This Bosch PSR 14.4 cordless drill (with charger and 2 batteries) was for sale at the local tip shop for the bargain price of $22. A quick look showed that it worked ok, and so I decided it was maybe worth $20. They accepted my offer, and the drill came home with me.
After looking at it a bit more closely, I wondered if maybe I’d made a mistake. The gearbox was loud, louder than I’d like, and the motor was arcing and sparking a lot. It seemed like electrical arcing happened mostly when the motor was starting and stopping. There appeared to be an electric brake engaged when the trigger was released. I don’t know how this works, but I expect it’s as simple as the windings are shorted. Perhaps in combination with a power resistor to dissipate heat.
I took a look inside and immediately noticed that the motor was kind of crimped shut - a non-serviceable part. That won’t prevent us making custom carbon brushes if necessary, it will slow the process down though.
This is a close up of the motor’s commutator. I think we’ve found the problem - all that carbon dust packed between the copper segments. Those are supposed to be electrically insulated from each other. The commutator also had some build up on it, but that cleaned off easily with a fairly rounded screwdriver. I used a worn out flat screwdriver because it’s better not to dig into or change the shape of the soft copper at all. The surface of the brushes looked smooth and clean too.
The drill is back in one piece, and 95% of the arcing has stopped, and the gearbox sounds like a new one! Hopefully this will give me some years of service and repay the couple of hours and $20 I put into it.
I still need to clean the outside casing of the drill, it has paint splatters and what looks like plasterboard jointing compound in the grooves. If I ever do that (ha!), I’ll post a final picture.