I’ve had problems with what appears to be z-wobble and I have no idea what to do now. I am running a sells Mendel(the original Mendel that came right after darwin), and so there should theoretically be 0 z-wobble. Any suggestions?
Also, the z-wobble appears to be consitantly worse on one side.
Also, as a side note, the electronics are mounted properly, but we’re dissassembled in the picture.
Clean and oil all rails. Tighten belts. See if your motors are overheating. Run filament at coolest temp possible. Add lots of fan cooling externally. All that should really help. Oh, and slow down. For small parts, try printing 2 at once to allow more cooling.
@Brook_Drumm nice crash course in 3DP
Maybe also try different slicers. Ive foudn the printers behave differently mechanically of various slicers. Ive had the best result with Simplify3D on my Prusa i3
The sells mendel doesn’t typically have less Z wobble. Because the threaded rods are rigidly mounted, and they’re just as thick as your guide rods, Sells mendels typically had worse problems with Z wobble.
Two of the biggest issues with this design is that the z leadscrews are too big, and the z linear rods are too small. I personally use 6mm leadscrews, and prefer 10mm z rods. If you change out your leadscrews to 6mm you’ll likely get a big improvement.
The second biggest issue is the triangle side pieces, they’re prone to twisting and bending. Best improvement I had made to my i2 was replacing those triangles with 1/2in mdf. You could go with 1/4in aluminum or steel.
A temporary fix is to decouple the x/y movement of the z leadscrews. An Oldham coupler is made exactly for this.
Switch from all-screws to trapezoidal screws. Fixed all my z-banding issues.
Thank You @Brook_Drumm , I’ve done most of those. The octopus still has blobbing from before I fixed some of that.
@Stephanie_A Do you know of any place to get oldham couplers inexpensively? A set of two would cost me over 60 from mcmaster carr.
Sells Mendels do have horrible Z-Wobble at times, the way the rods are rigidly clamped to the bottom bearings is a large factor there. You’ll likely never get them perfectly centered nor perpendicular.
Adding a coupler would be a good first step, i like “plum couplings” Electronics, Cars, Fashion, Collectibles & More | eBay as more rigid alternatives to the aluminum spiral couplers, but you might actually get better results with the cheaper and softer couplers.
Realistically, though, the Sells Mendel is a horribly outdated design with a massive heritage of bad design choices. Instead of pouring more effort and money into that design, why don’t you use the parts to upgrade to something like an i3 or even a Mendel90? Grab a bit of MDF and a few new fasteners (well, you’ve got plenty of them with the Sells anyways) and you should be able to carry over the rest of the components.
Yeah, looks like classic wobble. On the Sells Mendel, it can be caused by problems with the Z rod pulleys and the nuts clamping them.
Unfortunately, the threads in nuts aren’t usually cut perfectly perpendicular to the faces. If you want to fix the machine (rather than just putting it in a museum and replacing it), I would start by getting real pulleys with an 8mm bore (assuming you’re not going to replace them with thinner threaded rods as some have suggested) like the ones used by the Cupcake (around the same vintage, and more design flaws in common than people realize). Those should have a close-tolerance bore and a set screw that will keep them aligned better.
You’ll also want to keep the nut on top loose against the bearing so that it just rests on the bearing rather than clamping to it. You can counter-tighten two nuts to get them to stay in place without being tight on the bearing. This was standard practice on the Cupcake for minimizing wobble,by the time they were discontinued.
Ideally, you’d have another bearing on the other side of the pulley, but at that point you might as well just replace it with a motor and couple the screw directly.
I have an old sells mendel banging around with prusa X ends/X carraige and I get bugger all Z wobble on that machine (changed to GT2 belts too, plus replaced the bed assembly as it’s poorly designed too). I did make sure the Z threaded rod and Z smooth rod were VERY straight before using them (I tested them before cutting, rolling on a glass table I know is flat).
The standard sells X ends are prone to binding, which is why I switched then out, plus the X carraige. As mentioned tho, good quality parts make all the difference. If the nuts do not have flat faces, and you have access to a lathe, then you could always clean them up yourself. Just put a lot of nuts on a short piece of straight threaded rod in the lathe and square the end one off. Some nuts have raised text on the binding edges (which is just as bad), which you could just remove with a file.
Also, make sure that the Z threaded rod and Z smooth rod stay parallel for their entire travel with your chosen X ends. I found they were closer at the bottom, so ended up putting a piece of plastic in between the rod and where it is held (in the clamp) to move it out the necessary 1-2mm I needed.
As others mention, the triangle design can be a right pita to get correct. Start from the Z rods at the bottom, measuring front to back. Each side (front/rear) is a right angle triangle, and you need to get that as close to perfect as you can, otherwise the Z rods won’t be perpendicular to the Z rods. While you can probably adjust some of this out on the bed, it’s best avoided if possible.
