Re: explosive hydroforming for toroids and spheres

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.