Re: explosive hydroforming for toroids and spheres
Sent: Tuesday, December 02, 1997 10:39 PM
To: Tesla List
Subject: Re: explosive hydroforming for toroids and spheres
At 12:03 AM 12/3/97 -0600, you wrote:
>From: Jim Lux[SMTP:jimlux-at-earthlink-dot-net]
>Sent: Tuesday, December 02, 1997 10:50 AM
>To: Tesla List
>Subject: Re: explosive hydroforming for toroids and spheres
>> Explosive forming is much easier than metal spinning. Dig a dish shaped
>> hole in the ground. Lay a sheet of metal over the hole. Lay a 1/4"
>> layer of plastic explosives over the sheet of metal. Place the detonator
>> the exact center of the explosives. The shock wave travels out from the
>> center to the edges. The metal is blasted into the hole and it will take
>> the shape of the hole. It really works. The metal thickness has to be
>> calculated so not to blast a hole in it.
>Of course, digging the hole is the easy part. Actually, this sort of work
>is usually done under water. The water helps transmit the shock wave and
>also reduces the air blast. The air blast is what annoys the neighbors,
>since you don't need much explosive for this.
>The tricky part is getting the explosives (legally) and determining the
>amount to use. I have done some preliminary research in this area with
>respect to the Los Angeles region and have discovered that there is truly
>stunning amount of regulation. You need a permit from the ATF to purchase
>and use the explosives. To get that, you need either a pyrotechnician
>license or a blaster's license from the state of California. A suitable
>pyro card will take 4 years licensed experience, and wouldn't even be
>technically legal for this use. A blaster's license takes 3 years
>experience, and there isn't a box to check for "manufacturing" or "R&D" on
>the application form: you get to choose mining, excavation, or seismic
>work. Finally, the local fire department will look somewhat askance at your
>activities, not to mention the problems with storage, since most explosives
>dealers don't want to work in small quantities (i.e. < 50 pounds).
>Now, if you live in Nevada, or a similarly enlightened state, go to it. The
>regulations are much looser there.
>Actually, a few phone calls to the local explosives dealers found me a few
>firms that do this sort of work, have the water tanks, can do the
>calculations, etc. There is a substantial NRE cost, but the per shot cost
>is very low. Explosives are quite cheap, a buck a pound in small
>quantities, and, for this application, a pound is a lot. It is cheaper to
>do spinnings for very small quantities and simple axially symmetric shapes.
>Explosive hydroforming is more suited to complex shapes with lots of
>detail, rather than big spheres and toroids. Imagine stamping out a
>refrigerator door (the inside, with all the shelves) in one shot. For a
>production run in the 100 to 10,000 pieces range it is very attractive.
>When you start getting a lot of pieces, a conventional die and press system
>becomes more attractive.
>For larger shapes than can be done by spinning, why not go with internal
>ribs and structure covered by a thin flexible outer layer. This is how
>airplanes are built. Compound curves aren't too tough if the segments are
>chosen properly, and if you do a little hammer work on the pieces ahead of
>time to get the cross axis curvature. Any auto body guy could help here,
>and their hourly rates aren't too high, and you can learn to do it yourself
>in a few months practice, particularly with aluminum, which is soft.
In the state of TN anyone can buy explosives over the counter with out any
type of permit. I bought some Jellmax in 1 lb sticks which is basically
jelly nitroglycerine for a $1.00 a lbs. I tryed making a small dish shape
for a stirling engine project I was working on. I turned the dish shape in
a piece of thich steel in the lathe instead of diging a hole in the ground
because my shape is not very large like a toriod. I laid the sheet metal on
the piece of steel and put a thin layer of jellmax over the top of the metal
with a blasting cap in the center. It worked the 1st shot but not perfect
so I tried again. After several trys I layed a 1/2" thick sheet of black
neopream rubber on top of the sheet metal then put the explosives on the
rubber. This shot worked perfect without any inperfections of any kind in
the dish shaped sheet metal. I made a few more and found that lower speed
explosives like dynamite would work fine also. Hell with taking classes to
learn how to do this or getting permits. A few hours of practice and I made
3 good parts and it cost me less than $20.00 for everything and I had fun
doing it. And for ATF they can go to HELL, I have no use for those people.
This same method might work fine for making toroids.