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228                           THE  MACHINE-SHOP
and reverse, and pulleys of different sizes for the two functions, instead of
trains of gears. (6) Employing a large " bull wheel " for driving the table
rack instead of a small pinion. (7) A vast extension of electric driving, with
a corresponding multiplication of speeds, reverses, and feeds effected by
switches. In consequence of these improvements, modern planing-machines
hold their own in face of the keen rivalry of the piano-milling machines that
operate on the same classes of work.
An example of a motor-driven planer is given in fig. 61. Taking 84 in.
by 84 in., a 45-h.p. motor is required. It is one of a series by the Cincinnati
Planer Company of Cincinnati, Ohio, U.S.A. It is termed a " rapid power
traverse machine", because each tool-head is moved rapidly from one
position to another with power, derived from the motor mounted on the
arch, instead of slowly by hand. Power is transmitted down through a
splined shaft to a gear box at the side, provided with lever handles. Forced
lubrication is supplied to the table vees. The table is boxed, and open at
the sides so that dirt and chips can be drawn out.
Gear-cutting Machines.—Broadly all these fall under one of two
groups. In the first the teeth are shaped directly or indirectly from a
pre-existing form: directly, when the cutter has the section of the tooth
space; indirectly, when a reciprocating planer arm carries a single-edged
tool, controlled in its lateral movements by the edge of a former having the
desired tooth curves to an enlarged scale. This method is suitable for all
teeth, whether with single curves (involutes), or double curves (cycloids),
and spurs or bevels. But, since the degree of accuracy obtained depends
on the accuracy of the form, it is open to error. Though this may not be
wholly eliminated, the gears so made are good enough for most commercial
manufacturers. But they do not meet the very exacting demands of the
high-speed gears used in automobiles and the best machine-tools.
In the second group the teeth are generated from the basis of the involute
rack-tooth with straight sloping sides. A cutter having the section of a
rack-tooth is used for generation, or one flank only of a rack-tooth, or several
complete rack-teeth combined in one cutter, or, a pinion-like cutter, is gener-
ated from a rack basis, or a hob—a worm, with teeth of rack section, is
fluted in milling-cutter fashion. In some machines the rack-tooth is not
embodied in the cutter at all, but in the mechanism of the machine itself by
means of " roll cones " in one design, and in another by certain controlled
movements of slotted links.
Pressure Angles.—In order that all generated involute teeth shall
mesh together, the pressure angle must be the same for all. This corre-
sponds with the diagonal path of contact of the teeth to which the sides of
the rack-teeth on the pitch points are normal. This is 14^° in the B, & S.
system, the one until recently almost universally adopted. Its disadvan-
tage is that small pinions are much undercut below the base line, to