Strange behaviour



  • I just noticed that whenever printing hollow cylinders, specifically the inside wall of the cylinder the print head will slow down to almost a complete stop at the end of the run.
    It lasts almost 2 seconds.
    On first look I thought it was pausing at a complete stop, but it is actually moving at an incredibly slow speed.
    I put my fingers lightly on the filament and I can feel it retract ever so slowly at the distance (i'm guessing) that I have set for retraction in my slicer, during this slow down.

    When printing the outside of the wall, it behaves as normal.
    I have also noticed this sometimes on the corners of a square, on the inside wall.
    Printing speed doesn't seem to matter. It will do it at any speed I have set.

    Anyone ever experience this? I did a search but didn't find anything exactly matching my problem.


  • administrators

    Can you share the Gcode? If you are using pressure advance, does turning it off make any difference?



  • Surely. But forgive my ignorance.
    Do you mean the slicer generated gcode for the print?

    I haven't tried it with pressure advance off. But I surely will and report back!



  • Just in case you did mean the gcode for the print:

    https://dl.dropboxusercontent.com/u/15733846/20mm%20Cube_Cylinder%20Test.gcode



  • Hi Scott, you just recently enabled pressure advance. Share your jerk speed and accell settings.



  • [[language]]
    ; Drives
    M569 P0 S0 ; X goes forwards
    M569 P1 S1 ; Y Reversed
    M569 P2 S0 ; Z goes forwards
    M569 P3 S0 ; Extruder goes forwards
    M350 X16 Y16 Z16 E16 I1 ; Configure microstepping with interpolation
    M92 X99.5 Y99.5 Z399.7 E79.9 ; Set steps per mm
    M566 X900 Y900 Z12 E120 ; Set maximum instantaneous speed changes (mm/min)
    M203 X18000 Y18000 Z500 E1200 ; Set maximum speeds (mm/min)
    M201 X500 Y500 Z250 E250 ; Set accelerations (mm/s^2)
    M906 X1200 Y1200 Z1200 E1200 I30 ; Set motor currents (mA) and motor idle factor in per cent
    M84 S30 ; Set idle timeout
    M572 D0 S0.1	;Pressure Advance
    
    

    As coincidental as it was it never occurred to me that Pressure Advance was the culprit.
    I guess I was so giddy about being able to hit 100mm/s I got dumb.

    I am at work now, (I think you guys are 6 hours ahead) and will get off in 4 hours. I will disable PA, test it out and report.

    Thanks so much!!!



  • I've got pressure advance turned off at the moment because there is an issue using it with multiple extruders but when I did have enabled, I found I had to increase extruder jerk from 600 to 1200 so as yours is set to 120, I'd guess that would be the culprit.



  • I will change it tonight.

    What is a good speed to set it at? Is having a high speed setting, such as what I have on the X and Y detrimental even though I have no illusions of ever going that fast?



  • your settings are very low…. 3000 mm/m jerk and max speed = 50mm/s.... your slicer defaults to 55mm/s speed for retractions. so i recommend upping max speed and jerk for extruder to 3600 and even faster if you want to do faster retractions.

    You have a solid and light build, i do not see why you cant up your jerk and accell settings for x and y as well... just push it until the quality of print starts to degrade for your target speed



  • Honestly with such low extruder settings you probably dont know what your best x and y settings are yet because of how low those were.



  • @Scottbg1:

    I will change it tonight.

    What is a good speed to set it at? Is having a high speed setting, such as what I have on the X and Y detrimental even though I have no illusions of ever going that fast?

    Your X and Y speeds will mostly be limited to by the speeds you set in your slicer and also by your accelerations. So the maximum speed in config.g is largely irrelevant IMO. I just use it as sort of safety feature in case I ask the printer to do something stupid. One way to decide is to work out what maximum non-print speed is likely to be and set it to that. For example, I like to use a highish non-print move speed of 350mm/sec which is 21,000 mm/min. Another way of doing it is to calculate the maximum speed you could possibly achieve for the acceleration you have and the longest move you can do (bed size). So for example assuming the longest move is 200mm and you have your acceleration set to 500mm/sec^2 then the maximum speed you'll get up to is sqrt (2accelerationlength/2) = about 316mm/sec or 18,960 mm/min. So setting the maximum speed to anything above that will have no effect. BTW I'd say your acceleration settings are on the low side but it depends on what your motors torque characteristics are, the mass that you want to move, and how rigid the frame is. My X axis mass is a whopping 1.67kgs and calculating the acceleration possible with the torque available for the motor, the number comes out at around 1200 mm/sec^2.



  • Thanks for that. It's apparent I have so much more to learn.

