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Hi Vance & Frank,
I was quoting from "Stachiw" page 13 where as you
say Vance they had 4" thick cast acrylic sheets that they were
forming the segments from, but Stachiw then looked
to cast hemispheres to go thicker. I should have said the segmented
domes were superceded rather than abandoned.
Frank, maybe Greg could wade in on this if he's
about, he's the expert. However I'll explain my idea a bit more.
You start off with a 100mm thick acrylic slab &
cut a ring out that slightly exceeds the diameter & thickness of the
dome you require.
Then another slightly smaller in diameter &
stack it on top & repeat again till you have the desired dimentions of your
dome able to be carved out
of this crude block; then glue it together, anneal,
machine & polish to a perfect sphere and give it a a final
annealing.
This way would waste a lot of acrylic & a
better way would possibly be to form each ring in 2 segments thus utalizing the
acrylic block a lot
more efficiently. I'm picking that you could make a
dome with a 3000ft plus crush depth for under $2000- NZ & get it
manufactured locally.
The best price I've seen so far for a cast dome
would be $5000- plus to land in NZ.
One design criteria would be that you don't put any
of the joins at eye level.
Re the blowing of the domes; I could only find one
plastics manufacturer in NZ that would take on blowing a dome 25mm thick (have
now upped it to
35mm) & he's struggling. Apparently there's a
lot of pressure involved. When I cooked some acrylic I was surprized at how
rubbery & firm it was at
the forming temperatures.
Alan
----- Original Message -----
Sent: Tuesday, April 06, 2010 2:09
PM
Subject: Re: [PSUBS-MAILIST] alternative
dome forming
Alan,
I'm not sure I understand that last paragraph. Pentagonal arc segments of
4" thickness were formed into the original Johnson Sea-Link pressure hulls and
made many hundreds of dives. They were not abandoned, but rather superseded by
new techniques that allowed hulls to be fabricated without the visual
discontinuities.
Ultimately these hulls were made thicker, but only after many dives to
3000 feet did that requirement become evident. The hulls suffered some
unpredicted axial fractures around the penetration plates. There were no
catastrophic failures, aside from the nerves of a few people who had the sweet
ever-lovin' bejesus scared out of them at depth.
Vance
-----Original
Message----- From: Alan James <alanjames@xtra.co.nz> To:
personal_submersibles@psubs.org Sent: Mon, Apr 5, 2010 8:46 pm Subject:
[PSUBS-MAILIST] alternative dome forming
Following on from my previous post, where I
sugested it may be possible to form a dome
by stacking & glueing several rings of
acrylic on top of each other, I contacted my
acrylic fabricator & put the idea past him.
He thought that it was entirely feasable &
considered the joins invisible unless I was
looking straight through them. He said he had
100mm thick cast acrylic sheet wich would require
3 sections & 2 joins for the size I need.
He also had a large 2 axis router comming in June
& beleived the finishing of the surface
to be easy. His only apprehension being the
annealing time in his large oven; Once to set
the glue & anealed again after the machining.
The all up cost shouldn't increase much more with thickness,
just diameter.
Will add that early domes used to be fabricated
from pentagonal sections but were abandoned
because stock cast acrylic sheets that the
pentagons were formed from back then weren't thick
enough to attain the depths that Stachiw was wanting to acheive.
Alan
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