Fine thread for Z axis

Hey,

over the holidays I went to my local DIY store and tried to find replacements for the threaded rod of the Z axis. I had read somewhere that the metric M8 threaded rod and their respective half hex rivet nuts are better or tighter than the imperial ones we have in the standard kit.
However the shopkeeper reminded my that there are fine thread threaded rods with a pitch of 0.75cm instead of 1.25 with normal pitch. He also said that the fine thread was much tighter than the normal ones and considering how much play there is in the current model anything is better.
Also having gotten out most problem on my machine one remaining (and already much discussed) issue is that the layers are still noticeably not directly on top of each other resulting in that wavy wall pattern.

Now finding these half hex rivet nuts for metric M8 in the store was impossible and also online only certain specialized pages have them (mostly in 100 bulks). However I did not find any mention of a M8 half hex rivet nut with fine thread.

I´d love to give the fine thread a try since 2 nuts and 20cm of threaded rod should be cheap. Has anyone seen fine thread half hex rivet nut somewhere?

ummm ...... errrrrrr.....

On most of these printers, the Z axis drive is an M5 thread rod. The support rods are M8 or M10. The smooth rods are normally 8mm.

The Z axis has non trivial weight. Any "slop" in the threads isn't going to matter much. The nut will ride on the bottom of the threads.

What sort of printer do yo have with an M8 Z axis drive?

My base model is the Eventorbot, where the bed is mounted frame on only one side. At the moment I´m using 5/16 inch threaded rod.
You´re probably right with the weight letting everything rest securely in place, but the play between rod and nut could still allow some shifting from layer to layer.

From what I read on the "wavy walls problem" this seems to be the most likely explanation. After I pulling everything apart and realigning the coupler it got a bit better but I still have a noticeable, wave in my walls which repeats every 5-6 layers (with .3mm layerheight).

I had/have the same issue. the repeating "wave" every 5-6 layers with 5/16 rod. I am in the middle of upgrading my printer, so I can't tell you if it works or not, but it was suggested to me to decouple the rods from the motors with something like this [www.thingiverse.com] . also to put some sort of anti backlash mechanism on the Z axis

I already have a metal coupling with a flex part in the middle, but its not made for 5/16 its for M8. I taped electrical tape around the motor shaft to make it tighter but I still think that the coupling is not the problem.

Whats you upgrade plan? and whats your anti backlash idea? a tigher rod/nut connection would make a good start, do you think?

Thread rod is not very straight stuff. About the only way to get rid of Z axis banding is to use it only for height and not for X or Y position. It's just a matter of how it's made and what it's made for. You can get better stuff, but it'll cost you $15 to $90 a foot depending on just how good ....

"About the only way to get rid of Z axis banding is to use it only for height and not for X or Y position" what do you mean by that. Aren´t the x and y axis mostly beltdriven anyway?

and I noticed thats its not very straight but the wave is much smaller than the the noticeable bend. I would also assume that finethread is better produced than norm.

do you have a shop where to find the expensive stuff?

Most reprap printers before Prusa3 (i3) used M8 or 5/16 threaded rods. M5 threaded rods combined with 10 mm smooth rods do a better job of eliminating z wobble than M8 threaded rods with 8 mm smooth rods. This was the reason for the switch. Either way works. You can get prints without z-wobble or z-banding artifacts with ordinary and usually bent threaded rods if you design and build your printer to account for the rods. You don't need to spend a lot of money on lead screws. You don't need to get the fine threads.

In the meantime I got the fine thread and I did exactly what I hoped it would do. the gaps are gone and there is much less play.