I wanted to see if Alwynxjones's 6mm BB tank treads worked in real life, so I modeled a few things. Everything is my design except the treads themselves and the 9T and 11T drive sprockets; the latter are from haand001's sprocket generator, tweaked for my purposes.
I made these pieces to form a little tank (see video) that used a strangely-angled plastic box as the platform, so the inner walls of the tread...walls? are weirdly angled. Sorry. The axle-things (I call them "bumpers") match those angles, too. You'll need M3 or M4 (or #6 or #8) screws to hold things together, and you'll almost certainly need to drill all the holes out to the right size. I didn't make this to be a distributable model; I made it for a specific, not-very-generalizable purpose. However, I wanted to make sure the information is out there if people want it.
The drive sprockets are, again, a quirky choice: they have round holes through them ending in hexagonal shaft holes, to match the RC car I gutted to make this happen.
The 19T idler sprockets I designed to hold 8mm smooth rods (I get them from old printers and scanners). They slip into the 7x22 bearings held in place by the inner and outer treadwalls.
Video of proof-of-concept tank-thing working (with the guts of a cheap RC car as the motor): https://www.youtube.com/watch?v=6HEZ1bxChN4&feature=youtu.be
The "bumpers" (whether you choose the 2-piece or 1-piece model) have about a 3.8 degree angle where they meet the inner treadwalls, which are complementarily angled, as well. So make sure you lay things flat against your print bed (they won't do it automatically) unless you want to spend a lot of time cutting or sanding annoying supports.