UPDATE 13 April 2014
I have released details of Cherry Pi II with many improvements incorporated:-
This is my own variant of a variety of 3D printer designs available. I have used ideas from Kossel, Delta Pi, 3DR and, of course, Rostock. Many thanks to the designers of those machines for their brilliant ideas that I have shamelessly 'Cherry Picked' hence the name of my design!
The rods on my printer are 277mm from steel ball centre to centre. The towers are 800mm Bosch/Rexroth aluminium extrusion and the base frame extrusions are 250mm long as are, obviously, the top frame extrusions. You need nine in all.
The bearing carriers use 6 623 10mm x 4mm x 3mm bearings and slide directly on the extrusion. Nut traps for M3 nuts are included in the carrier. The rods connect to the carrier with a plate that fastens with 3 M3 bolts, again nut traps are included. The rod carriers and the effector plate hold 10mm x 5mm countersunk, cylindrical rare earth magnets and my rod end balls are 8mm diameter. My rods are 5mm carbon tube but you could use 6mm aluminium. My rod end balls have a 10mm M5 threaded rod and are the type used for the ends of gas struts, etc. the idea of the seperate rod carriers was to enable different rod attachment methods but the magnets and balls have worked out fine for me.
I have altered the delta rods and attachment mechanism and also designed a new effector plate. This time around I used 10mm spherical neodymium magnets and modified some M5 socket head capscrews with a 10mm ball end mill in my pillar drill. The magnetic attraction between the new spheres and the M5 SHCS 'cups' is about four times stronger than the previous version using 8mm steel balls and countersunk magnets. I've posted some images of the new design and also the .stl for the new effector plate. They should be self explanatory but drop me a line if you need further info.
I have had some issues with the idler bearing belt guides - they disintegrated on me! I've posted a modified design on the 'files' section