A gear is made in a chain of five steps: a blank, cutting the teeth while the steel is soft, heat treatment, hard finishing of the flanks (the working faces of the teeth) and measuring. The teeth are usually cut by gear hobbing, in which a rotating, worm-shaped cutter rolls against the blank. Internal gears are cut by gear shaping, power skiving or broaching, and after hardening the flanks are normally ground or honed because the part distorts during hardening. Each step needs its own type of machine. Below: the chain step by step, the cutting processes compared, internal gears, and what it means if you own one of these machines.
What a gear is: types and key figures
The word gearing covers the teeth of a part and their shape. The gear is the part that carries them, and a gearbox is a set of gears that mesh. For two gears to mesh at a constant ratio, their flanks have to follow specific curves. In mechanical engineering that is predominantly the involute, a curve that can be made with simple straight-sided tools.
By the position of the axes there are three families:
| Gear type | Axes | Typical features | How it is usually made |
|---|---|---|---|
| Cylindrical gear (spur or helical), external teeth | parallel | the most widely used gear type; straight (spur) or helical teeth | hobbing, shaping, grinding after hardening |
| Cylindrical gear, internal teeth (ring gear) | parallel | same direction of rotation, shorter centre distance, concave flanks, mostly in planetary gearboxes | shaping, broaching, power skiving; large rings also hobbing |
| Bevel gear | intersecting, usually at 90° | straight or spiral teeth | bevel gear cutting machine with cutter head; lapped or ground after hardening |
| Worm gear | crossing, not intersecting | one to four starts, ratios above 100 in a compact size, often self-locking | milling and grinding of worm and worm wheel |
Four figures describe most gears:
- Module m, the tooth size in millimetres, m = d / z (pitch circle diameter divided by number of teeth; the pitch circle is the reference diameter of the teeth). Only gears of the same module mesh, and modules are standardised in DIN 780.
- Pressure angle, the angle at which the flanks push against each other, 20° in practice. 14.5° and 25° occur, and 17.5° is found in some vehicle gearboxes to reduce noise.
- Helix angle, the inclination of the flanks on helical gears. Helical gears run more quietly and carry more torque at the same size, because several teeth are in mesh at once. The price is axial force, which the bearings must take.
- Undercut: with too few teeth the tooth root is undercut by the generating processes. At a 20° pressure angle the limit is 17 teeth. A profile shift allows fewer teeth without undercut.
A worked example: a spur gear with 40 teeth and module 3 mm has a pitch circle diameter of 3 × 40 = 120 mm. The module also shows why gear machines are rated by size: it decides how deep the tool must cut and what cutting forces arise, and so limits what a hobbing, shaping or grinding machine can process. The range is wide: Liebherr gives workpieces from 20 mm diameter for its hobbing machines; its LC 200 to 500 series works up to 500 mm diameter and module 7 or 12, and its largest series up to 12,000 mm and module 60. That is why there is no universal gear machine, and why the nameplate data matter so much for the value of a used one.
The process chain from blank to measured gear
| Step | Process | Machine type |
|---|---|---|
| 1. Blank | sawing and turning; in series also forging, casting or sintering | saw, lathe, forging press |
| 2. Cutting the teeth (soft) | hobbing | gear hobbing machine |
| 2. Internal teeth and awkward contours | gear shaping | gear shaping machine |
| 2. Internal and external teeth | power skiving | power skiving machine (power skiving explained) |
| 2. Bevel gears | bevel gear cutting | bevel gear cutting machine (gear cutting machines) |
| 2. Deburring and chamfering | chamfering, deburring | unit on the gear machine or a separate machine |
| 3. Heat treatment | case hardening | heat treatment plant |
| 4. Hard finishing | generating or profile grinding | gear grinding machine |
| 4. Hard finishing | honing, shaving | honing or shaving machine |
| 5. Measuring | profile, helix, pitch, runout | gear measuring machine |
Step 1: the blank
The start is a blank with machining allowance, typically sawn and turned. Gears can also be cast, sintered or, for plastics, injection moulded, and in large series, gears for differentials are die forged. Cast, sintered or moulded gears reach only the coarser quality grades 8 to 12.
