With the idea of using dual hobbled pulleys that are geared to one another

With the idea of using dual hobbled pulleys that are geared to one another gaining traction, has anyone considered mounting the gears onto a traditional lever/spring extruder? I was thinking you could potentially drill out the center of the pitch of a gear (if that makes sense) and tap it. In other words, instead of having the gear and the pulley side to side, have the hobbled pulley in the middle and the gear teeth on both sides. This would also make a more compact setup.

When the lever is pushed up by the spring, it engages the teeth of the other gear. I think acceptance to varying filament tolerance is necessary although it would still grip it in most cases. Any thoughts?

We have done it and it works great. Look on our Printrbot account in youmagine. You can get a gear set from us if you wanna try it. We are still undecided about mass producing them.

Thought about it a lot. For that, you’ll want a large pitch gear so that they will still mesh with some gap between them. With a large pitch, the valleys between teeth will be deeper, so you really don’t want to cut the teeth that grip the gear out of a those teeth, so you would need separate sections.

@Whosa_whatsis Not sure what you mean. Are you saying that tapping around the middle of a gear is a bad idea?

I also personally think that there’s not much advantage to having two gears in comparison to one. For bowden setup, if the filament doesn’t slip, then it doesn’t add much advantage in my opinion. I have yet to try using gears, but I can’t picture much difference aside from maybe extra grip when slipping is an issue, in which case I’d say that there is something else that is causing it to slip other than the extruder.

I was thinking there would not be any gap between the gears. They would align as per usual, just that the middle part of the gear would be tapped all the way around on both gears. I can design a model to illustrate this better.

I don’t think it’s a bad idea at all except that you really don’t want the biting portion to be tapped if you can help it in my opinion.

Folks use taps as a way of imitating hobbing, and the advantage is that it’s quicker/easier/cheaper to hob teeth into something than it is to cut them. The problem with using a tap instead of a proper hobbing machine is that you end up cutting angled teeth. And if you’re cutting the same but opposite angles into two sides of a geared driving mechanism you’ll be introducing a twisting force on the filament.

Will it be enough to cause a problem? I have no idea. But surely no good will come of it. It could be a reasonable compromise for production demands (or DIY accessibility) if it demonstrates to be non-critical, but I think I’d want to try it with properly cut (i.e. parallel to the axis of rotation) teeth if possible.

@John_Davis Oops, I meant regular hob teeth!

@John_Davis You can fix that angle by angling the tap when you use it. A real hob looks an awful lot like a tap, the real difference is how it’s used.

@Shai_Schechter If the teeth are fully meshed with the filament in place, you don’t have any wiggle room to account for filament getting any thinner (either due to manufacturing tolerances or due to stripping or compressing of the filament as retraction runs it back and forth. By leaving a slight gap so that the drive gears are a little too tight when the gears are fully meshed, you get this tolerance, which is the main purpose of spring-loaded idlers.

Of course, without the gears fully meshed, they may not turn together, but the filament will keep them both turning in most cases, and the second gear acts as a redundancy in case the first loses its grip. As it turns without the filament moving, it will engage the teeth of the other gear, which will push the filament to get it moving again, only possibly with a slight delay (where you would otherwise have a failed print.

Like I said, I’ve thought about this a lot.

@Whosa_whatsis Understood, it’s just more involved than “drilling out the center and tapping it” as described in the original post. Angling the tap used for hobbing will also widen the channel to some extent.

I think the other problem you’re referring to is also surmountable. As long as you use a tooth profile that has decent engagement/efficiency across the various depths required to accommodate filament diameter variations you should be fine.

I really think it just gets expensive to properly machine such a thing. Probably more expensive than just making the various toothed portions out of different pieces as has been done previously.

What about a crown gear set?

We use the gear head kit we sell in a new design on youmagine. It’s a mash up of our Alu extruder and the gear head. It uses machined pinions. The right idler is on a sprung lever but the gears mesh fine. Works great for even Ninjaflex… Although the new stiffer stuff is even more reliable. It’s a printable design. Still deciding on mass manufacturing it.


@Stephanie_A ​ I’m guessing you mean cutting the same tooth count into an angled channel like a bevel gear to drive the filament? I think one difficulty would be machining such a thing, especially into the center of a single piece. Also you’d probably get some radical speed changes depending on how deeply the filament was pressed into the channel, something that might make it hard to achieve consistent flow.

@Brook_Drumm ​ I like the idea as it sounds like the best of both worlds. I’m having a hard time understanding what the gear/pinion bits look like though. Are they the same as on the gear head kit?

I’ve been thinking of using two gears on one shaft. A filament drive gear and an off-the-shelf gear, like the ones sold for RC cars. In experimenting with this, I’ve found that the grooves on most drive gears are too deep to allow two of them to properly grip a piece of 1.75mm filament between them. I’ve been thinking for a while that the grooves were a bad idea anyway, and that they contribute to inconsistent steps/mm settings.

We might use the cheap chinese drive gears that have straight teeth and a rough coating similar a dremel grinding tool. I’ve also been considering using knurled drive. Ultimaker uses it and it seems to prevent plastic from clogging up the drive gear so that it needs to be cleaned before it will grip again. The downside is that it doesn’t grip as firmly, so the tooth marks end up closer together when you try to push faster, which means your steps/mm value isn’t consistent. I’m thinking that driving from both sides might be enough to mitigate this problem.


This shows our first beta .01… Now youmagine has version .02

It’s s little hard to see how we scoop out the pattern, but it works really well.


@Brook_Drumm Nice double thick lever. R&d done right.