Mass Dampers
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Hello.
Wondering if anyone has tried adding mass to the frames of their printers to try and minimise the amplitude of vibrations?
I'm suffering ghosting on a 300x300 version of the Spauda thingverse core xy d-bot. So far I've bolted the machine to an enclosure so there is little chance the base will move but the top is free to move with the flex in the frame. In my mind anything that is free to easily flex may do so and I'm slowly tackling the low fruit in terms of making the machine more rigid.
My mods so far are:
- Plywood sheets on the sides of the machine. Originally to bolt stuff to bu happily add extra rigidity to the system.
- Third z-axis screw to reduce the tendency for the elevator to pivot about the elevator/screw interface.
Soon to make mods are:
- Move original two screws toward the front of the machines, most importantly to the other side of the elevator c of g.
- Corner braces in the top left and right of the machine as you look at it front and rear.
What I'm now thinking about is cutting a couple of long lengths of 20mm steel bar stock and strapping it to the top of the frame! My inspiration is the large lump of concrete you often see in washing machines. As the force exerted on the frame by the accelerations of the carriage will remain equal if the mass they are acting against is significantly larger then the resulting accelerations the the weighted frame will be less. Granted the transmitted forces will still be equal but I though a lower amplitude of oscillation would be less likely to set other things in the frame resonating? Anyone tried it?
I realise the usual approach to reducing ghosting involves tuning the accelerations, max speeds, and Jerk, or machine rigidity mods. For the application here at work I really need to keep print speeds as high as possible. The first limit I hit is how fast the material can be controllably squirted out of a nozzle, then the maximising the accelerations become critical.
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I realise this is a bit of a bodge. Making the vertical sides out of sheet/plate, rather than extrusion would stiffen things up but the freedom to increase the size of the machine a little if needed may well be needed before the use case is proven viable or not!
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Are you sure it's the frame that is the problem and not vibrations or oscillations of the hot end within what may already be a rigid frame?
Just for info, I don't get any ghosting or ringing with my 4kgs of moving mass
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I know that within the 3D printing world there is an obsession with reducing moving mass far beyond that which can be usefully exploited, but there are side effects to this approach.
I guess what I'm trying to say is that if you have a really low moving mass, then the less likely it is to impart resonances to the frame, but the more likely it is to resonate of it's own accord or be affected by external factors (such as toothed belts meshing with pulleys and so forth).Edit. Easy enough to try. I know it'll go against the grain but try strapping something heavy(ish) to the hot end itself. I've been able to get up to 300mm/sec print speed and regularly use 350mm/sec for non-print moves with my 4kgs so you never know...........
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@deckingman printing speed have complete nothing to do with vibrations, zero. Only acceleration and jerk values have effect on vibration.
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@dragonn said in Mass Dampers:
@deckingman printing speed have complete nothing to do with vibrations, zero. Only acceleration and jerk values have effect on vibration.
Oh for goodness sake! The OP said he wanted to print at high speed. In order to attain a high speed, you need to set a high acceleration otherwise you won't reach the desired speed. So the two go hand in hand. i.e to attain a high speed across what is effectively a small area, you need to use a high acceleration. The higher the desired speed, then the higher the acceleration needs to be in order to attain that speed over a fixed distance.
So print speed includes accelerations and decelerations, in fact the print head is probably mostly either accelerating and decelerating.
I know I'm an old man but I did go to college and I haven't completely forgotten everything I learned, nor have the rules of physics changed that I'm aware of.
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@deckingman yeah I agree. But talking that you print with 300mm/s without posting any acl and jerk (and maybe bed size) info is just useless. As I said - 300mm/s itself doesn't make any vibration, to high jerk and acl settings can.
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@dragonn said in Mass Dampers:
@deckingman yeah I agree. But talking that you print with 300mm/s without posting any acl and jerk (and maybe bed size) info is just useless. As I said - 300mm/s itself doesn't make any vibration, to high jerk and acl settings can.
You do like an argument don't you? Did I at any point mention constant speed? I apologise if you find my information useless. I'm sure the OP finds value in it.
