EXAMPLES OF WORK
out of square arise. The first is checked with calipers as the work
proceeds before the glue has hardened, the second with a set-square
tried along a flank and working from a straight-edge.
To glue rough blocks on and work them in place with gouge and chisel
requires great care to get straight flanks. Using a small straight-edge, and
carefully glass-papering, the method is reasonably accurate, though tedious.
Since rough blocks, glued or dovetailed, have interspaces, these may be
filled in with wedge-shaped bits (fig. 54) to afford a continuous surface. It
is convenient for the turning, but not essential, since if light cuts are taken
with a sharp gouge, the teeth will not be knocked off nor the grain split.
And although a continuous surface is useful for locating tooth curves on,
the centres may equally well be set on a zinc templet piece as shown in the
upper part of fig. 54, worked round the periphery.
Tooth centres are pitched round on one side, and squared over to the
side opposite. The tooth thicknesses are set to right and left of the pitch
points, and the curves starting from these are described.
Bevel Wheels —These are based on precisely the same principles
and elements as the spurs, in regard to the shapes and proportions of the
teeth. But the pitch and related dimensions are always taken on the
major diameter, those on the smaller diameter being controlled by the
width of face of the teeth. The teeth are not developed on the real
diameters, but on conical surfaces at right angles with the pitch cone
Bevel gears are marked out as shown by fig. 55. The pitch cones ab
are the primitive rolling surfaces. The diameters A, B are the real diameters
for the actual pitches. Through #, the point of intersection of these, a
line is drawn perpendicular to ba, meeting the axes of the primitive cones
in c and d. Circles described with radii ca and da are the pitch circles
on which the teeth are drawn. In other words, they correspond with the
curves of spur gears of radii ca, da. As the teeth taper from the major
diameter to the apices of the cones, the tooth curves on the minoi
diameters are obtained on the developed surfaces having radii fe, ge.
The tooth forms for both are shown to the left, and those for the minor
radii are repeated at the right.
Pattern rims are built up with courses of segments that overlap sufficiently
(fig. 56) to include the cone section. Two chuckings are essential; whether
the back or the front is done first does not matter, since a straight-edge is
laid across the rim and each is turned with the aid of templets. Nails cannot
be used so conveniently to reinforce the glued joints as wooden pegs, though
the risk of segments starting after the teeth are attached is nearly negligible.
Teeth are fitted and worked in either of the ways described in connection
with spur gears. If fly-cutters are used, they are not selected for either
diameter, but for a location at about a third of the tooth length from the
major diameter, and two settings of a flank are needed.
Worm Wheels.—These are made much less frequently by the pattern-
maker than formerly, since the practice of generating has grown in favour,