Or a more general question, how do they make wire? Guitar strings seem more difficult, as there must be exact uniformity of thickness throughout.
Is there a mold involved? Any stretching? Any rolling? Are they cut from sheet metal?
Wire is formed by a prcocess called drawing. Rods are pulled (or sometimes pushed) through progressively smaller holes until the desired diameter is reached. Depending on the material of the wire, the process may be done either cold or hot. Cold drawing is used for particularly ductile metals like copper and gold, while harder metals, like steel, are normally drawn hot.
Wire manufacturing.
http://www.branfordwire.com/methods.htm
http://www.smorgonsteel.com.au/wireproducts/about/index.cfm?ObjectID=4
I love the Internet. 
Wire is made by drawing, which includes pulling on a heated bar and stretching it out to smaller diameters. Pulling the wire through a series of dies, each slightly smaller than the previous one, adds a great deal of fine control to the process. The wire is then tempered and annealed to the desired hardness as it is wound onto a spool. For guitar strings, some are then wrapped in another wire, either round, or flat and in some specific applications other materials are wrapped with the wire as well.
After finishing, the wire will be cut to lengths, and one end affixed to a small metal doughnut, to facilitate connection to the bridge of the guitar. Among the critical factors, such as diameter, are also temper, surface finish, and uniformity of tensile strength. All and all, it is a pretty well practiced technology, having been refined from the days when gut, or linen were the only available materials.
Xerox machines, on the other hand require very highly polished platinum wire, of a very high tensile strength and hardness. Expensive stuff, and man does the company hate having it lifted by technicians to show off to their friends.
Tris
“In my opinion, there’s nothing in this world, Beats a '52 Vincent, and a red headed girl.” ~ Richard Thompson ~
Thanks fo rthe info, everyone. Makes sense.
I was mostly talking about plain strings, but I guess wound strings would use the same process, x 2.
I’d also note that the wire thickness doesn’t have to be all that precise, any manufacturing variations between a set of strings will be compensated for by the tuning pegs. A slightly thicker string will only require a slightly higher tension to produce the same frequency.
Depends on the variations. Strings need to bend, and bend a known amount from known pressure. If I am used to using a certain gauge for my A string, and I know that force x produces that sweet sweet bend from A to B, and then a string saying it’s that same gauge is not, my next bend will sound wrong because of the thickness.
Different thicknesses also resonate differently. Mark Knopfler’s tone is partially due to using very thick strings, Pete Townshend used to a lot also. So it depends how big those variations are, as differences in thicknesses produce different sounds.
I understand the basic process. There’s a die with a hole through the middle. It’s big on one side and small on the other. The thick wire (or rod) goes in the big side, and comes out the small side.
How do they start the process? How would one go about pushing a 3/16" diameter hunk of steel through a 5/32" opening?
Is the die in parts, so that they can clamp it around the big wire and have something to start pulling on, or do they have to taper the end of the big wire to get it through the opening? Or is there a third solution I didn’t think of?
I don’t know anything about this process, but my guess would be that the material isn’t compressed or stretched but rather removed. Take a stick of butter and push it through a straw, you don’t have to get all of the stick in the tiny straw to get butter-wire. Just a WAG, could be completely wrong though.
K-Y. Lots of K-Y. 
Most likely it’s a tapered hole. Something like 10 feet long and it slowly narrows by a thousandth of an inch every 6 inches (or something like that) The whole narrowing tube is heated to make the wire more pliable. That’s my best guess anyway.
Precisely. And any variation within the length of the string can drastically and audibly alter the behaviour.
I toured the place that does the hot rolling for the titanium rod and wire that my company machines.
Picture a chunk of titanium, called a billet about six feet long and a foot in diameter. It looked like a chunk of old telephone pole to me. It’s put in an oven until it’s red hot. Picture a table that’s a couple of hundred feet long built out of rollers. The rollers are motorized. Across the middle is the gang of dies. Each one is a different shape, square, round, oval, triangular etc. The dies are split in half and operate by hydraulics.
The hot bar is propelled along the rollers and goes into the first die that clamps down over it as it slides through. This slightly squishes it and makes it longer. Parallel to the table 90° to the dies is a flapper looking thing that guids the bar as it goes back and forth. After each pass the rollers are reversed and the bar goes through a different die, reverse, different die, reverse different die.
It’s very fast and you see this six foot piece stretched out until it’s several hundred feet long about half inch in diameter and looks like a glowing hot stick with the consistancy of licorice.
By now it’s still red hot but too cool to work with. It goes along the rollers to an oven again. On the other end it’s pulled through a fixed die to the required size. After the die it’s wound onto a giant spool. Around 3000 feet long on average.
Off to the wire room. Looks like a bunch of mutant spiders run amok. Big wheels maybe a little smaller than a bicycle rim are in spindles a few feet apart chris-crossing the whole room. The wire wraps around the weels and through tension is slowly stretched out a little at a time as it goes from wheel to wheel to wheel.
Cheers.
Not necessarily so. A tuning peg can adjust the pitch of an open string, but once you lay a finger on a fret, an uneven string will give you all sorts of intonation problems.
And I imagine an uneven string would not ring very true, but that’s just my guess.
Another consideration is that with some guitar styles, you want the maximum tension possible without breaking the string. So you have to be able to get very precisely gauged strings. (These styles include Dobro, steel guitar, and some old-time country blues. In addition to giving a brighter sound, higher string tension means that the bar doesn’t bend the string as much, resulting in a ‘crisper’ response and less variation in pitch).
Inconsistent string diameter will indeed cause intonation problems. In fact, the consensus seems to be that nobody has managed to manufacture an unwound string over .020 gauge or so that plays in tune over the length of the string. Of course, you can always use a heavier-gauge wound string, but some steel players prefer the cleaner sound of an unwound string. (I have no knowledge of the details of the fabrication process – I’m speaking here solely as a guitar player).