Hi Michael,
I didn't realise that you called it a resistance welder ! Because the machine that he used had a pair of cables that went down to a transformer sat on the floor. He used a foot switch to operate it.
I didn't see anything that I would call complex about it. Two cams clamping the blade to a pair of copper blocks and another cam pressing them together, all on a 8 X 6 piece of Tufnol or Paxolin board.
I would have thought that it would be very easy to copy.
Attachment 383206
Just three cams, two clamping the blade and one pressing the ends together. The brown is the Paxolin insulator and the blue is the support. The two power cables from the transformer are shown by the orange dots in the copper blocks. Only one side actually moved to force the blade ends together. The gap between the copper blocks was about 1/2" to 3/4" of an inch.
The grinding jig, if you can call it that was just a curved steel section of a circle, probably of about two foot diameter and about the same length, about 50 mm wide, 1/8" flat bar. After welding and annealing, he just put the blade over it and held it down with his foot, ground it, flipped it over and did the other side.
If I ever break a blade I would be very tempted to build a device like this, and I've got an old welding transformer to do it with !
Oh I can feel another project coming on...... :)