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A hobbing machine is a special form of milling machine that cuts gears. It is the major industrial
process for cutting (as opposed to grinding) spur gears of involute form.
The machine forms the gear via a generating process by rotating the gear blank and the cutter
(called a hob) at the same time with a fixed gearing ratio between hob and blank.
The hob has a profile given in cross section by the fundamental rack for the gear tooth profile and is in the form of a helix so that
the sides of the teeth on the hob generate the curve on the gear. The helix has a number
of cuts parallel to the axis to form the cutting teeth and the profile is suitably relieved to provide
cutting clearance.
For a tooth profile which is a theoretical involute, the fundamental rack is straight-sided, with sides inclined at the pressure angle of the tooth form, with flat top and bottom.
The necessary addendum correction to allow the use of small-numbered pinions can either be obtained by suitable modification of this rack to a cycloidal
form at the tips, or by hobbing at other than the theoretical pitch circle diameter. Since the gear
ratio between hob and blank is fixed, the resulting gear will have the correct pitch on the pitch circle,
but the tooth thickness will not be equal to the space width.
Hobbing is invariably used to produce throated worm wheels, but it is not possible to cut all useful
tooth profiles in this way if any portion of the hob profile is perpendicular to the
axis then it will have no cutting clearance generated by the usual backing off process,
and it will not cut well. The NHS Swiss tooth standards give rise to such problems. S
uch small gears normally must be milled instead.
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