    I am sitting here thinking that most of the troubles I have experienced were most likely due to me trying to drive it harder than it could go.
    And yes, I haven't been able to get my retraction settings to work right yet. I have had very bad oozing and over extrusion at the beginning and end of a run. I have set my coast and wipe settings to some crazy numbers that I didn't feel good about.
    I was beginning to think that the PLA+ was more akin to PETG than anything. It was crazy.

    I guess I have some more tweaking to do tonight!
    With that, I want to disable Pressure Advance, and play around with my slicer once I up the jerk settings, and see how well I can get it to behave with retraction settings only. Then I will enable pressure advance and see exactly what the benefit it offers.



  • @Scottbg1:

    …..........
    With that, I want to disable Pressure Advance, and play around with my slicer once I up the jerk settings, and see how well I can get it to behave with retraction settings only. Then I will enable pressure advance and see exactly what the benefit it offers.

    Definitely the best approach. Cut down the number of variables as much as possible otherwise you'll just get lost.



  • @deckingman:

    @Scottbg1:

    I will change it tonight.

    What is a good speed to set it at? Is having a high speed setting, such as what I have on the X and Y detrimental even though I have no illusions of ever going that fast?

    Your X and Y speeds will mostly be limited to by the speeds you set in your slicer and also by your accelerations. So the maximum speed in config.g is largely irrelevant IMO. I just use it as sort of safety feature in case I ask the printer to do something stupid. One way to decide is to work out what maximum non-print speed is likely to be and set it to that. For example, I like to use a highish non-print move speed of 350mm/sec which is 21,000 mm/min. Another way of doing it is to calculate the maximum speed you could possibly achieve for the acceleration you have and the longest move you can do (bed size). So for example assuming the longest move is 200mm and you have your acceleration set to 500mm/sec^2 then the maximum speed you'll get up to is sqrt (2accelerationlength/2) = about 316mm/sec or 18,960 mm/min. So setting the maximum speed to anything above that will have no effect. BTW I'd say your acceleration settings are on the low side but it depends on what your motors torque characteristics are, the mass that you want to move, and how rigid the frame is. My X axis mass is a whopping 1.67kgs and calculating the acceleration possible with the torque available for the motor, the number comes out at around 1200 mm/sec^2.

    Thanks for that, I always wondered what the math was, I have only ever used the reprap calculator. Yeah he has a pretty light but what looks rigid setup with carbon fiber linear rods which is why I recommended he try out some better accell settings but only once he fixes his extruder settings so it can keep up.



  • @Scottbg1:

    Thanks for that. It's apparent I have so much more to learn.

    I am sitting here thinking that most of the troubles I have experienced were most likely due to me trying to drive it harder than it could go.
    And yes, I haven't been able to get my retraction settings to work right yet. I have had very bad oozing and over extrusion at the beginning and end of a run. I have set my coast and wipe settings to some crazy numbers that I didn't feel good about.
    I was beginning to think that the PLA+ was more akin to PETG than anything. It was crazy.

    I guess I have some more tweaking to do tonight!
    With that, I want to disable Pressure Advance, and play around with my slicer once I up the jerk settings, and see how well I can get it to behave with retraction settings only. Then I will enable pressure advance and see exactly what the benefit it offers.

    I forget which thread, but I believe it was DC42 who recommended that pressure advance get tuned with retraction off then slowly increasing retraction after tuning PA, but yes make sure you get good results without it because you have had some extrusion issues that should be sorted first.



  • Ok….

    I upped my jerk and acceleration speeds to this:

    [[language]]
    M566 X1000 Y1000 Z12 E1000 ; Set maximum instantaneous speed changes (mm/min)
    M203 X12000 Y12000 Z375 E9000 ; Set maximum speeds (mm/min)
    M201 X1000 Y1000 Z150 E1000 ; Set accelerations (mm/s^2)
    
    

    I also disable pressure advance.
    I set the retraction settings to this:

    I also disabled ooze control completely.
    This is what I got at 100mm/s

    https://dl.dropboxusercontent.com/u/15733846/Retraction%20on.mp4

    I then disabled retraction completely and enabled Pressure Advance.
    I kept upping the value until it started laying lines nice and neat with no breaking.
    I ended up with:

    [[language]]
    M572 D0 S1	;Pressure Advance
    
    

    This is the result:

    https://dl.dropboxusercontent.com/u/15733846/Retraction%20off.mp4

    What do you guys think?



  • I forget that it's after midnight for you guys when it's still evening for me.

    I have changed a few more things as my night has progressed.
    I turned retraction back on, yet at a much lower rate to start out.
    I tried 3mm retraction with the Pressure Advance rate I had set.
    I don't like it.
    I ended the night with bring the retraction length back up, increasing the retract speed a little more and dropping the Pressure Advance back way down.

