My main knowledge of , er, building nukes (hello GCHQ) comes from the widely accepted to be well researched book by Tom Clancy, The Sum of All Fears. Apparently it is a very good account of how to make a nuke, but with some very crucial details missed out of changed. And, yeah, very fair enough.
What I got from reading that book only once was that, even for a nuke bomb made from the remnants of an old nuke missile ("hacked, if you will") was that a fair amount of very, very high precision machining was necessary. In the plot, it was a massive big deal to get the right machine tools (Swiss, IIRC), and that alone was a massive task. Not just getting the tools, but getting one person with the skills necessary to do that machining.
Now, could well be the case the the precision needed to make a nuke is much higher than the machines mentioned in the article, don't know. Also could be that in that book plot, they needed very high precision to maximise the yield from old material, and of course perhaps a relatively small amount of material. So, its quite possible my point is redundant.
Another thought. Might be that the machine tools in the book were manual, not CNC. Might be that you needed high precision tools and a highly skilled machinist. So two things to required to make the bomb. Might be that now you dont need the skills because high spec CNC enabled less skilled people to do the work. So, that might be even more reason to keep tabs on the machines.
And thats all I know(, or think I know). Im merely suggesting that this might be a good reason to bolt down high precision CNC machines tools.
The detonation apparatus for a fission bomb is trivial. Students can build them, and they have. Someone even brought one onto the floor of Congress to prove a point in a committee hearing on nuclear proliferation. Without the uranium, of course.
Achieving fusion is tricky and requires extremely precise machining (you basically have to build a perfect sphere). The detonation condition for fission is simply a critical mass of U-235 sitting in one place. Typical design is two piles of uranium in a container, separated by a lead wall. TNT blows the wall and drives the piles into each other; nature does the rest.
Fortunately, enriching uranium is a massive (expensive, time-consuming, and not particularly subtle) operation. Really only within the reach of states. But if you got your hands on weapons-grade uranium, the bomb part wouldn't be hard at all.
"Typical design is two piles of uranium in a container, separated by a lead wall."
Even the simplest nuclear weapons design is slightly more involved than that - the gun-based design used in the "Little Boy" bomb that destroyed Hiroshima used an actual section of gun barrel to fire one chunk of uranium at the other.
More modern designs of fission bombs, either as stand-alone weapons or as primaries for H-bombs, are actually pretty complex - balancing the need to be efficient (plutonium and U235 are rather expensive), light, robust and safe is a non-trivial problem.
The W88 warhead with its prolate (egg-shaped) primary is the most advanced nuclear weapon that has had design details leaked to the public:
In the case of the Sum of All Fears the terrorists were trying to build a strategic level bomb (i.e. hundreds of kt) using a fusion boosted fission design to try and start a global war - so they were actually trying to build something relatively advanced, not just a basic bomb.
Edit: Another real problem with working with plutonium is that it is fairly nasty stuff to deal with - I can recommend the book "Making a Real Killing: Rocky Flats and the Nuclear West" to get some idea of the horrors of working with plutonium on an industrial scale:
True, but the prototypical modern terrorist doesn't need robust or safe, nor does he need to get the device airborne. He just needs a detonation, which (once you have the uranium) doesn't require sophisticated machining.
Actually, my understanding is that the tolerances, at least for implosion-type weapons, are extreme and required scientific advances to achieve during the Manhattan Project, not to mention the fact that Uranium and Plutonium are much, much denser than any material that would be encountered at a typical shop.
Ah yes, that was another point in the book. Its took them ages to very slowly and precisely machine the material to a very high tolerance. IIRC, the tools didn't last long at all.
But I suspect that the machining to make something that goes boom is probably well within the means of a traditional machine shop.