Semi-Bowden extruder?

Not sure if this is the correct forum... but...

It looks like a lot of people love Bowden extruder for the speedy printing. But as we all know, it has an inherent drawback of high hysteria.

So, I was thinking... what if we put a small stepper motor on the extruder? You know, it's like one of those direct drive extruder, but using much smaller stepper motor, like this one?



We will still keep the main motor and let it do the most of "pushing" (typical Bowden extruder setup), but this small additional motor will be right at the top of the hot end, not to push and pull the filament, but just to eliminate the hysteria.

So, it's like hybrid of Bowden and direct drive extruder, but it's more of Bowden principal...

Even with a mini stepper, it will add some weight to the extruder, but it will probably be still a lot lighter than typical direct drive extruders, so we may be able to achieve very good speed...


For this to be possible, two motors will need to work in perfect unison. I searched the internet a little bit, but the one I showed in the picture was the only stepper motor that was small (well, there were really really miniature ones, but I think they are actually too small), and this guy has a different step angle - 7.5 degree instead of 1.8 degree which typical NEMA 17 motors have. So, it may get tricky to make these two motors push and pull the filament the exact same length at the exact same speed...

I thought about using NEMA 8 motor instead of mini stepper motor, but I am not sure how light they are - I know they will most likely be lighter, but the question is if that's light "enough" - and also NEMA 8 motors seem to be quite expensive.. like around 200 USD.


Is there some extruder like this out there already? If not, what do you guys think? Do you think it would be feasible, or at least worth a try? Those mini stepper motors are actually pretty cheap, like 6-7 USD.

And if you know what you are doing (I don't) maybe it won't take that much effort to modify the firmware to take one E motor instruction and translate that to two separate drive signals...

Or, maybe you can create two sets of instructions for two E motors during slicing - I would guess you would either need a new or modified version of a slicing software or modify the g code instructions which were generated for a single E motor. The latter may not be too hard, since these two motors always have to work in unison, you probably can figure out the E step values for the second motor by multiplying certain ratio (if they need to move different steps) or simply by duplicating the same value (if they move the same steps).

Edited 2 time(s). Last edit at 08/24/2013 02:45AM by Yamster.

I had wanted to try this idea with some NEMA 11 motors. I think it has a chance of working, because the force required to retract the filament should be significantly less than the force required to extrude. The motor that is closest to the print head will have to do 100% of the work for retraction, because the hysteresis in the Bowden tube will not allow the main drive motor to assist.

I'm not sure you can run both motors in parallel off of a single driver...

iquizzle Wrote:
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>
> I'm not sure you can run both motors in parallel
> off of a single driver...

I have Prusa i3, and I believe my Z motors are driven off the same driver. Well, I actually know they are drive off the same driver, because they share the same connector - I wired them such way when I had to change the connector type; two of original connector type wouldn't fit to my RAMPS board.

However, in my Z motor case, they are the identical motors. I don't think we can do this for two different types of motors, especially when each has a different step angle...

As far as the motor closer to the hot end doing more work during retraction, I agree that probably is the case. However, while it has to do more work for retraction than extrusion, I don't know if it's going to be quite 100% of work.. Even with hysteresis, I think (and hope) pulling the filament out completely alone will be very different from pulling the filament while some big brother is also pulling it back there.

The reason I thought a small motor may work is that I thought the only actual work it has to do is just to relieve the hysteresis... I've never used Bowden extruder myself, so I haven't seen it in action in real life. However, I would guess the amount of hysteresis is quite small - not visible for human eyes to even detect, but has just enough presence to affect extrusion, which is really really delicate process.

Oh well, the only way to find out for sure would be to try out.

Keep in mind that NEMA isn't a specification about the torque output of the motor. It is a specification about the dimension of the motors and here their mounting holes are located. There can be motors with a smaller NEMA size that have the same torque as a larger one. Granted, the bigger the motor the more torque you can pack in there.