Thanks again guys, you seem like a very friendly bunch. Hobbing is really a method of producing gears in a production environment, due to the cost of the hobs, as lateatnight quite rightly points out. Some of the aluminium gears HPC sells are cut in less than 8 seconds. The hob is essentially a worm gear with flutes in it to give gutting edges and it comes in to mesh with the gear wheel (the blank) and cuts the gear. From a CNC point of view it is quite simple, the cnc contorols the rotation ratio between the cutter and blank (formerly done by gearing), the angle of the hob (pressure angle) and where the cutter feeds and stops.
Although I have been working cnc machines for years I still find it amazing that you guys build working machines in your sheds from bits you buy on the internet, respect to you all.
Ken