Gear hobbing is a machining process that cuts gear teeth with a hob, a cylindrical cutter with helical teeth, while the hob and the gear blank turn together at a fixed ratio. The machine that does it is called a hobbing machine, or hobber. Wikipedia describes hobbing as a process for cutting gears, splines and sprockets, and says it is especially common for spur and helical gears.
This guide explains how a hobbing machine works, what a hob is, how hobbing compares with other ways to cut gears, where it came from and who builds hobbing machines. It also explains the module and diameter ratings that come with a machine, and ends with what to note about a hobbing machine you own.
Machine Tool Exchange buys used gear hobbing machines directly from the shops that own them, so the guide closes with the details an owner needs on hand.
What Is Gear Hobbing?
Hobbing is done on a specialized milling machine, Wikipedia says. Its article on gears goes further and calls hobbing the most common form of gear cutting, though gear shaping, milling and broaching may be used instead. For the other machine families, see our guide to types of CNC machines.
According to Wikipedia, hobbing is used to make spur, helical, involute, cycloid and worm gears, and also splines, sprockets and ratchets. Cotta, a gearbox maker, adds a limit: standard hobbing cannot cut internal gears.
How Does a Hobbing Machine Work?
Wikipedia describes the sequence, and trade sources fill in the feeds:
- The blank and the hob sit on two skew spindles. One spindle carries the blank workpiece and the other carries the hob. The angle between them depends on the gear. For a spur gear the hob spindle is held at the lead angle of the hob, and for a helical gear at the lead angle plus the helix angle of the gear.
- The two spindles turn at a fixed ratio. The number of teeth sets the ratio. With a single-thread hob and a 40-tooth gear, the hob turns 40 times for each turn of the blank. A hob with several threads multiplies the ratio by its thread count.
- The hob is fed into the blank. It is fed into the workpiece until the correct tooth depth is reached, then fed through the workpiece parallel to the blank's axis of rotation. Cotta names three feed directions, axial, radial and tangential, and says the tangential feed is mainly used for worm gears.
- The control keeps the spindles in step. Cotta says CNC hobbing machines calculate the gear ratios automatically and keep exact synchronization during cutting.
Early machines did that job with gears. Grace's Guide says the 1856 hobbing machine was indexed by a series of change wheels that determined the tooth pitch.
What Is a Hob?
A hob is a cylindrical cutter with helical cutting teeth. Wikipedia says grooves run the length of the hob to aid cutting and chip removal, and that each hob tooth is relieved on its back side to reduce friction. The cross-section of the hob's teeth is almost the same shape as the teeth of a rack gear that would mesh with the finished gear.
Most hobs are single-thread. Wikipedia says double- and triple-thread hobs are used by shops with high production volume, and that multiple-thread hobs are more efficient but less accurate than single-thread hobs. Hobs are also made for particular jobs: Wikipedia lists spur and helical gear hobs, worm wheel hobs, spline hobs, roller chain sprocket hobs and chamfer hobs, and says custom hobs suit gears with modified tooth profiles.
What Are the Parts of a Gear Hobbing Machine?
Wikipedia says a hobbing machine typically consists of a chuck and tailstock to hold the workpiece, a spindle to mount the hob and a drive motor. Most hobbing machines are vertical, meaning the blank is mounted vertically, and horizontal machines are usually used for longer workpieces, such as cutting splines on the end of a shaft. Hobbers come in many sizes: tiny instrument gears are cut on small table-top machines and large-diameter marine gears on large industrial machines.
Builders' pages show what current machines add. Liebherr's LC 180 / 280 page says dry and wet machining are both possible, and its LC 180 / 280 DC version adds a chamfering device that works in parallel with gear hobbing. Gleason describes its Genesis 130H and 210H as vertical hobbing machines with optional chamfer rolling, and its 250HL as a hobbing and profile milling machine for long shafts, worms and rotors.
What Can Hobbing Cut, and What Can It Not?
| Process | What it is used for | Source |
|---|---|---|
| Hobbing | External spur, helical and worm gears, plus splines and sprockets | Wikipedia, Cotta |
| Gear shaping | Internal gears, or work where a shoulder keeps a hob from reaching the teeth | Cotta |
| Power skiving | Internal gears, cut with a rotary cutter in a process analogous to hobbing external gears | Wikipedia, Gleason |
| Gear milling | Prototypes and small batches | Cotta |
Cotta says shaping becomes necessary for internal gears or when shoulder restrictions prevent hob access, because the reciprocating shaper cutter reaches areas a rotating hob cannot. Builders treat these as separate machine families. Gleason's gear-cutting menu lists gear shaping, power skiving, profile grinding, threaded wheel grinding and gear honing machines beside hobbing, and Liebherr lists machines for hobbing, shaping, skiving, grinding and chamfering gears.
Where Did Gear Hobbing Come From?
