Was bumbling about with the 6840 earlier and cutting out a tower of Piza and I noted that the speed and feeds needed tweaking again to get a certain cut through every time.
Usually when this happens I start thinking a tube swap out is on the way so as I didn't have another tube in stock as yet I decided to give the mirrors a clean and the final lens just to be sure.
Mirrors cleaned: Check
Ahh now here is something I had forgotten to mention, I run a bigger belt drive compressor on the machine as I've found 25psi of air assist makes a BIG difference to cutting, since changing out the compressor I had fitted an air cleaner (to remove most of the moisture a compressor creates) but hadn't really thought about lens cleaning.
The final lens was way mucky, a quick clean with de-ionised water and a cotton bud and I'm back to running reliably at 12 speed 65% power on 4mm MDF with cut through every time using a 40 watt tube.
soooooo if you find your machine has gradually lost a bit of bang of late don't forget that final lens and keeping it clean it really does make a huge difference.
I forgot and nearly ended up buying a spare tube