Step 2: cutting the teeth in the soft state
The teeth are cut while the material is still soft. In every generating process the tooth form is the envelope of the cutting edges. In form milling and broaching, the tool geometry produces it directly.
| Process | Tool | Use and strength |
|---|---|---|
| Hobbing | worm-shaped hob | external teeth, internal only in special cases; very high stock removal; one tool for any tooth count in a module series |
| Gear shaping | cutter wheel | internal and external teeth; teeth without run-out, blind bores |
| Power skiving | skiving cutter with crossed axes | internal and external teeth; continuous, no idle stroke; good for interfering contours |
| Gear planing | rack-type cutter | very large gears; low stock removal, no internal teeth |
| Form milling | disc cutter in the shape of the tooth space | possible on ordinary milling machines; separate tool for each module and tooth count |
| Broaching | broach with staggered teeth | mainly internal teeth; high stock removal, volume production only |
Hobbing. The hob is geometrically a single or multi-start worm with flutes. With a single-start hob, the gear advances by one tooth per hob revolution. The cut is continuous with no return stroke, so even thick gears are made in a short time. Very small tooth counts can lead to undercut, and interfering contours restrict the process. Hobbing was invented in 1856 by Christian Schiele. Hobbing machines are built horizontal and vertical. Modern CNC machines couple tool and workpiece axes electronically instead of mechanically, which improves quality and tool life. More on the machines is on our page about selling gear hobbing machines.
Gear shaping. A cutter wheel, which you can picture as a gear with cutting edges, strokes up and down and rolls against the blank at the same time, with a radial infeed. Shaping suits internal teeth in smaller quantities, internal teeth in blind bores, and external teeth without run-out space where a hob cannot run out. See selling gear shaping machines.
Power skiving. The tool is synchronised with the workpiece over a crossed-axis angle and fed axially. Unlike shaping it is continuous and avoids the idle stroke and lift-off. It suits soft machining of internal and external teeth, and especially teeth with interfering contours. Our page power skiving explained covers the kinematics, tools and limits.
Gear planing, form milling and broaching. Planing uses a straight-sided rack-type cutter on very large gears and has low stock removal. Form milling needs no special machine but a separate disc cutter per module and tooth count. Broaching cuts the tooth form with several staggered cutting edges in one pass and, because the tools are workpiece-specific, is used only in volume production.
Bevel gears. Bevel gears are not cut on cylindrical gear hobbing machines but on bevel gear cutting machines. In the Klingelnberg system the Palloid process uses a conical hob and the Cyclo-Palloid process a cutter head. Gleason circular-arc spiral bevel gears are made on bevel gear cutting and grinding machines. After hardening, bevel gears are lapped or ground.
Chamfering and deburring. After pre-cutting, burrs remain on the faces. They are a risk of injury, disturb clamping in later steps and increase wear and noise in the gearbox. Gears are therefore often chamfered on purpose. That also protects them in transport, prevents brittle, over-carburised tooth edges during case hardening and is a prerequisite for some hard finishing processes such as gear honing. Chamfering can run in separate machines or inside the gear machine; methods that can be integrated into the machine include the Gratomat process with milling pins, press deburring (cold forming, mainly for large series) and the ChamferCut process with chamfer cutters.
Step 3: heat treatment and hardening distortion
After cutting, gears are often hardened, in practice frequently by case hardening. That makes the flanks more wear-resistant, especially against pitting, and the gear can take higher loads. The price is hardening distortion: the tooth geometry changes. The flanks therefore normally have to be finished after hardening to reach the required quality. For lapped bevel gears the distortion must be allowed for already when cutting, because lapping corrects it only slightly. Grinding removes more material and can eliminate much larger distortions.
