I’m sure there is a way of designing gears directly in Fusion 360, my current 3D software for choice, but instead I’ve been using the same software I use to design my laser-cut gears. The software is called Gear Generator and is available as a PC only download for a very modest fee here.

In this example I’m generating a 41 tooth gear. I seem to have settled on 6mm teeth for 3D printed models. Gear generator has plenty of variable for you to experiment with. Once I’m happy I export the file as a pdf. 

Time to transfer to Illustrator. I import the pdf straight into the workspace. I’ve actually deleted the spokes from the original file and added my own with a slightly larger hub. Once I’m happy with the design I export the file as an svg file but before I do that I scale the image by 133.33% This overcomes a quirk in Fusion 360 which seems to shrink everything.

In Fusion 360 I extrude the various sections of the gear to make a 3D model. The spokes are 3.5mm thick, the gear and hub are twice as thick at 7mm. From Fusion 360 I save the file as an stl (stereo lighography)

The stl file goes straight into Simplify3D where the tool path for the 3D printer is generated. The final file is a gcode file which is sent to the the 3D printer.

And here is the final result at the file is printed out with PLA plastic on an Ultimaker 2+