I created this for the Z-Axis of my robot arm. I am using a large torque servo to drive a heavy arm. Having an inner gear means there are more contact within gears hence less likely to skip or break off.
Models in the pictures are trial prints at low resolution (0.5mm). I suggest higher resolutions to accomplish more precise movements.
The smaller gear has no bore holes. Simply drill to your required size or epoxy it to your servo horn.