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BEVEL GEARS. 4OI
Measuring the back cone radius of the small gear, it is
found to be f| inch, and the diameter will be f£ inch ; conse-
quently the shape of tooth at a for the small gear is the same as
the shape of tooth in a spur gear of 21 teeth, sixteen diametral
pitch.
Therefore, if this pair of gears is to be cut by a rotary
cutter having a fixed curve, a different cutter is required for
each gear.
When, in a pair of bevel gears, both gears are of the same
size and have the same number of teeth, and their axial center
lines are at right angles, they are called miter gears, and one
cutter, of course, wilt answer for both gears. One cutter will
also answer in practice when the difference of the back cone
radius of a pair of gears is so small that it comes within the
limit of one cutter as used for spur gears of the same size. Bevel
gears may also be made with cycloid form of teeth, but when-
ever cut by rotary cutters, as usually employed in producing
small bevel gears of diametral pitch, the involute form of tooth
should always be used.
Cutting Bevel Gears.
When bevel gear teeth are correctly formed, the tooth
curve will constantly change, from one end of the tooth to the
other. Therefore, bevel gears of theoretically correct form
cannot be produced by a cutter of fixed curve ; but, practically,
very satisfactory results are obtained in cutting bevel gears of
small and medium size in this way.
When a regular gear-cutting machine is not at hand, the
Universal milling machine is a very convenient tool for cutting
bevel gears of moderate size, and is used in the following way
:
First, see that the gear blank is turned to correct size and
angle, and adjust the machine to the angle corresponding to the
bottom of the teeth in the gear. The correct index is set ac-
cording to the number of teeth in the gear. Adjust the cutter
to come right to the center of the gear, cut the correct depth as
marked on the gear at a (see Fig. 12), according to Table No. 49,
and when the machine is adjusted to the correct angle, and the
correct depth is cut at a, the correct depth at b will, as a matter
of fact, be obtained.
Second, when a few teeth are cut in the gear (two or three)
bring, by means of the index, the first tooth back to the cutter.
By means ©f the index, rotate the gear, moving the tooth toward
the cutter; but, by the slide, move the gear sidewise away from
the cutter, until the cutter coincides with the space at b ; then cut
through from a to b. This operation will widen one side of the
tooth space at a.
Note the position of the machine, and, by the use of the
index and slide, return the cutter to its central position and in-
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