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Cutters for gear hobbing

Gears can be cut by “hobbing”. In this a cutter in the shape of a serrated screw rotates as the workpiece rotates. The fact that gears can be made by hobbing is a property peculiar to the involute tooth form most gears use. Any other component that has involute flutes can also be hobbed. An example of this would be a spline.

Hobs can only be used to cut teeth on external surfaces.

262 Gear hob

fig 262 gear hob

The hobs of interest here are those that are made to cut involute gears. When looked at from the front of the teeth the shape of the teeth is that of a rack.

This shape is the outline of the teeth it would make on a gear of infinite radius. Though the teeth are rounded on the tops and the bottoms, the sides form straight lines. It just so happens that the angle of the sides is the pressure angle of the teeth it will cut.

A hob like this will cut a gear with any number of teeth but only for this size of tooth and this pressure angle. It will not only cut spur gears but it will also cut helical gears.

Hobs for wormwheels

Hobs with what appear to be truncated teeth will not cut conventional involute teeth but they will cut wormwheels.

A cutter like this will cut a wormwheel on any diameter with the appropriate number of teeth and where the diameter of the worm matches the diameter of the cutter.

Hobs that are not hobs

What might appear to be a gear hob but where the cutting edges do not form a screw is a hob for thread milling. It cannot be used for making gears.

Holding hobs for gear cutting.

Hobs look very much like they should fit on standard horizontal milling arbors.

fig hob on a standard 1 inch dia arbor

Some do fit standard size arbor but many do not. The reason is, that in industry, hobs are made to be used on machine specifically designed to be used for gear hobbing. On these machines the “arbor” is very short and is supported at both ends.

The solution to this is to make a short arbor to hold the hob and then hold this arbor in some sort of ER collet.


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