Dual drive gear extruder - interest and experience

Hello to everybody!

I'm interested in developing a single extruder with dual drive gears, and I'm curious about what has already been done. I believe that the twin drive gears should be able to sustain higher pressures in the nozzle, thus allowing faster and more reliable printing (hopefully with less "chewing through" the filament). I think a design like this would be easily adaptable to carriage-type gantry printers, such as the TAZ and the Robo.

Most current, open designs I have found just use two sets of the current standard: a drive gear paired with a pulley on some sort of spring assembly. This doesn't seem like the best design, IMHO - it doesn't change the real problem. If the pressure in the nozzle goes up too much, the spring deflects too much, and the drive gear chews through the filament. On some older Stratasys machines, where I've had the opportunity to look at the disassembled extruder, I've seen that they actually use dual drive gears.

Anyway, I'd like to know all of your thoughts on the following:
Would a dual drive gear extruder be beneficial?
What is the single biggest problem with home user printers?
Have you or would you consider buying add ons for a 3D printer?
Is it a problem for add-on producers that most things are easily reproducible or open-source design?

I'm not sure I'd like a dual drive gear, mainly for issues with weight and size. My x gantry which holds the extruder/hotend is not exactly light, and at some point I'm looking for dual extruders. I don't actually use a spring for my extruder arm, instead I just tighten it with a screw and I havnt had any problems with slipping or chewing the filament. It does however crush ABS filament slightly (no affect on harder PLA).

If you could make a dual drive gear design that was lightweight and was proven to be more consistent, it is certainly an upgrade I'd go for.

The motor should skip steps if the hot-end gets jammed or the filament gets tangled on the spool and it should never chew a divot into the filament. If your extruder is doing that you need to increase the pressure applied by the pinch wheel.

A single drive gear implemented properly works fine- my BullDog XL and E3D v6 combo has jammed exactly once in about 1 1/2 years of almost daily use, due to a foreign object in the filament.

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

That's very interesting to hear that you almost never have problems. I'd say that you don't really want the extruder to skip steps - you will extrude less than the Gcode calls for, which should lead to printing defects (even if they're too small to see). One solution is lower the printing speed (to lower the pressure in the nozzle), but nobody wants that.

I think you misunderstood the dentist. The motor should lose steps in case the force needed becomes to high because there is a problem that needs to be resolved.
Just building an extruder that continues pushing no matter what isn't a good idea in that case.

Edited 1 time(s). Last edit at 04/25/2016 12:44PM by Srek.

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[www.bonkers.de]
[merlin-hotend.de]
[www.hackerspace-ffm.de]

I don't understand. If the motor skips because the hot-end is jammed, the plastic isn't coming out of the hot-end and your print is finished. You're definitely not extruding as much plastic as the gcode calls for- you're extruding zero. If your extruder is chewing into the filament even as it is printing, you really need to increase the pinch wheel pressure. If it is skipping steps while it is extruding you need to adjust the motor current, or use a higher torque motor, or a motor with a gear box.

The BullDog XL has a 5:1 gear box and pushes filament hard. It takes a full-on hot-end jam for the thing to start skipping steps, which is as it should be.

Print quality and speed are inversely related. I usually print at 30-50 mm/sec and prints finish when they finish with consistently excellent quality. I'm not running a factory with a tight production schedule. I don't swap glass plates so I can start the next print without waiting for the machine to cool off, either. I grew out of that sort of impatience when I turned 9.

I recently saw something interesting: "if you don't have time to do it right, when will you have time to redo it?"

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

That may be true in some cases, but consider that the newest generations of printers claim to reach speeds of 100, 200, or even 300 mm/s. I agree that print quality generally goes down as the speed is increased, but that precisely indicates that the extruder isn't functioning as desired. That's another reason to consider upgrading the extruder in some way.

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tomMulholland
That may be true in some cases, but consider that the newest generations of printers claim to reach speeds of 100, 200, or even 300 mm/s. I agree that print quality generally goes down as the speed is increased, but that precisely indicates that the extruder isn't functioning as desired. That's another reason to consider upgrading the extruder in some way.

