[TowerTalk] Tower Grounding Questions

[David Gilbert]

Actually, I think that the starter material is simply the same copper 
oxide and aluminum powder ... just ground to finer particles.  I bought 
a lot of Cadweld powder on eBay for very low prices (typically 50 cents 
to one dollar per shot), but I thought it might be cool to mix my own so 
I also bought (on eBay) some copper oxide and aluminum powder.  It's 
easy to figure the correct mixture from the formula, so I weighed out 
the portions on my wife's gram cooking scale and thoroughly mixed them 
in a small rock tumbler.  Unfortunately, the only grit sizes I could buy 
were lab grade and much finer than the grit size of the Cadweld powder, 
and when I fired off my mixture it flashed like the stuff photographers 
used in the 1800's.  I was careful enough not to have gotten hurt by the 
flash, but I was half worried I had poisoned my lungs because the result 
was just a large misty cloud of green ... no solid copper to be found 
anywhere.  I've saved the materials in case I need more starter powder 
(I typically divide up 200 gram shots into three 65 gram shots for the 
molds I use), but I'm hoping not to have to use them.  In any case, I 
think a key aspect of the commercial Cadweld material is that the 
particle size has apparently been chosen to give a nice controlled reaction.

By the way, it's normally pretty difficult to fire off a Cadweld shot 
with a propane torch because it reduces instead of oxidizes.

In case anyone is interested, I fabricated my own Cadweld mold from a 
block of sintered carbon I bought on eBay.  The details can bee seen at:

http://www.ab7e.com/exothermic/AB7E_Homebrew_Exothermic_Mold.html

The only thing that I neglected to include there is that you need to put 
a small disk of thin aluminum (I think even aluminum foil would work) at 
the bottom of the powder cavity to keep the powder in place until it is 
ignited.

A very similar exothermic reaction is commonly called thermite, except 
that it uses iron oxide instead of copper oxide.  Thermite was 
originally used to weld steel rails together for railroad construction, 
and the army used it in WWII to burn a hole in the engine blocks of 
enemy vehicles they wanted to disable.  It's all kind of nasty stuff ...

73,
Dave   AB7E




On 12/21/2011 10:06 AM, Jim Lux wrote:
> On 12/21/11 8:00 AM, Richard Solomon wrote:
>> A friend is putting up an LM-470 and asked about grounding it. Who
>> makes the Ground Kit for
>> the tower ? I have a couple of DX Engineering PPC-UNI-KIT-2CT kits,
>> will they do ?
>>
>> Also, what's the name of those one-shot chemical "bombs" used to weld
>> the ground cable to
>> the ground rod ?
> Exothermic Welding, generically..
>
> Erico Cad-Weld is a very popular brand (OneShot is one of their models)
> Thermoweld is another
> UniShot is another
>
> If you go to a electrical supply place, they'll carry one or more
> brands.  They'll also have the stuff you need to use it (some kinds have
> a reusable mold and you load the charge in each time from a little
> container).  There are fancy igniters and such, as well.  Do some
> research (there's been stuff on this list over the years). I've used a
> small torch to light them off, but I'm not sure that's actually recommended.
>
> I've never seen this stuff at Home Depot or Lowes.. at least at the ones
> I go to, but that doesn't mean that some don't carry them.  You need to
> go to somewhere that really caters to "trade".
>
> You *could* make the stuff yourself, in the fine ham radio improvising
> way. It's just thermite...  Copper oxide and aluminum powder, piece of
> magnesium ribbon as fuse works nicely.
>
> What you are paying for with the commercial product is a nicely designed
> mold and consistent mixing of the weld material. The commercial ones
> also have a different mix at the ignition point to make it easier to
> start (I'm going to guess some sort of black powder type mix.. a good
> oxidizer and some fuel which gets good and hot, to get the metal
> oxide-aluminum reaction going).
>
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