I'm in the process of putting up a tower and need to make lots of
Cadweld connections (tower to rebar cage, tower to lightning radials,
lightning radials to ground rods, common ground point at entrance to
shack, etc), so I've spent some time trying to figure out economical
ways of doing so. While Cadweld makes those very nice One-Shots for
connecting heavy gauge wire to ground rods, they rather quickly add up
in price and don't lend themselves readily to connections to the tower
base (I have a heavy duty AN-Wireless tower with 5/16 inch thick angled
legs) or the horizontal rebar ties.
So I decided to check out the reusable Cadweld graphite molds and found
several varieties on eBay at very reasonable prices ranging from $10 to
$25 each for new units. Now then, to be honest, only one of those molds
(#4 wire to horizontal rebar) was designed for exactly what I wanted to
do, but this isn't rocket science and there are ways to adapt any
particular mold for other purposes. Narrow strips of copper foil can be
easily wound around thin rods or wire to fit nicely in molds designed
for larger rods or wire. I can also pre-weld wire to a ground rod while
it is laying horizontally on the ground and then pound the rod into the
ground afterward without ruining the weld. And although I haven't tried
it yet, graphite is soft enough that I think I could use a large drill
bit to ream out a mold for a larger size if I was careful to polish the
surface properly afterward.
The exothermic powder used for Cadweld molds is a thermite-type mixture
of copper oxide and aluminum powder, and it is often available on eBay
for less than $2 per shot. Some of the stuff is old, but even Cadweld
says the shelf life is virtually infinite. Most of the molds I bought
require either 65 or 90 gram powder loads, but since larger sizes are
often available for the same price (less demand for them, I guess) I got
to wondering whether I could divide, for example, a 200 gram canister
into three 65 gram loads. The trouble is, the Cadweld powder is
difficult to ignite without the special starter powder (a more volatile
version of the same copper oxide and powdered aluminum mix) they include
at the bottom of each canister, and there really isn't enough of it to
split three ways and be still assured of ignition. I and apparently
several other people have tried to use propane torches to set off the
powder but that doesn't work. A propane flame provides sufficient heat
for ignition but there is enough unburned gas to create a reducing
atmosphere in the flame ... just the opposite of what you want to do.
That's why a flint igniter (any standard version will do fine) is used
instead.
A little web searching brought me to a discussion forum frequented by
what might charitably be called "pyrotechnic aficionados", whereupon I
saw mention of using a standard 4th of July sparkler as an igniter for
thermite. I tried one with some Cadweld powder and it works ... you
just have to make sure the sparkler is only about half-buried in the
powder so that the sparkler doesn't self-extinguish before it does it's job.
I also considered buying copper oxide and finely powdered aluminum (both
also available on eBay) and mixing my own stuff. The reaction is simple
enough ... 3CuO + 2AL = Al2O3 + 3Cu + HEAT (lots of it) + LIGHT (lots
of it) ... so anything close to a 50/50 mix by weight should work, but
the cost worked out to be about $2 per shot so I couldn't see any
advantage to doing so. Also, the aforementioned crazies, er ...
"pyrotechnic aficionados", like playing with this stuff and one of their
videos (http://us.share.geocities.com/roguemovies7 ... download the top
link) leads me to suspect that Cadweld may have included some sort of
inhibitor to control the rate of reaction.
Lastly, Cadweld says you need to use one of their clamping fixtures
(kind of a large pliers with four prongs) with the reusable molds. I
got lucky and found a used one at a decent price, but after playing with
the molds a bit I can't see any reason why the clips that come with each
mold wouldn't work just fine as long as you're willing to wait for the
mold to cool down before grabbing it with your hands.
I've now got more money invested in molds and powder than I expected
because I didn't really know what I was doing when I started, but if I
knew then what I know now I could have made all the connections I wanted
for as little as $1 per shot.
73,
Dave AB7E
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