Very very weird issue, looks like z wobble, but....



  • Hi everyone! I just finished build my big HEVO, it's not using my duet wifi I got for it yet, but I will be soon, meanwhile just using a FYSETC F6 board for testing.

    My problem is this, since it's such a big bed (500 x 500mm) I decided to put in three lead screws, was originally going for 4 motors but learned that's not a good idea unless it's one motor controlling 4 through closed loop belt.

    So I did that, unfortunately some z wobble start popping up, but only after a certain height, 650mm (it can print to 800mm), and usually only one the sides near the back motor.

    So I tried a couple of things, some of which were dumb, but I'll list them here:

    • First I found out probably shouldn't be using 5 smooth rods 😛 cause it's build according to original, so 2 each for the side motor and the one at the back actually didn't have one, then I installed one to see if it made it better. No dice (duh)

    • Then I dumbly decided to put holders on top to keep lead screw from spinning too much, that seemed to help at first, but I would later find out it was just hiding the problem, not solving it, so they were removed

    • Realized that the nut bracket I design for the back was way too long (almost 80mm longer than stock) and since it's plastic, even with the bearing holders (which were just as long too) most likely would flex and cause and possibly hide problems,

    So I then moved the motor to one of the side, so it's like this now:
    https://www.dropbox.com/s/7vloq90e629zxld/2019-02-09 18.35.00.jpg?dl=0
    https://www.dropbox.com/s/ibaqeczr8mpyewk/2019-02-09 14.38.02.jpg?dl=0

    triple lead screw with 4 rods, had 3 rods only before (one for each motor) but then putting the 4th thing greatly eliminated tilt too.

    I realize that's not the best way, as best is distance between three motors should be almost or the same, but I can't do that using stock setup (I do have a design someone did that uses only one motor to pull 3 or 4 lead screws that I'll be doing for a second one) so this is the best I've got.

    three motors because I plan on using Duet's independent motor z leveling? If that's what it's called?

    later on I found out that I may have a bad coupling (some cheap ones on amazon) cause one of them, the back one, was making a weird zezezezezezeze soft grinding sound that I missed at first because of the board fan, until I was close enough to hear it, changed that out and it's pretty quiet now, though I am tempted to get oldham couplings and try those instead.

    So here's the current result (I have like 8 vases lying around now lol)
    this is a hexagon vase, from thingiverse scaled up to 710mm tall
    I should clarify, this "z wobble" (I have a feeling it's not that anymore) only happens after a certain height, usually around 650mm and higher, not before, no sign of any z wobble before that, and it builds up, pics below:
    https://www.dropbox.com/s/mx6tlnsenvydlpj/2019-02-11 06.52.48.jpg?dl=0

    you'll see it's there, but as you can see in this pic, while that corner had z wobble, the other corner, which is the same side:
    https://www.dropbox.com/s/90lg4sjqnhrrfb7/2019-02-11 06.53.07.jpg?dl=0

    does not or is barely noticeable, I have to be really close to see it and i know the light makes it looks worse but it's not feelable by touch

    Then I noticed this, this is a hexagon vase that's suppose to be smooth
    https://www.dropbox.com/s/ms6o5k7ccgdiltr/2019-02-11 06.53.28.jpg?dl=0

    the weird bulging,

    right now I'm going to try a cylinder, but this is very very weird, that's the only side that has the "z wobble" btw, above the 640mm height as mentioned (and no z wobble before that) the others are either no wobble or barely barely noticeable.

    I'm out of ideas and so are my friends so I am greatly appreciated if anyone know what the heck is going on here. The printer is usable, just can't really print above 650mm cause of this weird issue

    oh the vase is 200mm wide to prevent it from being too thin and causing sway from being too tall. Thank you very much!



  • looks like the entire bed is moving quite a bit.

    How do the lead screws look while spinning past 650mm?

    It may be that your steppers are actually shifting on their mounting points near the end due to the leverage.

    If you have pulleys on the steppers, it may be that they are oval shaped and are push and pulling on the steppers.

