Well, the strange noises and sparks would indicate to me, bearings and brushes. It would run $80 or so for a rebuild. I did try some fancy sealed bearings and didn't seem to get much more life out of them. Found out there are sealed and shielded bearings. I think the main thing that causes the drills to fail is that we are using them for grinders, and they are drills. To get maximum life out of them, I do use 3 inch discs almost all the time, and I run the drill at half speed max. This extended the life span from 300+ bowls per bearing set to maybe double that. The funny thing about sanding is that slower speeds cut faster than high speeds. Best guess is that it is about traction. If you are spinning at high speeds, the abrasive grit doesn't get a chance to dig in and cut. Think of a dragster burning out their tires, lots of heat and smoke but they are going no where.
I have been pondering on how I can make a flex shaft system that would run off of a mini lathe. It would mount in that articulated arm I made for when I sand. With a 1 hp motor, it could easily handle 5 or 6 inch discs. The Foredom drill set up just doesn't have the power for doing that other than on occasion...
robo hippy