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Printbed does not stay in same place

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Hi all

I've been using my UM2 for a while now without any major issues (lots of small snags though) but now I have come across the following issue.

Here's the pictureprintbedissue.jpg


These gears have been printed straight after each other... The one on the left was printed first and the one on right straight after. Before printing the left hand gear I have levelled the bed and I get quite nice tightly packed first layer. But for the second print the bed is not any more at the same position. It stays maybe extra 0.5-1mm lower resulting to the bad print quality as the print head "drops" the material on to the bed. This seem to especially affect the first 1-5 layers. The quality evens out after this.

Currently I have to level the bed before every print to keep the quality consistent.

Any ideas?




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If you look in the subject linked by robert:



Work around:


Manually do anything to cause the Z axis to re-home at least twice before you level the bed or print anything -- hitting "Advanced|Lower buildplate" a couple of times appears to work, for instance. Make sure you see the bounce before considering the Z axis to be homed.




It looks to me like Z_HOME_RETRACT_MM is set too low in Configuration_adv.h -- it's set to 3 mm, but the X and Y axes are set to 5 mm. When I change Z_HOME_RETRACT_MM to 5, homing works consistently for me so far; I see the little bounce each time. Patched version at https://github.com/stevegt/Ultimaker2Marlin/blob/consistent_z_homing/Marlin/Configuration_adv.h.


If there are no objections or corrections, I'll send Daid a pull request for this after running it myself for a few days.




I guess this will be added to a future firmware update? Meanwhile you'll have to do the workaround i'm afraid... i've been doing this for a while. Just check your first layer if it seems ugly abort and start the print again


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There are 3 solutions right now:

1) Modify firmware to go up 5mm instead of 3mm. This is advanced and if you haven't compiled the firmware will probalby take you a few hours.

2) Always raise the bed a few mm before you start a print. And before you level it.

3) Bend the Z limit switch a tiny bit so that the bed hits it 1mm lower than normal. I don't have this problem as the distance between all the way down and limit switch opens is 2.7mm. I measured this by twisting the Z screw with power off and I get 17 "stepper clicks" which is 2.72mm = 17/(6.25 full steps/mm).

So bend the z limit switch down until you have 17 or less clicks between all the way down and limit switch open.

edit: Oops - bend it *down* not up - you want the Z switch to get hit in a lower bed position.


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