On Thu, 08 Sep 2005 02:28:48 GMT, Ignoramus11916
wrote:

On Wed, 07 Sep 2005 20:55:03 -0500, Wayne Cook wrote:
On Wed, 07 Sep 2005 23:03:45 GMT, Ignoramus11916
wrote:

On Wed, 07 Sep 2005 16:16:45 -0500, Wayne Cook wrote:
On Wed, 07 Sep 2005 17:57:45 GMT, Ignoramus11916
wrote:





So, Wayne, what can I weld with such a welder?

Steel, stainless steel, actually in one form or another it would be
capable of most metals even aluminum. Just not in the most efficient
way.

That's very nice to know. Let's say, for instance, it should be easy
for a skilled welder to weld up a trailer frame for a serious
trailer. I do not consider myself a skilled welder and will not
undertake such a high responsibility project, but it is kind of a test
of a welder's capability for me.

Stick is a really good method for building a trailer as long as
you're not in a hurry. It's more likely to produce sound welds in
heavier metal than MIG. MIG is faster and in skilled hands a big
enough MIG is fully capable of welding on a trailer. However in
unskilled hands it can be a problem especially with the smaller
machines that most home shop welders will buy. The problem there is
that it's possible to get a weld that looks perfect but isn't stuck to
the metal.



Aluminum is most efficiently TIG welded with a square wave machine.
The good ones allow adjustment of frequency of the AC, and percentage
of cleaning verses heating polarity in the square wave (I know not
proper terminology but it's the clearest way of describing the actual
end product that I can think of). Next would be sine wave AC with
HF.

Wayne, I am a little confused. With a bunch of 30 A transistors like
these

http://www.onsemi.com/pub/Collateral/2N5302-D.PDF

ebay item 7543195186

I could relatively easily convert DC into square wave AC. I mean,
really, there is not much needed for this other than properly mount
the transistors (to cool them), add a blower/cooling fan and apply AC
voltage to their signal inputs, so that either half would be producing
either + voltage or - voltage.

Am I wrong here?

Unfortunately I think you are. Everything I've heard from the people
who build power supplies like this leads me to believe that it's
almost more of a art than a science to get one that doesn't blow up
components. You're talking about switching serious amperage here and
paralleling components like that is asking for problems with one of
them carrying more current than it's capable of while the others carry
less. This can lead to a cascade melt down where as each component
fails the rest take on more than they can handle.

Generally big welders like this use large high current SCR's and the
like which can be rather expensive. Mine has two SCR's rated at 200
amps in it and then there's two ultra fast blow fuses to protect them.
The catch here is that the fuses cost more than the SCR's (found that
out the hard way). All of this is just to control the reactor which in
turn controls the weld current. In other word they aren't even seeing
the full weld current.

It was the only method available for years and does work but requires
some more skill. However TIG welding aluminum started with reverse
polarity DC. The drawback here is that it puts so much heat into the
tungsten that you need a large torch and tungsten to do any real
thickness of aluminum. In truth sometimes reverse polarity DC is
better than AC. It has a much higher cleaning action for really dirty
aluminum. I've used it in the past a few times on troublesome
castings. But it really blasts the torch and tungsten to do it.

Very nice to know. Sounds like it could be done, with a big enough
tungsten electrode.

It can. The rule of thumb that I've heard is the tungsten needs to
be as big as the aluminum is thick.



This welder will likely make a superior stick welding power source.
Depending on the controls it has may well be one of the best stick
welding power sources out there in terms of wide range capability. I
can make my old CyberTIG 300 weld thin metal with small rods better
and easier than with any other power source I've ever used. Yet it's
also able to weld with large enough rods for 99% of anything I need
and will certainly be large enough for your needs.

Sounds great. I feel a little better about this project. I will ask
about square wave generation in a separate thread.

As stated above that's a rather big kettle of worms that you're
about to open. :-)

Wayne Cook
Shamrock, TX
http://members.dslextreme.com/users/waynecook/index.htm