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Gear Hobbing

Gear hobbing is gear cutting by use of a tool resembling a worm gear in appearance, having helically spaced cutting teeth. In a single-thread hob, the rows of teeth advance exactly one pitch as the hob makes one revolution. With only one hob, it is possible to cut interchangeable gears of a given pitch of any number of teeth within the range of the gear hobbing machine.

A gear hobbing machineis 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.

A gear hobbing machine forms the gear via a generating process by rotating the gear blank and the cutter at the same time with a fixed gearing ratio between cutter and blank. The cutter 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 cutter 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.

A gear hobbing machine is invariably used to produce throated worm wheels. It is not possible to cut all useful tooth profiles by gear hobbing; 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 will not cut well. The NHS Swiss tooth standards give rise to such problems. Such small gears normally must be milled instead of using the gear hobbing process.

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