    It also may be that your linear rods are not leveled near the top causing the steppers to struggle.

    I would forgo the belt idea, my machine (FT5 R2) uses a belted Z, but it is designed so the belt is perfectly aligned so as to not to impart any leverage issues.

    I'm not sure how to configure dual Z endstops, but I don't think it's that difficult, you would designate two steppers on one channel and the third stepper to another.



  • Hi wyvern, thanks for the fast response
    the lead screw on the side with two motors don't wobble much, just a little bit past 650mm, the one with one motor though does wobble quite a bit after that

    hmmm that is a possibility
    should I remove the pulleys? I kept them for now just cause I need them in sync, but should I try without them?

    when you said near the top, do you mean when it's getting closer and closer to the motors? hmm I really made sure the nut is pretty much centered to the jaw coupling, but I might just hafta recheck that. Especially since the one as mentioned above with one motor wobbles quite a bit after 650mm ish.
    here's a video showing the three lead screw in action from 600-700mm and back:
    https://www.dropbox.com/s/dkj2sf2hyv6adr5/2019-02-11 09.18.51.mp4?dl=0
    What should be my first step?



  • Yeah, they look a little wonky.

    Also your clamps for the Z rods won't do, they stand no chance at stopping any wobble, you need to install metal clamps.



  • oh dear 😕 where can I get metal versions of these clamps here in Canada? I have no idea how.



  • @iamthebest22
    https://www.amazon.ca/SH12A-Linear-Shaft-Support-Bearing/dp/B0718ZXJF8/ref=lp_11850906011_1_5/142-1468665-5733639?s=industrial&ie=UTF8&qid=1549906979&sr=1-5

    Assuming they are 12mm?

    The only sucky thing is you may need to alter your design a bit, I love these machines but sometimes it's a bunch of trial and error.



  • hmm okay well I'm not the designer, but I don't think there's a parametric bracket so that would be hard for me, I have a printer that can print metal filled or those types of filament, would that be just as good or do I absolutely must have full metal?



  • gah so close those holders you linked, they're 23mm away from the extrusions, the stock HEVO plastic parts are 22mm, are you kidding me >.<

    EDIT: Should I also try these oldham couplings? Heard great things about them
    https://www.aliexpress.com/item/-/32867555428.html?spm=a2g0s.13010208.99999999.260.67893c00yB3LTK

    I would be buying the 5mm to 12mm ones.

    EDIT 2: I just saw your edit on having an Ft-2, I have one too! Might get their Ft-6 in the future, but hmm okay I am using Precision Piezo as auto bed leveling, so many the belt isn't really needed after all. I shall try without belt and report back, so yeah I have ABL



  • Your lead screws look like threaded rods to me. If that is the case, you might like to re-consider as I doubt you'll ever get good accuracy and stability with threaded rods; particularly ones that long and that bent!

    I'd totally agree with running the motors from separate drivers and dumping the belt. The Duet and RRF is perfect for bed levelling via the Z axis motors and a piezo Z sensor is the ideal sensor for this.

    Looking at your machine overall, I'd say the frame is way too big for the motion components and frame itself is not mechanically rigid enough for the size. My CoreXY printer has a 500mm square frame built in a similar way to yours but with double (2040) extrusions at the top and bottom edges. Also, all the extrusions are bolted together with metal corner plates. It looks like a tank. Even though my frame is more rigid than yours, it's close to the limits when it comes to rigidity. Your frame is huge by comparison and seems to be made entirely of 2020 or 3030 extrusions. I don't see any bracing on the frame to help deal with how tall it is. With the twisting forces that a CoreXY exerts on the frame, I doubt it is rigid enough to provide accurate and repeatable results.

    Personally, if I was going to build something this size, I'd use doubled (2040 or 3060) extrusions everywhere and cross-braces on two or even three vertical faces. I would dump the threaded rods or lead screws and move the bed with belts. I'd also double the size of the Z axis smooth rods or maybe consider using linear rails instead. Certainly, at this size, all the motion components, including motors, idlers etc. would be mounted to the frame with metal parts, not plastic.

