Hobbing machine

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. Such small gears normally must be milled instead.

This technique can generate involute gears, ratchets and cycloid gears.

Hobbing cycloidal forms

For cycloidal gearing (as used in BS978-2 Specification for fine pitch gears. Cycloidal Type Gears) each module, ratio and number of teeth in the pinion requires a different hobbing cutter so the technique is only suitable for large volume production.

To circumvent this problem a special war-time emergency circular arc gear standard was produced giving a series of close to cycloidal forms which could be cut with a single hob for each module for eight teeth and upwards to economize on cutter manufacturing resources. A variant on this is still included in BS978-2a (Gears for instruments and clockwork mechanisms. Cycloidal type gears. Double circular arc type gears).

Tolerances of concentricity of the hob limit the lower modules which can be cut practically by hobbing to about 0.5 module.

Hobbing machines are characterised by their capacity of module or pitch of gear hobs that can be loaded on the machine.clarifyme

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References

*Machinery's Handbook, Industrial Press

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