Grace's Guide says Christian Schiele patented a hobbing machine for cutting gears in 1856. The teeth were machined by a profiled cutter in a worm and wheel arrangement, and the gear was indexed by a series of change wheels that determined the tooth pitch. Wikipedia also dates the hobbing machine's patent to 1856.
Several of today's builders trace to later companies. Wikipedia says Gleason was founded in Rochester, New York, in 1865, and lists Gleason-Pfauter Maschinenfabrik GmbH, originally Pfauter of Ludwigsburg, Germany, among its companies, acquired in 1997. So Gleason and Pfauter names appear together in this family of machines, and we keep a page for each: Gleason and Pfauter-Maag.
Who Builds Gear Hobbing Machines?
Several builders list hobbing machines. This table shows what each builder's own pages or the trade press name. It is not a ranking or a full list.
| Builder | What its pages name |
|---|---|
| Gleason | Hobbing machines in three size bands on its site: up to 300 mm, up to 800 mm and up to 10,000 mm. The lines named include the 100H Series, 250HL, Genesis, P-Series and Titan Series. |
| Liebherr | The LC 180 / 280 platform, which Liebherr calls modular. Its default hob head is listed at a maximum module of 6 mm and a maximum hob diameter of 150 mm, and an optional directly driven head at a maximum module of 4 mm and a 90 mm hob. |
| Samputensili | Horizontal hobbing and gear shaping machines from Samputensili CLC in northern Italy, which Star SU announced for the North American market in 2019. Star SU said in 2019 that the agreement helped complete its line of hobbing machines for shafts and long workpieces. EMAG acquired Samputensili Machine Tools and Samputensili CLC in February 2021, and they became EMAG SU. |
Model lists change, so check the builder's current pages for any machine you are describing. Our builder pages cover Liebherr and Samputensili machines as well, and our page on used gear machines lists what we buy.
What Do the Module and Diameter Ratings Mean?
Hobbing machines are rated by size. Wikipedia says they are characterized by the largest module or pitch diameter they can generate, and gives the example that a machine with 10-inch capacity can generate gears with a 10-inch pitch diameter and usually a maximum 10-inch face width.
Module is the size of the tooth. Wikipedia defines it as the pitch diameter in millimeters divided by the number of teeth, so a gear with a 40 mm pitch diameter and 20 teeth has a module of 2. Diametral pitch, used with inch measurements, runs the other way: it is teeth per inch of diameter. So a hobbing machine is usually described by the largest workpiece it takes and the range of module or diametral pitch it was built for. Read those figures off the data plate or the model's literature, because capacity differs from one model to the next.
What to Note on a Hobbing Machine You Own
To describe a hobbing machine accurately, collect these details from the machine:
- Model and serial number, from the data plate. A photo of the plate is more reliable than memory.
- Build year, or your best estimate.
- Control name and generation, and who did any retrofit.
- Capacity: the largest workpiece diameter, and the module or diametral pitch range the machine was built for.
- Layout: vertical or horizontal, and the hob head and hob diameter limits as listed for the model.
- Tooling that goes with it: cutters and cutter heads, arbors, fixtures, change gears and index plates.
- Gear-checking equipment that belongs with the department.
- Condition: whether it still holds tolerance and when it last ran a part.
- Photos of the plate, the control and the work zone, plus the machine from far enough back to show its footprint.
These are the same things we look at when we buy a used hobbing machine. Machine Tool Exchange buys gear hobbing machines, gear shapers, gear grinders and gear shaving machines, CNC and conventional, outright from the companies that own them and arranges the pickup. Request a quote or call 517-514-5190.
Frequently Asked Questions
What is a gear hobber?
A gear hobber is a hobbing machine: a specialized milling machine that cuts gear teeth, splines or sprockets with a hob while the hob and the blank turn together at a fixed ratio.
Can a hobbing machine cut internal gears?
Not with standard hobbing, according to Cotta. Internal gears are cut by shaping, or by skiving with a rotary cutter, which Wikipedia describes as a process analogous to hobbing external gears.
What is the difference between gear hobbing and gear shaping?
In hobbing, a rotating hob and the gear blank turn together at a fixed ratio. Cotta says shaping becomes necessary for internal gears or when shoulder restrictions prevent hob access, because its reciprocating cutter reaches areas that a rotating hob cannot.
What does module mean on a hobbing machine?
Module is the metric measure of tooth size: the pitch diameter in millimeters divided by the number of teeth. A hobbing machine is rated by the largest module or pitch diameter it can generate, so those two figures describe its capacity.
Does Machine Tool Exchange buy gear hobbing machines?
Yes. Machine Tool Exchange buys gear hobbing machines, gear shapers, gear grinders and gear shaving machines, CNC and conventional, directly from the shops that own them. We buy outright and arrange the pickup. Call 517-514-5190 or use the quote form on our sell page.
Have a gear hobbing machine you want to sell? Machine Tool Exchange is a direct buyer, so the offer comes from the buyer. We buy outright and arrange the pickup. Get a quote or call 517-514-5190.