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Mr. Fusion
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We have cage
November 29, 2002 — 17:08


Well,
I got the cage finally built.  Must say that it’s a nice milestone.  I
had ordered 12 great circles, laser cut, from
www.quickparts.com.  It’s like
casting a spell, or invoking daemons to carry out your will.  You upload a
CAD file of what you want, and about 5 days later it comes back to you in
manifest form.  On the right
is the finished product.


Laser
cutting does not stress the metal at all, so the pieces were beautifully flat. 
The process leaves a very nice finished edge, so they were wonderful to touch. 
I set about scratching the fold lines and then used some metal folding Vise
Grips and got some very adequate folds very easy.  For the previous cage
(the one from the great circles I had cut out by hand), I used two pieces of the
quarter inch stock held by some vice grips.  Heck of a lot easier, and far
more accurate.

Oh, and there was something really stupid I did to the hand cut cage pieces. 
Since they were covered with a lot of oil I decided to clean them.  I put
them in the dishwasher and forgot about them.  D’oh!  They came out
with a very fine layer of rust on them, and some rather large spots – guess I
should have used Jet Dry, eh?  Oh well.  I tried cleaning them with
Naval Jelly, but that was just sad.  Yes, Naval Jelly does get rust
off.  It took off the fine layer with little effort.  But the surface
was quite pitted, and I had to use steel wool to get some of the larger rust
spots off.  Obviously, you don’t stand a chance in hell in recovering from
rust damage.  Just don’t even think about getting things rusted in the
first place.  Geesh.  So that’s the reason for the laser cutting. 
Surface is perfect.  Cuts are precise.  No stress.  And most
importantly, no rust.


I
took the old cage and spot welded it together, so I could get a feel for what
was going to happen.  From my experience building these butterfly polyhedra
with paper card stock, I knew there was going to be some fun.  Because all
things have thickness, and because the bends in the great circle have a radius,
assembling the cage introduces stress into the system.

I filed down the tips on the spot welder tongs to make the weld long and
thin.  I think they worked out pretty well.  I was worried that the
weld size would be too large, but the whole system has a lot of slop in it. 
I worry too much.  So lucky thing I practiced first.  Little errors go
a long way.  By the time I got three of the circles welded, things were a
little egg shaped.  Lots of stress in the final product.  I set about
with some pliers and was able to work out a lot of it by adjusting edges. 
This was a practice cage, after all, but the result was acceptable if not a
little wavy.

The laser cut great circles had little stress going into the welding. 
Using the Vise Grips, I was able to fold the great circle very accurately with
very little added stress into metal.  I took my time setting up the welds
as well as careful about the order in which I did the welds – i.e. joining
opposite vertexes rather than progressing along a line.  When I had three
great circles welded (i.e. one half of the icosidodecahedron), the result had
much less distortion than the practice cage.  Some of this had to do with
the slightly thicker gauge of the steel in the laser cut circles – 22 gauge. 
Most of the reason is just the practice I had previously, and the more accurate
cutting inherent in CNC machining processes – duh.

However, let me warn you.  Some welds had a lot of set up time. 
Due to the thicker metal gauge, the whole structure wants to expel outward, like
a balloon.  This is actually a very cool property of any of these
structures and what makes geodesics so strong.  Each edge of the structure
is actually being expelled outward along radial lines.  Thus, the whole
structure is actually under a lot of tension, rather than compression. 
Very cool.

Anyways, by the time I was joining the last vertexes of the two hemispheres
of the cage, there was a lot of stress built into the system.  Since the
blasted thing is pretty much impossible to clamp and I was working alone, there
were a lot of entertaining positions I was in trying to force the metal
together.  My fingers are going to be numb for at least a day.  But
the result was worth it.

I also broke down and ordered Neodymium Iron Boron (NdFeB) magnets for the
cage.  I found the perfect size, 0.107" x 0.107" x 2".  NdFeB has a
residual field strength of 1.23 Tesla, but the downside is the lower temperature
stability – 80 degrees centigrade.  But I figure if the system is operating
as designed, there shouldn’t be much grid heating to speak of.  I’ll have
to calculate it out exactly, but it looks like the cage should be at least 80%
transparent with these size magnets.  If everything works, then none of the
plasma should be anywhere near the grid, confined down in the center of the
system.  I am worried about the poissors, though.  If I understand the
dynamics of the system, they should be directed straight down the cusps, right
into the grid – after all, the point cusps form very nice magnetic quadrupoles. 
This could heat things up.  Then again, the poissors might form on the face
centers.  It’ll be interesting to find out.


So
here is what the new cage looks like. 
I modeled it in AutoCAD as a single part, rather than going the route of using a
sheet metal model.  I did manage to figure out the sheet metal features of
Inventor.  However, I couldn’t figure out the constraints to get the cage
built.  Odd.

Anyways, I’m working with Huntington on the chamber.  Hopefully that
will go reasonably well, although they got the angles wrong initially.  I
saw what their drawings looked like and sent them back my drawings in similar
layouts.  Fingers crossed.

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