Keith, you think that two cooling fans, blowing on each other, will cool better than a one fan mounted on the cooling fins?

 

Are you planning on dual extrusion at the same time? Given you have two NEMA 17s as actuators. That is a lot of extra weight that you will be carrying. And that extra weight needs be supported with a stiffer frame, which in turn slows down your gantry acceleration or it will introduce unwanted vibrations. Or are you gonna keep your keep your print head locked and move your bed, which will result in a much smaller buildvolume. Either way it might be worth it to see if you can get away with one actuator for the direct drive.

 

Good to know you weren't planning on using flimsy printer constructions ;-) Just keep in mind that bowden-tubes, with all their flaws, are used for a reason. A common mantra we often say at Ultimaker is: "You have to carry and accelerate every gram on the printhead". Extra mass can result in a degraded print quality but at the very least requires an upgrade of the motion system as a whole. A lessen I learned when working on a couple of prototypes for our next gen. printhead (also direct drive).

 

True its all about compromises, and every design choice has implication. One big interesting puzzle.

 

I've been working on a dual extrusion setup similar to you, but the one I used is based on the bondtech process:
Dual BMG Chimera+ for CR-10 by Festivejelly

I modified it so that instead of the 2 50mm radial fans for parts cooling, I'm using a CPAP blower attached using a tube:

 

Those are pancake nema 17s. Pretty light

 

Well the Bondtech BMG Extruder includes a 3:1 gear reduction which provides not only better accuracy, it also triples the strength of these small steppers. I really like gear reduction on both z axis and extruders since they don't move very fast during prints.