Necromancer's notes

5/16 screw tap?
Just take this heresy to Mordor and drown in the lava of mount Doom!
And use only metric stuff we have a plenty right here.
(c) A salesman at the local hardware store

 

Okay, fixing Z axis wobble, take 2, the proper fix.These instructions are quite complex, so make sure you have all the instrument and skills required. Otherwise, go for simpler lawsy’s fix.
And take care not to kill yourself trying to do it.
It all took about 12 hours of experimenting to get it working properly + a week or so seeking out the 5/16 screw taps throughout the city hardware marketplaces.

So far 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

If you have access to a CNC capable of machining the whole thing out of brass – go for it, it would be a lot better solution.

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 external dia is about 7.7mm (on mine).

• Neither the nut, nor the rod are produced to be THAT precise. Even more, the rod looks like it is overcut (See below for explanation).  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

Why brass? I had some in my workshop, and it will be more durable, than a steel nut on a steel rod.

Putting things together

I had a few 11x11mm chunks of brass, so while the holder was printing, I started by cutting an 11x11x11 brass cube. Try to make those cuts as precise as possible. We’ll drill through the sides we’ve cut and leave the factory sides to align the part vertically in the nut holder.

Mark the spot you’re going to drill, and carefully align the brass cube at the drill press.

The drill bits I had were designed to drill steel, so they have the angle of 120 deg. Brass needs about 116 deg. So it’s a good idea to either get hold of the proper drill bit set, or sharpen them a bit, if you have the right skills. This is optional.

Carefully drill the brass. Start with something like 2-3mm drill bit and go up to 5.7mm (I only had 5.9 drill bit, it worked as well)

Once you’ve done that, it’s time to make use of those screw taps. I nearly ended up replacing the rod, because I couldn’t find any 5/16 screw taps for sale here, in Russia. One salesman even suggested taking that heresy to Mordor and drowning all the 5/16 stuff in the lava of mount Doom and using metric instead.

Adding kerosene while tapping is a good idea, but that’s not 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’. Applying # III will introduce the same wobble, so DO NOT use it unless the nut doesn’t fit.

Now, time for assembly. 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. Or just in case you decide to take it to Mordor later).

(optional) Insert M3 nuts into the holder, insert M3 screws 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. See if it is properly aligned with the hole. Use side screws to adjust if needed. The more precise you machine and align it – the better quality you’ll have.

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.

Use longer M3 screws to assemble everything. When motor’s off tripple check that the rod moves easily throughout the threaded rod, you should be able to rotate it by hand easily. It’s better to tighten the screws when the bed is at the very bottom.

Check for wobble by hand, like when you adjust it via lawsy’s hack. Can’t find no wobble at all? Then everything is fine.

Once done, add move the bed around via pronterface, to see if everything is working, check the Z motor temp and finally print something. On the pic below you can see printed cylinders with layers  .3mm (left) and .15mm (right) height respectively.

And the result is a BIG change compared to what I’ve had before – either with or without weight applied to the bed

Final thoughts

• A very slight wobble on the sides remain. Mostly due to the fact that I print with top cover removed, since it causes occasional filament jams for me. Despite the frame is quite solid, the best way to get rid of those – attaching your printer to some very solid surface. Or you can just lower the head travel speeds. It will take longer, but parts will be A LOT better.
• Making the nut even higher might improve quality, but will introduce a LOT more load on the stepper motor. So it’s not a very good idea to do so.
• Making the nut holder a little bit higher (there are a few spare mm of rod remaining) may be a better idea, since the table will be way more solid this way.
• Originally I wanted to go after a split lead nut, but after testing this one out – it looks like a split one is not necessary after all.

The nut holder design

Get the scad file for openscad at my github, adjust to your sizes.