    Also, if you can't accurately verify how well aligned the output shafts of your Z steppers are with the drive screws, it's quite likely that the misalignment is beyond what a semi-rigid coupler can cope with. You may be able to improve the wobble somewhat by using flexible couplers. In the end though, they won't solve the underlying problems and you'll keep discovering new weaknesses as you print different things.

    In short: The frame is too big for the way it is constructed and the Z-motion components are not up to the job. You're just saving up lots of pain here with the current design. Going really big requires exceptional mechanical components.



  • @iamthebest22
    You can always sand the bottom down to fit!

    And yes, you are asking a lot from the frame, even on the FT-5 I had to brace it to the max and go to 12mm z rods to get rid of ossilations. My FT-5 looks more like a aluminium ladder now.



  • @grizewald thank you very much for the suggestions, damn.... I guess I went too big >.<

    the lead screws are from Zyltech and they said it's ACME screws, so I hope they are as they said.

    Unfortunately I'm on a tight budget for another month or so, is there any way I could make mines even more rigid then it is right now? I do have external brackets printed out and attached on the bottom, these ones:
    https://www.dropbox.com/s/y17zvr8yb6z4gx9/2019-02-07 19.14.29.jpg?dl=0

    should I do it also for the top?
    damn had I realize this I would have stopped using L bracket as the stock design said and go metal corner brackets all the way 😞

    I'm doing a print right now without the belt, so I'll report back when it finishes.

    So I guess the first thing I should do is to try the best I can right now to further brace the frame more with un more 3d printed brackets? or go for metal ones like these ones:

    http://www.zyltech.com/3030-extrusion-90-plate-bracket/

    Thanks!



  • hmm I know this is really frowned upon, but would 4 motors help the rigidity, or no just gonna cause more binding/walking/ syncing issues?



  • Leads should not be used for anything but bed movement, using them as the primary support for the bed is asking for issues because no two lead screws are perfect or identical.

    That is why people get massive linear rods/rails and skinny 8mm leads.

    You could solve most your issues by getting new 8mm lead screws, replace the Z rod brackets with metal, and chop your Z axis down to size.



  • okay gotcha, lesson learned, thank you very much!



  • Good luck on your build!



  • Thanks! I'll still be reporting back here, first thing I'm going to do is like buy 20 of those 3030 plate brackets and attach them to the printer, then try again and see if it improves. do I need 2020 plate brackets also for the bed or no? I can buy them too just wondering if that's needed.

    Update on removing the belts, doesn't seem to help much, though there is slight improvement, but still there.



  • @wyvern The lead screws are the only support for the bed. The guide rails don't support it- if you took out the lead screws the bed would happily slam to the bottom of the printer on the guide rails.

    I think people buy skinny 8mm screws because they're the cheapest ones available.



  • @mrehorstdmd I meant support as in stability and alignment of the bed.

    And a big fat lead screw is going to overcome the rods/rails alignment faster than a skinny pliable one.

    You would need thicker leads for very heavy beds or whole gantry assemblies where a skinny leadscrew would actually deflect under the weight.



  • Hmm so maybe it was a mistake for me to use 12mm lead screws? that's what I'm using at the moment, maybe I should switch to 8mm lead screws?



  • @iamthebest22

    You can try, 8mm screws and nuts are cheap as dirt, I had to get 4 just to have two that were straight.

    You have a tall platform, but you also have 3 leads so it should be fine.

    Double check when you alter your machine to see if there is any binding in the axis without the leads installed, putting the machine on it's side and moving the bed is a great way to find out- it should slide with almost no resistance, a out of whack Z rod will usually cause the bed to have resistance or spring back- The way I set my FT-5 was to get one rod perfect by checking with a square, then for the other rods, loosen one rod clamp slightly and tighten back down with the bed as close to it as you can get and repeat on the opposite end. I do this usually twice on the other 3 rods until it's perfect.

    It was a PITA to get it perfect, but it's super important- just make sure the reference rod is square. Next machine I build/design will have 3X linear rails and 2020,


 

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