I'm getting really tired of trying to help people but ending up having arguments with you about wording.
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And ..............Oh never mind.
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Well I only wonted that you post yours acl and jerk settings. I am just interested in those.
https://www.dropbox.com/s/06um8lldsadv4x4/VID_20180419_204852.3gp?dl=0
Here is my printer printing with travel 550mm/s. Travel acl is set to 5000mm/s and jerk 200mm/min but I need to verify acl and jerk because now I am writing from memory. I hope this is usefully too for OP.
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And directly about mass damping I still have an idea in my mind to fill all free space in my printer with epoxy. Something like that https://hackaday.com/2017/03/27/casting-machine-bases-in-composite-epoxy/
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@dragonn said in Mass Dampers:
Well I only wonted that you post yours acl and jerk settings. I am just interested in those.
https://www.dropbox.com/s/06um8lldsadv4x4/VID_20180419_204852.3gp?dl=0
Here is my printer printing with travel 550mm/s. Travel acl is set to 5000mm/s and jerk 200mm/min but I need to verify acl and jerk because now I am writing from memory. I hope this is usefully too for OP.
Why didn't you just ask instead of just being rude and saying that my information was useless?
Anyway. Mass in X is around 2.7kgs (2,700 gms) and in Y it is around 4.0kgs (4,000 gms). Printable area is 360mm x 360mm. I have the accelerations set to 2,000 mm/sec^2 for both X and Y. I can go higher but with 4kgs suspended over a metre above the floor, the entire printer can start to rock about a bit because the floor has some "give" in it.
So with acceleration set to 2,000 mm/sec^2 and a distance of 360mm, the maximum speed possible is (sqrt(2 x Acceleration x length/2) = 848.5 mm/sec. The time to reach 300mm/sec is ((Vf-Vi)/a) = 0.15 seconds. The distance travelled to reach 300mm/sec is (1/2at^2) = 22.5 mm.
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Sorry, you a right. I could just ask. I checked my settings, acl is like I posted 5000, but jerk is higher X700 Y500. Small BTW it is possible to have different jerk for printing and travel? Like accelerations. Would be a nice idea to have such option.
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@dragonn said in Mass Dampers:
Sorry, you a right. I could just ask. I checked my settings, acl is like I posted 5000, but jerk is higher X700 Y500. Small BTW it is possible to have different jerk for printing and travel? Like accelerations. Would be a nice idea to have such option.
I didn't give you my jerk settings but for info I currently have them set to 600 for both X and Y but I need to revisit that when I get time. Non print moves tend to be a single straight line move so I don't see any value in having a different jerk setting. I'm not sure it would be possible in any case but I need to think it through.
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The biggest improvement of stability I found for the DBot was making the bed a square frame secured front and back on 3 screws. That kept the bed from bouncing as it tends to do on the stock cantilevered design. The wheels and plastic mounts just have too much flex to support much weight.
After that I replaced any plastic part I could with aluminum. Corner plates and brackets. That reduced flex quite a bit. There are no other frame modifications than this.
Aluminum wheel spacers and shims allow for proper tightening of the wheels without binding them. That combined with 3 wheel tensioners keep them nice and tight. No slop.
Then I replaced the idler bearings with actual smooth and toothed idlers. That removed some of the belt vibration. I've tried both steel and glass belt. The glass belt was a bit quieter and had less vibration. The diameter of the toother idlers are still too small to use with the steel belt, but interestingly I've noticed less ringing with the steel belt than with the glass when properly tensioned. I also switched to a carriage with belt tensioners built in which make it much easier to get equal tension on the belts.
The printer sits on a very heavy (300 pounds) bench which was originally designed to be a fish tank stand capable of holding 1000 pounds (double the weight of the tank), so there is no sympathetic vibration in the stand.
Since the Dbot has the XY movement at the top, it tends to wobble a bit. At first I tried to add some weight at the bottom of the frame to clamp it down to the table more. This worked ok, but was louder. Then I sat the printer on some thick dense foam blocks. The printer sways a bit more on fast travel moves, but is far quieter, and ringing seems to be less. It's as if the foam is absorbing the force rather than springing it back, so there's less oscillations.
