Planetary Gearbox Ratios for extruder

Can anyone help explain how the ratios(5:1, 17:1, 19:1 etc.) correlate to performance of a Bowden extruder? Do the higher ratios trade speed/step angle for higher torque? What ratio is the better option for a Bowden extruder?

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Erik S

Use no more than 5:1, otherwise retraction acceleration will be low. I reckon a gear ratio of about 3:1 is optimal for a geared extruder.

Edit: this applies to extruders for 1.75mm filament.

Edited 2 time(s). Last edit at 08/08/2015 01:27PM by dc42.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

My BullDog XL has a 5:1 gearbox and works fine for the relatively low print speeds I use (50-75mm/sec). I recently tweaked retraction settings and found that 20mm/sec with retraction acceleration set to 3000 mm/sec^2 provides very clean layer starts and stops.



If you're trying to print at 250mm/sec, 5:1 is probably too high a ratio. You need to know your controllers maximum step rate, and the microstepping you will use to calculate the maximum speed the extruder can handle in your printer, taking into account the gear box drive ratio. I'd say that in general, you want to use the highest gear ratio you can consistent with printing at the speeds you want because the torque multiplication you get from the gearbox helps prevent extruder jams by providing very high downward force on the filament.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

Dc42: I haven't finalized my choice of extruder, but I'm pretty sure it's going to be a direct drive.
The parts kit I bought comes with a generic version of the mk8 type extruder, but I'm thinking about using an M2 extruder or something similar. I think that having as much support as possible guiding the filament into the hot end would be best with the high torque of the planetary gearbox.

Dentist: I took a look at your Instructables page and have to say that I like the way you think, that's a pretty nifty machine you've made. The bulldog, besides the M2 mentioned above, was the other design I've been looking at. Have you run into any trouble with the filament backing up?

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Erik S

I've had the BullDog XL and E3Dv6 on the machine since December, printing almost daily, so far always ABS, and have not had a single extruder jam or print failure caused by anything extruder/hot-end related. I have not had to take it off the machine for any reason in all that time. It just works.

I'm in the middle of a new machine design and build but have some parts (blower and mount) set aside to allow PLA printing. I hope to get to that soon. I also have a spool of nylon filament I'd like to try...

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]