    I printed out a very small part for my webcam.
    I brought the speed down to 40mm/s because it has very small snap/fit pegs on it and I wanted them as uniform as possible.
    That slow a rate and that small of a part, the lines were separated and inconsistent with the PA set so high.
    And the part was pretty weak. The pegs snapped off when I was lightly cleaning stringing off with my fingers.
    I printed it PLA+ which is surprisingly strong compared to regular PLA.
    I printed it again but this time I upped the retraction settings because it was stringing badly, and I lowered the Pressure Advance.
    This time it was much, much better. It was minimal stringing and much stronger.

    I'm tempted to print it again with Pressure Advance disabled, as I really don't think I am gaining anything from it. At least on small stuff.
    If I understand it right, PA increases the extruder speed exponentially when beginning a run to compensate for the PTFE tube giving under pressure and causing under-extrusion at the beginning of the run. And then the opposite as it is coming to the end.
    My simple mind figures that would only be beneficial on larger parts with a lot of flat real estate to print.



  • You can actually observe the effects of pressure advance if you start a print without it… let it go and every quarter inch send pressure advance to different levels using the gcode console... it shows its best results on large parts but does affect small parts.

    Test with settings like 1.0 1.2 1.32 etc

    the best way i have found to dial in PA is at 60mm/s printing a 40mm cube with 1 parimeter and 0 top or bottom layers and 0 infill.

    You can really see the affect doing so.

    As far as your settings go, DC42 is going to have to chime in here because my understanding of jerk settings is for short speed changes which is what retraction and pressure advance do.

    Personally i have jerk and max speed set to the same for my extruder because if jerk is taken into account for these short movements then there is no way you are achieving 55mm/s

    But DC42 will have to clarify if my assumption is correct on this.



  • I couldn't sleep so I delved further into experimenting with settings.

    I've since changed my config settings to this…

    [[language]]
    M566 X1500 Y1500 Z12 E1500 ; Set maximum instantaneous speed changes (mm/min)
    M203 X12000 Y12000 Z375 E9000 ; Set maximum speeds (mm/min)
    M201 X1500 Y1500 Z150 E1500 ; Set accelerations (mm/s^2)
    
    

    Pressure Advance back to 0.2.

    I went higher on the jerk and acceleration settings and but I thought the printer would detonate.
    And the print quality went down. So I lowered them to what I have shown above.



  • Is your extruder a geared extruder?


  • administrators

    XY jerk and extruder jerk do different things:

    • XY jerk is a compromise. Too high and the printer will sound rough and may miss steps. Too low and circles and curves with small radius won't print smoothly.

    • Extruder jerk is mostly needed to handle pressure advance. Too high and you may see the extruder skipping steps.



  • @Whitewolf:

    Is your extruder a geared extruder?

    No sir. Not geared.

    I included the extruder when I changed the settings for X and Y because:
    1. I figured if the axes were behaving in a certain manner then the Extruder would need to match to extrude the plastic properly.
    2. I don't know what the hell I am doing.

    I understand what acceleration does. It dictates how fast it comes up to speed after a stop, and then how fast it decelerates before a stop.
    I know that this effects printing time, but I am not sure how it effects quality, unless it is set ridiculously high.

    I don't really have a full understanding of jerk. I THINK it has to do with the amount of time is dictated that the print head stops for that split second and begins travel in another direction. I can see how this could effect quality if too much jerk is applied because inertia that isn't bled off right before the stop could cause vibration, thus showing up in the print.

    I have no idea how either effect the extruder.

    I have googled the subject and most conversations are a bit advanced. I wish there was a 3D Printing guide for Dummies that explained most of this in layman's terms.
    I think I have an average level of intelligence, and capable of doing most anything I set my mind to. And I will get to a point where I understand more. I learn a little every single day just being on this forum. You guys are my only resource for knowledge. I think I am the only guy in the Deep South of the United States that has a device such as this.
    If I tell my comrades around here about it, I get: "You got a three-dee whuuut??". So I am a lone man on an island when it comes to this stuff.

    In short, I made the changes not really knowing what I was changing and what to look for other than failures in the print, or the printer vibrating off it's table.
    But I am seeing improvements every day in getting the most out of what I THINK this printer can do. Hell, I watch youtube videos of guys giving instruction on printing with a $2000 printer and their models look like hell with uneven layers, blobs all over it, and it needing a shave from all the stringing. And I rest easy knowing that my $500 printer with Chinese parts assembled by a Redneck produces far better results than that. And most of the credit goes to the great electronics I have purchased for it, and the invaluable knowledge I receive from you guys. I just hope you guys have a truck load of patience until I reach the level of understanding that I want to be at.



  • @Scottbg1. The settings you posted above don't look to be a mile out.

