Direct Drive extruder with 3mm filament?

I am designing a new printer that I would like to have dual extruders. All my hot ends and filament are 3mm, so I would like to stay with that size. I want to minimize my X carriage footprint but I do not wish to use a bowden setup - I tried it once already and it did not work well for me. I was thinking that to minimize the footprint I would like to go with a direct drive extruder. Everything I read, though, discusses direct drive in conjunction with 1.75mm filament.

My question is this: Is there something inherent in the direct drive where it would not work well with 3mm filament? Are the motors too fast without the gear reduction? Or perhaps is there not enough torque for the larger filament? I don't want to go down this path only to find that there is some basic flaw to the design.

BTW - the printer I am hoping to build will have Core XZ design.

Thanks.

Edited 1 time(s). Last edit at 09/09/2014 10:22AM by jbernardis.

It's the torque. It comes from basic hydraulics, which probably doesn't apply completely. Given that you need a certain pressure at the nozzle, a smaller diameter filament will require less force/torque to drive than a larger, in proportion to the cross sectional area. In this case the 3mm filament has a cross sectional area of ~7mm^2 while the 1.75mm filament has a cross sectional area of `2.4 mm^2. So to get the same pressure (P=F/A) you need three times the force or torque (Assuming the same diameter hobbed bolt). Most motors are not up to it. But a geared stepper could do the job. I think that's what makergear uses. You might get away with using a non-geared stepper if you used a smaller diameter drive (Perhaps hobb the stepper shaft directly?), a stronger motor, and maybe run the printer slower.

Thanks

If I hadn't just purchased 2 3mm J-Heads, I'd probably switch the new printer to 1.75.

I have a couple of PG35 geared steppers. Maybe I'll design the extruders around those.