Z lead screw nut thread stripped

Hi guys just for information so it doesn't happen to you.

After a fair bit of printing (lost count but have been through nearly a kilo of PLA) the thread of the nut of the Z-axis screw sheared off.

I was not printing at the time but raising the extruder to wipe it before a print. The z-axis assembly (X-axis carrier, extruder and the rest) weighs a fair bit and came crashing down on the bed.
No damage hopefully but I would advise you keep an eye on this and replace with a better quality nut and/or if it exists a nut with a taller profile to share the load across several threads. Aside from possibly breaking the bed glass plate, it could do some damage if half way through a print the z-axis decided to stop climbing or worse decided to drop.

I was using silicon lubricant every now and then on this but have now put some copper grease now. As it is a fairly quick and easy part to remove and replace it may be worth changing every so often before it is too late.

I guess the issue is that having only one small nut taking all this load is probably not ideal.

Hi arnaud31

We do get this problem, very occasionally, on our Mendel production machines, but it usually takes at least a year of 20 hours per day/6 days a week printing to cause this, and they use the same M5 BZP rod and nut. We usually find it's because something has stuck, and wear is exaggerated, even then.

It could have been a poorly manufactured nut, inconsistent threading on the rod, or foreign objects on the rod. I'm not a fan of the PTFE lubricants; they tend to coat and dry onto parts, then wear off and ball up. A light machine oil, like 3-in-1, is better in my opinion.

There should be a spare nut in the kit, and you could turn the rod over, to use the other end, until a replacement is sought.

Ian
RepRapPro tech support

as long ad the oil film is between the moving parts there will be no wear (and clean from abrasive parts that is), an oil film has a limited carrying capacity, the bearing surface of the standard nut is quite small, you could distribute the pressure by using a longer nut (what's nut to like)

[www.ebay.co.uk]

Erik

There have been quite a few threads on this in the main forum, eg [forums.reprap.org]

Stainless steel threaded rod and stainless steel nut are not a good idea - they'll want 'gall', and seize together. Our forefathers realised this, and tended to use a screw with high hardness with a soft nut, usually brass, on old lathes and mills, before modern materials took over. The nearest, easily obtainable, equivalent is stainless steel threaded rod with a brass nut. The brass will wear, but this actually helps; it spreads the load along the nuts thread, and will adopt the shape of the stainless rod. A longer brass nut would probably be ideal. A normal BZP nut and stainless rod is the next best choice.

Cleaning the leadscrew before assembly will help wear - I'll add this, and maintenance advice, to the instructions. I also think that the PTFE lubricants can trap debris under their coating, rather than help remove it, like oil does.

Ian
RepRapPro tech support

The screw has no visible damage at all so it might have been a dodgy nut. I have used the spare nut and resumed printing.
With the copper grease it is really smooth so will keep an eye on accumulation of dirt and revert to oil if need be. Again it is a very quick and easy part to remove for inspection/cleaning anyway.

Hi Guys,

my "carrying nut" is gone, too.
I have been printing approximately 300g of PLA so far, so it happened pretty fast. I saw the first signs some days ago, when the drive had difficulties moving the X-axis-support and lost steps. The nut was greased, so I am a little bit supprised.
I had planed to exchange the rod and the nut, but I could not yet get hold of a brass nut and better rod.

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RS Ormerod #374

Polish the rod with lapping compound, take a close look at it through a magnifying glass, repeat
Mount a longer nut, should of cause be of other material than the rod, I think I would chose bronze

Erik

Quote
Kilpikonna
my "carrying nut" is gone, too. I have been printing approximately 300g of PLA so far, so it happened pretty fast.

Wow, yes, that is quick, and I am surprised too. You should have a spare nut, and give the rod a good clean and degreasing. Any grease or oil should help long life, so long as it doesn't actually stick contaminants together, to become a grinding paste!

Ian
RepRapPro tech support

Strange that, after test print about 200g of filaments and the nut have weird sound , and shaky.

