Mark wrote:
An old PM article uses a homebrew transformer similar to the one I
wound in 10th grade for my from-scratch soldering gun. A long time ago!

I recall someone's suggestion that a "real" soldering gun could form
the basis for a spotwelder - all I'm interested in doing is making
better bandsaw blade joints than my crappy Silver soldering has
produced to date. My bandsaw only has 10-inch wheels, and I haven't
been able to make a lasting joint in HSS - 1/2 inch stock....

Before I rush out and buy a cheap soldering gun, has anyone done this
successfully?

I tried building a bandsaw blade welder. I got close, but still had to
deal with the whole mechanical aspect, where one jaw of the fixture needs
to move closer as the ends fuse. I eventually snagged one on eBay when
eBay's computers gagged at a fortuitous time.

Anyway, you need LOTS of amps. Enough to make steel particles fly through
the air like sparks from a grinder. I'm guessing that you need something
like 1000 amps to do it right. These welders take close to 15 A from the
wall socket during the weld cycle. (Some take even more, or need 220 V)
So, a slodering gun is hopeless, they get around 40 A output.

I got a 1 KVA isolation or control transformer and cut the secondary windings
off it, and made a 1-turn secondary of #4 stranded wire, that I connected to
the ends of the blade. I was then able to make adjustments to the voltage
by adding 1/2 turn on the transformer core as needed. I think I found 1.5
turns worked best on this particular unit.

One feature of all the commercial units is a tricky linkage that applies
inward pressure to the blade ends, and automatically turns off the current
when the ends have moved together a certain amount, maybe .050".
Too much upset and you have too much of a blob to grind off. Not enough
upset and you have a weak weld that is not fused all across the blade
width.

Jon