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isphording90

Small steppermotor for Direct Extruder

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Your motor needs too be up to the task:

It needs enough torque to operate your extruder. I don't know how much that will be, but my guess is it can be much smaller than the big guys used in the UM axes...

Then, it must be compatible to your stepper drivers which means, it must have the same principle of operation (sorry this may be a strange formulation - I'm no native english speaker...).

There are several different types of stepper motors, like 2-phase or 5-phase. And motors may differ in step sizes.

Basically, if you want to drive your smaller stepper motor with the UM electronics, you need one that is the same as the stock UM motors but just smaller. In that case you can just use it without making any changes to the firmware.

You'll probably have to lower the driver current but afaik that's no problem. (My UM is still shipping...)

Then of course, with your new setup, you'll have a different extrusion speed because your setup will probably have a different gear transmission ratio. You'll need to recalculate that when setting up the new extruder. This shouldn't be much of a problem though, maybe contact Daid if there's a simple way to tweak that in Cura?

I'd have a look at these:

http://de.nanotec.com/produkte/250-st4118-schrittmotor-nema-17/

(These are industrial grade so they're not the cheapest, but they should do a good job)

I read somewhere the UM uses NEMA17 (or similar) motors. So they probably have a smaller size version of the same type which should work fine.

 

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At least the current draw is similar - so you probably won't need to make any changes to the stepper driver.

I don't know about the step size, maybe there's something at the RepRap wiki...

2-Phase, bipolar is correct. That makes 4 wires. Unipolar should be 6 wires and is less effective, less popular (I believe I have never seen an unipolar stepper motor before...).

Don't know about the driver's voltage, but as they're current controlled, that shouldn't be an issue. They just need to be able to supply 3.6V at 1.1 A. The larger motors should have the same or higher specs (given they're bigger and stronger) so that should be fine.

It's not necessarily a problem if your motor has a different step size than the original one - that just adds another factor to your motor speed equation. And of course yes, the more steps, the more accuracy. And less maximum possible speed (not an issue on the extruder...).

About the software stuff:

What I wrote before was assuming that the calculations for the motor driver are made in Cura (or whatever other PC software you use) and not inside the Marlin Firmware. That's probably wrong because these programs aren't machine specific so they can't know what the actual equation for the motor speed is.

So probably the PC software just gives a command like "go at 50mm/s" and the Marlin FW does all the calculations. Then you will have to change the firmware OR do the calculations yourself and enter the result in your PC software.

- But those are still only assumptions, I don't know the UM firmware...

 

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...

So probably the PC software just gives a command like "go at 50mm/s" and the Marlin FW does all the calculations. Then you will have to change the firmware OR do the calculations yourself and enter the result in your PC software.

- But those are still only assumptions, I don't know the UM firmware...

 

The gcode specifies the amount of filament to be fed into the printer for each printing head move. Based on the speed and distance for the move, the firmware determines the rate per second at which the filament has to be fed in, and the steps-per-e setting determines the rate at which the firmware has to fire the stepper to achieve it. So, no firmware change should be necessary, if the motor is connected in the same way as the original, to the same pins etc.

The steps-per-e can be set in Cura, so that it gets specified in each print via a gcode command in the header, or it can be stored in EEPROM using the Ulticontroller, or other gcodes.

Note that the number of steps per e - which is a function of the steps-per-revolution of the motor, the microstepping setting, and the gearing for the extruder - does affect performance in a couple of ways. Firstly, for retraction, speed is currently limited by Marlin's limit of 40,000 steps per second (which limits the standard extruder to a speed of about 48mm/s - which is probably more than anyone needs with that design, but in an ideal situation, higher speeds might be beneficial). Lower steps-per-e would help with this.

On the other end, especially with 3mm filament, slow/fine printing requires very small rates of infeed. For this, a very high number of steps-per-e is beneficial, since it allows finer grained control over the amount of filament being fed in. So it's a balancing act between the two use cases. However, with a Bowden-less set up, retraction may be less demanding.

 

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There will still be a tube of some sort, I presume, but since it's not between the drive and the head, there's a lot less plastic under tension, less variability in the energy in the system, and probably you can get away with shorter retraction distances, as you don't need to worry about taking slack out of the filament in the tube.

 

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illuminarti, what does a tube do for you in this case?

It seems that the primary problem with a head mounted extruder is the aggregate weight of the motor and the gearing, and of the two, the motor weight is the more difficult to reduce. I wonder about the possibility of using either a flexible drive shaft or a sliding square rod to connect a rear mounted stepper to extruder gearing on the head.

