A Spindle Nut is a nut typically used in conjunction with a threaded rod to make a carriage move. Many RepRap designs use rods and nuts with fastening threads, like M6 or M8 for their low price. If you want to move more than just the Z axis up and down once per print, trapezoidal threads like Tr8x1.5 or Tr10x3 are much better, though. These trapezoidal nuts usually require some rework to be usable, so let's see what we can do with them.
The classics are nuts made from brass, they excellently pair with rods of steel, if some lubrication is available. There are also nuts made from plastics, which usually run dry just fine. Surprise, surprise, good plastics nuts are known to be more expensive than their brass counterparts.
Brass nuts are often much longer than we need them. They're made for heavy duty applications, after all, but 3D printers typically operate with rarther low forces. As such, using a nut of half length is entirely sufficient. Another reason to cut a nut into two pieces is to use these two pieces to assemble a play-free movement block, like Traumflug's Movement Block.
First step is to mark the center of the nut. It's also a good idea to make a cut with a handsaw. A depth of about 1 mm is sufficient, it's just a marking.
We could saw this nut by hand all the way through, of course. But we're lazy, so let's use a table drill, one of the most essential devices in a RepRappers' workshop.
Only very large drill chucks can hold a 22 mm diameter part, so let's use a trick: mount a short piece of trapezoidal rod into the chuck, then screw on our nut. The nut won't loosen, as the cutting force will try to tighten it further.
Now we have a setup almost like a turning machine and can cut the nut with a handsaw without too much effort. Brass can be cutted at pretty high speeds. When cutting these 22 mm diameter nuts, I ran the drill at 1700 RPM. For plastics, you'd use much lower speeds, perhaps 100 RPM, else the plastic could melt in the cut.
In any case, be careful and expect your handsaw to bind and jump away any time. Put almost no pressure on the handsaw, their teeth will do their work at low pressure even better. Also, move the handsaw slowly back an forth to avoid clogging or overheating the blade in one place. Other than that, just have some patience. It'll take a few minutes. As long as you see chips falling, the cut advances.
To start the cut, hold the blade where you see the mark rotating. Hold it to the right side, else you unscrew the nut (and use your saw the wrong way). If you hit the marking cut precisely, your blade will wobble a bit, that's why we did this marking cut. The wobbling will confirm the blade's correct position and also go away as soon as the cut advances a bit.
Just before you're through, the handsaw will start to wobble again, this time a lot. Try to continue holding it steadily, at low pressure. A few seconds later, the lower part will just fall down.
For bonus points it's a good idea to de-burr the cut with a simple knife. Your matching rod should slide through snugly, just as easy as before the cut.