What would stop people from forming a coop and buying such machinery in common?
Nothing would sop people. They cannot do a thing to stop self made (and now printed) guns. People are arming themselves to the teeth. They see what's happening.
“What would stop people from forming a coop and buying such machinery in common?”
Why bother? You can buy a very nice bench top mill for a few hundred bucks! That’s really all the “machinery” you need.