This may sound like a naive approach, but isn't the turntable overkill in this case?

Did you try a simple Technic <strike>pin</strike> peg to connect both parts if the robot, or do you actually need some transmission to go from one half to the other?

If a Technic peg isn't robust enough, maybe an axle would do the trick. I'd suggest using one [Technic Axle 8 with Stop](http://www.peeron.com/inv/parts/55013) (![55013 — Technic Axle 8 with Stop][1]), on which you first slide 3 regular [Technic, Brick 1 x 2 with Hole](http://www.peeron.com/inv/parts/3700) (![3700 — Technic, Brick 1 x 2 with Hole][2]) which will be attached to one half of your model, then 5 [Technic, Brick 1 x 2 with Axle hole](http://www.peeron.com/inv/parts/32064) (![32064 — Technic, Brick 1 x 2 with Axle hole][3]) which will be attached to the other. Maybe the strain on the axle will be too much (I read it's [prone to breaking](http://bricks.stackexchange.com/q/360)), but it could be worth trying.

And if that's not solid enough, build a staple of 2x2 round plates around an axle, fix that to one half of the model, and build a hole on the other half to have it slide in (a square hole is fine and easy to build: just a two bricks wide, two bricks high gap and you cover the studs underneath with a tile. Use archs if you want for the higher half, but don't bother for the bottom one). Of course you have to finish the pseudo-axle with something bigger than 2x2 to lock it in place.

And if THAT's not enough, there are 4x4 round plates now with which you could do the same.


  [1]: http://i.stack.imgur.com/a4Pas.png
  [2]: http://i.stack.imgur.com/gtv3a.png
  [3]: http://i.stack.imgur.com/6QoN4.png