Modifying the Z assembly

Hi,

I have some Z wobble problem on my printer. I am not sure if the M8 threaded rodes I've used are straight enough or adequate. I have tried two different couplings, one plastic and one metal (helical) and I still have this wobble. I have attached my Z elevator's picture and a picture demonstrating its location and relationship with other pieces on the printer. The wobble is more pronounced in the lower levels and less pronounced in the middle sections. I assume that's a proof that there is a problem with the Z threaded rods. The hexagonal part contains two M8 nuts and a spring in between for the backlash. The nuts have some minimal empty space around them, it's not a snug fit.I suppose this could be a reason for the wobble. I am trying to correct this poblem.

1) I can try to wrap some kind of band around the nuts to make a snugger fit (but probably this won't be enogh).
2) I can try to find better threaded rods but some hardware stores I have visited tell me that they cannot be perfectly straight as they aren't designed for this purpose.
3) I can try to find better couplings (as it seems to me they aren't perfectly flat. But isn't the helical design supposed to wobble a little bir, as it is flexible for this reason)
4) I can buy trapezoidal leadscrews (metric). This option is the one I'd like to try the most as I have heard good things about them. However there are some problems I need to solve. As you can see on the pictures, there is a hexagonal part with nuts inside. The best solution would be to find M8 leadscrews which would be able to fit there, and also I'd need to find hexagonal nuts to go with the leadscrews. But it seems to me that it is almost impossible to find M8 leadscrews (M10 seems to be available online in foreign web sites. On hardware stores in my city the minimum size I've been able to find is M16). I have seen a few M8s, but their length (300 or 340 mm) is inappropriate for me (I need 380-400 mm). Also, there are no hexagonal shaped leadscrew nuts (there are a few but it is not recommended to use them for motion as they are not made from brass or bronze). If I want to use a flanged nut, I'd need to change the design of the Z elevator (for which I only have an STL file and I am not really used to engineering software ). If I have to use a M10, I'll also have to modify the design as probably it won't fit.
5) M10 leadscrews have a thread pitch of 2 mm. Would the stepper motors have any difficulties driving them?

Thank you very much for your suggestions.

You can try without the bearing around the upper extremity of your z screw so it can move more freely. I heard that worked for somebody here !

i have a i3 but yes your right you should allow the z to to move and not be constrained at the top unless your using proper lead screws threaded bar isnt perfectly straight

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I have removed the top constraints. You are right, keeping them in place when you don't have straight rods is causing more problems, or at least masks partially the problem itself. Now the wobbling movement is easier to see. I have triedba few things: I have removed the threaded rod from their elevators and also removed a motor. I have rolled the rods on the table to see if they are straight and I haven't seen any discernible curve on them. I have tried mounting the rod to the motor with a plastic coupling, without putting it in the Z elevator to see if the wobbling was present in this minimalistic setup. It was wobbling. So this observation limits the problem to three possible parts: Either the motor's own rod is bent, or the coupling is problematic, or the threaded rod is bent. I have changed the motor by the X motor but there is no difference (while it might be possible to have a bent motor, it seems to be the least likely possibility as the motors look very solid). I have two different couplings, one metal and one plastic which requires some tubing for the motor. The wobbling is present with both of them, altough it isn't possible to test the metal one in this setup as it is springy and needs some constraining.

I have ordered some lead screws from China and will try them but it will require some modifications. Can I use the same couplings? (The leadscrews have a diameter of 8 mm, the same as the rods). Should I try other threaded rods? Other couplings? Or even buy a different brand of stepper motors?

Possibly move down to 5mm threaded rod? Even unconstrained, perhaps the mass of the wobbling 8mm rods is moving the printer. Smaller rods won't have as much affect on your smooth Z rods.

Any inconvenience on changing to 5 mm?