Hi,
After a print I saw that the bed goes up and down in shocks and not fluent.
I took the Z-axis apart to inspect it and tried to put it back together, however I still have those shocks.
I removed the 4 bolts from the big black ‘thing’ that holds the IGUS nut and the holders for the bearings to test if the rod has some vibrations or the build platform but both are vibration free and the build platform can move without resistance.
I don’t know what it can be…
Dylan
Hi,
the problem is how the lead screw is integrated and supported in the printer - it is not supported by a fixed bearing but sitting on the motor coupler which in turn acts as a spring.
Normally in any type of cnc machine a lead screw has a fixed bearing and a floating bearing with the former holding it in axial direction. The floating bearing is not fixed to eliminate the risk of binding as the lead screw lengthens and shortens with temperature.
The motor coupler is just there to compensate for misalignment between the motor axis and the lead screw axis. It is not designed to take any axial load and neither are the bearings in the motor (although they’ll survive it)
I took some measurements with a micrometer and was able to compress the coupler by ~0.6mm by just pushing slightly on the lead screw with a finger. The opposite is also true, pulling on the rod will expand the coupler by the same amount. Every bearing has some initial stickiness on the gilde surface unless it’s not actually touching (i.e. magnetic bearings, etc.) and the result is the stick-slip effect. With a continous movement or just slack it just happens once, but with a spring like the coupler it happens again and again.
This is also the reason why the floating bearing on top of the rod as one of the most popular modifications to the printer has almost no effect on the print quality. It may add some drag and therefore reduce the “bounciness” of the coupler, but not eliminate it.
The two possible solutions are to either use a coupler that can not be compressed or expanded in axial direction or add a fixed bearing (ideally on the motor side of the rod). I prefer the second because a coupler that is really rigid in axial direction has also reduced flexibility in the radial direction.
There is enough room in the back of the 8400 to include such bearing by designing a new motor mount that has the motor sitting 15mm lower and a flange bearing above the coupler. I already designed and printed the part but did not find the time yet to install it. I will upload a rendering later (have it one another computer).
Well that’s interesting… I have a Z-stabilizer on the top only.
Have you checked this one out:
https://www.thingiverse.com/thing:775478
Please keep me posted 
Dylan
Yeah, I also had that stabilizer on top for a while until I figured out that it doesn’t actually do anything during the print when the bed moves only very slowly. It reduces the wobble during fast travel though, so I probably will install it again at one point.
I have seen the one from thingiverse you linked. It is a step in the right direction, but as the upper bearing does not really lock onto the lead screw, the lockring on top unfortunately only eliminates compression of the coupler , so it only improves travel upwards which means it’s pretty useless for printing. Now, if you added a second lockring below the bearing, that might actually work.
But in my opinion it’s just overkill, apart from the fact that the parts are too big to print with the Vertex.
Here is my solution:
The new red motor mount moves the motor 15mm down and adds a flange bearing (~8EUR) on top of the coupler which locks onto the lead screw with two set screws, which eliminates any play in the vertical direction.
Three of the four old mounting holes line up with the new mount and I added two additional holes as an option. The fit is really really tight though. I have about 2mm of room left between the motor mount and the Z-Carrier, so it might be to close together if a heated bed was added as this moves the whole bed assembly down. It might be possible to move the mount further down, but then none of the old mounting holes line up anymore.
I will upload the .stl once I can confirm that it actually works.
I have a heated bed 
Your idea seems solid So you do the same as the top one but at the other side
I understand what you mean now with the coupling.
Would a solid coupling without the ‘spring design’ not help?
Dylan
Can you measure how much thicker the heated bed is than the standard?
It’s only similar to the top bearing, not the same, because a flange bearing really locks onto the lead screw. You could move it to the top and I almost did that, but then the (small) flex of the case itself is still there.
A solid coupling has the problem that the motor axis and lead screw have to be aligned perfectly or you get a much larger Z-wobble. It would be the same as if you pushed the lead screw so far down in the coupler that it sits on the motor axis, as is described in the Wiki
I’m not aware of a coupler design that allows for radial play and angular misalignment but has no axial play whatsoever
I know that you haven’t got any time to install the design but maybe you can share it already?
I could print it and mount it and see if it has any effect?
Dylan
I’m sorry that it took so long, but I lost track of the project after I had to redesign the motor mount several times to make it actually possible to attach the damn thing, because there were some screws that wanted to be in the same spot, but recently got back to it, finished it and uploaded it to thingiverse a few minutes ago.
Link: http://www.thingiverse.com/thing:2121687
Here is what it looks like:
It’s a bit fiddly to mount, but it makes it possible to not loose any build-height and the motor is still above the table. The pillow bearing I used is linked on Thingiverse, I hope it still fits, because the image in the link looks different, but the technical drawing is still the same.
I also redbuilt the bearing clamps and replaced the linear bearings for high quality ones. It’s a bit annoying to actually find good ones, because no matter what most shops claim, they might be the same. I ended up with 20 of those cheap ones and only 4 were somewhat smooth running. In the end I bought them from Misumi for 8€ a piece, but what a difference.
Rule of thumb: If the bearings come in packs of two in a little plastic bag, throw them away. The good ones are individually packed.
Check for mechanical problems. For instance, if the Z coupler is not securely attached to both the motor shaft and the threaded rod, the motor will spin without raising the print head. The threaded rod should also be securely pressed to the motor shaft by the Z coupler.
Other possible mechanical solutions
Power your bot on. Hold something metal (screw driver, knife blade, etc) directly under the end of the probe and see if the light on the top of the probe lights up.
Check your probe extension wire. Always match the wires from the plug end of the cables (as if the probe were being plugged straight into the board) before plugging them together. Do not just assume that matching the colors is the correct way. In the picture below, you would actually connect the black cable on the left plug into the red wire on the right plug. [url=https://vexmatech.com/fdm-technology.html]3d printing service fdm technology[/url]
Check your wiring for fatigue or a stress point. Move the extruder just above the bed close enough for the light on the probe to turn on. Move the bot around as well as the bundle of wiring coming off the extruder and see if the light goes out at any point. It it does, find the break in the wire and replace the part.
Make sure that the probe has not moved over time. Through movement and vibration, the probe may have shifted after the nuts holding have loosened. You may need to tighten those nuts and re-calibrate your Z probe.
Also check to make sure you hot end is not loose. If your hot end is not secure, it might drop far enough to make contact with the bed.
If you have a Rev F board, make sure the probe voltage select jumper (pin connector) is in the right location or not missing.