This project has been put on hold, as I've been pretty busy lately with finals and robotics. But here's a quick update.

The new design seems to be pretty good. The reshaped sliders work great, and the tensioning is the right strength, and it all moves smoothly. The enlarged gears are also big enough that they mesh smoothly. 

The one problem has been manufacturing the screws. Even with a center drill eliminating wandering, I haven't been able to get a good center. I think I'm going to try to put the screw in the chuck of my drill press and use a jig to hold a center drill so that it can slide but not rotate - like a planar joint in fusion. Hopefully, this will self-center, and I can drill it normally after it is started. 

If that doesn't work, my next plan of action is to use the CNC to drill a centered hole by measuring the position of the screw on several sides. However, the CNC (from my robotics team) isn't great for this kind of small-scale stuff, as it has significant backlash and no probing abilities, so I'd have to compensate for all of that manually.

Another update will come once I manage to finally drill these holes... which might be a while :(

And no, I don't have access to a lathe. My robotics team is an FTC team with no need for one, my school's FRC team is small and poorly funded (despite the exorbitant tuition), and my friends from another school who actually have a lathe aren't able to use it for outside projects. There is another team I could ask, who I will ask if none of the above strategies work, but ideally, this extruder should be accessible to us lay hobbyists without lathes. This is why I'm hoping the drill press idea will work - so you don't need a mill to make this.

Anyway, wish me luck. I'll need it.