We've had a number of customers over the past year building their very own BB-8 astromech droid. One of the more popular designs for the internal drive mechanism comes from Edsjunk.net. We have had a number of other people inquiring about the design, but who haven't had the complete design ready for our online quoting and ordering system. So with Ed's permission, we've added all of the drive parts as a single design to our Astromech Droid Parts page.
You can see in the video below how the whole thing comes together.
Check out all of the Astromech part designs on the Astromech example parts page. And if you're building your own droid with parts from Big Blue Saw, be sure to let us know!