update:
Oct 03: New version with a lot of improvements:
http://www.thingiverse.com/thing:1050574

Thanks for smartfriendz to share this revolution of their great design to Onshape!
We now have the source files to change the design for our needs.

Changes I made:

• adapted the parts which have rods and bearings from 6mm to 8mm size
• beefed up some parts to keep their strength with the 8mm holes
• removed the L and R signs from the parts (they are clear to me)
• changed the wall thickness to 13mm to fit the HobbyKing 12.8mm alu square tubes they sell (400mm and 600mm length available)
• shortened the overhanging part of the wall pieces to fit the tubes
• changed diameter of the wall screwholes to 3mm and moved them up to the middle of the tubes

The parts for the walls should fit 12mm MDF fairly well, but I do not have any at hands to test this.

I only uploaded the stl files which I have adapted. When you need all stl files of the Smartrapcore standard design, I urge you to signup for Onshape and get them by saving your own copy of the design to your documents.

Go to the following link for the Smartfriendz design and parts: https://cad.onshape.com/documents/086229ea796640d29acd68ff/w/2030a3358fd641da896d20a5/e/1e4ef00263ee4eae9316731a

[edit]
17 sept:

• improved parts after printing and assembling
• added Z axis top and bottom adjusted for mounting on alu square tubes (not tested yet, currently printing)
• added profile corner braces for top and bottom
• added photo's of assembly XY gantry