Some days ago I tried to mill a part out of 10 mm aluminium like I did it several times before. I knew I had to be very careful because there is so much flex in my CNC router. Unfortunately my Proxxon "spindle" (hand grinder FBS 240/E) started to sound unhealthy and I mentioned more vibrations than on my earlier milling jobs. Playing around with the rpm made it a bit better. Then, at a cutting depth of about 9 mm it happened. The end mill got stuck and Spindle started to smell strange. The end mill didn't take any visible damage but the Proxxon vibrated so strong I decided to open it. (Always disconnect machines from power supply if you do something like this!) The bearing was still ok, but the ventilation wheel was loose. So I think the cooling was insufficient and therefore the plastic coupling between the motor and the shaft got soft and deformed. Touching the motor was still impossible because it was so hot. I thought I can repair it by replacing the plastic coupling with one out of metal. When I tried to press an aluminium tube onto the gearwheel of the motor, the gear slipped back. Turning the motor was now impossible.
End of the story: I bought a new Proxxon (not a bad tool but the load was far too heavy) and decided to re-engineer my router. The new router should be much more rigid so I can use my Kress 800 FME, which is too heavy for my current mill.
I'm currently designing my new router on CAD. I will write a separate post in the next few days to show my progression.