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.
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
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
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!
In message ,
Zorro writes
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
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.
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
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.
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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