[~misfit~[_3_]]

Somewhere on teh intarwebs Greg Wormald wrote:
I don't know anything about doped cloth surrounds on speakers, but I
had a friend who built large model airplanes and in the '60s they
doped their own cloth for them.

Maybe try a model airplane shop for materials and shaping advice.

Thanks Greg, I also used to build model airplanes (with my older brother, in
the late 60s / early 70s), mainly large gliders that were covered in doped
cloth. They flew for ages! We lost more than one - even using a fuse to burn
through a rubber band so that it released the 'flaps' and theoretically made
it stall and descend.

That was a different process - with the 'planes the light cloth was fitted
over the wings as tightly and neatly as possible and glued in place. The
'dope' was then applied and it tightened and stiffened the cloth making a
very light but rigid covering for the wings that also pulled all of the
(balsa) wood framing tightly together strengthening the whole assembly.

That's the first thing that I thought of myself when I first heard the term
'doped cloth' applied to speaker surrounds. However the desired result with
speakers is different to the result for 'planes. For the speakers the 'dope'
is more like a rubber in a sovent that (as far as I can tell), as the
solvent evaporates it leaves the rubber compounds behind which then hold the
cloth on shape, 'build it up' a bit and also make it air-tight - essentially
making a light rubber surround using the cloth as a scaffold.

I have successfully repaired rips in this type of surround using diluted
'contact adhesive' and fabric patches. However the shape was already there
for me, I merely patched it. The bit I'm struggling with mentally is how to
get the cloth to hold it's shape while the first application (and later
applications) of 'dope' dries - without sticking to whatever I use as a
former. I figure that I need some type of releasing agent - but then that
means the solvent won't be able to evaporate from one side of the new
surround....

I guess I'm going to have to make it in stages, one side at a time -
alternately. Googling and searching forums a few months back I found where a
guy had done this using silicone as the 'dope' (as I mentioned earlier). He
used a large lathe to make a wooden mould of the 'dipped' side of the
surround, sanded it back and coated it with a release agent. He then used it
to cast a plaster mould (which he also coated with release agent) so he had
both a positive and negative former.

However he then put silicone / cloth / silicone on the one and bolted the
two together until it cured. With a rubber dope I won't be able to do that,
I'll need to paint layers of it on/in to the cloth and leave it open to the
air to let it dry - and I'm struggling over how to hold the cloth in shape
while I do that. I guess I'll just have to 'suck it and see' - maybe just
make the wooden part of the mould, coat it with release agent, paint it with
diluted rubber cement and stick the cloth to it. Then paint the other side
and hope that when I peel it off (if it even works) that it retains it's
shape.

So now, other than getting the use of a lathe, I just need to work out what
cloth to use, what rubberised solution and solvent and what 'release agent'.
This is the info I hoped that someone might know already and be able to help
me with. I'd rather not have to make a whole bunch of 'failures' first - I
have limited mobility. I'm guessing that 50 years ago doped cloth would have
been the best way to make prototype drivers surrounds. These days all of
that sort of thing is done inside a CPU then ordered from China. :-/

You know, it the drivers didn't have inverted surrounds that are impossible
to get to I would try to use the one that isn't broken up as a mould.
However the outer side of the curve that I'd need to use is under the lip of
the cast alloy basket - inaccessable. I don't think using the inside of the
curved surround would work as the result would have a smaller curve... But
I've started thinking....... What can I coat old 'rubber' with that will act
as a release agent for 'rubber cement' and won't destroy the cone if it gets
on it?

Thanks again.
--
/Shaun.

"Humans will have advanced a long, long, way when religious belief has a
cozy little classification in the DSM."
David Melville (in r.a.s.f1)
[Sent from my OrbitalT ocular implant interface.]