Hi fellow reprapers, I would like to ask question...

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(Jan Koniarik) #1

Hi fellow reprapers, I would like to ask question…

I am thinking about making worm-screw based extruder (I should be able to make worm-wheel on lathe with screw-cutting tools, so for DIY should be enough)

and there is one thing about worm-screws, they got big gear ratio, like 1:40? which seems fine because you can make extruder with ridiculous strenght.

But I was wondering, what is maximal speed i should be able to achieve with Nema17 (2.5A) and reprap’s stepstick ? have somebody every really stressed it to it’s maximum in terms of speed?

I mean, it would be pointless to got such a big strength with pulling with really slow speeds…

Of course I can compesat for gear ratio with making gear that pushes material bigger (again, possible for me on lathe), but actually calculations said that to get 1000steps/mm (which just doubles my actual settings), my pushing gear should be 3 times bigger than worm-wheel :slight_smile: (which is too big to be practical)(P.S: assuming that pitch on the worm-wheel would be 1mm)

question is, what is maximum steps/mm for me to be able to work?

(Whosawhatsis) #2

The gear ratio will be too high for good retraction. For 1.75mm filament with a 6-12mm effective-diameter drive gear, the resolution of a 1.8-degree stepper is close to ideal. For 3mm, you want a gear ration in the range of about 3:1-6:1 for additional resolution and torque.

Worm drives give a high gear ratio, but they have a lot of friction and low efficiency, and for this application, the gear ratio is just too high. There are reasons that (outside a few of experiments in the early days) nobody uses them for extruders.

(Jan Koniarik) #3

I actually have 1:5 gear ratio for 3mm and it works… but entire mechanism is too big … any links/tips where to look for failed experiments? :slight_smile: (I don’t want to give up until I am sure I really can’t make it better -> more study)

(Whosawhatsis) #4

Look here: http://hydraraptor.blogspot.com/
and here: http://blog.reprap.org/

Start at the beginning, the more recent stuff will be of less interest.

(Whosawhatsis) #5

@Mark_Rehorst Short answer: You don’t. The augers typically used for pellet extruders are terrible for retraction, or even speed changes. It’s difficult to even make them reliably maintain a constant output rate without external feedback.

There are other mechanisms that might work better, but most look more like a pinch-wheel extruder design than an auger. See discussion here: https://groups.google.com/forum/#!topic/3dp-ideas/_ER48Oucl60

(Markus Seidt) #6

Well, with respect, I’ve been using small direct drive extruders with a 1:40 worm gear for 3mm and a 13:60 for 1.75 for more than a year now and I’m very happy with the performance. https://www.youtube.com/user/wokbrenner

(Jan Koniarik) #7

wait! this doesn’t aim to be pellet extruder… “worm gear” != worm to push pellest :slight_smile:

@Markus_Seidt awesome! will look into that :slight_smile:

btw: don’t you have more details about entire setup? I am interested about gear that pushes the material given the ratio and esteps settings in software… thanks a lot! :slight_smile:

(Jan Koniarik) #8

@Branden_Coates awesome, given that flex drive uses that gear ratio, I do not have to worry about it beiing ‘too big’ … just use smaller microstepping and hi esteps :slight_smile:

(Markus Seidt) #9

@Jan_Koniarik More about the worm gear setup can be found here: https://ultimaker.com/en/community/view/9802-um-original-with-direct-extruder?page=1

(Jan Koniarik) #10

@Mark_Rehorst awesome!

@Whosa_whatsis yeah, all prototypes I saw in that links got wrong shape for wheels in form drive… which explains it being non-effective enough :slight_smile:

(Whosawhatsis) #11

A worm drives makes sense for the flex shaft because flex shafts are more accurate and efficient when they are moving at high speeds. If you’re using one of those, you want to gear the motor up on the input to the flex shaft so that it turns faster than the motor, then gear it way down on the output to get back the torque and resolution, and to make up for the losses to friction inside the flex shaft. It’s a very special case, and just because something works with a flex shaft doesn’t mean that it’s a good idea in general.

(Markus Seidt) #12

I’d consider carrying around 70 gr instead of 250 gr on a direct extruder a good idea if it’s capable of +10mm3/s with perfect retractions. There is a sweet spot of torque and weight. IMHO many designs don’t take this into account sufficiently. Perhaps because the required steppers are roughly three times as expensive …