Adjusting MIG for sheet metal

However, I always have trouble getting things set right when I have to work with thin sheet metal, especially if it is rusty. I use the chart on the welder as a guideline, but sometimes I can't get their settinsg to work well.

It seems like I either burn through the stock, or I don't get reasonable penetration. It is hard to find a good current/wire feed setting, and hard to find a voltage setting to match.

I have a MillerMatic 250, which has wire speed (current) and voltage settings.

My thinking is this- tell me if I am wrong. I assume you should set the wire speed so that you get the right penetration without burn through. Then adjust the voltage so that it makes a nice "sputter" sound. To low voltage will give very slow pops, and too high voltage gives continuous arc that tends to use up the wire too fast.

Tell me- when adjusting, do you keep a hand on the voltage knob while drawing an arc to find the best setting?

Keep in mind that the voltage is effected by arc length in addition to the setting you control on the panel. The longer the arc, the higher the voltage. The higher the voltage the more spatter and the less penetration. You really can't get to the manufacturers spec on voltage w/o a voltmeter option.

MIG machines penetrate when pushed. This is the opposite of the stick machine.

I assumed you were using a gas shielded wire. That isn't going to weld dirty. You need to remove the rust from the parent material. Rust is also a source of hydrogen which can lead to hydrogen induced cracking and it is also a source of oxidation which is also detrimental to sound weld quality. The rust may also effect your ground which will make the wire speed erratic.
Steve

Also- just a technique question- do you typically start an arc and then turn knobs with one hand while welding with the other? Or do you adjust, weld, adjust, weld...

The thing you don't want to adjust while energized is the type that clicks from one setting to the next like the Lincoln 225S for SMAW or the old Miller 35 wire welders. It burns the contact points.

You might want to check your polarity. The person that had the machine before may have been running open arc wire. You'll find the polarity shims under the side door just above the inlet guide where the wire goes thru. There should be a chart that shows you how to position them.

If you mainly have trouble when the arc starts like burn back, you may need to have the trimmer pot adjusted. That lets the motor start a bit slower as you strike the arc and lets the speed come up afterwards. I've had trouble with those before on brand new machines right out of the box.
HTH,
Steve