Gear Hobbing and Shaping For Precision

Gear Manufacturing

Gear Blanks

Gear blanks can be manufactured by most processes (casting, forging, extrusion, powder metallurgy, blanking) but as a rule, machining in the form of hobbing and milling are applied to achieve the final dimensions, shape and surface finish in the gear.

Gear Hobbing

Gear Hobbing is a machining process for gear cutting, cutting splines, and cutting sprockets on a hobbing machine, which is a special type of milling machine. The teeth or splines of the gear are progressively cut into the material (a flat, cylindrical piece of metal) by a series of cuts made by a cutting tool called a hob. Compared to other gear forming processes it is relatively inexpensive but still quite accurate, thus it is used for a broad range of parts and quantities. The hob is basically a worm with gashes cut across it to produce the tooth face cutting edges. All motions in hobbing are rotary, and the hob and gear blank rotate continuously as in two gears meshing until all teeth are cut. Hobbing is one of the most fundamental processes in gear manufacturing. Its productivity and versatility make hobbing the gear manufacturing method of choice for a majority of spur and helical gears.

SPINE SHAPING

Gear Hobbing From Source International

The hobbing machines that Source International uses are fully automated and come in many sizes, able to produce anything from tiny instrument gears up to 10 foot (3.0 meter) diameter gears. Each gear hobbing machine typically consists of a chuck and tailstock, to hold the workpiece or a spindle, a spindle on which the hob is mounted, and a drive motor.

Most hobs are single-thread hobs, but double, and triple thread hobs increase production rates. The downside is that they are not as accurate as single-thread hobs. Depending on type of gear teeth to be cut, there are custom made hobs and general purpose hobs.

Types of hobs we use include:

Roller chain sprocket hobs
Worm wheel hobs
Spline hobs
Chamfer hobs
Spur and helical gear hobs
Straight side spline hobs
Involute spline hobs
Serration hobs
Semitopping gear hobs

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Gear Hobbing Machine process – Overview – CNC Vertical Gear Hobbing Machines

As the hob feeds across the face of the work piece at a fixed depth, gear teeth will gradually be generated by a series of cutting edges, each at a slightly different position. The number of cuts made to generate the gear tooth profile will correspond to the number of gashes of the hob. Simply put, more gashes produce a more accurate profile of the gear tooth.

The hobs several cutting edges will be working simultaneously, which provide significant potential for fast cutting speeds and/or short cycle times. With this realization, one can see the hobbing process’s advantage over other cutting processes.

All gear hobbing machines, whether mechanical or CNC, consist of five common elements, which are listed below and shown in Figure 2.

A work spindle to rotate the work piece (shown in blue)
A cutter spindle to rotate the cutting tool, the hob (shown in yellow)
A means to rotate the work spindle and cutter spindle with an exact ratio, depending on the number of teeth of the gear and the number of threads of the hob (shown in red)
A means to traverse the hob across the face of the work piece (shown in green)
A means to adjust the center distance between the hob and work piece for different size work pieces and hobs

The following schematic diagram shows a horizontal mechanical hobbing machine: