I am no expert on this front, but those look good to me. They are similar to the ones on the open builds store. I don't know what the STH versus HT refers to though. Different manufactures seem to have a slightly different naming conventions.

 

I've added an "Assembly" doc to the files section. More work then I figured. I skipped the wiring/programming/tuning sections but if you have questions just ask.

Also, inspired by JCPhlux I installed this mod just today. Goal was to use the same concept as I used on the CoreXY portion for the Z axis. Since the 20x40 extrusion is oriented in the opposite direction I had to come up with a modification. I used 20mm M5 stand off to resolve this (25mm stands would of saved me a 1/4" spacer but I couldn't source any). What do you think? I had to move the extruder to the front to accommodate.

 

The printed extension would work, but they require using 55-60mm bolts, right? Sourcing metric is surprising hard in Canada which is on the metric system!
I think I could get them on Element 14 but would have to order 50, if not 100, of them. It would definitely be a better way to go then what I've done with the standoffs.

It was mainly about symmetry and consistency with my CoreXY design as the cantilevered design was a carry over from my H-Bot. Also, there were some concerns mentioned about the Z setup so I wanted to present an alternate. This mod can be extended to work with 20x60 extrusion as well (I had created it to share but somehow deleted the files).
The cantilevered design does work and I had my extruder mounted and wired to that leg so I didn't want to move anything. Then JCPluux got me thinking more on this when he started planning a shift to 20x20mm extrusion. This is what I came up with that enabled me to use my current 20x40 leg design. Did a test print yesterday and all is running well. I'd say either design is fine.

 

I am pretty happy with the Z axis setup and I am getting excellent quality prints out of it (much better than my OB1.4 which does suffer from intermittent Z wobble). The biggest factor though is using lead screws instead of 8mm threaded rod and being carefully squared and aligned.
For a 12x12 print bed I would recommend upgrading the Z frame assembly to use 20x60 extrusion (20x80 if you want to be extra cautious) instead of the 20X40 I use on the rear section. The idea here is not to prevent Z wobble (that still shouldn't be a problem) but to prevent deflection at the far side of the bed and reduce the force on the mini V wheels for printing large objects.
I hope to add dual extrusion down the road. The C-Bot setup is symmetrical, so other than designing a new hotend holder there will be no extra design work required.

 

I used a long spacer instead of adding another 8 v-wheels for a cost savings. I knew from my previous design that they could handle the loading of my build plate. It would work though and would increase the rigidity as Derlin has some give to it.

Two lead screws in the back and one in the front (or vise versa)? That would eliminate the cantilever forces and take the torque loading off the V-wheels. You wouldn't need to upgrade the rear Z bar if you did that as the V-Wheels will be acting for XY stability and alignment. Someone earlier was looking to use an 8lb heated bed so your mode would be perfect for them! Any concerns about belt backlash? What size stepper would you use to drive three lead screws?

I was thinking of putting both hotends on the same side (which actually represents the Y axis with how my endstops are configured- see end note). It only creates a small imbalance in the bowden configuration, which I was eventually going to counter balance with print cooling fans. I see no reason why what you are suggesting wouldn't work. Just make sure you size your extrusion to handle the extra range of motion required for dual extrusion.

End note- my configuration treats the 12" (left to right) as the Y axis and the 8" side as the X-axis. Why? It has to do with my endstop locations/the auto home feature/ and Marlin restrictions. I wanted to keep the endstops stationary, the auto home feature homes X then Y, Marlin doesn't let you home to X-max or Y-max for a CoreXY setup.

 

Are you using sli3r? If so, just go "split parts" and it will drop everything to Z=0

 

The benefit with 1.75mm filament versus 3mm filament is the force required on the drive is less. For the same pressure at the tip of your hotend you need to apply less force with the smaller diameter (area). P=F/A ... Pressure = Force / Area.
It is also helpful in a bowden setup as the filament can bend and flex easier.
You don't necessarily need to change your hotend though, but I would recommend a geared extruder with 3mm filament.

 

The M3 Nylocs aren't required, but I was planing on using them for the XY bar idler pulleys.
As for the spacers, it seems Open builds sends out two types, ones that work with your mini V-wheels and ones that wreak your mini V-wheels by pressing on the seal shroud! The ones on the bottom right are the correct ones. If you have the ones on the left then you will need to use the shims you mentioned.

 

****, you are correct. I'll update ASAP.

 

Looks good, thanks for sharing. What size bed are you planning on using?
I am sure folks with a front to back bed size of greater than 8" will be considering using these.

 

Yep, as soon as I get home. Another thing you can do is "split" the two ends apart in slic3r, delete one, and then export the stl file instead of G-code.
Do you still have a question about extrusion lengths or did you figure that out?

Aside: I have some new parts to post.
-New XY bar ends that allow you to run an LED strip behind it. Otherwise identical.
-A dual hotend carriage
-An alternate Z endstop