If you take a round wire, and twist it, you are distorting the
metal, but the overall shape stays the same, since although the
round wire is being twisted, there is nothing in the shape to
show that fact, so it still looks round, for the most part.
The twisting straightens it because you are also putting a bit
of tension on the wire pulling it straight as you twist. What
happens is this:
When you bend or distort metal, up to a certain point, it is
elastic and springs back when you release the force. But once
you exceed the elastic limit of the metal, it permanently “flows”
into it’s new shape. (Actually, atomic planes in the crystal
structure are slipping past each other, somewhat like taking a
deck of cards and pushing the top of the deck sideways, so the
side of the deck, formed of all the card edges, is now angled) ,
distorting the shape of crystal grains) that final shape it
takes on is the result of ALL the forces acting on the metal, not
just the strongest ones. So the twisting provides enough force
to overcome the elastic limit of the metal, and it takes a
twist. But since at the same time, there is tension on the
metal, it also becomes straight, even though that tension would
not have been enough to overcome the metals elastic limit by
itself, and there is not enough tension on it to cause much
stretching…
Obviously, the twisting method only works well with round wire,
unless what you want is twisted square (or other shape) wire
that’s nice and straight…
You can also do as I, and several others advised, instead of
twisting, just pull harder, until the metal’s elastic limit is
exceeded. When that happens, you’ll feel the metal “give”, and
it stretches.
Both methods work, but there are practical differences that may
favor one method over another.
Drawn round wire, at least from most drawplates, still has some
marks on it from the plate. Usually these are not obvious.
Twisting makes them more obvious. However, twisting does not thin
the wire at all. In fact, it can even sometimes very slightly
thicken and shorten it in a few metals, though usually the
surface roughness created negates the practical useability of
that minor effect. A straight pull is a bit more effective at
getting truly arrow-straight wire, and can work for any shape
wire. But for heavier guages of wire, especially in harder
metals, unless you’ve got a draw bench, it may not be possible
to pull hard enough by hand. Twisting generally is still
possible with these heavy wires and hard metals.
One other method has also been mentioned, simply pulling through
a drawplate. If the wire starts out annealed, and is pulled
straight, it MAY come out straight, since the plate, in thinning
the wire, is overcoming the elastic limit of the metal, and the
drawing tension is pulling the wire straight too. But this
depends very much upon the drawplate. The hole must be
absolutely uniform, and the hole must be completely concentric
where the wire leaves the plate, and the draw must be absolutely
perpendicular to the hole in the plate, or the drawing operation,
in work hardening the metal, will build up unevel stresses that
will cause the drawn wire to curl again when tension is
released. Few plates, other than drilled diamond dies, are quite
accurate enough to do this reliably time after time, and even
then, it’s not quite a sure thing. But the degree of curl left
after a first draw from an annealed state, is usally pretty
minor, and is often quite sufficient in practice.
Also… isn’t a wire that breaks while being turned
by a drill a fairly dangerous loose cannon? Clearly I’m
missing something; pls. illuminate! thanks Ryr
Well, if it’s a heavy wire, and you’re straightening with a
straight pull, and it breaks, the “whiplash” effect can turn the
straight wire into a tangle of crumpled wire around your ankles
as you fly across your shop… THAT’S the loose cannon… If
you’re twisting a wire, and the wire breaks, it doesn’t
immediately fly around. It just flops down and spins on it’s
axis. Also often gets a bit bent up, but it’s not exactly a
major disaster most of the time. Besides, you run the drill at
the lowest speed you can, in short bursts if needed, to keep it
under control. It’s not like you crank it up to several thousand
rpm and lock it on till it breaks… That might be dangerous,
though I’d imagine the wire would just break away at the drill
chuck with such abuse.
Hope this helps.
Peter Rowe
