Step 6: Make the jaws
For the jaws, I used two blocks of wood measuring approximately 3.5" by 1.5" by 5", a 4" hinge, and two lengths of 19mm steel tubing about 10" long. The blocks of wood were cut from an old pallet, and the steel tubing came from an old rotary washing line.
Although I could have run the wire through the tubing, I chose not to for reasons of simplicity, heat dissipation, and strength of the jaws (as extra holes would need to be drilled). Also I would like to leave open the option to use thicker wire at some point. I have seen a design which uses copper tubing for the jaws which then form conductors between the transformer and electrodes, however I thought the conducting area would probably not be enough. I used cross dowels to enable quick and easy removal of the electrodes for renewal and re-shaping.
Lay the two blocks of wood with the long edges together, lay the hinge on them with the pin along the edges which meet, and draw around it.
Using a chisel, cut a rebate just inside the line you drew on each block for the hinge. Slope the rebate off a bit towards the edge of the wood to allow for the slope up to the hinge pin. Check that the hinge fits correctly, and screw it into place.
Measure the centre line at right angles to the hinge pin, and mark the centre at the outer edge of each of the two blocks. Fold them together and check the centre marks meet. It isn't important that the marks are in the exact centre, but it is important that they are aligned. Use your square to mark lines across ends of the blocks at these marks, and then find the middle of each of these lines.
Remove the hinge and make a 19mm hole (or whatever size your tubing is) in the end of each block at the place you just marked. Go a couple of inches for each one - they do need to be about the same depth. The exact method you use depends on the tools you have available. I used a hole saw, first making a pilot hole with a wood boring bit. Drilling a pilot hole enabled me to check that the hole was square (to the face of the wood), as I was doing this using a hand-held drill. Apply the hole saw in stages, pulling it out frequently to remove sawdust. Break the core of the hole by inserting a drill bit into the centre hole and pushing it sideways to snap the wood across the grain.
Drill a pilot hole in each piece on the hinge side - this is where the screws will eventually go which lock the tubing in place, so they need to be partway down the depth of the big holes you made. Don't drill the tubing yet. I used a wood boring bit for these, with the tubing in place. This gave a nice shallow hole with a clear centre point, but not drilled through, precisely positioned as the drill doesn't wander. Offset the holes from each other along the pipe so the heads won't short the pipes (which will have the welding voltage across them)
Insert the two pieces of tubing into the holes as far as they will go, and fit the hinge back on.
Position the jaw assembly in place in front of the transformer, and mark where the electrodes will be. They both need to be exactly the same distance from the hinge, so use your square to mark both tubes together.
Make a second mark on each tube far enough from the first to allow for the thickness of the electrode, a gap of a few mm, and a cross-dowel, then a few mm again to allow enough steel to retain the dowel. Screw the retaining bolt into the dowel and hold it and the rod you intend to make the electrode out of, against the tube to check the position of this mark. This is where you will cut the tube to length. Don't cut it yet though in case you make a mistake or decide to change something.
Mark the position for a hole in the bottom wood block so it can go from the hinge-pin side to the tubing side without fouling the hinge screws, and also misses the mounting screws which will be underneath. The hole needs to be big enough for the wire to the bottom electrode to pass through (I used 13mm)
At this point, you might want to tidy up the ends of the wooden blocks so all is flush and nice.