First off there is at least one dual geared extruder like you suggest on the market. I believe it's called "bondtech" or similar.
Secondly I'm not sure that you can directly equate that print quality is dropping only because of the extruder. Printers have trouble at those speeds as a result of design/cost/etc compromises in the frame or guide systems. These show up as you push the system to faster speeds with higher accelerations. While the extruder plays a role in this, it is likely not what is holding anything back.
I don't think it's the worst idea, but I will tell you this. I cut gears all the time, from little .5mod to gears 18" or so in diameter that drive large industrial equipment. I cut drive rollers as well, so it wouldn't be very difficult to cut the gear teeth in the same setup. But as far as I have seen there really is no benefit to the added complication, and I haven't bothered to implement this in anything I've built. Your mileage may vary.

When testing the Merlin with some new extruder designsi reached some nice extrusion speeds. 200mm/s were possible, the hotend and extruder worked fine, but the print quality was rubbish. So the extrusion speed is only one lmiting factor and afaik not even the worst.

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[www.bonkers.de]
[merlin-hotend.de]
[www.hackerspace-ffm.de]

Hi Koko,

Thanks for your reply. I agree that the print quality is certainly affected by more things than just the extruder having trouble at high speeds. However, I would suggest that the problem is more real than people realize. For example, a colleague of mine did some extrusion tests with a temperature sweep. He used a FlashForge Creator 2, and sent the plain Gcode command to extrude at a given speed for 30 seconds, then measured the mass of the extrudate. You can then compare the expected mass (using motor turns and the filament diameter and density) to the extruded mass. The test was repeated at increasing temperatures.

If you look at those curves, you'll obviously see the importance of getting the right extrusion temperature. However, we also saw that higher speeds never actually got up to the same extruded mass as lower speeds, even at higher temperatures. This is surely related to the highest pressure you can achieve through your extruder / nozzle setup.

I'm not sure that a dual drive gear setup is the right answer - but I'm sure that the extruder is very important in determining the final part quality.

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tomMulholland
Hi Koko,

Thanks for your reply. I agree that the print quality is certainly affected by more things than just the extruder having trouble at high speeds. However, I would suggest that the problem is more real than people realize. For example, a colleague of mine did some extrusion tests with a temperature sweep. He used a FlashForge Creator 2, and sent the plain Gcode command to extrude at a given speed for 30 seconds, then measured the mass of the extrudate. You can then compare the expected mass (using motor turns and the filament diameter and density) to the extruded mass. The test was repeated at increasing temperatures.

If you look at those curves, you'll obviously see the importance of getting the right extrusion temperature. However, we also saw that higher speeds never actually got up to the same extruded mass as lower speeds, even at higher temperatures. This is surely related to the highest pressure you can achieve through your extruder / nozzle setup.

I'm not sure that a dual drive gear setup is the right answer - but I'm sure that the extruder is very important in determining the final part quality.

I'm certainly not suggesting that the extruder isn't vital in part quality. I would say that you should be careful drawing specific conclusions from the results of those tests, although it is interesting data. The motor could be missing steps as well as not grabbing, nothing shows one way or the other. If the motor is missing steps, then the drive arrangement isn't the issue. There are more variables in play then you have accounted for.
Most of what exists in this area, especially in terms of what is commercially available at prices regular folk are willing to pay, is not an ideal solution. People in the hobby space are reluctant to pay what it costs to really "do it right", and so the same lower cost and lower complexities will tend to dominate, as they are good enough for most. I have a lot of machining resources available, and I can make just about anything, but it's not the case for most people here. I'm working to develop the "best" extruder for my purposes too, but have zero plans to commercialize it as I think no one would pay for it.

It's an interesting idea that lower nozzle pressures may lead to lower mass of a printed item, does this also come with significant quality reductions to suggest missed steps and/or slippage? Or does it affect quality on a smaller scale? (eg, a more 'porous' extrudate?)

Either way I think designing a test to account and control for these confounding variables would be very easy, and would certainly prove if dual gears are needed.