Are the M5 nuts supplied with the Ormerod a 'half' nut?

I noticed RS Components have M5 'full' nuts [uk.rs-online.com] they also do stainless steel grade A2 at 1m lengths
[uk.rs-online.com] at £2.63 each, but the min order is 5 pieces. Good price for a meter though!

I also found Macc Models [maccmodels.co.uk] supply M5 stainless steel grade A2 studding 12 inches long for £1.63, but no full nuts.

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Ormerod #007 (shaken but not stirred!)

Quote
tru168
Strange that, after test print about 200g of filaments and the nut have weird sound , and shaky.

This is how it started with mine. You probably should check the Nut before it is completly worn out.
What I also realised is that there is swarf on the gears and in the grease from the nut, this is a strong indicator that something is wrong.
On mine the spare Nut is in, Rod was thoroughly cleaned and it works for now. Replacing the Rod and Nut I will tackle tomorrow and keep you updated.

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RS Ormerod #374

Quote
Kilpikonna
What I also realised is that there is swarf on the gears and in the grease from the nut, this is a strong indicator that something is wrong.

I had this too. I cleaned the gears and applied light oil to the threaded rod, and I haven't seen any swarf since then.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Yeah, found a spare nut in plastic bag. I will clean, oil and replace it by tonight.

Edited 1 time(s). Last edit at 01/20/2014 01:14AM by tru168.

What is not helping is that the screw is not straight or at least not rotating straight. I wouldn't be surpised if the fact that the screw is a little bent makes it chew into the nut faster.
A more sturdy screw would have been better I think.

The problem I can see is that the bottom of the rod is wobbling, so it evidently isn't concentric with the rotation point of the bearing. I reprinted and replaced the z-driven gear but the result is the same. So something is not quite right:

- Wonky bearing?
- Error in the z-gear-driven STL file, so the nut hexagon doesn't line up with the bearing hole?
- Nut drilled off-centre?
- Something else?

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Z gear wobble, I even suspect my 623 bearing worn out ?!

Well spotted dc42.

I took some rough measurement between the vertical Z aluminium extrusion and the Z threaded rod at the bottom by the gear the gap is ~34mm, the top is ~30mm, by the Z trapped nut is ~32mm.
I've made a couple of printable shim's to put between the Z nut trap and the X motor mount to test the affect, i'll post extended versions on the full nut trap after some further tests.

Matt

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Limited Edition Red RS Ormerod 1 #144 of 200 - RRP 1.09fw
iamburnys Ormerod Upgrades Github
Follow me on ThingiVerse My Designs

The risk highlighted is that if this thread fails the entire thing can fall onto the bed and really break stuff. Suggestion. Add 2x nuts and a wide washer between them a cm or so below the working nut. This way, if the nut strips ...and things start to fall, then the 2x nuts and washer will catch everything. Dropping only a cm or so it better than maybe dropping 10cm.

The z-gear and z-gear-driven are definitely concentric in the model: [github.com]
So it's either an effect of printing (seam in the print, or not perfectly round), or the z-gear-driven is not straight on the z-leadscrew, or the z-gear is not meshing well with the z-gear-driven, pushing it over at certain points during revolution. Lifting the z-gear on the motor shaft a little may help. We probably should have done the gears with one of them with one tooth less, so the gears aren't 1:1.

Regarding the motor corner bearing and nut trap being in line, the model looks spot-on to me, looking at the STEP file: [github.com]
However, this doesn't always convert into reality. Looking at the Ormerods here, I can't see any divergence between aluminium and leadscrew. Possibly if the aluminium extrusion isn't vertical, it would be a problem, or some error may be introduced if the lower Z clamp and motor mount are not quite done right. But even then, I'm not sure we'd see this much wear on the Z nut.

Check you haven't done the z-runner-mount adjustment up too tight, which WOULD cause wear, as it would increase the resistance of lifting the axis.

Otherwise, it's a bit of a puzzle where the wear is coming from.

