This is a gear design to add onto a large bearing.

Quick and dirty writeup of something we made at the Robots and Dinosaurs hackerspace to show off the capabilities of the lasercutter and also 3D printer

The gear is designed to be a module 2 equivalent:
They are module 2 gears.

The internal spur is 80 teeth and the pinion gear is 12.
http://en.wikipedia.org/wiki/Gear#The_module_system

Parts List:

12k6809 $22ea $33.45 shipping

Lee Valey Engineering
http://www.leevalley.com/en/wood/page.aspx?cat=1,250,43298,43316&p=44014

yg2738 gearhead motor from Jaycar. We used the 160rpm version but there are a few others.
http://jaycar.com.au/productView.asp?ID=YG2738&keywords=gearhead&form=KEYWORD