There are two big reasons why print quality goes down with speed. First, controlling the motion of relatively heavy moving parts requires a lot of power and requires very rigid construction, most printers have neither. Second, you're squirting liquid plastic out of the nozzle. Once it leaves the nozzle it is out of control. It will go wherever its momentum takes it.

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

Funny that someone opened this tread, I just finished designing one. It is printable and uses standard stuff. since I don't have access to milling equipment, the minor modifications needed will be done by hand....

I do think that extruder needs to be improved (not precisely my design...) but improvements needs to come as a whole and not to individual parts, you can have a great hotend but flimsy frame and you would not gain any quality in the printing, a great extruder but a crappy stepper will do the same.

This sounds like a very interesting idea. I have had chronic slipping and grinding issues, especially with the long retraction that accompanies a Bowden tube, and I'd love to increase reliability.

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ggherbaz
Funny that someone opened this tread, I just finished designing one. It is printable and uses standard stuff. since I don't have access to milling equipment, the minor modifications needed will be done by hand....

I do think that extruder needs to be improved (not precisely my design...) but improvements needs to come as a whole and not to individual parts, you can have a great hotend but flimsy frame and you would not gain any quality in the printing, a great extruder but a crappy stepper will do the same.

That's a nice clean looking design. What's the ratio between the stepper gear and the drive shafts? The filament gears look quite large diameter, so I'm guessing you'll need something like 3:1 on the drivetrain to get back to parity with a direct drive extruder with a smaller hobbed gear. One problem is that adjusting the tension is going to change the meshing of the gears that drive the second side. Maybe with drive from both sides you won't need much adjustment travel and it will be ok, but it might be a bit finicky. It also looks like the tensioning will have to be push rather than pull, but I guess that's fine.

I can't see any problems with dual drive gear - I am presuming we're talking about both drive gears actually being driven rather than using one as an idler? It seems logical it would help to control filament more tightly. It is more complicated to build though.

If I can pick up on one point for clarification: My understanding with idler tension was that the filament grinds when the tension is too much, whereas the drive gear slips on the filament producing the click from the extruder if it is lighter and that this is recommended, assuming the motor current is not too high. Here we seem to be saying the idler should be very tight to prevent grinding and that the clicking is the motor skipping steps.

I don't have a preference but I have heard both versions and am interested to know which seems to the consensus view?

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Simon Khoury

Co-founder of [www.precisionpiezo.co.uk] Accurate, repeatable, versatile Z-Probes
Published:Inventions

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DjDemonD
I can't see any problems with dual drive gear - I am presuming we're talking about both drive gears actually being driven rather than using one as an idler? It seems logical it would help to control filament more tightly. It is more complicated to build though.

If I can pick up on one point for clarification: My understanding with idler tension was that the filament grinds when the tension is too much, whereas the drive gear slips on the filament producing the click from the extruder if it is lighter and that this is recommended, assuming the motor current is not too high. Here we seem to be saying the idler should be very tight to prevent grinding and that the clicking is the motor skipping steps.

I don't have a preference but I have heard both versions and am interested to know which seems to the consensus view?

My experience is that grinding of the filament happens when idler tension is insufficient. Very high idler tensions increase load on the stepper and can cause stalling to happen earlier. In general the path to more reliable extrusion is more torque plus more grip, coupled with reduced friction in the filament path.

Thanks James.

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Simon Khoury

Co-founder of [www.precisionpiezo.co.uk] Accurate, repeatable, versatile Z-Probes
Published:Inventions

Thanks for the comments James,

In order to reduce the size of the design as much as possible, all the gears in the system are the same : 40 tooth gear 40 tooth 21mm in diameter, with these: bearing

Some filing is needed on the gears moving the filament for better grab, but they are small enough by themselves. the final ratio will be given by the stepper motor gear, that most likely will be a 5:1 like this one: stepper motor
thanks

Edited 1 time(s). Last edit at 04/26/2016 01:09PM by ggherbaz.

Ah, I see. I hadn't realised the drive chain runs through the pivot point, that makes much more sense. Nice render