tl;dr 4mm ply laser stock is really warped, can cause serious flame-age due to laser focus if not careful - don’t use if the other thicknesses will work!
Noticed this week when using the laser that the 4mm plywood in the store of materials is really curved, probably a (perfectly understandable) humidity/moisture issue, but it caused quite a few issues with cutting and engraving given the deflection was about 20mm at its worst, making focusing the laser basically impossible for large objects being cut/engraved.
It ended up being ok for me as I didn’t need a particularly close tolerance in my cuts and I just ran a second pass where needed, but given that the focus issue can cause serious flame-age when not calibrated right, I thought it was worth highlighting!
Also, I know this is know to the laser techs, as a couple of them stopped by while I was working on my project, but someone jammed the old clamps to the aluminium bed and basically smashed it up pretty badly in places, making it tricky to get a perfectly flat project on the bed - they’ve removed those clamps, but obviously need a new solution!