How to properly clean an adjustable lens

I unscrewed the lens of my N 40630 laser after about 4 months of use and tried to clean it, tried a few things with cotton swabs, glasses cloth, lens cleaner.
but there always remain small fluff or stripes on the lens,
well the hole is only about 4mm
Or do you have to unscrew the small lens from the round black part???
Can any of you help me or give me a tip
Another question
Why does the focus shift after cleaning
Originally I had 35mm
after the 1 attempted cleaning it was 41mm
after the 2 it is 45mm
what is changing there???

you shouldn’t have unscrewed this. clean it with a cotton swab and alcohol. on a fixed focus laser I would never unscrew the lens. Also I would recommend to clean it more often. I clean mine every week and after every cut.

Thank you for the tip
I have an adjustable focus and a second lens
do you have another tip why the focus is always shifted in height ???

probably the threads are cheap/unprecise. could be many reasons due to unprecise material

If there is a lot of lateral movement between the barrel and the housing then the laser could be passing through a different part of the lens depending on where the barrel is angled. This could affect focal distance.

In this case I suggest applying some thread tape to the threads of the barrel, enough to reduce the gap enough where the barrel fits snugly. This can also help reduce the barrel unthreading from the housing through vibration. Make sure the thread tape is tidy and that nothing is obscuring the path of the laser.