To cut metal, the most basic tools are the drill press and band saw. The use of a drill is obvious, but a band saw can easily be used to nibble at edges, and cut pretty much anything you can fit on it. Some might prefer a jig saw, but 9mm/.45ACP. Doesn’t make too much difference. I’ve been told you can finish an AR lower on a drill press, you don’t need a mill, but it’s harder and longer. There are designs out there for a lower that is made of plate metal, an ideal project for a press and saw.
Level one would also include a welder. Welding is stronger and easier than bolts, and looks a bit nicer on top. Gas will let you do some cutting as well, without buying two machines, but has some disadvantages, like being slower. It’s very versatile however, and the bead and puddle management you learn will also be useful with other welding techniques. I’ve been told by some people stick and gas welding are going the way of the dodo, but every welding truck out here has an oxy-acetylene rig as well as the electric welder, and most of those are capable of stick welding. Welding deserves it’s own post, probably by a welder(any volunteers?), but you need to be able to cut, drill, and assemble metal with reasonable accuracy, for which you don’t need machine tools for on a basic level.
Almost all of the stuff we do on CNC machines these days were done on manual machines in the past. Time was the limiting factor, and some stuff was done differently until CNC machines made it worth the time and effort. Manual machines are also still very useful for modifications and repairs—by the time a CNC operator has a part set up, tools set up, and program written, a lot of operations can be set up, done, and torn down on a manual. Even in the age of Star Trek replicators, there will likely be a Bridgeport around the corner from the warp core for those times when just tapping a new hole will be enough.
A variety of companies make small bench top mills and lathes, some of them can actually be useful. For machining, the two big points are vibration and flexing—an accurate tool won’t do either. This generally means mass, and lots of it. Even small machines will have thick, heavy parts, and “bench top” machines can weigh hundreds of pounds. Most of the small cheap machines are made in China, so quality can vary. The manual Sherline machines can be a good buy for starters, but for most work, will be a bit small and slow. Sherline is also one of the companies blurring the line, since their machines can be had in CNC versions. It’d take a while to make an AR lower on it, but most of it could be done.
AR lowers are a good example of how not to design something for making at home—the magazine well has nice sharp inside corners that you can’t just mill out with the same tool as the rest. Big shops use a process called broaching to finish out the well. This requires a specialized machine and special tooling that you’re not going to have at home, or at most hackerspaces. It can be worked around, but will be a pain.
Most of the smaller machines in the bench top size can be modified to be CNC, but quality varies. Larger manual machines can be modified too, but aren’t nearly as good as real CNC designs. Part of this is machine geometry—hand cranks are a different beast than motors, and are placed differently. The other part is the “motion control” that actually moves things. The smaller and the cheaper machines use stepper motors, which are limited in torque, and aren’t controlled the same way.
I’ll let someone else explain the difference:
We can get to Tormach and similar machines at this point, which offer actual CNC control, though I’ve heard complaints the stepper drives are rather slow, and servos are expensive. You can get servo machines in the price range however, but I have had little luck finding many with a whole lot of features. If you look carefully, you can find good deals on higher end machines, but keep in mind you still have to ship it, and since most of the machines are well above five thousand pounds, they’re generally out of reach of 5K forklifts and LTL shipping, which means flat beds and more or less hiring a whole truck, which is rather pricy.
The next level is to buy a real CNC machine, like a Haas or Mazak. While a Tormach with bells and whistles can be had for $12-25 grand, real machines start well used in the upper part of that range, and go up, bigger or fancier machines can be a million a piece new. If you can afford it, it would be the king of the hackerspace, but are really only needed when you’re running the machine 16+ hours a day in production, and making heavy cuts in hard alloys.