, 7-in. pitch, 26-in. face         pattern, which Stands higher

than a tall man, is shown

in fig. 58. The teeth are shrouded to the pitch circles. There is a joint
in the pattern along the face of each shroud. These are built up with,
segments, as are also the bosses. Fig. 59 shows a segmental pattern, from
which large toothed rings are built up, being bolted together by the end