I am the local mechanic of the family and friends and would never put anyone in danger. New drums over here are a lot more than over their, and some of these people are struggling to stay a float. No one wants to pay the mechanics high prices so they come to me, and then I get some more tool money to spend.
I would never machine a drum if it's going to be past it's limits, but most have never been done or have plenty left. Like I said with the disc's, it's easier to take them somewhere or do them on car.
As for helping them with no money, I remember once my daughter and her boy friend ran there car out of oil and seized the overhead cam on one of the bearings and snapped the timing pulley dowel. They asked how much to fix it and I said a new head etc, etc and they said we don't have that sort of money. So I said I would fix it at no cost but no guarantees.
I then lined the timing marks up and took the cam out on the side of the road, took it home and put it in the lathe where I removed the aluminum off the journal and then used some emery to make it smooth. I returned to the car with a new machined dowel, a new 5ltr of oil and my battery drill with a drum sander on it and gave the head bearing and cap a light clean up. After cleaning it up I re installed it and it ran like that for 2 years without any problems. At the 2 year mark the radiator let go, warped the head , but they where a bit better off and bought a second hand head cheap.
A few months after fixing the cam they sprung a leak in the exhaust valve port of the aluminum head, about 1 1/2 inches in on the bottom. They went around to all the places to see if it could be welded, and got the answer of we cant get to it you need to buy a new head. So once again no cost, no guarantee (but they had nothing to loose anyway) I stuffed a wet rag down the port, cleaned it up with the die grinder and aluminum mig welded it up, this also held until they took it off.
I have been heavily into cars, machining etc for around 27 years and have a show truck here built from the ground up that has a mile of my own custom mods that are all engineer approved after inspection, so there is no way I would let something out the gate if I thought it was dangerous.
I have also done panel beating repairs right up to replacing nearly 1/4 of cars from accident damage for other people, and have done a few power steer conversions with chassis mods for people with the same truck as me that wanted it, as it's not available anywhere and they have seen mine at shows.
I have also built pantech bodies and tail gate loader for trucks, and they have all passed engineering.
I understand your concern about safety, but as you can see I am not just your average back yard guy endangering people, I have a lot of experience behind me.
As for machining brake drums, as long as they are lined up to run true in the lathe, there is nothing to it. The worst thing that could happen is they would be out of round and lock up the wheel, which you would find out on the first small drive.
Dave
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