Hi Skinner,
You need to be sure you have the coils in the right
phase:
from the "start" of the 460v winding, at the 230v tap you need to wire the newly-free end that connects through to the "finish" end to the "start" wire, then the other new end (which will show continuity through to the "start") connects to the finish - so both windings coil the same way around the transformer core.
I've attached a Crap-o-CAD that should explain it better!
You can do a Proper Test to confirm if you're unsure, attach a small AC transformer to the secondary (arc) side of the transformer, not too many volts (you'll get quite a voltage increase), maybe 3v AC - 6v AC but with a few amps behind it (a good rating would be whatever the welder's minimum AC weld current is), and put the meter on AC on the mains side windings.
Connect what you think are the "starts" of the windings together, you should see little or (ideally) no AC voltage across the two "finishes" - if you see more than a few volts, swap one winding end-for-end and test again, if you suddenly have no volts showing - perfect, or you didn't join the wires up... If you have a lot more AC volts showing, swap it back how it was before!
Once you've got them what should be the right ways around, the 3v - 6v of AC into the secondary winding should deliver maybe 25 - 30v AC from the primary side as a final check.
As someone said, bear in mind that at full chat it's going to pull DOUBLE what it would on 460v, so size for 60+ amps - or keep the wick turned down!
Dave H. (the other one)