RRF 2.03 pressure advance causes 20% overextrusion
-
i think the biggest problem is that they cant do a full revolution
-
@Veti said in RRF 2.03 pressure advance causes 20% overextrusion:
i think the biggest problem is that they cant do a full revolution
On one hand, that shape of servo can be had in continuous rotation, on another hand, it’s not a servomotor, what is meant when some people say they want a servo on a CNC type machine like a 3D printer. Hobby servos are very limited on what they can do for that kind of use because the type of input doesn’t allow for the level of control needed. Hobby servos also use deep multistage gearboxes, which induce excessive backlash.
-
This thread is long and getting hard to navigate. I think what is being said is that using RRF 3.1.1, a particular print causes 3% over-extrusion (it was more in earlier firmware). Is that correct?
My suspicion is that this is caused by the firmware rounding pressure advance to the nearest microstep. If this is the case, changing extruder microstepping will affect the amount of over-extrusion. @jschall, are you able to test this? What extruder microstepping are you using at present, and what is the extruder steps/mm ?
-
@jschall Can those servos meet the speed/acceleration achieved by a stepper? Not saying they can't I just have no idea of what they can do.
Yes. Regardless, not suggesting the use of those specific servos.
@Veti said in RRF 2.03 pressure advance causes 20% overextrusion:
i think the biggest problem is that they cant do a full revolution
That's just because of mechanical stops in the gearbox to prevent the potentiometer from having problems. In an extruder application, you'd have a somewhat different configuration. Remember, this is just more of an example of how cheap, small and light an integrated servo motor could be, not suggesting that these are options for our application without significant modification.
@JRDM said in RRF 2.03 pressure advance causes 20% overextrusion:
Hobby servos are very limited on what they can do for that kind of use because the type of input doesn’t allow for the level of control needed.
@JRDM said in RRF 2.03 pressure advance causes 20% overextrusion:
Hobby servos also use deep multistage gearboxes, which induce excessive backlash.
I'm not talking about using hobby servos. I'm using hobby servos as an example in a first-principles argument that steppers are dumb in extruders. Backlash not important for extruder. Just means you need very slightly more retraction to take up the backlash.
@deckingman said in RRF 2.03 pressure advance causes 20% overextrusion:
So if carriage mass isn't a limiting factor, then why reduce it? In fact I have demonstrated this by printing at up to 300mm/ sec with a moving carriage mass of around 2Kgs driven by modest NEMA 17s. I'll make a other contentious statement that adding mass reduces the resonant frequency - I don't get ringing -ever.
It's more about acceleration than speed. A NEMA 17 could accelerate a locomotive to 300 mm/s, assuming low enough friction. It'd just take longer.
-
@jschall said in RRF 2.03 pressure advance causes 20% overextrusion:
@deckingman said in RRF 2.03 pressure advance causes 20% overextrusion:
It's more about acceleration than speed. A NEMA 17 could accelerate a locomotive to 300 mm/s, assuming low enough friction. It'd just take longer.
That's a ridiculous statement because even with zero friction, the acceleration would be so low that the distance travelled would be from here to the moon or perhaps even the other side of solar system.
In order to attain a speed of >300mm on a 3D printer implies that the acceleration must be sufficiently high to attain that speed over a distance of a few centimetres.
If a Nema 17 is perfectly capable of attaining printing speeds of >300mm/sec with a 2Kg mass, then it is implicit that it must have sufficient torque to accelerate that mass up to that speed in less than half the axis length.
-
@deckingman said in RRF 2.03 pressure advance causes 20% overextrusion:
If a Nema 17 is perfectly capable of attaining printing speeds of >300mm/sec with a 2Kg mass, then it is implicit that it must have sufficient torque to accelerate that mass up to that speed in less than half the axis length.
So you are actually getting achieved speeds of 300 mm/s with a NEMA 17 pushing around 2kg?
Still, acceleration matters a lot. It dominates when printing smaller parts. What kind of accelerations are you achieving?
