With this method of setting up a MIG machine you lead the setup by choosing a voltage. This is the first method I was shown.
This is the "voltage-lead" method I have been shown for setting up an unknown MIG welding machine with arbitrary markings on voltage-tap and wire-feed-speed dial This setting method can be iterative as in the first time around on a machine you know absolutely nothing about you will know much more about the machine but can be off the conditions.Guess your voltage. You want plenty of fusion, without burn-through (melt-through) of workpiece. If the machine is rated as being for 1mm (small) to 6mm (large) thickness plate and you are doing 3mm plate, the voltage setting is going to be about half way up the voltage settings range. The voltage to thickness range can generally be assumed to be linear (eg. for 3/4's of the machine's rated plate thickness capacity, use about 3/4's the way up the voltage-tap range). Else, when no given information about the machine's capacity - take a guess on the machine's capability based on its size (is it huge or small?) or other clues like the size of its torch (is it big suggesting large current-carrying capacity or is it small?) -- and use this estimate of the machine's top capability to judge what part of the voltage range to use. In either case - set your voltage tap. Set your wire feed speed. The middle value of wire feed speed is a good guess for lower voltage settings. If you are likely to be up in/near the spray transfer mode, because of a high voltage, it is a good idea to start high on the wire feed speed, as erring too low can result in a burned-out contactor tip in a fraction of a second as the spray-arc burns back to the contactor tip. For dip-transfer: You want to be in the range between burn-back (too low w.f.s.) and stubbing (too high w.f.s.) - usually up in the higher part of the "good" range close to onset of stubbing. So have a quick test to find the w.f.s. which is the onset of stubbing. It is informative to find where burn-back is starting, where you get pauses in the dipping. If the range is wide, you know you are in for an easy time, whereas a narrow range suggests settings are going to be difficult to get exactly right. A narrow "good range" can indicate that the voltage is too low for the thickness of material -- does other evidence suggest this, like cold welds with little weld pool fluidity and lack of willingness to fuse along the root and up the sides of the weld? For spray transfer: Come down from w.f.s. high settings until you get a steady spray transfer arc. Evaluate your result When you try a weld, is the evidence for good fusion? Do you see discolouration and flaking of mill-scale on the the back surface, without any melting? (usually, that's good - what you want). If you are getting melt-through, your voltage is too high - so return to stage-1 trying a lower voltage-tap If the weld seems cold or you see no evidence on the back face of any heat coming through and you decide you want more heat (you usually do - you are out to get fast easy-to-control welds), go back to stage-1 trying a higher voltage tap. Remember that for T-fillet joints your "combined thickness" of heat flowing off in three directions means you expect higher settings than joint configurations where two plates meet (butt, outside-corner)