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Tooth lengths are proportioned to pitches, but teeth are always made shorter
now than they were formerly. Proportions are given in many textbooks,
and they are standardized in the shops.

Spur Wheels, Pattern Construction.—For these, two materials are
used chiefly: yellow pine and Honduras mahogany, or Baywood, the first
for the bodies, and the second for the teeth. Yellow pine is suitable for
teeth when only moderate numbers of moulds have to be taken.

Only very small pinion patterns are made solid, that is, with the teeth
in one with the centre body, and the grain running longitudinally. Pinions
of over 6 in. or 7 in. diameter must have their centres built up. In the
smaller sizes, courses of sectors are glued up, the grain running radially.
In those above say 8 in. or 9 in., seg-
ments are used, the grain running
tangentially. Thicknesses will range
from \ in. to I in. in small and large
patterns respectively. Gluing is done
carefully, and nails or wooden pegs
reinforce the joints against the rough
usage of the foundry. The rims are
turned and finished before the teeth
are taken in hand, these being always
made distinct from the rim to get
longitudinal grain.

Methods of Constructing
Wheels.—The larger pinions, and
smaller wheels, have solid-plated
centres, built into the rims. All large
wheels have arms made separately
from the rims, which are built up.
„ Plated centres are built up with sectors
having the grain radiating, in not less
than two thicknesses. Or narrow strips with open joints are prepared, and
the courses of rim segments are glued up on the discoid centres. Rims for
armed wheels are built up and turned as separate elements into which the
finished arms, usually of T-section, are fitted. When they have the section
of a + they are built into the rim at the half-way stage of the courses of
segments. During the fitting, care must be taken not to drive them into
their recesses so tightly as to distort the rim. Only light hand pressure is
employed, with glue and fine screws. Though the locking together of the
arms at the centre is rather flimsy, the screwing on of the central boss and
the fitting of the vertical arms provide additional strength. The latter abut
against the boss, or fit in shallow grooves cut in it. They also abut against
the rim. Fillets or " hollows " glued in all angles further stiffen the struc-
ture. With arms of -(--section, the ribs that come in the top should be
do welled loosely with that boss portion, for reasons previously stated
Fig. 52 shows a wheel pattern with split lugs.

VOL. I.                                                                                                                         6

Fig. 52.—Portion of Pattern Wheel with
Splitting "Lugs