The video below was produced by a German guy who calls himself The Post Apocalyptic Inventor (PAI). I don’t know his real name but he appears to have an engineering/technical background—and he knows tools and how to repair them. Watch, and you’ll see him repair a Fein D-handle drill, a Bosch angle grinder, and a pair of unusual tools from Fein and Bosch—tapping drills specifically designed for threading metal. This video is not for everyone, but if you’re a gearhead, into tools, or like to fix things, then you’ll enjoy watching it.

My comments below:
02:12 You can tell this Fein D-handle drill is old because the housing is held together with slotted, rather than Phillips head screws.
02:20 It’s cool someone (the manufacturer?) thought to store a pair of replacement brushes inside the tool. Who, nowadays, even thinks that far ahead?
03:25 I had to look up what an isolation transformer is; now that I know I’m even more impressed with this guy.
04:40 It was a good idea to have opened the gear case, because he found metal shards that could have caused damage.

08:28 Next up, a heavy duty Bosch angle grinder
10:45 The brushes are frozen in place, but the holders are solid and the PAI is able to clean off the rust and install replacement brushes.
13:40: Hard to believe how good this tool looks and how well it works once it has been cleaned and repaired.

14:20: Fein tapping drill—designed to hold taps for tapping metal. I wasn’t aware such a drill existed. But then I’m not a metal worker.
16:00 The tapping drill has a very odd drive mechanism; it runs counterclockwise with no load and clockwise under load—an instant reverse feature that allows you to avoid over-tapping and stripping the threads.

16:50 A Bosch tapping drill.
18:00 The rotor looks like any other, but the stator is very odd—with the windings off to one side. Weird!

19:25 I like sanding blocks he uses to clean the metal square. I have some metal sanding blocks and they’re great for removing corrosion. His are from 3M, but other companies make them too.
21:00 The PAI gives an interesting explanation of how certain brands of hand tools were named. I particularly liked the discussion of Gedore because my first set of mechanics tools were from Gedore, though my tools were a “budget set” from India—not German tools like his.