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Full text of "Modern Mechanical Engineering Vol-I"

82

PATTERN-MAKING

Tooth   Formation   and Fitting. — Teeth  are  either  shaped first
and attached to the rim afterwards, or they are worked in their places.

r—~,  r™!

Fig. 53.—Method of Fitting Teeth with Dovetails

The latter is to be preferred when a dividing-machine and a fly-cutter can
be used. But if not, the best way is to fit each tooth with a dovetail (fig. 53).
Turn, pitch, and mark the teeth out in place, remove them to be shaped

with planes, return and glue them permanently.
The best pattern wheels, apart from those
machine-made, are constructed in this way.

Other methods are, to plane each tooth sepa-
rately, fit and glue it to the rim, or to glue rough
blocks on the rim, turn, pitch, and mark out
(fig. 54), and cut the shapes through with chisel
and gouge. These methods are however not
entirely satisfactory.

Using a fly-cutter, the rim is turned J in.
or A in. below the roots, and rough blocks, each
wide enough to include three or four teeth, are
glued on in contact. Material is thus left below
the roots on which the radius or fillet is cut
without leaving a " feather edge ". The blocks
being turned, the tooth spaces are shaped in a
machine, which also divides for pitch. No taper
is given, and since the teeth are accurate, they
can be drawn through a stripping plate, in
hand-made moulds, or on a machine.

The chief advantage of planing  the teeth
previously to attaching them to the rim is that
they can be shaped accurately.  They are planed
in a box made of hard-wood, having the cross-
section of a tooth.     The difficulty lies in gluing the teeth to the rim.
Setting is done by centre lines, or by the edges of the flanks, to lines
pitched and scribed around the rim.   Errors in pitching and in getting

Fig. 54.—Method of Fitting Teeth
to be Shaped in Position