The bottom corners (feet) are also annoying because it’s quite possible that even on a flat surface they won’t sit flat (even if the rest of the machine is correct). If you’re invested in getting the machine running, then there are vertex pieces with an embedded adjustable foot on them up on thingiverse that you can use to replace your existing parts.
Fwiw: Unless you feel you have to get it running, I concur with others that there are newer designs with a lot less issues to address that would be more worth your time.
Thanks for the help everyone.
@Stephanie_A @Thomas_Sanladerer , would using flexible couplers with m8 threaded rods work to get rid of the wobble? Is that reasnable, or should I just replace the m8 rod with m6? Either of these wouldn’t be insane, and are not super cost intensive either.
@Stuart_Young I do see the possibility of scrapping the machine as many have suggested, but I would like to get it running well if possible.
@Aaron_Spaulding , don’t get the spiral couplers – they have a tendency to not hold the X Carriage stable in the Z axis. That’s why the suggestion of oldham couplers. Those will allow the rod to turn/wobble, while keeping the Z positioning error free.
@Aaron_Spaulding Since you want to keep it, my first suggestion would be to swap out the T5 belts on X/Y and go to GT2. Firstly, the printed T5 gears from the default Sells Mendel were too small for T5, causing the belts to excessively wear and stretch, which leads to gaps between the teeth being too large (especially if over-tightened). Second, when the belts go around the Y idler bearing, the belt “bounces” a bit on the teeth (watch the belt height around the idler bearing as you move), which leads to inaccuracies over time. Lastly the GT2 belt pulleys you can buy now are really round, so they’re more likely to be central to the motor axle (unlike some printed pulleys).
With the X ends, check that the nuts on the Z threaded rod move smoothly, and that your Z threaded rod is completely clean. If you can’t wind them from one end to the other of your threaded rod with pretty much zero force (no matter what size threaded rod you have), you’ll have issues.
@Aaron_Spaulding the couplers really depend on which one you buy. Some will do better than others. The ones that Thomas recommended are a cheaper alternative.
At this point There are a few things that you can do:
Buy couplers and hope they fix it enough.
Buy smaller threaded rods for the z. (most i3 designs use 5mm, I found the threads were too weak and prone to dust and dirt jamming it, or careless dents/bends in the threads, thats why I use 6mm). This route will be difficult because it requires a redesign of a lot of components.
You could buy a kit for an i2 or i3. Use your experience here, I’ve found most i3 frames to be terrible, only slightly better than a sells mendel. If you can twist or distort the frame at all when assembled, then it will do the same when printing. (linear rod size is a different issue).
Upgrading to an i2/i3 will require new belts/pulleys and belt idlers. The easiest upgrade is to an i2, but that will still have the old triangle frame.
I’m a very picky person with my 3d printers, been burned too many times by not paying enough attention to the design. You fix one problem and another problem becomes apparent. Couplers might fix the z wobble, but so might new belts and pulleys, or a stiffer frame, or perhaps you used inch threads instead of metric. Pick a goal, what quality do you want, what speed, what cost, then plan it all out before touching your first screw.
Thanks @Stuart_Young , I am running GT2 pulleys with GT2 printed drives, I don’t think that’s an issue right now, but I can all ways change them out for some aluminium ones.
I think I will go with 6mm rods for now, but keep in mind the couplers If I need them. Upgrading to 6mm rods shouldn’t be insane, and might be all I need.
@Stephanie_A @Thomas_Sanladerer Thanks for the detailed response. If I wanted to upgrade to a different machine what machine would you suggest? I could go to an i3, but then I’d just have another set of problems to deal with. Would an iTopie i3 be ok with the stress, or should I go for a Mendel 90 style machine like Tom’s old Mendel 90?
Honestly I don’t have any good recommendations for a reprap at the moment. All of them will need some work to make reliable and fix issues. Almost all have issues with the z axis being underrated, and have poor frame design. But there is no perfect answer. I have a p3steel and I’m annoyed with it, going to scrap it for my own design. If you want a printer that just works, get a printrbot simple (I found it to be better than the printrbot play).
If you decide on a reprap, a few things to look out for: do not get acrylic, It’ll break. If its mdf, paint it before assembly, mdf absorbs water like a sponge. Look closely at belt paths and idlers. A cantilevered idler (that is, supported at only one side) will eventually fail or bend. The belt path should be perfectly parallel to the axis of movement and the mounting points to the carriage. That means that from the pulley to the point it is mounted to the carriage is parallel with its axis of movement. Also note belt tension. If you tension the belt on most i2/i3 x-axis, that tension will pull the x-axis ends inwards, bending the z rods. A good design would prevent that by putting the tension on the x axis rods instead.
@Aaron_Spaulding You’re using GT2? The pic in your post shows T5, which is what the sells mendel had by default.
@Stuart_Young , those are not GT2 belts, they are XL belts, so they are similar I think. I had thought they were GT2 for a moment.
