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From: Louis Boyd <>
Newsgroups: rec.crafts.metalworking
Subject: Re: Project: Help - forming rifle brass hydraulically
Date: Sun, 30 Aug 1998 21:08:08 +0000

Chris Stratton wrote:

> Louis Boyd wrote:
>>The suggestion of using capacitor discharge sounds like a good idea to
>>me. It would not be expensive or complex if made from surplus equipment,
>>although it would not be very practical for commercial sale. A 1000
>>microfarad capacitor bank at 1000 volts gives 500 joules which is in the
>>order of the energy of powder charges used for fire forming typical
>>cases. The medium could be water with a trace of acid or alkaline or
>>salt to optimize the conductivity with a simple wire and ceramic
>>insulator which fits the case neck, (possibly a spark plug) and the case
>>set in a standard sizing die. Switching could be a triggered spark gap
>>or an ignitron.
>What type of capacitors would you use? The really high capacity ones I'm
>aware of don't have dielectrics that can take more than a few volts. I'm
>not saying they don't exist - just that I don't know of any economical
>capacitor type that would work.
>(I assume these components are smaller than the ones that would gain
>attention from the agencies watching for front companies trying to buy
>parts to trigger shaped charges to do implosive forming of somewhat
>heavier metals...)

Simply using gunpowder is probably the cheapest way to go for
fireforming. There's no question that it would be more compact.
You don't have to use your fancy match rifle, just
use the same chambering tool on a cheap single shot with a
worn out bore.  A Thompson Contender pistol would make a good
fireforming gun for many cartridges. The hydraulic and capacitor
discharge methods might be useful in a production environment,
such as a company which reloads military fired brass.  Probably
not worth the effort for an individual hobby shooter.

Back to the capacitor discharge methods, if anyone still cares...
At least in the Phoenix area you can find large capacitor fairly
cheap.  Oil caps which are at about the limit of what a person
can lift are used for power line correction.  A few of them in
parallel should do the job nicely.  They're designed for AC but
will handle DC ok.  Some have built in bleeder resistors, but
that wouldn't hurt this use. One might use electrolytics in
series/parallel with diodes and resistors to prevent reversing
the polarity, but they aren't the best for pulse work.

It's not hard to build a triggered spark gap which will handle
thousands of amps.  Three metal disks in a row. THe middle one
is connected to a spark coil and held at a voltage halfway between
the other two until time to fire.  THen a pulse of either polarity
will break down one size, with the other following immediately.
Plain air will carry the current.

No, the components would probably have to be much larger than
the triggers for a nuclear bomb since they have to carry the
actual energy of the "explosion" rather than just initiate
a chemical explosive.  I doubt it would alarm anyone unless
they know you have a stash of plutonium too :-)

Lou Boyd

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