Initially I had different brackets connecting the 2 linear bearing next to each other. I was disappointed with how (not so) smooth this was running.
With limited tools and space available, I don't always manage to get everything 100% straight or holes drilled in exactly the right spot. This easily leads to over-constrained mechanisms: the top and bottom planks try to keep the vertical rods at 49 mm apart while the plastic bracket tries to keep them at 50 mm apart.
In the original Sells-Mendel printer, proper design was used to make sure that no mechanisms were over-constrained. Somehow nowadays most printers seem to be produced precise enough to allow for parallel linear bearings without causing too much pain.
I decided to modify the bracket to make sure only one linear bearing was used for guiding the bracket in 2 dimensions and the other bearing is only used to make sure the bracket can not turn around the first rod. This means the second linear bearing needs to be able to move slightly sideways and be allowed to be slightly non-parallel. By attaching the bearing with only a single cable tie and creating a small "bump" where the bearing is touching the bracket, enough free movement was achieved. The bracket looks like this (red is old, white is new):
This seems to work much better, installed the bracket looks like this:
The previous picture also shows the idler wheel bracket at the bottom of the rods which is used to guide the spectra wire (0.46 mm, 55 kg breaking strengh) back up to the stepper motor.
I have started on the electronics part as well. I use the Megatronics 2.0 board from www.reprapworld.com and so far it seems to work fine.