-
@jschall said in RRF 2.03 pressure advance causes 20% overextrusion:
@deckingman said in RRF 2.03 pressure advance causes 20% overextrusion:
If a Nema 17 is perfectly capable of attaining printing speeds of >300mm/sec with a 2Kg mass, then it is implicit that it must have sufficient torque to accelerate that mass up to that speed in less than half the axis length.
So you are actually getting achieved speeds of 300 mm/s with a NEMA 17 pushing around 2kg?
Still, acceleration matters a lot. It dominates when printing smaller parts. What kind of accelerations are you achieving?
Yes - 2Kgs with Nema 17s all the time. My travel speed is set to 350 but I've done some crazy high speed printing at up to 300mm/sec actual print moves using multiple melt chambers. https://somei3deas.wordpress.com/2018/10/14/real-3d-printing-at-high-speeds-and-even-higher-melt-rates-with-a-large-nozzle/ and the accompanying YouTube video https://www.youtube.com/watch?v=rUV5IZxfAxU
It's a CoreXYUV with the 5 Bondtech extruders mounted on the UV gantry powered by Nema 17s. Both gantries move in sync. The XY gantry just has 5 Colour diamond hot end so weighs somewhat less than the 2 Kgs that the extruder gantry with the 5 BMGs weighs.
IIRC, back then I was running at a modest 1000 mm/s^2 so to attain 300 mm/sec would take 0.3 seconds and the head would travel 45mm to reach that speed. Obviously with a "normal" but still high hot end mass of say 500 gms, the acceleration could be pushed to 4,000 mm/sec^2 with those same Nema 17 motors.
But it's all academic because as I have said many time before, the limiting factor is how fast one can melt and extrude filament. High carriage acceleration for print moves is even more pointless because one can't accelerate the extrusion flow rate at anything other than a modest rate. All that happens is that you get a pressure pulse inside the hot end because of the combination of viscous filament and small nozzle diameter.
-
@deckingman So why the hell is bowden so popular? It isn't simpler, it isn't cheaper, it isn't better.
-
@jschall said in RRF 2.03 pressure advance causes 20% overextrusion:
@deckingman So why the hell is bowden so popular? It isn't simpler, it isn't cheaper, it isn't better.
I can't say for sure but I suspect it's because the 3D printing community in general has a (in my opinion misguided) perception that less mass is always better. As a statement on it's own that is true. That is to say, it takes certain amount of torque to accelerate a certain mass at a certain rate. So reducing the mass will allow for faster acceleration.
BUT (and it's a big BUT) if the limiting factor is not how fast one can accelerate the carriage but rather how fast one can extrude filament (or accelerate the rate of extrusion) then why reduce the carriage mass?
An analogy I like to use is that one could do the school run in heavy 4x4 SUV. One could also buy a light weight Ferrari which has a much higher top speed and much faster acceleration. But what's the point if you are going to be stuck in traffic and/or travelling in built up areas with 30 mph speed limits for entire journey? You won't get the kids to school any quicker, no matter what acceleration rate the Ferrari is capable of.
But the 3D printing community has this fixation with reducing mass, possibly because there are many "armchair engineers" who have read on the internet that less mass = faster acceleration and from that they deduce that this will therefore lead to faster printing. But as I've said many, many times, one can't use that acceleration except for non print moves due to the way we melt filament and force the resultant viscous fluid through a small orifice.There are situations where it is physically difficult, if not impossible to mount a "direct" extruder. My own printer is one such usage case because I have six extruders feeding into single hot end. But I manage to keep the Bowden tubes as short as possible by mounting the extruders on a separate gantry (the UV gantry) which sits above the hot end (the XY gantry).
One last contentious comment is that as mass increases, the resonant frequency decreases. Is it purely coincidence that I never get "ringing" or "ghosting" when using my heavy hot ends but people with lightweight hot ends offen have those problems?
-
@deckingman said in RRF 2.03 pressure advance causes 20% overextrusion:
One last contentious comment is that as mass increases, the resonant frequency decreases. Is it purely coincidence that I never get "ringing" or "ghosting" when using my heavy hot ends but people with lightweight hot ends offen have those problems?
What is your jerk set to?
