Direct Drive (Gearless) Extruder
Description
I made two versions: One uses the MK7 pinch wheel and the other uses an M8 hobbled bolt that's been cut and drilled to mount on the motor shaft. The advantage of the smaller diameter 8mm bolt/rod is that it can apply more force to the filament.
It can surely go faster if 1.75 mm filament is used. The design for the hobbed bolt should work for 1.75 mm filament if a MK7 gear drive is used instead.
I've also included a 40 mm fan bracket.
Instructions
You'll need to either buy a MK7 pinch wheel. The better choice is to take an 8mm hobbed bolt, cut it, drill a 5 mm hole down the center and then tap across for a grub screw to fix it on the stepper motor shaft.
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I think drilling the 5mm hole in the 8mm bolt would be difficult with home tools?
Direct drive works just fine even for 3mm ABS. Been there, done that. The motor was operating at the very limit, though...
(I'm running a 5:1 geared extruder right now and am planning to reduce that to 2.5:1 because the retraction is painfully slow with 5:1.)
There is one advantage of this extruder over a wade style extruder. I've never stripped a filament since the motor runs out of torque before that happens. With a Wade styl extruder, it's very difficult to adjust the stepper controllers to limit the torque. With a lower gear ratio, perhaps you'll strike a more optimal balance.
With a 0.5 mm Budascnozzle, I can print ABS with the 8 mm bolt at 0.4 mm layer thickness reliably at a print speed (feed rate) of at least 60 mm/s using the Marlin firmware. I typically print parts that are 100 mm long/wide so I'm sure it's getting up to that speed. I'd like to print much faster than that but in my experience, the infill gets thinner in the middle due to extruder lag using either Sprinter or Marlin.
Recently, I started using a 0.35 mm J-Head with the MK7 pinch wheel on a Prusa 2 and I found that I can print up to about 70 mm/s feed rate at layer heights below 0.3 mm. But I haven't print anything really large with this printer yet. I need to have another 8 mm bolt made to get more force.
I th
ink the MK7 drive gear is probably best used with 1.75 mm, which is what it was designed for. But, it can still work with 3 mm if an 8mm hobbed bolt/rod is not around.
how well does this work without the 4:1 ish mechanical advantage of a wade style extruder?
Does this actually work with 3mm filament, a Mk7 drive gear and that tiny motor? That shouldn't have nearly enough torque for direct drive with the back-pressure of a 3mm hot end... at least not without a nozzle bore of about 1mm or more...
I'll have to try the direct-drive gear from 8mm bolt/stock thing. Actually, it's the first thing I tried after the dremel-hobbed stepper shaft thing failed when I was first trying to build a repstrap, but I didn't have any of the right tools at the time.
License
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For those asking about whether you can extrude 3mm feed fast enough with a direct drive NEMA 17, the answer is yes. Paired with a good hot end like the J-Head Mk-V that requires little force, I can extrude up to 160 mm/s of 3mm ABS feed through a 0.35 mm nozzle using a drive that's around 6.5-7 mm diameter.