Ballbot skeleton

It's been quite some time since I last wrote something here, before the last post that is 🙂 so it's about time I do so!

It all started when I saw a post on Instructables about a Remote controlled basket ball robot. I immediately started to plan how I could make my own - who doesn't want a remote controlled ballbot!

Since I've got a lot of R/C parts laying around I figured it wouldn't be too expensive to gather the rest of the electronics needed, a DC motor and an ESC.
In the instructable the author used a 7" (17.8cm) hamster ball as the housing for his ballbot. 7" is quite small if you want to fit sensors and other fun stuff into the ball and really make it into a robot, not just a radio controlled ball.
I began to search the net for a bigger one and soon realized it wasn't an easy mach to get a ready-made ball in a suitable size, around 30cm (11.8")!

Finally I came up with the idea to make a CNC-milled ball skeleton, so I started to sketch on post-its how to make my own ball. Wood is a cheap material to prototype with and it's not too heavy if properly chosen.
The way I first drew my sketches, the ball would consist of 16 bows mounted in two center-hubs, one on each side. There would also be three circles, made out of four pieces/circle, in the rolling direction to smoothen the ball and to stabilize the bows.

Ballbot sketches

ballframe - main bow

ballframe - center-hub

ballframe - stabilizer

After some calculations I realized that the gaps between the bows would get too big for the ball to roll smoothly, so I added 16 more bows to the drawing.

When I finished my sketches I tried to figure out how I would be able to manufacture the parts as cost effective as possible. Searching the net, I stumbled upon GHS - Gothenburg Hacker Space. On their IRC channel I found a guy, "Dubbear", who was willing to help me with the CNC-milling!

Next step was the most difficult one (so far): I had to learn how to use a CAD program! The natural choice is of course FreeCAD for an open source hardware guy like me 🙂
Said and done, I started to learn, and I'm still learning!

Anyway, after endless hours of CADing (or should I say trying to CAD) I had finished the main bow and the center hub, so it was time to talk with the guy from GHS again. "Dubbear" asked if we could 3D print the parts instead of CNC-milling and I thought that was a great idea! The only problem was that the 3D-printer had much smaller working area than the CNC-mill, 200x200mm instead of 200x300mm, which meant that I would have to cut up my main bow into two pieces and my center hub into four pieces - lots of more work!
The good thing was that I didn't have to start from the beginning, I just had to find out how to cut them into pieces and how I could fit them together again, learning by doing 😉

Yet again, many, many hours later I've finally finished the part designs and have also been able to figure out how to assemble them into a ball.
Below you can see the parts and how they look like assembled: