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Layer shifts after changing heatbed  

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NinaS
(@ninas)
New Member
Layer shifts after changing heatbed

Hello,

I have a MK2/S MMU1 (got it second hand about half a year ago). Recently I changed the heatbed to the magnetic one (for the MK2.5), and ever since I've been getting layer shifts in the y-axis. They seem to occur only when the object is above a certain size along the y-axis, so very small things usually print fine. The shifts aren't very large, about 1-2mm.

I've tightened the belt, tried with different power settings, cleaned and lubricated the smooth rods - I've run out of ideas. Only thing I can still think of is completely exchanging the rods and the bearings...

Maybe noteworthy, before changing the heatbed I had some issues with bed adheasion, prints getting knocked/dragged off the plate midway through. Haven't had anything like that since switching. I'm also aware that MMU1 with magnetic heatbed is not an intended combination - could that be related?

I'm grateful for any tips or hints as to what could be causing this or what I could still try to fix it.

Posted : 27/04/2021 2:44 pm
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(@)
Illustrious Member
RE: Layer shifts after changing heatbed

Photo can help ... but common issue is the belt drive gear is loose (improperly tightened set screws). This makes a variable shift back and forth. If the skip is random, and usually one direction, then bearings are generally the issue. Tightening the U-bolts too much crushes the bearing case and keeps them from rolling smoothly.  With power off, the bed should slide front to back without any force. Even a slight tilt of the printer should send the bed rolling.

Posted : 27/04/2021 7:19 pm
NinaS
(@ninas)
New Member
Topic starter answered:
RE: Layer shifts after changing heatbed

I have checked and tightened the set screws.

The shift happens at different heights every time, and tends to either shift back and forth or at least shift back eventually, so not always in the same direction The distance of the shift seems pretty constant too.

With power off but belt tight the bed doesn't slide at all when I tilt the printer. I can push it but there is some resistance.

I have also noticed grooves at the top of the smooth rods, running most of the length. Looks like it might be the bearings...

This post was modified 6 months ago by NinaS
Posted : 27/04/2021 7:38 pm
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(@)
Illustrious Member
RE: Layer shifts after changing heatbed
@ninas
 
The bed should slide easily back and forth. As should the extruder - but the extruder takes a bit more angle before it starts moving. Marks on the rod are an indication a bearing is seizing up, and over tightening the u-bolts that hold them is a common problem.
 
Go back to the U-bolts (I am making a presumption here that you have u-bolts holding the bearings) - and make sure they are snug, not tight. The u-bolts should be tightened until they no longer wiggle, and maybe 1/8th turn more. I'd rather a bearing wiggle a bit than be crimped.

=== on the other note ===

If the set screws were loose, that's a clue the gear is wiggling free.

Gear set screws must be tightened flat first. Start with both set screws loose enough the gear can spin. Then, slowly align and tighten the set screw closest to the flat of the shaft; wiggling the gear as you tighten to ensure the set screw lands fully on the flat. Once you are satisfied the belt and gear are aligned, torque the flat set screw tight. Now, and only after the flat set screw is set, tighten and torque the set screw on the round. 

To remove the gear, loosen the set screw on the round first - otherwise you might strip the flat set screw. 

Posted : 28/04/2021 12:21 am
NinaS
(@ninas)
New Member
Topic starter answered:
RE: Layer shifts after changing heatbed

Ok, I will check the u-bolts, thanks. Any idea why this problem only started after exchanging the heatbed? I didn't change anything about the bearings or the y-carriage while exchanging it.

Set screws were fine.

Posted : 28/04/2021 8:05 am
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(@)
Illustrious Member
RE: Layer shifts after changing heatbed

@ninas

I wasn't there to see what was done, so no, not really any clue. Most of this peer to peer help done without photos that show the problem is mostly guessing.

Posted : 28/04/2021 4:52 pm
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