Different offset at each first layer

[ManAtArms 59]

Greetings!

I got a problem since a few weeks wich i am unable to solve.

I started a thread in the german section already, but i thought it would maybe help if i ask this here too.

 

Problem: Everytime i start a new print, the nozzle has a different offset to the glas on my Ultimaker 2+.

The most time i had this problem after changing to Tinkergnome, now i use the custom FW from Bondtech, because i have a DDG Extruder installed.

It seems that the problem occurs less, but the first print today had almost contact with the printbed!

 

I do not know why it happens, i dont changed anything related to this and i make a maintenance every month.

The only hardware i changed a little time ago were the linear bearings in the printhead and a new ptfe coupler.

The rails are all oiled with Unilube and the Z-Screw with Magnalube.

 

This all started in late summer 2020, but it only happened at a few prints and never occured again till November 2020.

I checked everything that i could think of causing this problem, but nothing.

Everything works fine (besides a little more noise than usual, maybe need to change the bushings) and the print quality is also like on day 1.

 

Also nothing suspicious in the Gcode of my files, i have Cura 4.8 in use.

 

It is the only problem i can not solve and that bothers me, because i have to sit in front of the printer every time, hoping the offset is in acceptable distance.

I have this printer aber more than 2 years now and can tell when something is wrong and what causes it, but this....no idea.