Well the Rostock isn’t even finished and I’ve begun upgrading parts of it.
The printed arms to me aren't the best, even in a tinted PLA (one of the more rigid colours) it is still quite flexible, I also didn't like the idea of PLA bearings, these will wear out quickly.
The following solution was suggested to me on the #reprap IRC channel
The inside diameter of the Plastic part is 3.5mm and 10.5mm deep. The inside diameter of the Steel is 3mm (not documented but needed by us)
Assembling these together was a little tricky. One side of the plastic is bevelled (shiny circle around the hole) they go in that side. I held the bearing with some long nosed pliers just sitting on the bevelled side then snapped them in place using the plastic as a lever.
Next is the arms between the ends. Some people are using m4 thread rod as their arms with these, but this is heavy and easy to bend.
I found a local supplier of 3.5mm solid carbon fibre rod. Now this rod was not the best quality, ie it was a little bent, but when cut up into 6 * 236mm lengths I was able to discard the worst bits. (236mm is what is required to make the arms the same length as the originals)
I glued the arms into place, the fit was so snug I had to ensure there was no air trapped in the end (ie you would let the arm go and it would spring out due to compressed air being trapped). Also make sure that the two ends are on the same plain. Ie The flats are level.
The top original arm weighs 16g, the new carbon fibre arms weighs 6g, and is a lot stiffer and in my view a lot better looking.
That also a weight saving of 60g! (6 x 10g) (important when you considering putting an extruder directly onto the carriage vs a Bowden cable type system)
The arms fit where the old ones did. There are new single lugged plastics if you want to reprint, but I didn't want to.
NB this was originally connected with 2 * 12mm m3 bolts. Here I am using one 20mm m3. To be exact the order pictured here is M3 bolt head, 3m washer, Printed lug, embedded m3 nut, Traxxas steel bearing, m3 lock nut. The second m3 nut is still embedded in the other side but the thread is just sitting indented in it, not threaded in.
NB this is still a work in progress, looking at its range of movement a second m3 nut before the bearing to move the arm towards the centre of the gap will be required.
Update: I’m not happy my with my attempts to attach these to the original plastics
So I am printing a set of http://www.thingiverse.com/thing:31642