Converting to an Ultimaker style gantry system?

Sounds like a good project; can't speak to the bowden...but just make sure the rods are hardened and ground...

I've thought of this, but worked out it would probably be easier to implement on a new aluminium extrusion frame anyway - alongside your working solidoodle to print all the bits!

The tricky part of the Ultimaker system is that the linear rails double as the pulley driveshafts - this means you have to use bronze bushings (as the ultimaker does, albeit likely with a bit more consideration than is shown with the SD variety - you'd have to be just as careful in your selection to make sure you get high quality matching bits!) or hilariously expensive / much harder to obtain radial+linear ball bearings: http://www.euro-bearings.com/lin16.html. I love bearings, so not being able to use them put me off a bit.

In the solidoodle frame, you already have holes for mounting shaft bearings at both the front and back of the case, so that's one ultimaker axis sorted (haven't checked whether you can get 13x8mm flanged bearings; you'd want at least 8mm diameter shafts). The motors can go on the outside of the case with a bit of effort (and it's a bit ugly hehe) - I do this with my y axis already; big ol' printed adapter. You'd need to duplicate all this from scratch in the other axis just below, so be prepared to carve out some accurate holes in the sheet metal (the alignment of the rails is a bit critical). All the carriages etc. would need to be custom designed, as you're probably aware (and looking forward to!).

The other thing that put me off the ultimaker style system was mixed reviews on the print quality impact of the bowden feed - it sounds to me like it will sacrifice print quality a touch for the lightness (and thus speed).

I'd appreciate others' thoughts on all this, as it's the closest I've got to wanting to build a second machine...