Step 4: hard finishing
Hard finishing restores the required gear quality after hardening.
| Process | Principle | When it makes sense |
|---|---|---|
| Generating grinding | continuous; grinding worm with rack profile, the involute is produced by rolling | high runout and pitch accuracy, short machining times, long tool life |
| Profile grinding | discontinuous; profiled wheel | profile, pitch and runout tend to be less accurate than with generating grinding |
| Gear honing | continuous, with an abrasive (geometrically undefined) cutting edge; internally toothed tool ring meshes with the workpiece | quality 1 to 6, named for high noise requirements |
| Shaving | before hardening, external gears | best price-performance ratio, large series with little part variety |
| Skive hobbing | finishing hardened flanks with a hob-like tool | quality 6 to 7 to ISO 1328, soft and hard machining on one machine |
Generating grinding rotates a grinding worm in sync with the workpiece. A trade portal for grinding technology reports pitch accuracy below 2 µm on modern machines. Profile grinding works tooth by tooth. Modern machines also produce deliberate profile and helix modifications, known as topological grinding. Without a combustion engine, gearbox noise is heard more clearly, which is why grinding and honing are gaining importance for electric vehicles. Shaving improves the quality of externally toothed gears before hardening by at least two DIN grades, but has quality limits because of hardening distortion; for higher demands after hardening, generating grinding is recommended. Ground gears are not necessarily quieter and need higher investment. Our page on selling gear grinding machines covers those machines.
Step 5: measuring
At the end of the chain comes inspection. On cylindrical gears, profile, helix, pitch, runout and tooth thickness are measured, and the deviations give total, form and angular deviation. Results are not only for release. In a closed loop, as Klingelnberg describes it for bevel gear grinding, the deviations are recorded, correction data calculated and the grinding process adjusted. Measuring reports are also the document that proves the quality a machine actually delivers, which matters on a used machine.
Internal gears: why they need a different process
An internal gear has its teeth on the inside of a hollow part. The best-known example is the ring gear of a planetary gearbox, which meshes only with the planet gears. Other cases are coupling hubs and the hub of a shaft-hub connection. Internal teeth allow compact designs, spread the load over several teeth in mesh and can save parts.
They are harder to make than external teeth, because the tool must enter the bore and has little room there. Hobbing, the standard process for external teeth, is largely ruled out: according to Wikipedia, small internal gears cannot be hobbed, only helical teeth of large diameter. Klingelnberg writes in a white paper that rings with internal teeth depend on broaching, form milling or shaping, and that power skiving opens up new options at exactly this point.
For splined hubs, Wikipedia lists three profile forms and their standards: involute (DIN 5480, ISO 4156), straight-sided (DIN 5464, ISO 14) and serrations (DIN 5481).
| Process | Typical use | Advantage | Limit | Machine type |
|---|---|---|---|---|
| Gear shaping | ring gears, coupling hubs, blind bores, single parts and small series | straight and helical internal teeth, also in a blind bore | needs a relief groove of 5 to 10 mm; thin, long teeth are difficult; return stroke without cut | gear shaping machine |
| Broaching | larger series with fixed geometry, spline and keyway profiles | usually a single stroke, tolerance IT7 to IT9, calibrating broaching also on hardened parts | profile-specific, expensive tool with long lead time | internal broaching machine |
| Power skiving | planetary ring gears, teeth against a shoulder, from one piece to medium batches | shorter times than shaping, no relief groove, turning and cutting in one clamping | needs stable clamping and high speeds; longer times at small axis-crossing angles | skiving machine or turn-mill centre with skiving cycle |
| Wire EDM | very hard workpieces, single parts, special profiles | works on very hard workpieces | limited working area | wire EDM machine |
| Hard finishing (profile grinding, hard skiving) | hardened ring gears with high quality | compensates hardening distortion when hard skiving | needs an internal grinding arm or a skiving machine | gear grinding machine with internal arm, skiving machine |
| Cold extrusion | hollow shafts with internal and external teeth | teeth in ready-to-fit (net shape) quality, according to research | usually needs machining afterwards; pressed teeth fall in DIN 3961 grades 8 to 12 | press |
When to use which. One piece, blind bore or changing profiles: gear shaping. Large series with fixed geometry, spline and keyway profiles: broaching. Teeth next to a shoulder, turning and cutting in one clamping, one piece up to medium batches: power skiving. Hardened workpieces with high quality: EDM, profile grinding, hard skiving or calibrating broaching.
