Thank you, Larry. Since the other replies have been long winded, and yours the most enthusiastic, I though I'd take the time to acknowledge it here.
The reason for the pairs is, of course, in the abscence of computers, CAD files, CAD systems, NC machines, and probably even writing paper, self-reproduction of a shop full of machine tools is possible only when there is two of everything. And your post indicates you understand this.
For those that do not, in the absence of a drawing or schedule, one might be busy repairing things all day, but if more operators became available, it would probably be most effective to work directly from each machine tool as if it were broken.
That is, disassemble, extract one part, reassemble to be sure that nothing was lost, then focus just on the one part, with calipers if need be, and duplicate it.
A whole machine shop might have a lathe, mill, surface grinder, and cylindrical grinder, all powered by some BIG theoretical power supply that I am not including in my quantum mechanical scribblings. Assume unlimited power, limited resources, limited time, and limited labor, but a steadily growing demand as things wear out and new Mars colonists arrive.
So you pull the spindle out of the mill, put everything else back on it, hang an Out of Order sign on it, UNPLUG IT AND LOCK IT OUT, and measure the spindle with calipers. You pull some stock, or, having unlimited power, melt some rock, refine some metal, pour a slug, and rough it out on the lathe, file or finish turn, and head for the cylindrical grinder.
You grind it close, maybe using a long-since-worn out micrometer with no graduations as a direct mesurement device, and you get it close enough. Then you notice a problem.
It's of hardened steel, now, since you hardened it in between the lathe and the cylindrical grinder. But you forgot to slot it! It's going to spin if you put if back in the mill, so you put a narrow wheel on the grinder, a wheel that happens to be made from Martian rock held together with synthesized hydrocarbons, and, grossly overloading the tool, you grind a suitable slot, and you are ready to disassemble.
Now, if you were an experienced machinst, and I am not saying I am, you'd have thought BEFORE roughing the slug, finishing it, and hardening it, that you could have slotted it efficiently on the mill.
But wait a second, the mill is OOO. So you have to have a second mill.
Now one of the first finite algorithms we have to apply to self-reproduction is sort of a combination of old wisdom and something extremely specific. We "do the hard part first", which is the old wisdom, and specifically, we take the slug, center drill it, and then, on centers, on the mill, machine the slot first, before even turning it.
Why? Because, by the old wisdom, if there's something that needs doing in parts, and you do the hard part as soon as possible, not necessarily first, you've got a little working room and you can avoid mistakes like hardening the spindle, then grinding the slot.
Now this is easy to incorporate into a computer program, but to a person, it is wisdom that cannot be taught unless they learn all about finite math and the critical path method. (Gantt charts. Hate 'em) But an appropriate, sufficiently deep theoretical understanding is the equivalent of wisdom, and usually costs as much in tuition and labor as you'd earn learning it. And this theoretical understanding can be taught without knowing anything about finite problems or applications. Just a few simple things to learn, and you're ready to fly to Mars, and if one of your fellow colonists has to take over when you drop that spindle on your foot, no problem. Everyone is multi-trained without seventeen lifetimes of apprenticeships.
How's your foot?
Remember this is human-facilitated on-demand reproduction, so there's no grey goo. You're probably the only person on this thread so far that gets that bit.
On Mars we can synthesize the carbon and rubber from atmospheric CO2 and water, draw up a couple thousand yards of wire, and make a tire in an appropriate time. It all changes when you have access to energy, and that means nuclear.
Yours,
Doug G