JohnM wrote:

Ignoramus25850 wrote:

Someone offers me a hobart mega-arc 300 with 200' of cable, tig torch,
ground clamp etc. Supposedly can be used for tig welding.

No HF starting.

Question, is that a good deal at $500.

It is a 3 phase machine, which does not bother me terribly too
much. If it demands more power than my 10 HP RPC can produce, I will
add another, second idler motor. These motors cost nothing, relatively
speaking, and can be kept in some unused corner. I would spin up the
first idler and only then start the second, and the load after that. I
have 60A in my garage, easily upgradeable to 75A.

Any thoughts?

i


That should be a pretty good machine, but he's asking a lot of money
for it. If the HF is the old fashioned sort, a cleaning of the spark
gap or replacement of the transformer should be all that's required, I
think any ordinary furnace lighting transformer would work fine.

Offer him $200, see what happens.

Can't be sure, I was thinking Ignoramus meant that it did not have the
HF system at all,
rather than it was not working. Anyway, you really WANT HF. Now that I
have
my Lincoln Square-wave TIG 300, I can't imagine using anything without HF.
And, you will have a horrible time doing Aluminum without HF. Since it is
a 3-phase machine, it either has to be an inverter square-wave system, or DC
only. You can do practically anything BUT aluminum with DC. The whole
reason
I got a square-wave machine was to have aluminum capability. (And, my whole
system of acquiring tools was to get only ONE of any type machine, and
always
try to get the most versatile machines I could.)

Jon

On Wed, 07 Sep 2005 17:57:45 GMT, Ignoramus11916
wrote:

On Wed, 07 Sep 2005 11:06:15 -0400, JohnM wrote:
Ignoramus25850 wrote:
Someone offers me a hobart mega-arc 300 with 200' of cable, tig torch,
ground clamp etc. Supposedly can be used for tig welding.

No HF starting.

Question, is that a good deal at $500.

It is a 3 phase machine, which does not bother me terribly too
much. If it demands more power than my 10 HP RPC can produce, I will
add another, second idler motor. These motors cost nothing, relatively
speaking, and can be kept in some unused corner. I would spin up the
first idler and only then start the second, and the load after that. I
have 60A in my garage, easily upgradeable to 75A.

Any thoughts?

i


That should be a pretty good machine, but he's asking a lot of money for
it. If the HF is the old fashioned sort, a cleaning of the spark gap or
replacement of the transformer should be all that's required, I think
any ordinary furnace lighting transformer would work fine.

No, it is supposed to be the "new sort", with more electronics and
lighter. Also comes with 200 feet of welding cable and tig gun and
stinger.

Probably a good machine but parts are almost none existent for these
old Hobart welders. If it has square wave AC capability then it would
probably make a pretty decent TIG welder. If not then aluminum
capability is going to be ok but not as good as the newer welders.


Offer him $200, see what happens.

A 10 hp convertor should power this machine well.

My calculation, based on 300 amps at 40V, gives me energy consumption
at least 12 kW. That's a little bit too much for a 10 HP RPC. Am I
mistaken?

No you're not mistaken. Mine takes 80 amps single phase to run full
out (though I've only got it on a 60 amp breaker).

I doubt that I will need 300A very often though, although I have no
idea.

Not likely.

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

Ignoramus11916 wrote:
On Wed, 07 Sep 2005 11:06:15 -0400, JohnM wrote:

Ignoramus25850 wrote:

Someone offers me a hobart mega-arc 300 with 200' of cable, tig torch,
ground clamp etc. Supposedly can be used for tig welding.

No HF starting.

Question, is that a good deal at $500.

It is a 3 phase machine, which does not bother me terribly too
much. If it demands more power than my 10 HP RPC can produce, I will
add another, second idler motor. These motors cost nothing, relatively
speaking, and can be kept in some unused corner. I would spin up the
first idler and only then start the second, and the load after that. I
have 60A in my garage, easily upgradeable to 75A.

Any thoughts?

i


That should be a pretty good machine, but he's asking a lot of money for
it. If the HF is the old fashioned sort, a cleaning of the spark gap or
replacement of the transformer should be all that's required, I think
any ordinary furnace lighting transformer would work fine.


