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