 

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I meant the bowden tube specifically. I wouldn't want a bowden adding any friction to retraction if the drive motor was downsized severely, so something with at least a 4mm ID would be a minimum. For the electronics, a lightweight sleeve or coil that stays out of the way seems sufficient. If the extruder gearing is on the head, it may make filament swaps easier to separate the electronics from the filament, or just feed the filament naked to the head.

What do you think of using a flexi-shaft to transmit mechanical energy from a motor mounted on the frame to the head? Do you think that given the torque and low speed that there would be much hysteresis or backlash? There was a RepRap forum thread on this, but it petered out inconclusively, and someone posted a rough design in Feb 2011 as http://www.thingiverse.com/thing:6653

Under the "so crazy it just might work" category, I wish I had the parts to try a sliding square shaft like here http://garyhodgson.com/reprap/2012/01/experimental-off-carriage-extruder-motor/ but with the extruder motor mounted behind a pivot at the right rear corner, the shaft cantilevered in front of the pivot, and the sliding shaft coupler on the head similarly able to pivot. The coupler could potentially bind, but the arc is less than 90 degrees, so perhaps not.

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The flexshaft won't work so don't bother trying. I'm developing a small direct drive extruder for around a year now and I've tried many power transmission types. Sometime last year I tried out a flexshaft based extruder. Everything worked, except the flexshaft itself. The hysteresis causes very very bad walls because of the constant fluctuation of extrusion.

Bear in mind while doing all this that a single NEMA17 motor is much heavier than the whole UM printhead. And that's without a hobbed bolt and all other parts. So if you don't have an effective way of keeping the weight down, you are basically sacrificing all that makes UM faster and more precise than other printers. You get a bit less oozing and retraction problems, but you lose speed and precision.

 

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Agreed. The only real reason I see to make a direct extruder would be printing in highly compressible filaments like TPE, which I haven't tried, but seem like they would bunch up in the bowden tube and have a very long lag time between desired extrusion rate changes.

Nylon does this to some extent because the filament is very soft. The lower speed of a heavy print head wouldn't be a huge problem because I'm getting my best results printing nylon at 30-40mm/s anyway...

 

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Thanks, chopmeister, for confirming my worry regarding flexshafts. My reason for exploring direct extruders is to come up with a more elegant design for paste extrusion than all of those designs where filament is used to push a syringe plunger or whatnot.

Another design that I have been mulling would be to:

 

  1. mount the extruder motor at the top-right-rear above the case,
  2. which drives a splined shaft running right-rear to right-front,
  3. which drives bevel gears mounted on the right y sliding block
  4. which drives a second splined shaft mounted onto and above the y sliding blocks, crossing above the head,
  5. which drives an extruder assembly mounted on the head.

I figure that that even when well lubricated, this assembly adds enough carriage weight and friction that I wouldn't want to even try this without first switching to snowygrouch's direct drive mod. An alternative to splined shafts would be square rods with captured square linear bearings, except that square linear bearings are so expensive in comparison to splined shafts.

 

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Well, if that's your goal, why not just use compressionless housing from a bicycle and make a "reverse bowden" - that acts in compression instead of tension, with a metal or aramid wire rope running down the center - which is compressing a plunger when pulled - and is getting wound up onto a spool by a stepper motor on the back of the Ultimaker.

Or, if you're trying to get away from the plunger entirely, you could probably use two compressionless tubes and power your paste-pushing mechanism by wire drive with the stepper still on the back of the machine.

 

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Yes, I'm trying to get away from the plunger entirely. I don't want to drag around 60cc of material that may be as heavy as precious metal clay, and I want a more continuous printing process than syringes would allow.

I do like the idea of using two bike bowden cable housings to constrain wire drive cables! I plan on feeding the paste to the head through tubing under pressure, but I want to finely control the feed rate with a small moineau pump on the head, and a wire drive would turn the rotor rather nicely. That's so much easier to provision, build and maintain than the Rube Goldberg contraption that I was coming up with. Thank you!

I apologise for hijacking isphoarding90's thread, but it seems that your solution may serve him as well. Only one question: How would one best cleanly splice the wire (or synchromesh cable) into a closed loop after feeding it through the compressionless tubes?

 

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Hmm, yeah, that's an interesting problem. I'm sure it has been solved by some industry... but my first thought would be to use a braided aramid, like this 2mm spectra: http://www.backpackinglight.com/cgi-bin/backpackinglight/aircore_1_spectra_cord.html#.UfsW55K1Gzk

and then splice it together with some old school rope technique. They actually describe doing this on the product page. Could work well.

 

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