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T8 leadscrew slop

Posted by n9jcv 
T8 leadscrew slop
March 16, 2017 07:23PM
I am working on a new printer. It is a corexy with a single lead screw. I am finding that the bed will tilt side to side due to the slop in the nut for the T8 leadscrew. I am wondering if there are any easy ways to deal with this issue. Has anyone else solved this?

Re: T8 leadscrew slop
March 16, 2017 10:39PM
Been looking all over, not sure I have any solutions yet. I have 4 lead screws and nuts and they all have the same amount of backlash.

So, thinking, maybe just go to 5mm threaded rod and a nut, still backlash, but not nearly as stiff and maybe the bed will be enough to counter it?

Thinking maybe use a belt for the z, maybe no backlash?

As I said right now this is a cantilever bed, 2 smooth rods and a single motor driven T8 Lead Screw. When the bed goes up and down, it sometimes tilts to one side or the other due to the backlash, there is no sticking of the bearings, they are smooth. Also there seems to be a fair amount of bounce. Neither of which can be good. HMMMM???? When I move the bed without the lead screw from the center, everything seems smooth, but I still see bounce. Seems to me it is the backlash from the nut doing this. Perhaps a belt solution is best?

Any other ideas, I would love to hear them. My bed is fairly light weight. I used aluminum angle and an MK3 200x300 heat bed, no glass. The aluminum angle is less than 150 grams.

Re: T8 leadscrew slop
March 16, 2017 10:42PM
maybe an idea, from Digital Dentist in another post

"The biggest problem with cantilevered beds is that the edge opposite the side with the guide rails tends to bounce when the XY mechanism is flying around causing the machine to shake. Flexibility of the guide rails, the printer's frame, and the cantilever itself all contribute. If you put a lead screw or two under that floppy edge the bouncing can be almost completely eliminated."

He may not like my interpretation, but how about 2 smooth rods on back and stick the lead screw on the front? Would that smooth it out? I would need to build a new frame to test the theory out, hmmmmmm

I tried it, that did not work, it made it worse.

Edited 1 time(s). Last edit at 03/17/2017 09:02PM by n9jcv.
Re: T8 leadscrew slop
March 17, 2017 06:42AM
Your bed is more likely tilting side to side due to slop in the guide rails, not the nut. Or it is your bed levelling mechanism giving unwanted movement. Cantilevered beds give unwanted movement in Z (assuming the guide rails are good) when the unsupported end of the bed flexes up and down or 'bounces'. Ultimaker users often say that weighting the bed down helps stop this bouncing although it is an awful fix...

The nut can have as much slop as it likes in the XY axis, and in theory it can even have slop in the Z axis if the weight always takes away the backlash. The guide rails are designed to constrain the movement to only one direction, the nut is just the mechanism to move the bed. I suggest you consider why your bed is moving so much, as otherwise your fixes will make no difference.

EDIT: This is why in theory you can use M8 rods for Z screws instead of leadscrews and still get decent quality out of them. They just rely on the guide rails removing unwanted XY movement. Not that I suggest you use M8 screws... that really is a backwards step.

Edited 1 time(s). Last edit at 03/17/2017 06:46AM by Origamib.
Re: T8 leadscrew slop
March 17, 2017 08:29AM

thanks for the suggestions. Let me provide some more detail. At this point I do not have the mk3 installed, just the frame for the bed. I double checked and there is NO movement side to side or back to back with the frame x or y. The movement is in the z direction. In the center z movement/slop is very tiny. But at the left or right rear bearing, the z movement can be up to 2 mm. If one side of the z goes up the other goes down. The z nut is center to within 1mm of center, close as I can get it to exact center with my measuring tools. It is this movement that I am trying to eliminate, but I can't seem to find a solution.

Any other ideas or suggestions?
Re: T8 leadscrew slop
March 17, 2017 12:32PM
Could you post some pictures?

For you to have a bed that slants in such a way, I would still suggest that there is unwanted XY movement with your bed. Perhaps the bearings are sloppy allowing it to angle (XY slop). How do they move on the rod with nothing attached? Can they 'wiggle'? If not, the bearings may be loosely attached to the bed mechanism, allowing one to move lower then the other. If you are using cable ties to attach bearings to plastic parts, my money is on this. If you still believe it is unwanted 'bouncing' of a cantilevered bed, a simple test would be to print a large-ish cube on the bed at multiple points. If the bed bounces, you will see more banding the further away from the bearings you go. If it appears on the left and right more then the middle, it is XY movement. Or perhaps it's both, but then you should see the worse banding at the end and on the left and right.
Re: T8 leadscrew slop
March 17, 2017 01:28PM
Here is a pic at any given point at z, the bed will tilt with even slight pressure. There are no zip ties, and again i checked, if you grab the purple bearing holder you can not move it in the x or y direction.

Re: T8 leadscrew slop
March 17, 2017 06:20PM
So, I think I found a simple solution, but I am not sure why it works. I had the long side of the T8 nut facing up, to maximize my lowest z position. I now have the long side pointing down and 90% of the tilt is gone. It would seem to me that the bed weight would be the same on the T8 nut regardless of which direction it faces??? Strange

So that was not the problem, I think at this point the only solution I have is to go to 2 z steppers, one positioned by each smooth rod.


Edited 1 time(s). Last edit at 03/17/2017 09:01PM by n9jcv.
Re: T8 leadscrew slop
March 17, 2017 09:24PM
if the extrusions where placed in a T shape you might get some different results,
the top of the T going between the 2 purple blocks(which would need turning 90 deg either way, screws near back perhaps), and the other bits in the center, 2x 2020 might be good for that part, then try to get the z running as close to the join.
you will still have 3 points for the bed leveling. might be easier to make it stiff,
gives you an alternate arrangement to try.(also try decouple Z so it only provides lift
and no other motion.

Edited 2 time(s). Last edit at 03/17/2017 09:28PM by MechaBits.
Re: T8 leadscrew slop
March 18, 2017 08:30AM
You've got no height to your bed support, so you're asking too much of the linear bearings. You need to add 3 or 4 inches of depth to the support with a second set of bearings at the bottom, then form a triangle out from the bearings to support the bed. Rather than trying to prevent twisting with a single 1" lm8uu you prevent it with the relative motion of two bearings separated by 1 or 2 inches of Z.
Re: T8 leadscrew slop
March 18, 2017 09:35AM
Yeah all my Z rods are 60mm apart smiling smiley
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