A while back, I posted this on another woodworking forum after having a table mounted Hitachi M12v explode under my chin due to the motor fan failing ( blades sucked into the motor, splitting the casing and ejecting parts in various directions) during operation:
""But seriously, do yourself a gigantic favor; use a flashlight and closely inspect the fan blades where they are pressed at the base. I found (during the postmortem) that the fan blades appear to be highly susceptible to stress fracturing in exactly the same way a fan fails in gas turbine (jet) motor, at the apex of the bend where the fan blade connects to the sleeve (in this case pressed) . The majority of the blades had stress fractures which ran over half their width, that is, the ones that were still attached after the catastrophic failure... Definitely worth a quick peek.""
Inspecting the fan will most likely require cleaning away some very fine dust. this can be swabbed away partially and/or carefully blown clear with compressed air through the vents, do whatever it takes to get a good look at the fan itself, it might save you a bad day in the shop. This unit was made in the early '90s and "died" in the Fall of '05, less than 15 years old and was only used as a back-up for most of it's life.