I wasn't expecting to be getting into PrecisionPefect (aka low clearances between parts) modelling so early in the game.

The M3 mounting bolts were cutting into the space of the 605 bearing, so I've changed all rails from 9C to 9H and placed the bearing bolts where the top 2 carriage threads are. The 20T teeth diameter was too large, so I've gone to a 16T. The outer diameter of this is 14mm, so I'm probably going to use some kind of tape to seal the difference between it and the inner diameter of the tube. 

The reason why I've gone with the belt going under the pulley is so that the belt can be as high as possible, but more importantly, so that if the R linear axis is the only one that's moving, the R rotational axis rotates in the direction it is intended to roll in. If the belt went over the top, the rotational axis motor would have to work against the rotation brought in by the linear axis motor.

I haven't yet modelled in the 5mm dowel, R axis belt/belt holder or the 2ish mm cleaning string.