Figure Wheels and Sector wheels

The figure wheels are made from 40-teeth gear. Each figure wheel can store one digit (0 to 9). 

The nerigboring figure wheels are connected by a sector wheel, which is a section of a 40-teeth gear.

The combination of figure wheels and sector wheel can perform additions.

I chose the size of figure wheel to be 60mm, so that it is not too small and hard to print, and not too large to fit in my 3D printer.