Probably easier to just scavenge stuff, if you're fortunate enough to survive the initial fallout/disruption from say a nuclear war there's going to be a lot of stuff and not so many people.
David Gingery's book "Build your own metalworking shop from scrap" is a very fun read though
There will be lots of stuff, but what are the odds that exactly the thing you need will be available and in working condition? If you can make something entirely from scratch, you can make any subset of it from scratch. More generally, developing the means to do something yourself gives you a much deeper understanding than simply utilizing someone else's solution.
David Gingery's book "Build your own metalworking shop from scrap" is a very fun read though