You know who is going to be your "Best buddy" in that type of Long Term survival stuff (where this thread should really go)

That strange guy down the block - who builds strange things - I'm what is known as a Home Shop Machinist - yeah, I have a manual machine shop (and some CNC) in the basement. Can I build a crossbow? Yeah - and have, when I was like 10.. Even designed my own release. Never made a high powered one from a leafspring, but no big deal. Can I build a boiler? Yeah, as long as I can get the raw materials (don't want to trust making tubing myself) - machine a steam engine? I've made small ones, - big ones are just bigger. Build a small to mid sized foundry? In theory, and I've helped, but not actually done it - neighbors would get a tad annoyed. Morse code? Yep Electronics? - well, if I can scrape 15-20 years of rust off my skills, I did it for a living. Iron and Steelworking? That's waht I did during college for my money. Butcher a steer/hog? I guess, as they are not that different than a deer. I'd figure it out. Medicine. Now that would be tough.

Want some tricky onces, that very few people know?

With NO other reference, make a truely flat surface - say, 2ft square, to within .0001"

Generate a reference thread - we all know "20 thread/inch" or whatever, on screws. MAKE a machine to do that - you need to make a threaded rod that is more accurate than the final product - do it - from scratch

Make a right angle

These are all skills that are needed to build a machine society from NOTHING, and all skills that I know folks who can do, starting with no more than your basic hand tools (knife, hammer, saw etc)

BTW learning HOW to do these things is actually fun, and so is actually DOING them