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what is a 'normal' feed force required from extruder motor?

Posted by shadowphile 
what is a 'normal' feed force required from extruder motor?
November 10, 2014 10:21PM
Hi All.
Still trying to get to the point where I can stop fooling with a reliable extruder feed and move on! This is my first printer and been stalled for weeks on this.
I finally got a reasonable 10mm test cube printed last night, my first almost-acceptable print. And then it started crapping out again.
Without ways to measure forces it seems I have to rely on either crowd feedback or trial and error. The latter is becoming unacceptable at this point when I don't even know what is normal.

I have read a lot about the extruder-motor and gear issues, both skipping and chewing, but not enough to solve the problem socratically. (thinking only)

It's a Kossel Mini kit.
-I upgraded to a 3mm e3d V6.
-The feed tube is pure teflon, about 24 inches, bent in a U-shape so the tube shoots up about 30 degrees off vertical. The tube bends almost 180 depending head position but this is literally the most relaxed arrangement I could work out and still mount the extruder motor on the frame. I say this because it was mentioned in other places as a source of friction.
-Feed motor is NEMA23, about 2 inches long. It's chinese so I have no idea of quality. Feed gear is just that: a gear with straight teeth and flat ends (about 10mm diameter). The gripping mechanism is from a custom printed design (not mine) that uses a spring and idle pulley.
-Drivers are standard Polulu's with tiny stick-on heatsinks. (I think is A4988). The setting is up to about .7 or .8 volts now.
-3mm PLA
-200 degrees.
-0.4mm nozzle

I used a Dremel tool with diamond blade to grind a grove into the teeth which gives the gear great bite compared to before, which is why I finally got a decent print last night, but it didn't last for some reason. I cleaned it out, replaced the filament, the sharp teeth corners now only seem good for chewing parallel slots. smiling smiley

Anyway, even if the gear-bite is tight (pinching with my hand), I can't get more than about 10-20 mm/min feedrate before the extruder motor starts skipping.

Assuming the hotend is ok and not plugged, the only two problems are motor too weak, and/or drive gear grip too lame.
I ordered a Mk7 stainless gear so will hopefully fix the gripping issue.
I still have the skipping problem though, and the driver is mostly maxed out.
I wanted to order the Mk8 gear because it would almost double my torque (6mm diameter) for nothing, but then it would not align with the filament guide holes to- and away from the gear. I would have to redesign the extruder assembly.

This seems like a lot of trouble (especially from a 'kit') just to get something working. Is this kind of headache typical of bowden-tube/3mm pla direct-drive/all-metal hot-end setups? And what would it take to eventually get some real speed, ie 100mm/sec? At some point the forces within the filament path are going to get beyond the original 'specification', whatever that is. I've looked everywhere for tables of back-pressure vs feedrate/filament/temperature type data,no luck. I'm an engineer, I'm used to voltmeters, oscopes, pressure-gauges, things that MEASURE. I'm not good (or patient) with this kind of guess-work.

(BTW, if feeding by hand it requires a strong grip and very stiff push on the filament to get a reasonable (ie "it looks good") filament from the nozzle. But my hand cramps up pretty fast. That is the best I can quantify the feed-force for you.)
Re: what is a 'normal' feed force required from extruder motor?
November 11, 2014 01:33AM
I suspect, from what you have written, that the problem lies with your nozzle. I have two machines that have an E3D V6 and I have built two others for clients. It should be fairly easy to, manually, push filament through the nozzle. That said mine were 1.75 versions. I have no experience with 3mm. Are you
Positive your motor is a NEMA 23? Most people use a NEMA 17
Re: what is a 'normal' feed force required from extruder motor?
November 11, 2014 04:24AM
Re: what is a 'normal' feed force required from extruder motor?
November 11, 2014 11:20AM
I run all my motors at .41-.42v ~ 1amp with my drivers. I also run a geared nema 17

Edit: Also what drivers are you using? What do you have them set to? Its possible you have an underpowered motor. Also the temperature of the hot end changes the extrusion rate.

for me anything under 220c with ABS seems to be more dificult. Sweet spot for abs seems to be 2225c - 230c. Much higher with the larger orafice head. Also what size nozzle do you have? .4mm has a top end of 100mm/s but the smaller nozzles have significantly less.

