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The short answer is. whatever hammer works. I think I'd probably
reach for a reasonably sharpish cross-peen. Say 1/8" radius on the
If you're trying to make half-round wire, bashing it out against a
mandrel is just about the last way I'd think of to do it. (just
about guaranteed to ding up your mandrel, for starters.)
A better answer is to make half-round (D section) wire *first* then
make a ring out of it.
Couple of ways to do that, depending on what gear you've got.
If you've got one of the big Durstons, you've probably got the studs
for outboard rollers. Yes, half-round rolls are expensive, but so is
time. Just get the rolls, if you can.
Next step is a round wire drawplate. Take a bit of wire a bit larger
than the thickness of the 'D' you want to end up with, and fold it
in half. Solder the joined end together, for about an inch. (splay
it out into a 'Y' to prevent it soldering beyond the point you
want.) Forge/file a taper onto the soldered bit, so you can get it
through your drawplate. Draw it down a couple of times, until it
becomes the "D" section you want. The trick to it is that you stick
a bench knife between the wires on the backside of the drawplate.
Set the back edge of the knife against the back of the drawplate,
and set it so that the blade is either vertical or horizontal. The
reason for the blade is to keep the wires from twisting around each
other as they go through the drawplate. (you'll end up with a
barberpole without the knife.) You will have to do some test runs in
copper to see what size starting wire you need in order to end up
with the size you want.
I've never heard of running wire between the flat rolls and a sheet
of copper, but there's no conceptual reason why it couldn't work.
I'd use pretty thick copper. The point of the copper is to cushion
the bottom ("D") part of the wire, so it needs to be at least as
thick as the 'D' section. You'll probably get a sort of squashed "D"
or fat-sided oval, rather than a true half-round profile.