Minimalistic Mk8 replacement

The Minimalistic Mk7 replacement works so nicely, I wanted to put them on my Replicator as well. It required a slight redesign in order to fit under the piece that holds the guide tubes. I use 693 bearings on this, but you should also be able to use 623 bearings like on whosawhatsis' original.

I get much better prints with these than the stock Mk8s because they grip much better on a variety of filaments due to the compliance and there is much less friction thanks to the bearing. I was using Protoparadigm's PLA and having all kinds of problems with missing filament after reversals, but not anymore. No more lost steps or stripping on the extruders, and an added bonus is you can now clean the drive gear without disassembling the unit. Also, switching colors is quicker since you can just squeeze and pull instead of going through all that unload/reload filament business.