When I first started down the road to green sand metal casting, I figured I'd use the lost foam method. To cut the foam, I built a hot wire foam cutter.
The principle is easy - a wire is headed electrically and melts it's way through foam. In use, it is similar in function to a jigsaw or band saw, though with the addition of a fence it can be used like a table saw. Mine was built entirely out of stuff I had in the basement - a chunk of blue particle board, a brass plated pipe (originally a ceiling fan support), a unidentified metal rod of some sort, springs, wires, etc. The usual crap you've got rolling around in the bottom of your tool box (well, maybe not the particle board). Even the transformer was something I had. (The only thing I had to buy was the light dimmer, and I only had to buy that because the one I had got put to use in the dining room and my wife wouldn't let me have it back.)
The first step was to drill a hole in the pipe to insert the metal rod. To hold the rod in place, I drilled a third hole in the pipe at the same level as the others, but 90° to the side, and inserted a set screw.
To isolate the metal rod and pipe from the circuit, I installed a small piece of 1/8" rubber hose through a hole in the metal rod, and threaded the eye hook into it. A single strand of 0.010" stainless steel picture hangar wire is formed into a loop and placed over the hook.
I backed up the particle board with a piece of 3/4" pine, and drilled a smidgen-smaller-than-the-pipe hole through it all to hold the pipe with a tight fit. The hole is 1-1/2" deep, plenty deep enough to hold the pipe rigidly. Between the pipe turning and the metal rod sliding in and out, it is easy to adjust the wire to be perfectly square to the table.
The wire passes through a 1/16" hole in the table. As the wire will lengthen and shorten as it heats up and cools down, I used a spring to take up the slack. The wire is pinched between two washers on the end of a lever, which is pulled against the spring. There are a million ways to do this - this is just the one I cobbed up first. I guess the wire shouldn't touch the wood like it does, but I haven't had a problem with it.
Power is provided by a 120V to 12.6V 3A transformer. I tried a door bell transformer (16V, no current listed) but it just didn't have the oomph to heat the wire. This transformer will heat the wire red hot if you let it - a light dimmer keeps it in check. (Note: 120V can kill you.)
The fence is just a couple of pieces of 3/4" pine, held in place by C-clamps.
© 2003 W. E. Johns