What kind of replacement Y rod?

+1 on McMaster

I am also curious about that, and further curious about what the drawbacks are to the direct drive Y-axis. I don't see many people using it but I'd like to know more.

Edit: IronMan you need to get a vape!!! love my mech mod, cig free for 4 months

If you loosen the set screws on the pulleys they will align themselves even if the rod is stiff. That's because the belts have trapezoidal teeth and they don't like to be misaligned one bit!

Ah, but I have fishing line :-). When I installed it, it worked out very well to position just one pulley so there was no tendency to slide, then do the 2nd one so I knew if any sliding happened it was all due to the 2nd being misaligned, so I only had to tweak one pulley at a time.

Grob, what are the disadvantages to this mod? What are its quirks? Assuming you're using fishing line, what are the fastest speeds you can achieve? (Belts have natural backlash so high speeds with them isn't all that interesting to me)

Well, the obvious quirk is the motor sticks out. Can't avoid that. You'll probably have to adjust the shape of your side panels to accommodate it. No big deal.

Using a rigid coupler (as you described Claghorn) worked in the end, but it's really not great. The motor still flexes around a tiny bit (the sheet metal frame bends to accommodate this!!).

I haven't had a chance to try out the flexible coupler for real yet, so not sure whether this introduces any issues (e.g. if the torsional stiffness is still not quite good enough). With a rigid coupler, I got rid of most of the hysteresis from the y axis. I should probably print out a circle test some time and post it... Aaah in the land of free time haha!

I'm on GT2 belts, and haven't really got to properly speed testing. I'm not sure this would reliably lead to speed increases, but if it's done well and the drive efficiency is better than the original setup (one less radial force applied to the rod, less friction) then I'd expect some ability to up the speed before the y motor skips - then there's the question of whether you're now limited by the x axis and it's tiny wee motor!

I really like the results from my direct drive.  I upgraded to an 8mm shaft though, it's much easier finding couplings for 5mm to 8mm.

Hey ward, neat idea - if you can get your hands on a lathe, just use a piece of 8mm rod and bore a 5mm hole in one end. Then use skate bearings (608zz) wherever you can fit them, tap a set screw (or two) to clamp up to the motor shaft if the press fit doesn't go so well, and let a slightly flexible motor mounting constrain the axial direction.

Ironman, good idea - if you want it to be properly flexible though, use two gaskets to make a real flexible mounting:
Motor | Gasket | Frame (oversize the holes) | Gasket | Plate (alu would be good)
Chop pieces of M3 rod, thread them in to the motor with loctite to make studs, and then bolt the assembly together with M3 nyloc nuts so you can adjust the precompression.

Maybe a bit OTT but it might do away with the need for a flex coupler, and be a bit more stiff in torsion?
The flexing of the frame kind of achieves the same thing though, so might not be much benefit, I don't know!

If you really like to make a direct drive for y carriage,
move y drive motor to outside frame (left side),
make a U shape abs printed motor bracket,
use 4 holes for mounting the motor, a big hole
For motor shaft w/alum. Coupling, from McMater
buy a small alum. Coupling, one side w/5mm
Hole for the motor shaft, one side/6 mm for
6mm rear rod, driving rod.
The U shape has another 4 holes on its foot,
drill 4 holes on frame wall(left), mount the
Bracket to frame.
So no any extra bending force in between two
Brass bushings.