How much pressure are you applying at the electrodes?  Miller's online data on RSW (resistance spot welding) talks about electrodes clamping with a kN or two (1 kN is about 225 lbs).  Without the high clamping force, you won't fuse the heated metal.  Also, that 400A measurement is somewhat suspect.  I've run a bunch of numbers to compute the current my setup with a 3-turn secondary (35:1 turns ratio), and I can't my primary current above a couple of amps, and that's assuming that I have a heavy-duty winding made from bar-stock and the arms of the welder are 25mm x 37mm aluminium bar stock (50cm long), and further assuming that the metal I'm welding has no resistance!  Like I suggested in an earlier post, measure the current on the primary side and use the turns ratio to calculate the current at the secondary side.  I'm setting up a spreadsheet to figure out the best turns ratio to maximise welding current, and I'll post it when it's done.

I suspect that one of the things that we haven't been considering is happening when you weld, Tyler.  I hadn't even considered it until you told us your results (particularly the dropping of output voltage at the secondary when welding).  I think that the transformer core is saturating.  We all know how a wire has a current rating, and if you try to jam too much current through it, it will start to heat up, and this in turn will limit the current as the resistance of the wire increases with temperature.  Well, permeable materials (i.e., those that conduct magnetic flux, like our transformer cores) also have capacities, and just like electrical wire, these are set by the material and how much of it is present.  It's possible (likely?) that the transformer core is saturating and you're losing a lot of your input power through the core's reluctance (the magnetic equivalent of electrical resistance).  It's the one resistor in the system that we can't change :(

However, there is a solution.  Get a second core and wire the two in parallel on the primary side and in series on the secondary side (mind the phase).  This will half the net reluctance, quartering the losses (they follow a square law just like electrical losses).  I'll post my results with my transformer.  If they agree, I'd start looking at reluctance losses.

-Chris

I have decided to buy a spot welder rather than make 1.  They are not very expensive at all.  Here down under they sell for only about $1100.  Not a bad price.  I will however have quite a bit of use for it, especially doing panel work.  I am only probably going to get it about June/July.

Mtw fdu.

Yes Tyler it was $1100 for the spot welder.  I tried to post a pic but it didn't work.  If you go to http://www.hareandforbes.com.au   follow the link on the "Metalworking" section then click the "Welding" section then click the "Spot Welders" section.  All the info is there.

They are rated at 40kg to 120kg clamping force good enough for panel work.

Mtw fdu.

Will do!!!

Mtw fdu.