CNC Gear Cutting Method: How to Choose and Run One

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  Key takeaways
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  What the phrase CNC gear cutting method actually covers
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  How each cutting process removes metal
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  Material and module limits you should respect
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  Step-by-step: choosing and running a gear cutting job
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  Comparing CNC gear cutting methods

People use this phrase for two different things. The first is the tooth-generating process: milling, hobbing, shaping, broaching, grinding or wire EDM. The second is the machine platform and the setup that carries the blank. Both matter, because a good process on a loose fixture still produces a bad gear.

A gear is defined by four numbers before any cutting starts: module or diametral pitch, number of teeth, pressure angle and helix angle. Module is the pitch diameter divided by the tooth count, in millimeters. Get one of those wrong in the CAD file and no machine can save the part.

The blank also has constraints. Face width, bore diameter, hub height and the distance from the tooth to the nearest shoulder decide whether a standard hob or cutter can reach the root without colliding. A 20 mm face width on a Ø30 mm blank is easy. The same tooth on a 200 mm face between two shoulders usually is not.

Finally, tolerance drives money. A 3D-printed visual prototype can hold ±0.3 mm and nobody cares. A gearbox input pinion at ±0.005 mm and Ra 0.8 μm needs a different machine, a different cutter and a different inspection plan. Decide the grade before you decide the method.

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  Module (m)Pitch diameter ÷ tooth count, in mm. Sets cutter size and depth of cut.
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  Pressure angle20° covers most industrial gears; 14.5° still appears in older drawings.
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  Helix angle0° for spur, 15–30° typical for quiet-running helical gears.
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  Quality gradeDIN 6–7 is normal commercial; DIN 5 or better needs grinding.

Milling uses a rotating end mill or a form cutter that follows the tooth gap. On a 4-axis or 5-axis machine the tool tilts to match the helix, so a single setup can cut both flanks and the root. It is slow per tooth, but it needs no dedicated tooling and handles internal gears and non-standard profiles that a hob cannot reach.

Hobbing is a generating process. The hob and the blank rotate in a timed relationship, and the hob feeds along the face width. One hob with the right module and pressure angle cuts any tooth count of that module. That is why hobbing dominates automotive and aerospace volumes.

Shaping works like hobbing but with a reciprocating cutter instead of a rotating one. It reaches shoulders and internal gears that a hob physically cannot enter. The trade-off is speed: shaping is roughly three to five times slower than hobbing on the same tooth count.

Grinding removes a small amount of hardened material with a profiled wheel. It corrects heat-treatment distortion and pushes the gear to DIN 5 or better. Wire EDM is the outlier: it cuts the whole profile through a hardened plate, which suits low-volume internal gears and prototypes in tool steel.

Aluminium 6061, 7075 and 2024 cut cleanly at any module. The risk is built-up edge on soft aluminium, so use sharp cutters and air blast rather than flood coolant where possible. Aluminium gears are common in robotics and instruments, rarely in power transmission.

Stainless 303 and 304 cut well but work-harden. Keep the feed per tooth above 0.05 mm so the cutter does not rub. For 17-4PH, machine in the solution-treated condition and allow for shrinkage after aging.

Steel 1045 and 4140 are the workhorses for gears. Below 300 HB they hob and mill fine. Above that, plan on grinding. Case-hardening grades such as 20MnCr5 are not in our stock list, so tell us the equivalent you want and we will confirm.

Small module is a separate problem. Below module 0.5 the hob or cutter flute gets thin, chip clearance drops, and the risk of tool breakage climbs fast. Keep depth of cut under 0.15 mm per pass and use a rigid setup. If the gear is also internal and small, wire EDM often becomes the practical choice.