EDIT2 : Found your problem.... I had to reread the info. you said your running .7 - .8v referance? Are you sure your stepper drivers are not overheating? They may just overheat at those settings as I think that around the maximum they can provide.

Edited 2 time(s). Last edit at 11/11/2014 11:27AM by jaguarking11.

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Re: what is a 'normal' feed force required from extruder motor?
November 12, 2014 02:28PM
This is brand-new V6 e3d.

@AndyCart: At first estimate it should be close to the same

@hercek: I came across this article and I would LOVE it, the exact kind of sensor I would devise at work, and a really useful live gauge to monitor and tweak over time and changes.
Alas, I'm not prepared to go that route right now, it should not be necessary to just get the setup working. But definitely something I would prioritize if I get to the 'pimping' stage. (or would that be 'hot-rodding'?)

@jaguarking11: I started at about .4, then tweaked higher when the skipping issues were confronted. Then I discovered that I could adjust that value live while the motor was turning so I just turned it up until it started skipping then backed off some. (at a slow feed so that it was possible at all)
It is supposed to be a NEMA 23, part of the 'upgrade' of the kit, but I will check. I'm fairly sure though.

Is there a chart somewhere of viscosity vs temperature for PLA? Google search is not succeeding for me. I'm already at 200 deg, found it made a big difference from the default 185 in Pronterface.

I read another forum post where somebody actually rigged the head to push on a scale while feeding, using a spool of tape as a standoff. Sounded like a good hack to get some force numbers to start with.
Re: what is a 'normal' feed force required from extruder motor?
November 13, 2014 06:07AM
Hi shadowphile,

I also had this problem with my extruder motor on a kossel mini,
in my opinion a direct drive is not powerful enough for this setup.

Quote from myself from this topic:


My problem was solved by replacing the motor with an expensive geared extruder motor,
if you want a cheaper solution you can probably also use a 3D printed gearbox design!

The problem in the other thread was solved by increasing the voltage, but you already did that.
I guess that our bowden tube is too long, or the bend curve increases the resistance too much?

RS-Online Ormerod #263, Kossel mini with Minitronics, Prusa i3 MK2
Re: what is a 'normal' feed force required from extruder motor?
November 13, 2014 05:26PM
I just bought a small digital weight scale so I can do some force tests.
I thought my extruder motor was a NEMA23 but I was wrong! It's a long (~2") NEMA17.
In wonder what the other built- and kitted- Kossel Mini's are using....

edit: did some wandering. The Kossel Mini is native 1.75mm design. People using 3mm have had to basically redesign the extruder setup, especially the addition of gears. Found several designs for this on thingiverse.

Edited 1 time(s). Last edit at 11/13/2014 05:37PM by shadowphile.
Re: what is a 'normal' feed force required from extruder motor?
November 13, 2014 07:01PM
Nema 23's are not supposed to be driven with standard Polulus, you probably will need an external driver, such as a Gecko driver.

23's are typically higher stepping rate and need more amps, sometimes different voltages as well.

Edited 3 time(s). Last edit at 11/13/2014 07:08PM by sheepdog43.
Re: what is a 'normal' feed force required from extruder motor?
November 16, 2014 05:47PM
Ok, I think I got this worked out, thanks to a number of scattered pieces of info:

-Friction of 3mm PLA through close-fit PTFE tube of ~24 inches was not huge, but significant. I cut the tube in half. Strain on the effector is higher but I have a chain of rubber-bands pulling on the top of the loop to help it out. Might be a problem once I start trying to dial in some accuracy.
-I raised the temp to 240 for PLA. I tried to do some manual estimates of the temp vs push force and it seemed to get better at higher temps but its hard to quantify.
-I replaced the really lame feed 'gear' with a real drive pulley, a MK7. Big difference.

I'm now able to calibrate the feed rate reliably and can print out objects if I keep the extrude speed to about 15-20 mm/sec, for a 0.4mm nozzle and .2mm layers. Even then I keep tickling the stepper driver to higher drive levels (bias = ~0.6 to 0.7) and the driver IC starts getting hot unless I force air onto it. Even THEN I still get random stepper skips on the faster (infill) moves.