Travel moves are comfortable up to 220mm/s. I usually keep them to 180, but 250 worked, but felt a little violent. Perimeter and infill print moves up to 150mm/s are doable and I use that for my draft settings. Good quality with no artifacts up to 90mm/s.
I'm still tuning my accell and jerk settings. I think I'm going to switch over to Cura again to have more fine grained control over the jerk and accel settings for different moves. 2000 accel is fine for infill, but for outer walls 500 keeps ringing down while still letting me hit 90mm/s on long walls.
This is all on a 24v setup with 2a 0.9 degree motors with a Titan Aero.
Reducing the mass of the extruder is tempting. The stock Dbot uses a short bowden tube setup and reduces weight compared to direct drive substantially, but all of the carriages I've seen tend to be tall and stick the hotend fairly far out from the plane of the cross bar, whereas the Titan aero carriage i'm using is very short and compact and slings most of the weight directly under the crossbar.
The new CoreXY printer E3D is working on is very interesting. Extreme rigidity from the tooling plate top frame and carbon fiber cross bar and bowden setup for light weight. They haven't really talked much about print speed from what I can see, but I would think it could be quite the mover.
Ha, sorry for the essay, but I've been playing with the Dbot design for the passed year. It's a great design in the stock form and leaves the door open for a lot of improvement. A perfect tinkering platform.
I look forward to see what you come up with on your Dbot.
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Thanks for the comments. So so far no one seems to have tried strapping heavy static weights to the top of the frame.
Thanks also for the current settings people are using. I'm on about 450 x/y on jerk and low on acceleration at 1000. I realise the system could be capable of performing faster moves in the x direction but I don't want it to. Parts suffer non-isotropic strength enough already with the weak z. My understanding is that allowing different x and y accel/jerk would mean there would be a subtle difference in the processing of predominantly x or y direction vectors.
My understanding of the print speed issue is the duration of the acceleration (+/-) periods will be greater with higher print speeds. As there is mass in the static system as well as the flex the motion of the head will cause opposing accelerations in parts of the frame the creater the duration the greater the displacement and peak speeds. The mass on the frame aims to reduce the accelerations and so displacement and peak speeds attained.
Running 3 P3Steel with Duet 2. Duet 3 on the shelf looking for a suitable machine. One first generation Duet in a Logo/Turtle style robot!
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...and timely reminder abouth the toothed belt running on the smooth bearings. I had wondered about that. I think that is one of the potential causes of a constant amplitude wobble.
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Just to add that if the print head and frame move together, then there is unlikely to be a problem. It's only when one moves in relation to the other that there is an issue. When I've been experimenting with high speed tests, throwing 4 kgs around at 300 plus mm/sec, at a height of over a metre above the floor, I've had the entire printer rocking. However, print quality didn't really suffer as the head and frame all rocked together. (Sounds like a concert I once went too
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@doctrucker my friend did try it https://reprapy.pl/viewtopic.php?f=50&t=1766 (it is not English but I am sure you can interpret it only by looking at the photos). The question is if this helps when you printer frame is rock solid since K8400 is a only an acrylic light weight frame. Did you looked at my post with casting epoxy to dump vibrations?
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I appreciate that, but don't have any equipment that I can use to see whether I've got movement of the bed relative to the nozzle so just trying things out and seeing what happens to the ghosting.
Think belt tension wasn't great. Next itteration of the carriage will have a more continuous adjustment. Belts themselves not likely to be class leading either.
No sure if I said it in my first post but essentially trying what the do in washing machines!
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@dragonn Thanks for the link. Looks like it made quite some difference for him. The extrusion based core xy are far from rock solid. No triangulation in the frame work so relying on the printed corner pieces to maintain rigidity and they're a balance between needing rigidity and needing to print quick.
Do you mean filling the extrusion voids with epoxy of larger areas within the machine?
