This issue has been going on for a long time and I always blamed it on the Z height but now I have eliminated that so I am at a loss.
Here is the setup.
I engrave about 80 different anodized aluminum parts on two different fiber lasers, they are separated by the Z height of the engraving area - 60mm or 16mm. I use my 3D printer to create fixtures to hold the parts, some one part and others hold multiple parts, at the correct height so there is no moving of the Z axis. Some of the parts have an engravable area as small .2" x .35" so alignment is critical, .001" off and you can see it very easily. Fixing the Z height and using fixtures eliminates any possible height differences and should eliminate size/position errors by not having the Z exactly the same every time I run the part.
For the last several months I have noticed a bit of movement of the engraving area, usually no more than about .1mm, or less, every week. That doesn't sound like much but with the tolerances we need to maintain, it's a lot!
There is nothing that moves, the fixtures are solidly held in place on an aluminum plate using dowel pins that they are held against, and the parts don't move in the fixtures. Nothing with the laser is changed, it doesn't get moved, adjusted, etc. The parts are all within .0001 tolerance.
I have done everything I can imagine to stop this movement but something is moving. I'm guessing it's the mirrors because that's the only moving part, but I can't understand why they would move at all, and if they did move, why so little and fairly consistently.
Any ideas would be welcome!