[TowerTalk] Cad Welding tips

[David Gilbert]

I found it impossible to fire off Cadweld powder with a propane torch, 
and I tried many times.  Cadweld powder is basically an oxidation 
reaction, and the hot part of the flame from a propane torch has lots of 
unburned fuel, making it basically a reduction environment.  Sparks seem 
to work best, although I've also heard that people were able to use 
strips of magnesium as a sort of fuse. When I do my shots I just make a 
small trail of the starter material and hit the end of it with the sparker.

I bought LOTS of Cadweld powder off eBay for the equivalent of 50 cents 
per shot.  It takes about 65 grams to bond a #4 wire to a ground rod, 
and I'd just buy the 200 gram ones and divide them by three.  You can 
also occasionally find used molds for various applications in decent 
shape there at modest prices, but the ones for ground rods in good shape 
are much harder to find.  Hence ...

I decided to make my own Cadweld mold out of a graphite plate I bought 
on eBay.  Graphite can be easily worked with standard wood working 
tools, but you want to be very careful not to breathe the fine dust that 
results.  Check out 
http://www.ab7e.com/exothermic/AB7E_Homebrew_Exothermic_Mold.html for 
step by step directions on how to make your own.

Just for grins, I also tried to make my own Cadweld powder.  Cadweld 
powder is simply the copper version of the iron thermite that was used 
to weld railroad rails together .... the same iron thermite that was 
also used to burn through the engine blocks of captured tanks and trucks 
during World War II.  The Cadweld formula is:

3CuO + 2AL=Al2O3 + 3Cu + HEAT (lots of it) + LIGHT (lots of it)


I bought the copper oxide and aluminum powder (again on eBay), measured 
out the compounds per their molecular weights and the formula above, and 
mixed them in a small rock tumbler.  The problem was that the only 
copper oxide and aluminum powder I could get was fine grain laboratory 
grade, so what I ended up with was essentially the starter powder and 
not the slow burning coarser material that you need for the actual 
weld.  When I fired it off it flashed like the stuff they used for 
photography back in the 1800's.  There wasn't a trace of anything left 
so I had to hold my breath for fear of breathing in the vaporized 
residue until I was sure the wind had blown it all away.

Ham radio experimenting isn't limited to electronics.  ;)

73,
Dave AB7E



On 11/1/2016 6:23 AM, William K Carr III wrote:
> I am about to embark on use of cadweld for bonding my ground wires to copper ground rods.
>
> Has anyone used a pencil tip propane torch to light the cadweld? It would offer a little more distance to protect knuckles, etc.
>
> Also, I have a K0XG rotating tower base. Richard recommends large diameter (in my case 4AWG) stranded welding cable to ground the tower (above the rotating base) to the ground rods. Has anyone used cadweld on stranded welding cable successfully?  Or, just go with a bronze clamp?
>
> Your experience and/or suggestions are appreciated.
>
> Kim K5TU
>
>