Wigging out: Output amps -vs- arc volts (MIG)

Long-time lurker, first-time poster. Be gentle. :)
I've spent years hobby-welding with Tig, Stick, Oxy/Acetylene, and plasma cutting and rather thought I was way-cool-bitchin'... then came the MIG machine a few weeks ago. Now I'm back to tripping over my own two feet, making a mess generally... and having loads of fun in the process. Up to a point.
For the record this discussion revolves around a Miller 251 running 0.032 (70S) wire and a 75/25 Argon/CO2 mix at 30ish SCFH on mild steel. Nothing exotic.
My problem is *really* basic, ie, how to convert from a current-based rule of thumb such as "one amp per .001 inch of base metal" into arc **volts**. The MIG machine is CV, not CC (like my TIG, buzzbox, etc), so how the heck do I convert? Funny thing: even the Miller 251 manual itself references output amps (and even mentions the well-worn rule-of-thumb above!), but says absolutely nothing about how to convert a current value into arc **volts**. Looking at the Voltage/Current curves in the manual isn't productive either. These curves specify a possible operating *range* at a given voltage, which, when you work it out to watts gives you an actual min/max heat range in excess of 40%. That's huge.
Can I get some help from the illuminati, please? Maybe a "volts to amps" Rosetta stone for MIG newbies? :)
Roark

No need to worry over amps too much with mig. Yes they are there but as you already know mig is based on volts. Speeding up the wire puts in more amps. But I bet you knew that also, I think your making it harder than it is.
Want to use amps? Put a LN25 on a tig box. I used that for two years and it sucked mostly. Got a Ranger 250 to power the feeder and it is great now. This drove home to me the various power supplies way of dolling out electricty.
Les

Here is a re-post of Mike Graham's 1/1/2002 reply on the subject that I find useful:
Here's the deal:
To get deeper penetration you do this (in order of usefulness):
Increase wirespeed, decrease stickout, drag the torch, switch to straight CO2, use skinnier wire.
To get shallower penetration you do this (in order of usefulness):
Reduce wirespeed, increase stickout, push the torch, switch to argon/CO2 mix, use thicker wire.
To get a larger bead you do this (in order of usefulness):
Increase wirespeed, decrease travel speed, increase stickout
To get a smaller bead you do this (in order of usefulness):
Decrease wirespeed, increase travel speed, decrease stickout
To get a higher, narrower (colder) bead you do this (in order of usefulness):
Decrease voltage, drag torch, increase stickout
To get a flatter, wider (hotter) bead you do this (in order of usefulness):
Increase voltage, push torch, decrease stickout
To get a faster deposition rate you do this (in order of usefulness):
Increase wirespeed, increase stickout, use smaller wire. To get a slower deposition rate you do this (in order of usefulness):
Decrease wirespeed, decrease stickout, use larger wire.
Note that when you change the stickout you change the current, so you might need to adjust the wirespeed to compensate. Same with changing wire size, obviously.

MIG circular slide rule made by Lincoln or miller might help.
If you must have amp readings buy a DC clamp on amp meter. Sears sells a lower priced model. Most clamp on amp meters only work with AC so make sure to buy DC model.

Great input! Thanks guys!
The real thing which was holding me back was the apparently inverse relationship between wire speed and heat/current/penetration. Once I got it right (higher wire feed speed = higher amperage, hotter weld), things turned around quickly. Between that and a "nuts to the tables, lets just goof with this thing until we get a feel for it" ethic, we got past the problem. (It's possible to get too hooked on the manuals I'm finding. At some point you have to open it up and just let it fly).
FWIW, at 18.5 volts (and some currently unrememberable feed speed), all the cracks and pops became the sound of a high pressure water jet. I'd stumbled onto the "sweet spot" and the bead became much nicer. From there I started going up with the voltage and feed speed while still staying in what I'll call the "zone". Again, nice welds with the completely different sound. Now that I know what I'm looking for (with kudos to this group!), it's a whole different world.
Thanks for the advice!
Roark

With CV wire feed welder voltage is adjusted to maintain desired arc length. Then amps will change as you accidently increase or decrease contact tip to work distance to maintain constant arc length with different stickout. Volt setting supports wire size and feed rate. If you used CC welder the wire feed speed would have to change to maintain constant arc length but it doesn't react as fast so need to hold steady stickout. Voltage tracking feeder is really poor with smaller diameter wires.
Stick welders current is more constant and voltage changes so you can maintain arc as arc length changes. This also lets you hold longer arc putting less heat into puddle so it freezes.