A few figures to put it in proportion. Typical gear shaping machines have stroke lengths of 80 to 200 mm, less often up to 400 mm. Shaping reaches quality 5 to 9 on the DIN 3961 scale. One German contract gear cutter gives quality 7 for shaping. For broaching, a contract shop sees series up to roughly 2,000 pieces and profiles up to about 60 mm diameter as still competitive; another contract gear cutter rates broaching small internal teeth with profile broaches as much faster than shaping.
Replacement gears. If a ring gear or hub is worn and there is no drawing, one contract gear cutter states that the profile can be reconstructed from the existing part and remade.
Gear quality: what each process reaches
Gear accuracy is given in quality grades. The classic DIN 3961 has twelve grades, with 1 the finest and 12 the coarsest. The standard has been withdrawn and replaced by DIN ISO 1328-1:2018-03, so the table is an order of magnitude.
| Process | Quality (DIN 3961) |
|---|---|
| Honing | 1 to 6 |
| Grinding | 1 to 7 |
| Shaving, cold rolling | 5 to 7 |
| Hobbing, gear planing, gear shaping | 5 to 9 |
| Form milling, form shaping, broaching | 7 to 12 |
| Stamping, pressing, sintering, injection moulding | 8 to 12 |
One gear maker gives qualities down to DIN grade 2 for its chain of cutting, hardening and grinding. What a particular machine really delivers is shown by an inspection report.
Which process for which quantity
- Single parts and prototypes: simultaneous 5-axis milling with unprofiled tools is a newer method for single parts and small quantities. Form milling on an ordinary milling machine is also possible. Reproduction from a sample, for example spare parts for older equipment without drawings, is common among contract manufacturers.
- Small and medium series: hobbing covers many tooth counts of one module series with one tool; gear shaping is used for internal teeth and smaller quantities. One contract manufacturer gives batch sizes of 1 to 1,000 pieces.
- Large series: hobbing for external teeth, broaching for internal ones; in automotive manufacturing, shaving before hardening is widespread for finishing.
Other routes to a gear
Besides cutting there are alternatives. Precision forging with calibration makes straight bevel gear sets in large numbers by warm or semi-warm forming, and in one process chain studied, the grain flow stays unbroken. Other routes are the classic powder-metallurgy chain and binder jetting as an additive process.
Splines, serrations and Hirth couplings
Not every toothed part is a gear. Gear makers also supply sprockets, timing pulleys, worms and worm wheels, splined shafts and hubs, and racks. Shaft connections such as serrations are made on the same machine types as gears: the shaft is hobbed, the hub broached or shaped. Face couplings such as the Hirth coupling are made by milling and profile grinding on the end faces.
If you own one of these machines
We buy your gear cutting machine. Sale through our buyer network: commission stated upfront; you are paid before pickup. We assess the machine from your data: nameplate, year, module and diameter range, control and condition, plus photos and inspection reports if you have them.
The offer usually follows 24–48 hours on working days after we have all the details. On a direct purchase, payment is made by bank transfer before pickup, and we organise dismantling and transport; whether those costs are included is stated in the offer upfront. The whole process usually takes 7 to 15 working days on a direct purchase.
Start with the type that matches your machine: hobbing, shaping, grinding or broaching machines. The overview is on selling gear cutting machines, and what your gear machine is worth explains the value factors.