How it works

Proper hobbing generates the involute gear shape essentially by putting the proto gear through every possible permutation of positions against a rack shaped cutter and cutting away all the bits that interfere. The result is a perfect involute.

For a lot of practical applications, we can use approximations to the involute and not notice much difference. Single point cutters may use circular arcs for example.

The hob is a bit tricky to make well although many have done so. It is like a large tap with a lot of gashes. You need to cut a circular pitch thread. More significantly, the rotation of the gear must be synchronized with the cutter.

This method simplifies things a lot. You still only need one cutter for all sizes of gear of a given pitch and pressure angle and the approximation to the involute is made by a series of straight lines.

Essentially, a rack shape for the gear in question is cut, in circular for on a rotary cutter. The cutter is then gashed to make teeth and the teeth used to make the gaps in the gear blank.

You can see here the result of a single cut on the wheel blank.

Cross Section through a Rack Gear

Straight sided gaps are cut. The next cut is rotated by one tooth. It cuts another full sized gap in a place that has already been partially formed by the previous cut as well as putting some extra shape to the previous gap.
Result of two passes of the rack cutter

Repeat this once for every gap and you are done.
Several passes of the rack cutter

For small gears, say 12 teeth, the approximation is not so good but then it often isn’t and the tooth shape on commercial gear that size will often be messed with to get good performance in the intended application.

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