Ian
RepRapPro tech support

Just back from office, I check the nut / m5 rod, wipe it with clean towel, i can see thick dark goo on the rod. that was 3 + 1 oil that I applied few days ago when I install it. Indeed , dark goo means the metal rub each other and worn out. I will try to find a brass nut and try it.
The M5 rod takes the load heavier than other design I think, because its the only metal rod that takes the load from entire X axis assembly . other than that , I believe some other reason on it yet to be discover.


EDIT : I ended up with print new Z-nut trap plate and also a Z driven gear. The M5 threaded bar now held in place solidly without any wobble when I print something. My prints seems swollen a lot, can't find reason yet. I file the screw holes for a tight fit.


Ew

Edited 1 time(s). Last edit at 01/20/2014 09:46AM by tru168.

Regarding the wobbly z axis lead screw. I found that though everything seems central, there is a fair amount of play between the gear and the ballrace that it sits on (the one screwed to one of the z axis stepper mount screws). The gear can therefore rotate slightly around the x and y axes. The downward load forces it to rotate until the slack is taken up. I found that wrapping tape around the ballrace so its a tighter fit on the gearwheel had a good effect. ( Not a cure but better)

I think a better design would be two ballraces on a longer screw with a spacer between, which would reduce potential for rotation around the x and y axes. Alternatively to use a threaded rod connector on the end of the m5 threaded rod and make the gear much longer so the threaded rod connector locates into it. Or any other means to prevent the gear rotating around x and y axes . Its just meant to rotate around z !

regards
Andy

Edited 3 time(s). Last edit at 01/20/2014 10:28AM by kwikius.

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Ormerod #318
www.zoomworks.org - Free and Open Source Stuff

Take a look at my topic it might fix it...
Longer nut

It might be to long so you might have to cut it down.

I have just had the z nut seize completely on the threaded rod. Awfully grinding of gears. Managed to remove the nut and the threaded rod seems to be ok. I will however order another piece of rod to replace it. In the mean time I have machined a piece of brass bar and drilled and tapped it. The new nut is now 15mm long, and running very smoothly. There is definitely an issue with the nut supplied.
John
Ormerod 096

If your in the UK, I think HomeBase stock M5 stud bar.... (It might be M6 so don't travel miles to get it.... I bought some years ago for a submarine I was making).
Kim

Kim
I suspect they would be streel and not brass. However my machined brass nut is working great. The layers are much better, I suspect this nut has been failing for a while

johneato: nice. But you didn't take the protective cover off the acrylic?! You must have been in a hurry...

Ian
RepRapPro tech support

Nice job John. My lathe is sat at home waiting to be played with. I'm going for the long nut I found. I was thinking of a little dab of grease or a drop of 3in1 on it as hopefully there won't be to much crud gets on the lead screw. Where did you get the brass stud bar from? Or did you turn it up your self?

Kim..

Without access to a lathe, I have just tried new brass nuts, but with no visual differences!

From my examination of the cog (looking from the top), the hole is asymmetrical to the nut faces.
Also the new brass nuts are slightly looser, but I don't think that is an issue, however the nuts are sitting on a taper which is not perfectly printed (a small bump) and may add to the effect, however it is the asymmetry that is the issue on my cog, even the outer edge is observed to be asymmetric.

The bearing is a very tight fit, so no adjustment possible there.

But at least now have brass on steel.

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Ormerod #007 (shaken but not stirred!)

johneato, is that the new style nut trap, or one that you've modified for your square bar? Any chance of a link to the stl please?

Cheers

Ray

Ian
>> johneato: nice. But you didn't take the protective cover off the acrylic?! You must have been in a hurry...

Hadn't really bothered me until now, but as you have you pointed it out, I have felt obliged to remove it.

Kim
I machined the nut on a CNC and tapped it by hand. I have lubricated it with high performance bearing grease that I use on the CNC.
It was very nice to design the nut and the modified nut trap on Turbocad. Send one part to the CNC and the other part to the printer and then bring them together and they fitted together like a hand in a gloove.
John