Perfectly align z axis and rods to counter z wobble?

Hi,

we have printed new x-end's for our Prusa v2 and switched from stiff z couplings to flexible aluminium ones.
[www.emakershop.com]

We expected improvements regarding z wobble but it got way worse. As I understand it this has to do with the alignment of the threaded rods to the smooth rods. The stiff couplings prevented some wobbling but transfered much of the exibited force onto the motor bearings.
So is there a "proper" way to perfectly align the threaded and smooth rods? I have read about loosing up the z motors a bit or using some sorts of "anti wobble" contraptions. But as so many other Prusa users are getting good prints without this hacks, I wonder how they did it.
One thing to note - our printed parts are bad. They have some ridges and maybe this inaccuracies are the source of the trouble.
[nicolinux.org]

Thanks.

Stefan

Use a flexible coupling and make sure one end of the threaded rod is free. This way you don't need to worry about making a perfect alignment of screws which are probably not even straight. Also, make sure your layer height is an even multiple of a full step on the Z. Fractional step positions are not guaranteed on Z, and this is another cause of banding.

But your prints look like you're losing steps on X and Y. Either from too little current causing too little torque, or from too much current causing momentary thermal shut-down of the stepper drivers..

We have already used a flexible coupling with one end of the threaded rods free (that's the default prusa build regarding the threaded rods).
Yes, the prints look like that. Right now we would like to at least get to that stage again. I thought that flexible Z couplings are better, but this is not the case and that's why I am asking for advice.

thx.

The problem with flexible aluminium ones is they stretch like springs. As the X carriage shifts its weight from one end of the axis to the other it will see-saw slightly. They are supposed to be used between two shafts supported by bearings so that they have no axial load.

You need something that is stiff axially but can flex angularly, like these: [hydraraptor.blogspot.co.uk]

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[www.hydraraptor.blogspot.com]

Thanks for the info.
We have reverted to the stiff couplings and the wobble is completely gone. In the process of upgrading we also tightened the x-belt and added grease to the linear bearings. I guess all this factors helped fight wobble.