No, it is supposed to be the "new sort", with more electronics and
lighter. Also comes with 200 feet of welding cable and tig gun and
stinger.


Offer him $200, see what happens.

A 10 hp convertor should power this machine well.


My calculation, based on 300 amps at 40V, gives me energy consumption
at least 12 kW. That's a little bit too much for a 10 HP RPC. Am I
mistaken?

I doubt that I will need 300A very often though, although I have no
idea.

thanks John...

i


The last statement is the question; how much are you going to ask from
the machine? Try it out, it'll either suit you or it won't, no big deal
either way.. I'm guessing you'll find it's more than adequate.

HF *should* be easy to repair, regardless of the age of the machine, but
there's sure no guarantees. Go over and ask on sci.eng.welding, I think
there's some guys there who don't hang out here, one of them may have
dealt with that very machine.

John

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:


Probably a good machine but parts are almost none existent for these
old Hobart welders. If it has square wave AC capability then it would
probably make a pretty decent TIG welder. If not then aluminum
capability is going to be ok but not as good as the newer welders.

It is from mid 90s and does not have AC capability.

Ok. Still hard to get parts for since Hobart got split up in the buy
out.


Offer him $200, see what happens.

A 10 hp convertor should power this machine well.

My calculation, based on 300 amps at 40V, gives me energy consumption
at least 12 kW. That's a little bit too much for a 10 HP RPC. Am I
mistaken?

No you're not mistaken. Mine takes 80 amps single phase to run full
out (though I've only got it on a 60 amp breaker).

Yep.

I doubt that I will need 300A very often though, although I have no
idea.

Not likely.

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.

Without HF you would be limited to scratch start TIG (if you'd kept
the arc stabilizer it would of fixed this problem). The real problem
with this is tungsten contamination. However it's been done this way
for years and is certainly doable.

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.
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.

Most other metals will DC TIG just fine on straight polarity so the
only thing you'll have to watch out for is contaminating the tungsten
when you start.

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.




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

On Wed, 07 Sep 2005 11:06:15 -0400, JohnM wrote:


That should be a pretty good machine, but he's asking a lot of money for
it. If the HF is the old fashioned sort, a cleaning of the spark gap or
replacement of the transformer should be all that's required, I think
any ordinary furnace lighting transformer would work fine.

This is not a good idea!

Ignoramus25850 wrote:
Someone offers me a hobart mega-arc 300 with 200' of cable, tig torch,
ground clamp etc. Supposedly can be used for tig welding.

No HF starting.

Question, is that a good deal at $500.

It is a 3 phase machine, which does not bother me terribly too
much. If it demands more power than my 10 HP RPC can produce, I will
add another, second idler motor. These motors cost nothing, relatively
speaking, and can be kept in some unused corner. I would spin up the
first idler and only then start the second, and the load after that. I
have 60A in my garage, easily upgradeable to 75A.

Any thoughts?

i

For my nickel, a DC only machine without HF and 3 phase input that is
hard to get parts for? I would offer him a case of beer and hauling it
out of his shop. $500 will get you lots more useful machines if you have
interest in alumuinum or TIG welding.

As for building your own inverter, Don is right. That is a serious
project and by the time it is working well, it would have been cheaper
to buy a Miller Dynasty!

Save your money,
Bob

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

Don Foreman wrote:
On Wed, 07 Sep 2005 11:06:15 -0400, JohnM wrote:



That should be a pretty good machine, but he's asking a lot of money for
it. If the HF is the old fashioned sort, a cleaning of the spark gap or
replacement of the transformer should be all that's required, I think
any ordinary furnace lighting transformer would work fine.


This is not a good idea!

I haven't replaces a HF transformer with one from a furnace, but I've
opened up a few old welders to blow them out and the transformer looked
for all the world like the furnace ignitor type. I think I remember two
Hobarts that had those transformers.. If there's a fundamental
difference between the two I'd certainly be interested in knowing it,
the more I know the better off I am.. I think...

John