For as long as I've been playing with electronics, there's been bundles of wires held together by twisting the individual strands together. It's so ubiquitous that I had never given it thought. It seems to be perfectly obvious how they are made: lay wires out alongside each other, hold the ends, twist, done. Right?

Today I learned: yes... mostly.

Certainly the simple straightforward way is sufficient for my daily life, because I only ever need short segments to be twisted. Household electric projects using twist nuts only deal with 1-2cm worth of wire. Hobbyist projects can also get away with this kind of thing - sometimes assisted by a cordless screwdriver/drill - because we rarely need more than a few meters of wire. There are pliers designed for twisting, but again they are only for a meter or less of wire.

When the wires are twisted simply, the individual strands also receive a rotational torque tension. Each strand will want to relieve this tension by un-twisting the bundle. For short runs, this tension can be mostly ignored. It is also less of a factor when the individual strands are relatively rigid: they'll want to hold their shape more than they want to untwist. (Even more if the strands are hammered together.) But for longer twists of flexible cable, it's easy to see the wire bundle trying to untwist itself.

When the twisted wire bundle needs to be longer - much much longer - this tension will become unacceptable. So wire twisting machines that make long runs of cables (hundreds of feet or longer) have added complexity. For example, the twisted pairs in our CAT-5 networking cable. As the wires are getting twisted together, the individual strands also must rotate with the twisting motion to relieve the tension before being merged into the twisted bundle.

A simpler way to approximate this is to let the individual strands move freely while twisting. The built-up tension at the point of twist will be relieved in the form of the individual strand rotating about. This can seen in video of some wire-twisting machines. (Pay attention to the individual strands rotating in the feed tube.)

A twisted wire bundle built using this technique is less likely to fight to untwist itself. In this picture, I hand twisted about 5cm of wire while letting the individual strands rotate to relieve the torque tension. I then held both ends and twisted another 5cm in the naive method. As soon as I released the stranded end, the second half of the bundle untwisted itself. The first half stayed twisted.

Wire twist test

UPDATE: I didn't have any luck finding YouTube videos illustrating the twisting that needs to be done for the wiring bundle to stay together. At least, not machines that twist wire. I found one that twists yarn, but illustrates a similar principle.