What Is Hobbing?

Hobbing is a type of metalworking process that falls under the category of machining—using a machine to remove material from a workpiece to achieve the desired shape. Hobbing is used primarily for cutting grooves that form teeth in gears and sprockets and the splines on drive shafts. The cutter used in the process is called a hob, while the machine tool itself is generally referred to as a hobbing machine, a gear hobbing machine, or simply as a hobber.

A hob somewhat resembles a metal pinecone, being a cylinder with a screw-like helical ridge running around it from top to bottom, and the ridge divided into cutting teeth by a series of straight flutes or channels that are cut into it at intervals around the diameter of the tool.

The How of Hobbing

A gear hobbing machine is a fully automated device that utilizes two spindles with an adjustable angle between them. One spindle holds the hob, while the other holds the workpiece. Both spindles rotate continuously at specific speeds at the set angle so that the hob can make a series of cuts into a blank to designated depths to produce the desired gear or other part.

Gear hobbing can be used to make a wide variety of gears and gear-like parts, including:

Bevel gears, which are shaped like cones so that they can engage at right angles.
Helical gears, which have angled teeth that help them run more smoothly than spur gears.
Ratchets, which are gear-like objects with fin-shaped teeth that can only turn in one direction when paired with a spring-loaded pawl that clicks into place after each tooth passes.
Splines, which are teeth on the end of a drive shaft that look like a gear when viewed end-on.
Sprockets, which are spoked wheels that engage with a chain (like on a bicycle).
Spur gears, which are also known as straight-cut gears and are the most common type. Their teeth run parallel to the direction of the gear’s axis. Two kinds of spur gears are involute gears and cycloidal gears, each of which have teeth with a specific type of rounded curve to their edges.
Worm wheels, which have the appearance of spur gears, but mesh with worm gears (which are cylindrical, screwlike gears).

There are three classifications of gear hobbing based on the direction that the hob feeds during the process:

Gear Hobbing with an Axial Feed. Used for cutting helical gears and spur gears, this procedure feeds the hob along the surface of the workpiece, parallel to the axis of the blank.
Gear Hobbing with a Radial Feed. Used for cutting bevel gears and worm gears, this process feeds the hob against the surface of the workpiece in a radial direction, perpendicular to the axis of the blank, moving in towards the center of the blank from the outside.
Gear Hobbing with a Tangential Feed. Used for cutting teeth on worm gears, this method feeds the hob across the surface of the workpiece at a right angle to the axis of the blank.

To visualize the three types of feeds, picture the gear blank lying flat as a disc on a surface in front of you, with the center of the gear—the axis around which the gear will turn—pointing up and down. If the hob is to the left of the gear blank, then the three feed movements are as follows: axial feeding moves the hob from top to bottom against the left side of the blank; radial feeding moves the hob directly into the side of the blank from the left; tangential feeding moves the hob towards you across the left side of the blank from the back to the front.

A History of Hobbing

The production of gears dates to at least the 4th century BC, but the methods for producing them have varied over the centuries. The earliest gears were made of wood and carved by hand with knives, saws, awls and/or files. When metal started to be used for gears it was shaped by file or by forge, but over the centuries other manufacturing techniques were developed that could be used in gear making, such as grinding, broaching, milling, and extrusion.

In our day, the most widely used gear-cutting process is hobbing. The modern hobbing machine can be traced to December 6, 1856, when Englishman Christian Schiele filed a petition at the Office of the Commissioners of Patents for his invention of “Certain Improvements in Machinery or Apparatus for Cutting Nuts, Screws or Bolts, and Toothed Wheels.”

While technology has advanced in the nearly two centuries since Schiele developed his mechanism, his proposed hobbing process for cutting “toothed wheels” (spur gears) is essentially the same that is used by modern metalworkers today.

Advantages of Hobbing in Gear Cutting

While there are certain limitations to gear hobbing—inward facing gears and certain tooth profiles cannot be hobbed—it is generally regarded as an efficient method when compared to other gear forming processes. Gear hobbing can be considered:

Accurate. Hobbing is a high-precision process for cutting gears to tight tolerances.
Cool. Hobs have multiple cutting teeth, allowing heat to dissipate from each of them between rotations, keeping the hob from overheating.
Economical. As a continuous process, gears can be generated at a much more productive rate.
Fast. Gear hobbing is faster than other methods of gear cutting. Multiple gear blanks can also be mounted on the same arbor and hobbed simultaneously.
Flexible. Hobbing is a versatile process, useful for creating other parts besides gears. Various types of hobbing machines—and interchangeable hobs—also permit for a wide variety of gears to be cut. In addition, hobbing machines are flexible in their use, allowing anything from a lone gear to a large batch to be cut in a single operation.

Now the Gears Are Turning

As laser cutting and 3D printing technologies advance, better and faster ways of producing accurate gears may well be found, but the time-honored tradition of gear hobbing will likely never completely disappear. Christian Schiele’s genius in inventing the gear hobber has never been equaled by any other type of machining process, so, as long as there is a need for gears to be put in motion, there will be a need for the esteemed gear hobbing machine.