CNC Machining for Custom Gears
While traditional gear cutting (hobbing, shaping) remains the most efficient method for high-volume production, CNC machining has become a viable option for prototype gears, small batches, and custom designs. Modern 3-axis, 4-axis, and 5-axis CNC machines can produce accurate gear profiles directly from CAD files.
CNC Processes for Gears
- CNC milling (3-axis): Using a ball-end or tapered end mill, the gear profile is cut by following the tooth space contour. The gear blank is indexed to each tooth position. Suitable for larger module gears (module 3+) and spur gears. Accuracy depends on the end mill diameter relative to the tooth space
- CNC milling (4/5-axis): Continuous multi-axis machining can cut helical, bevel, and complex gear profiles without indexing. Higher accuracy than 3-axis but requires more sophisticated programming
- Wire EDM: Uses a thin electrically charged wire to cut through hardened steel with extremely high accuracy. Produces excellent surface finish and can cut any 2D profile regardless of material hardness. Ideal for spur gear prototypes and thin gears. Limited to straight (non-helical) profiles
- CNC turning + live tooling: For internal gears and splines, CNC lathes with live milling spindles can cut simple tooth profiles in a single setup
Design Constraints
When designing gears for CNC machining, consider these constraints:
- Minimum fillet radius: Limited by the tool diameter. For end milling, the root fillet cannot be smaller than the tool radius. Use the largest fillet that the meshing gear can accommodate
- Surface finish: CNC milled surfaces typically achieve Ra 1.6-3.2 μm — adequate for most applications but inferior to hobbed or ground surfaces
- Tooth accuracy: CNC milling typically achieves AGMA Q6-Q8 accuracy. For higher accuracy, post-machine grinding or lapping is needed
- Internal corners: End mills cannot create sharp internal corners — always include a radius at least equal to the tool radius
File Preparation
To prepare gear files for CNC machining:
- Generate accurate involute profiles using GearForge's generators
- Export as DXF or SVG
- Import into CAD software (Fusion 360, SolidWorks, FreeCAD)
- Extrude to the desired face width, add bore holes, keyways, and chamfers
- Export as STEP or IGES for the CNC programmer
- Specify material, tolerances, and surface finish requirements on the drawing
Material Selection
- Aluminum 6061: Easy to machine, lightweight, good for prototype and light-duty gears
- Steel 1045: Medium carbon steel, good balance of strength and machinability for functional gears
- Steel 4140: Alloy steel for higher strength applications, can be heat treated after machining
- Brass: Excellent machinability, good for worm wheels and decorative gears
- Delrin (POM): Easy to machine plastic, self-lubricating, good for light-duty gears
Cost Optimization
CNC gear machining is most cost-effective for batches of 1-50 pieces. Above that, traditional hobbing becomes more economical. To minimize cost: use standard module values, keep the number of teeth reasonable (more teeth = more machining time), and avoid unnecessarily tight tolerances.