CNC Gear Handling: How Gear Teeth Get Cut

• 1
  What happens on the machine
• 2
  Which gear cutting method fits
• 3
  Tolerances, materials and heat treatment
• 4
  When CNC gear handling is the wrong answer
• 5
  Gear method comparison

Gear handling on a CNC machine is not one operation. It is a chain: hold the blank, cut the tooth space, control the index, then measure the result. Each link sets a limit on the next one. A gear that wobbles in the fixture will not pass a lead inspection no matter how accurate the cutter is.

The blank is usually turned first. The bore, the face and the reference diameter get machined in one setup so they stay concentric. Runout here is the root of most problems later. A 0.02 mm runout on the bore becomes a much larger error at the tooth flank once the part spins.

Cutting the teeth comes next. The tool path depends on the module, the pressure angle and the tooth count. A small module with fine teeth needs lighter passes. Coarse teeth on a large gear can take deeper cuts. The machine does not know the difference, so the programmer sets the depth per pass.

Indexing ties it together. A rotary table or a fourth axis turns the blank by one tooth pitch, then the cutter repeats. Errors stack up tooth by tooth. That is why the index mechanism matters as much as the cutter itself on high-tooth-count gears.

• 1
  One setup for the boreBore, face and reference diameter in the same operation keep concentricity.
• 2
  Runout firstFix the blank before blaming the cutter.
• 3
  Index error stacksEvery tooth repeats the same positioning error.

Hobbing is the common choice for external spur and helical gears. A hob rotates against the blank and generates the tooth form continuously. It is fast, and the same hob covers a range of tooth counts at one module and pressure angle. Setup is simple once the hob is on the arbor. The catch is that hobbing needs a clear path for the hob to run off the part, so shoulders and flanges can get in the way.

Shaping uses a reciprocating cutter that strokes along the tooth. It handles internal gears and parts with a shoulder that blocks a hob. The trade-off is speed. Shaping is slower per tooth, and the cutter is more expensive to maintain. Use it when the geometry leaves no other option, not as a default.

Milling a gear on a three-axis or five-axis machine is different. Here the tooth space is cut with an end mill or a form tool, often with the part indexed between passes. Accuracy depends on the machine and the CAM path rather than on a dedicated gear machine. This route makes sense for prototypes, low volumes and gears that also carry other features like flanges, keyways or mounting holes.

Grinding comes after heat treatment. Hardened gears need the tooth flanks finished to hold tolerance and surface finish. Grinding removes the distortion that quenching leaves behind. It adds cost and time, so design the gear so grinding is only needed on the flanks that actually run.

• 1
  HobbingExternal spur and helical, medium to high volume, clear run-out needed.
• 2
  ShapingInternal gears and shouldered parts where a hob cannot reach.
• 3
  MillingPrototypes and low volume, especially when the gear has other features.
• 4
  GrindingPost-hardening finishing for tolerance and surface finish.

A gear drawing usually carries several callouts: bore tolerance, runout, tooth thickness, lead and profile. They interact. Tightening the bore does nothing if the fixture lets the part move during cutting. On our machines we hold ±0.005 mm on critical diameters and Ra 0.8–1.6 μm on functional surfaces, but those numbers only matter when the datum scheme in the drawing matches how the part is held.

Material choice drives the process. Mild steel and 4140 cut well and are common for general power transmission. 17-4PH stainless brings corrosion resistance and higher strength, though it work-hardens and needs conservative feeds. Aluminum gears suit low-load applications and prototypes, where weight matters more than wear life. Plastics like POM and PA run quiet and need no lubrication, but they carry far less torque.

Heat treatment changes the part after machining. Carburizing, nitriding and through-hardening all move dimensions. A gear cut to final size and then hardened will not stay there. The usual fix is to leave grinding stock, harden, then grind the flanks and bore back to size. That sequence needs to be on the drawing from the start.

Surface finish on the flanks affects noise and wear. A ground flank at Ra 0.2–0.8 μm runs quieter than an as-machined one at Ra 1.6–3.2 μm. That difference is audible in a gearbox. It also changes the contact pattern under load, which is why prototype gears are often ground even when production parts will not be.

• 1
  Datum scheme firstMatch the drawing datums to how the part is actually held.
• 2
  Leave grinding stockPlan for post-hardening finishing before the first cut.
• 3
  Finish affects noiseFlank Ra drives gearbox sound and wear rate.

Not every gear should be milled. High-volume spur gears in one size belong on a hobbing machine or a gear shaper. The cycle time per part is far lower, and the tooling cost spreads across thousands of pieces. Milling the same gear one at a time on a five-axis center works for the first fifty units and becomes expensive after that.

Very fine modules are another boundary. Teeth below module 0.5 get fragile, and the cutter needs to be small enough to fit the space. Tool deflection then dominates the result. At that scale, a dedicated gear machine or even a different manufacturing route often holds tolerance better.

Internal gears with a closed housing can be impossible to reach with standard tooling. Shaping handles many of them, but some geometries need the housing split or the gear made in sections. Say so on the drawing rather than letting the shop discover it at setup.

Finally, consider what the gear does. A low-speed hand-crank gear with loose backlash is not the same as a gearbox pinion running at 8,000 rpm. The first can be milled and shipped. The second needs grinding, inspection and probably a matched set. Spend the tolerance where the function needs it.

• 1
  High volume, one sizeUse hobbing or shaping, not milling.
• 2
  Module below 0.5Tool deflection dominates; reconsider the process.
• 3
  Closed internal gearsCheck tool reach before committing to a design.