Gear Hobber

A gear hobber 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 gear hobber forms the gear via a generating process by rotating the gear blank and the cutter, the gear hob, at the same time with a fixed gearing ratio between the gear hob and blank. The hob of the gear hobber 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 gear 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.

When making a gear it is important that the fit between the gears is proper and that the teeth are of good quality. If this is not done then it will result in inefficient energy transfer and will ultimately wear and break down much quicker. A popular way to build gears is by form cutting. This is done by taking a blank gear and rotating a hob of a gear hobber machine, with the desired tooth pattern, around its periphery. This ensures that the gear will fit when the operation is finished.

There are a few different types of cutters used when creating gears. One is a rack shaper. These are straight and move in a direction tangent to the gear, while the gear is fixed. They have six to twelve teeth and eventually have to be moved back to the starting point to begin another cut.

Another is a pinion-shaped cutter that is used in a gear shaper machine. It is basically when a cutter that looks similar to a gear cuts a gear blank. The cutter and the blank must have a rotating axis parallel to each other. This process works well for low and high production runs.

Lastly, there is a cutter called a hob used in a gear hobber. This is like a worm that turns and cuts the gear. The angle must be set up at minus 90 between the gear hob and the gear blank, but then the lead angle of the hob threads must be accounted for. The hob of the gear hobber must make one revolution to create each tooth of the gear. Used very often for all sizes of production runs, but the gear hobber works best for medium to high.