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Hobbing Manufacturing Review
Hobbing manufacturing is a special form of tooling and manufacturing that cuts tooth type geometries. It is the major industrial process for cutting (as opposed to grinding) spur gears of involute form. It forms the gear by rotating the gear blank on a spindle and the cutter on a different spindle at the same time. The cutter is similar to a worm gear, except that it has gashes to form a cutting edge. The hob will make a single revolution for each tooth if it has a single pitch.
The hob cutting tool teeth are helical, and when cutting a spur gear form, the axis of the hob tool is inclined from the perpendicular orientation by the helix angle of the hob cutting tool.
Hobbing is employed to produce spur gears, involute gears , ACME threads, worm gears, and cycloid gears , however hobbing cannot produce bevel or internal gears.
Modern hobbing machines, also known as hobbers, are fully automated machines that come in many sizes, because they need to be able to produce anything from tiny instrument gears up to 10 ft (3.0 m) diameter marine 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.
For a tooth profile which is a theoretical involute, the fundamental rack is straight-sided, with sides inclined at the pressure angle of the tooth form, with flat top and bottom. The necessary addendum correction to allow the use of small-numbered pinions can either be obtained by suitable modification of this rack to a cycloidal form at the tips, or by hobbing at other than the theoretical pitch circle diameter. Since the gear ratio between hob and blank is fixed, the resulting gear will have the correct pitch on the pitch circle, but the tooth thickness will not be equal to the space width.
Hobbing machines are characterised by the largest module or pitch diameter it can generate. For example, a 10 in (250 mm) capacity machine can generate gears with a 10 in pitch diameter and usually a maximum of a 10 in face width. Most hobbing machines are vertical hobbers, which means the blank is mounted vertically. Horizontal hobbing machines are usually used for cutting longer workpieces; i.e. cutting splines on the end of a shaft.
The hob is a cutting tool used to cut the teeth into the workpiece. It is cylindrical in shape with helical cutting teeth. These teeth have grooves that run the length of the hob, which aid in cutting and chip removal. There are also special hobs designed for special gears such as the spline and sprocket gears.
The cross-sectional shape of the hob teeth are almost the same shape as teeth of a rack gear that would be used with the finished product. There are slight changes to the shape for generating purposes, such as extending the hob's tooth length to create a clearance in the gear's roots. Each hob tooth is relieved on the back side to reduce friction.
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. Custom made hobs are different from other hobs as they are suited to make gears with modified tooth profile. The tooth profile is modified to add strength and reduce size and gear noise.
This list outlines types of hobs:
- 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
Hobbing is used to make following types of finished goods:
- Cycloid gears (see below)
- Helical gears
- Involute gears
- Spur gears
- Worm gears
Hobbing is used to produce most throated worm wheels , but certain tooth profiles cannot be hobbed. If any portion of the hob profile is perpendicular to the axis then it will have no cutting clearance generated by the usual backing off process, and it will not cut well.