One day, a long time ago, I walked into a small machine shop after hours and asked the owner what the words "Screw Machine" in the business' name meant. There were lots of small shops around called "x Screw Machine Co." and I was curious.
He took me on a short tour of his shop and showed me what a screw machine was. Most of the equipment was 40+ years old and many of them still had the stencil, "Property of War Department."
In short, it's a bar-fed high-speed automatic lathe with all the automation done with cams and gears. I watched it make some kind of specialized spacer, each piece taking about 40 seconds, and as a software engineer familiar with CNC, I was amazed that this could be done so fast without a single computer in sight.
And yes, there are now CNC versions that can do even more mind-blowingly complex operations, but I'm still impressed by cams and gears.
Having recently bought a metal lathe, and in general having a long time fascination with manufacturing, broaching is freaking mind blowing. Imparting form into metal by extremely strategic shaving off thousandths until you get to what you're looking for. It just doesn't seem like it should actually work at all, yet we wouldn't have things like allen bolts, splines, or keyways without them. Turns out all you need is the right geometry and super hard metal to make it work.
Rotary broaches are just so cool in that getting the right angle and wobble to the tooling is incredibly important, but it just seems like magic when it actually does it's thing. https://youtu.be/GWyHJVOxKK4?t=444 It's only when I saw what it was doing with the rotary motion stabilized that it started to make sense. https://youtu.be/4-3gPWl6wfU?t=95
There’s loads of good books like Machinery Handbook but I generally just keep a set of Roebuck Zeus Tables in my pocket.
I love the Workshop Practice Series books including this one on the screw cutting lathe[1] is a good introduction.
As much as possible I stick to the ISO Metric Course thread standard. That’s the M sizes.
I can get away with just one set of change gears, taps and dies and one set of tapping drills.
E.g. to tap an M8 size hole you need a 6.8mm drill so that when you tap it it will have the correct size. You generally won’t find that a set of drills from a DIY store.
I avoid it but if I do need Metric Fine threads I can cut them on the lathe with the same set of change gears.
For other non-metric standards for a specific purpose I buy one off taps and drills but I really try to avoid this.
e.g. camera mounts are usually either Whitworth or UNC but you won't find these much else where.
Quality drills and taps really help with small lathes where motor power is limited. I’ve tried cheap stuff but it's somewhat frustrating when a cheap, no name tap breaks off in something you've spent days making. Now I just use Presto[2][3] for everything.
Ever since I've found my self looking a small metal parts and wondering if/how I would make them on my lathe.
i.e. what order of operations I would use, how would I hold the work, do I need extra milling operations?
I can happily waste an afternoon looking at a toilet roll holder working all that out.
A real puzzler for me was, how do you make hex sockets at home? Answer: The magic that is the rotary broach[1].
1. https://en.wikipedia.org/wiki/Broaching_(metalworking)#Rotar...