R. Duncan wrote:
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Creating the "stack of dimes" effect, which leads many to believe incorrectly that mig welds without the freezelines are no good.

I think the other answers were complete but I would like to comment on the frustration I also have.
I believe this is another example of where the designer of a product and it's documentation has never used it and never will. All MIG machines are sized & priced by their Maximum Current rating. Most documentation I have read concerning MIG uses the term "Current" during much of the discussion yet no MIG has Current settings. Furthermore, volts and "inches per second" are used at times but my baby MIG panel has letters for volts and numbers (1 through 10) for feed speed. To use the machine, I "must" rely on the chart on the inside of the machine. It *would* be easier if they just used common machine terms, Volts and IPS (in USA).
To me, it is just like going to the store to buy something and they tell you the price in Pesos or Yen. (directly proportional as are volts and current)

Maybe it would be better to understand the actions of constant current ( SMAW or stick) versus constant voltage ( GMAW) The constant current machines gave you a set amperage that was sort of regulated by the design of the transformer. You could slightly change amperage by pulling back and lengthening the arc when using stick but the effect was limited. Constant voltage machines hold voltage to your selection and you increase amperage by increasing wire feed. Because you set voltage on a wire feed machine you are selecting a particualr mode of metal transfer. Most of the hobby machines don't have the versatility that allows the user to select his voltage / arc transfer of his choice. He will get short circuit transfer and globular transfer when the machine is maxed out. Conceivabley we could have a machine that the user selects short circuit transfer, globular transfer, or spray transfer then adjusted his amperage from there. These machines now exist but we are talking high end in cost. You program what type of wire, gas mix and transfer mode. The integral computer does the rest. And if for some reason you still are not happy you can hook up your laptop and start tweaking. Some 250 amp machines can get into spray transfer ( around 26+ volts) Personally I use nothing but spray unless of course I am forced to weld thin guage and I can't keep my speed up. When people describe "frying bacon" or some other term they are really talking voltage and transfer mode. If you think two adjustments are bad I should tell you that the first machines I used had voltage, wire feed and inductance. You could get way off if you didn't know what you were trying for. Randy
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Randy Zimmerman wrote:
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Spray is great. Ed Craig's www.weldreality.com has some good explanations of wirespeed and transfer modes, and really corny clip-art graphics.

Thanks Randy. One of these days I'm going to play around with a real MIG machine to see what we can do. I've measured the voltage on my SP-125 at max setting during welding and it goes to about 26V w/.023 but can't do nearly what the school owned Lincoln CV-250 could do at 20V w/.035. There's got to be a current difference hiding in there somewhere. Those electrons are just so gol-durned invisible!

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The wire size also has an effect.There is a limit how much current .023 wire can carry:-this increases with diameter of wire.Above 200 amps ,one should be using 1.0 or1.2mm wire.

At 26 volts you are just at the cusp of spray transfer. with very high argon mixture you just might get that type of transfer. With 20 percent argon you likely are firmly established in globular transfer which gives a fair amount of spatter. Randy
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Spray transfer is tempting to try but I'm just too cheap to get the right gas and try it on this baby. I've been told spray won't work on this (SP-125) even though it can weld at ~26-27 volts.
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That sounds like what it does at times. I'm using C-25 by the way. The machine really does amaze me with what it can do at it's limits. I've never overheated it but I have blown the breaker while "testing" it at it's max output.

It is not realy possible to produce true spray transfer with C-25. You need 20 percent or less to develop the paticular type of arc. A 125 simply does not have the poop to do it. I have a millermatic 250 clone at home and I can just get there with .035 wire and C-20. Randy
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Thanks Randy. Your comments inspired a Google search which came up with an interesting description. http://www.thefabricator.com/xp/Fabricator/Articles/Welding/Article254/Article25 4_p1.xml

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I wont claim to be a welding expert but from a physical point of view the current follows Ohm´s law. Voltage divided by resistance equals current. Assuming the figures you state refers to wire thickness, a thicker wire has less resistance so will give more current at similar voltage. It might be possible that the transfer from gun to wire is different as well. It is also possible that the small machine doesnt hold the voltage as well under load. I assume you mesured the voltage without actually welding at the same time.
Appart from thatr this has been one of the most useful threads so far for me. Thanks all you experts! Henning