Section 13. qxd

Gear Grinding
Machines for the grinding of spur and helical gears
utilize either a forming or a generating process. For form grinding, a
disk-type grinding wheel is dressed to the proper shape by a diamond
held on a special dressing attachment; for each number of teeth a spe-
cial index plate, with V-type notches on its periphery, is required. When
grinding helical gears, means for producing a helical motion of the
blank must be provided.
For grinding-generating, the grinding wheel may be a disk-type, double-
conical wheel with an axial section equivalent to the basic rack of the
gear system. A master gear, similar to the gear being ground, is attached
to the workpiece arbor and meshes with a master rack; the generating
roll is created by rolling the master gear in the stationary rack.
Spiral bevel and hypoid gears can be ground on the machines on
which they are generated. The grinding wheel has the shape of a flaring
cup with a double-conical rim having a cross section equivalent to the
surface that is the envelope of the rotary cutter blades.
Gear Rolling
The cold-rolling process is used for the finishing of
spur and helical gears for automatic transmissions and power tools; in
some cases it has replaced gear shaving. It differs from cutting in that
the metal is not removed in the form of chips but is displaced under
heavy pressure. (See Sec. 13.2.)
There are two main types of cold-rolling machines, namely, those
employing dies in the form of racks or gears that operate in a parallel-
axis relationship with the blank and those employing worm-type dies
that operate on axes at approximately 90
to the workpiece axis. The
dies, under pressure, create the tooth profiles by the plastic deformation
of the blank.
When racks are used, the process resembles thread rolling; with gear-
type dies the blank can turn freely on a shaft between two dies, one
mounted on a fixed head and the other on a movable head. The dies
have the same number of teeth and are connected by gears to run in the
same direction at the same speed. In operation, the movable die presses
the blank into contact with the fixed die, and a conjugate profile is gen-
erated on the blank. On some of these machines the blank can be fed
axially, and gears can be rolled in bar form to any convenient length.
On machines employing worm-type dies, the two dies are diametri-
cally opposed on the blank and rotate in opposite directions. The speeds
of the blank and the dies are synchronized by change gears, like the
blank and the hob on a hobbing machine; the blank is fed axially
between the dies.