1 (edited by Necromant 2013-01-15 20:30:51)

Topic: [MOD] Z axis custom nut replacement (fixes wobble, stabilizes bed)

Well, this is my cure of the Z axis wobble at solidoodle 2nd generation. It also alters the construction to make the bed more solid, and less affected by fast print head moves.
The idea is a custom brass nut + a 3d-printed holder for that. Please note, that this hack requires some skill, and is a lot more difficult to pull through, than the existing z axis wobble fixes I've seen here.
Why brass? I had some in my workshop, and it will be more durable, than a steel nut on a steel rod.

The theory.

So what’s causing the wobble?
The solidoodle guys fix a usual 5/16 nut on a threaded rod that pulls the bed up and down. The rod’s actual external dia on my solidoodle was 7.7mm (sic!).

    Neither the nut, nor the rod are produced to be THAT precise. Even more, the rod looks like it is overcut.  It is a common issue with those, and that sucks pretty much. The same goes with the nuts, so to make things precise you either have to machine the threaded rod yourself or the nut. ( I went for the second one).

    Second bug is the nut height. Theory says that the nut should be at least 1.2 * rod diameter to avoid wobble. Ours is not high enough, so we’re doomed for Z axis wobble. Just don’t make it TOO high, or your stepper motor will have problems rotating it.

To complete this hack we'll need:

  • 5/16 screw taps I, II and III (Finding those was the most difficult part)

  • A 11x11x11 brass cube (More or less precise)

  • Solidoodle to print the nut holder (use 1.0 infill to make part solid).

  • A drill press with a set of drill bits.

  • A way to properly align the brass cube when drilling (IMPORTANT!)

  • M3 screws and nuts

  • Proper skills to work with all of that.

  • Several hours of time


The stl file for the nut holder is attached, the openscad design can be found on my github page here:
github.com/nekromant/thingies/
You may want to customise it a little bit.
If you have access to proper equipment to fully make this one out of brass - go for it, it would be a lot better.

Implementation

So, start by cutting a 11x11x11 brass cube. The more precise you make it - the better.
Then, carefully align it on the drill press, and carefully drill.
Brass requires different drill bits, then steel. For steel the angle is about 120 deg., while for brass it's about 116 deg. You can sharpen them, if you have the skill, but be careful. If you don't have the experience - just ignore that - that doesn't make a big difference.
Carefully mark the spot, and start with something like 2 or 3 mm drill bit and carefully go up to 5.7 (I only had 5.9, these also worked fine)

Adding kerosene while tapping is a good idea, but that’s not strictly necessary. I didn’t have any around. Just don’t go very fast, and be extra careful when tapping with #I screw tap. Apply tap # I and tap # II. Now stop at this point. Try to put our nut on the threaded rod to see if it fits. Mine fitted on nicely, so I didn’t have to use # III screw tap at all. This indicates that our rod is a ‘overcut’ during manufacture. Applying # III will introduce the same wobble, so DO NOT use it unless the nut doesn’t fit at all.

Remove all the original stuff holding the 5/16 nut. Place it somewhere safe (In case something goes wrong or our part breaks – we’ll have a backup. ). Now take the nut holder we've printed.

Insert M3 nuts into the holder and check if everything fits. These are optional, since M3 screws tap their way in the plastic and get quite a grip.

Now insert the brass nut we’ve machined, you can use a small hammer, but don't go extreme. See if it is properly aligned with the hole for the rod in the plastic. Use side screws to adjust if needed. The more precise you machine and align it – the better quality you’ll have as a result.

I drilled a few holes in a small metal disk to secure everything on the bed. You can use a 3d-printed part here, but that looked faster to me.

Attached is the photos of the resulting prints of .3 and .15mm height.
Since this this ensures that the nut is a little higher against the top of the bed, then it used to be, it make the the bed less shaky, when printing at high speeds.

Sorry, since it looks like I can't intermix text and images in a readable way here, it's mostly text. More photos can be found in my blog post here: ncrmnt.org/wp/?p=1260

(I'm too lazy to upload them all here one by one).
Comments welcome

P.S. Be extra careful when assembling, tripple check that everything moves smooth with your hands with the motor turned off - don't be afraid of the grease. It's better to tighten the screws when the bed is at the very bottom.
P.P.S. Oh, and does it void the warranty? Was there one? Well, if there was one, I think it's now void.

Post's attachments

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solidoodle_nutholder.stl 68.87 kb, 208 downloads since 2013-01-13 

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2

Re: [MOD] Z axis custom nut replacement (fixes wobble, stabilizes bed)

A most excellent job.  This should be valuable for the SD staff for sure.