This is a dumb question, but can someone help me? I want to
fabricate a hollow 3D donut shape.
This is certainly not a dumb question. It's an interesting problem,
but you don't quite give us enough for me to suggest
which of the following solutions might be more appropriate.
First, the tubing solution might work, but bending tubing is an art
in itself. You are somewhat limited by the diameter of the donut in
relation to the tubing diameter and metal wall thickness. For
example, if the donut were to be 2 inches in diameter, the tubing 1/4
inch, and the wall thickness around 20 gauge, you could probably do
it with a coil type tube bender that slips over the tubing. But you'd
be near the limit of feasibility for this approach. You'd bend it in
a slight spiral, going past a complete circle, then cut both where
they overlap and re-align the ends and solder (leaving a pin hole
somewhere for expanding gases to escape. As the donut required
becomes smaller, the diameter tighter, the wall thickness greater,
you reach a point where it will take too much force for the tube
support of the bending coil to keep it from collapsing, or it gets
simply to hard to bend it.
If the donut were, say, 2 inches in diameter, the cross section of
it an inch, and the wall thickness 20-22 gauge, your best approach
would be to carve half in the donut in wax and use that to make
devcon dies and resort to a hydraulic press. With the press, you
wouldn't actually need Devcon dies, you could probably use dies sawn
out of plywood and faced with masonite (see masonite die forming
below). The dies would be a thick plate, faced with masonite, with a
circle cut out. This would be glued down to a piece of plywood, and a
plywood "plug" the diamter of the donut's hole, also masonite faced,
would be centered in the larger hole and glued down. A soft durometer
rubber pad would push it down into this die. Much thicker metal and
this wouldn't work well. But this could form two halves, which could
then be soldered together, again, the pin hole for expanding gases.
If the donut were much smaller, say, half an inch, with a cross
section of 1/4 inch, the simplest approach would be to carve a wax
donut, allowing a little extra thickness at the "equator" and saw it
in half, then hollow it out, cast the halves and solder together.
Now, the hard way(s)... If you've got good hammer technique, you
could form two halves in pitch by chasing and repousse, and better
yet (a way I've used to make these sort of things) is a technique
called "masonite die forming". There isn't room here to go into that
except to say it uses the aforementioned plywood and masonite die
system, but the metal is clamped down to the die and free form
hammered down into the cavity. Another variation would be to use
anticlastic raising techniques, again, it takes some metalsmithing
skills, but it can be done (yep, done it myself).
If the donut were really large, say, as big as a Jell-O mold, you
would probably be best off sub-contracting it to someone who did
metal spinning, but I doubt that's what you're after.
So, whereas all these approaches are a lot of fun, really, what you
can resort to will depend on your skill level and your equipment. So
finally, can you tell us more about the dimensions of this proposed
donut? Diameter of the donut, diameter of the cross section,
thickness of the metal wall, etc? And what metal are you using,
silver, gold, brass? Give me that and I'll point you in
the right direction. That said, it wouldn't surprise me if there is
someone else here who can cut to the chase better than I can.
David L. Huffman