    Jerk as it is implemented in 3D printers isn't the same as it is the sense of physics. In physics jerk is a change in the rate of change of acceleration. It's difficult to put into layman's terms but try this. Imagine a locomotive pulling a long line of carriages and there is a bit of slack in the couplings between the carriages. If the driver starts out accelerating the engine at a fixed but slow rate of acceleration he experiences smooth motion but what happens is that the engine moves smoothly from standstill but for the first few inches, the front rail car doesn't move because of the slack in the coupling. By the time that slack is taken up, the engine is doing say 2mph. So the front carriage has to move instantly from 0 to 2mph to catch up with what the engine is doing. Hence the term Jerk. If the driver continues with his nice smooth acceleration, by the time the slack in the second coupling has been taken up, the engine might be doing say 4mph so the second carriage gets jerked instantly from 0 to 4mph. If the driver carried on accelerating nice and smoothly, the jerk for the 100th car would be like going from 0-80mph in zero seconds which is just going to snap the coupling (or some ones neck). So, if the driver knows his job, he'll start slowly but not accelerate further until the slack has been taken up in all the couplings. (Next time you are on a train, take the car nearest the front and you'll get a smoother ride).

    So Jerk in the physics sense is bad and it isn't used in 3D printers. That is to say, a longish move will start from zero speed, accelerate up to the demand speed, maintain that speed, then decelerate down to zero. All without any jerk being applied. The problem occurs mostly with circles and curves because slicers don't do arcs. Instead, a curve is made up of a lot of very small straight lines. Think of a hexagon that has 6 sides- it's kind of roundish. An octagon has 8 sides and is even more roundish so if we go up to say 100 sides, it'll look more like a circle. If we try to print that, for every one of those little tiny straight lines we'd have to start at zero speed, accelerate up to speed then decelerate down to zero and move on to the next little line. The trouble is that because each move is so short, it'll hardly start moving before it has to slow down again and the result would be that the curve would simply take too long to print. So in this situation, an instantaneous speed is applied (what is referred to as Jerk in 3D printer terms) which allows small moves and small changes of direction without having to slow right down to zero then accelerate up to speed again.

    You'll note that M566 is referred to setting the "allowable instantaneous speed change" which is a more accurate description but most people refer to it as "Jerk" (which in the physics sense, it most certainly isn't).

    Did that help or have I confused you even more?



  • @deckingman:

    Did that help or have I confused you even more?

    You sir, are a scholar and a gentleman.
    Yes indeed. The jerk settings are applied where the acceleration begins and ends, enabling a constant and fluid move throughout the run.
    Explaining how a slicer sees an arc was the kicker. That is very useful information.

    Now, I can see how there might be a 'sweet spot' in jerk settings. Too much and you experience a hammering effect which would of course be detrimental. Too little and you'll lose speed throughout the print. But is there a 'sweet spot' for acceleration? Can there be?

    I can see myself setting acceleration for say a large flat print to tweak a perfect speed setting. I can't even imagine getting that dialed in on a small print with lots of moves.
    Would the setting I have for long continuous runs be left and it work ok for smaller stuff? Or would I dial it down on purpose to have more of a one size fits all setting?

    I love this forum. Not only do I get top notch information on 3D Printing, but invaluable travel advice as well. If I ever find myself on a train, the first car is where I will be!
    I just hope the Lounge car is located in Carriage #2.

    Deckingman, a million thank yous for taking the time to break it down like that.
    I hope that someone like me runs across this conversation and they find it as profound as I have.
    I can't wait to get home tonight so I can get to work on this!



  • Great explanation Ian,

    Scott by what DC42 says my understanding of jerk was correct. Without a geared extruder you should be able to set your extruder jerk to 3600 without issue which will allow it to do 55mm/s speed changes for these small quick movements. Just make sure your tensioner is set correctly to avoid slipping.

    There are situations where this will not be beneifical, like when printing with filaments such as nylon or acetal pom during retraction heavy prints.

    You can compensate for those in your slicer by either turning off pressure advance or slowing down your retraction speed and or print. ( i am still playing around with what settings work best for these slippery filaments so sorry i cannot give a definitive on them at the moment)

    Everything mostly looks good in your settings as Ian said, but there is one more thing i would try changing. By the looks of your settings you have a Z screw similar to mine (assumption based on your settings) You might try changing your jerk to match your top speed 375 this will allow snappier z hop movements (if enabled in your slicer) i prefer not to use z hop with a screw because its rough on small parts but if you do this will help.

    Your z accell is fine really it can never achieve that because of the speed limit of the screw (accell is in mm/s not mm/m like the others) but because of that your layer changes will be as quick as they can be which is really all that matters with a slow screw (snappy as possible layer changes)

    You can always play around with your xy accell up or down until you are satisfied with print quality vs speed (lower values will decrease ringing and higher values will make the printer faster but increase ringing)

    I hope that all this helps you get a better understanding of how these settings affect everything.


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