Z Axis Assembly

Theres a captive M8 nut beneath both the X-Motor-End and the X-Idler-End , but the topside of the threaded rod location is is in a hex format as well...is there meant to be an additional M8 above both X-ends and/or has anyone tried this? I wondered if there would be more structural rigidity with top and bottom nuts (and less sideways movement of the threaded rod) and there shouldn't be a risk of binding as the top nut wouldn't have to be clamped tight to the X-ends.

>is there meant to be an additional M8 above both X-ends and/or has anyone tried this?
The BOM I had has just 2 nuts , one for each end, but I used 2 on each end in the way you say. Brass on stainless
Theres some Z axis wobble but the prints are good.
I havent tried with just one each so cant compare, you could just try it & see.

Mike

Thanks Mike, my BOM had just the one each end as well but two looked as if it might be better and I thought I'd ask before I get any further in the build and then want to strip it down!

My (limited) understanding is that there's little need for such an additional nut, although if you want to print upside down (!) you can install one and a spring. Not having one has the advantage that it's much harder to have a head crash, because if the Z axis rotates too far, all that happens is that the nuts pop out of the bottom of the X carriage. If you have top nuts too, it'll drive those downwards and your hot end will crack your glass and perhaps damage your heated bed as well.

I've not heard the stability argument. I'd like to see comparative testing done to see if it's valid. :-)

Gerv

Thanks Gerv, I'm leaving the upside downprinting till I have finished manufacturing my space station [why I needed the Mendel90 in the first place ] In terms of the risk of a head crash I've actually installed both top AND bottom Z axis limit switches (though none of the electrics are connected to the Sanguinolulululu yet). Does anyone know can I just wire the two Z limit switches together so that if one is activated then the motors stop or does Marlin require me to specify which i'm using so it just stops going any further in that direction?

I strongly suspect you need to distinguish. But I don't know.

Gerv

Logrod,

I rough tested 2 nuts and 1 (lower) nut on the idler end.

M8 brass nuts on stainless. Nophead z axis couplers / plastic sleeve on the stepper shaft. The brass nuts have some tape around to fit better in the idler. No springs.

result: with 1 nut less z axis wobble. Surprised me as I expected worse, intuition said 2 = more stable.
Maybe changed something else in the process, but looks the same.
Pretty quick to test anyway,

Mike

MMT

Ok thanks Mike, I might take the extra two off then...haven't got around to testing yet as i'm still soldering all the electric up (hoping to test it all in next couple of days if I don't get distracted by real life too much). How did you assess the wobble just visually or...?

The second nut is for anti-backlash, so only required if gravity is not sufficient. The trap for it is deliberately loose so as not to over constrain the lead screw. The idea is it can flex angularly at the coupling, so even if it is bent, or off centre, it doesn't disturb the X axis as it can pivot slightly at the nut bottom nut. Having two nuts, both rigidly constrained horizontally, would not allow the rod to pivot, so any bend would in it would put force on X end.

It is a common misconception that the stopping the rods wobbling is beneficial. Hence there are Z stabilisers on Thingiverse that put a bearing at the other end of the rod. That is a bad idea as again it will bind or impart wobble to the X end if the screw is not perfectly concentric and perfectly aligned.

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Logrod Wrote:
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> How did you
> assess the wobble just visually or...?

yes just visually. I had idea to measure with digital caliper at the top, but instead just did a quick look.
Looks like I reduced the constraint, but have less wobble anyway. Maybe before pivoting on the top nut?
Going to repeat for the motor end, I needed to take apart to add the pointers anyway.

Mike

MMT