I now have a MK8 drive pulley which is supposed to give me 30% more torque but I have to redesign the existing filament-feed parts because the gear radius is smaller than the original design.

So it really appears that the Kossel Mini was designed for a 1.75mm which requires roughly 1/3rd the force.

Fortunately it's working enough that I can print my own upgrade parts. I already printed out an adjustable three-point spring-mount design to support the bed so I can do simple mechanical bed leveling. It also makes the bed nozzle-crash-resilient and allows me to manually remove the glassbed/heater plate stack with three simple twists.

Software leveling is just a pain. I gave up on the auto-level until I can design a better probing method than the custom (and not working) design that came with the kit.
BTW, the obvious easy-to-do adjustment would be to move it close to each post, put it exactly where I want it, then send a command that 'here be zero'. Do that on all three upright posts as needed. I can't find a way to do that. Right now I'm tweaking the endstop adjustments but that is still rather fiddly.)

thanks for all the help, I'm finally printing, yay!

I want to eventually get the speed up which probably means moving up to a geared stepper.
Re: what is a 'normal' feed force required from extruder motor?
November 18, 2014 03:43PM
I have tried 3mm PLA through a Bowden setup using a Gregs geared extruder with my own homemade 8mm hobbed bolt, I found the pressure required was enormouse and had issues with the drive grinding the filament, I have since designed an extruder with twin hobbed bolts which did not slip but used to build so much pressure in the Bowden every now and then the motor would recoil backwards, this blew up the stepper driver, I then upped the large gear size and tried again and failed again, the extruder can push that hard that I stretched the PTFE tube out of the air fitting it grabbed the filament, end of that print, still don't know how anyone can push 3mm through a Bowden, have bailed out on the bowden and have now gone for an effector mounted extruder pushing directly into the hot end, works but would be speed limited due to the mass I am swinging around.
Good luck
Re: what is a 'normal' feed force required from extruder motor?
November 18, 2014 09:22PM
yeah Paul, I don't see a future with this setup. The Bowden is cool and a cleverly simple and cheap mechanism, but I don't like the idea of using the filament as the force-transmitting mechanism, and it's worse with the newer spongy materials. Operation of the machine should not have to compensate for the material, ideally.
I'm toying with how I might put the feed-grip on the extruder platform but physically drive it from a remote motor. (I see lots of flying pulleys and swinging trapeze artists..)
Re: what is a 'normal' feed force required from extruder motor?
November 19, 2014 08:30AM
This thread might be of interest to you, a remote drive for extruders:


Re: what is a 'normal' feed force required from extruder motor?
November 19, 2014 01:49PM
I am using the geared stepper motor and hobbed gear for my extruder setup. It's a NEMA 17 with a 5.18:1 gear drive. Works fine - no issues with pushing filament. I'm also using the E3D v6. I've tapered the end of the Teflon tube that is seated in the cold end, up against the heat break. It has fixed ALL of my jamming issues with the E3D. FWIW, I'm using the FSR's for bed leveling, and it works every time. I think I need to do a write-up on my printer though so folks can see how this setup is working. I'm able to get 30 micron prints very consistently, as well as very clean 200 micron prints. Without the tapered Teflon tubing, it was failing a lot.
Re: what is a 'normal' feed force required from extruder motor?
November 21, 2014 02:41PM
triplanedave: it sounds like you are running 1.75mm filament. If so, you still felt it neccessary to go to a geared motor?
I'm also toying with going the FSR route, after looking at someobody's setup last night.
When you say 'tapered', you mean you sharpened it like a pencil, or reamed the inside out like a hollow cone?
Re: what is a 'normal' feed force required from extruder motor?
November 23, 2014 11:42AM
@Shadow - I bought the geared motor as I was starting to get into 3D printers. So, it was more of a purchase under lack of information, rather than a specific need. That said, it works great. By tapered - like a sharpened pencil. Like Life cereal - "Hey Mikey, try it, you'll like it."
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