We found a load of polystyrene in the garage this afternoon, so we decided to make a hot-wire cutter for it. Basically, you take some wire with an ohm or two resistance and pass a few amps of current through it. The wire is held tight and the polystyrene melts as it passes around the wire.
The only tricky bit was scrounging some nichrome wire from my son’s electronics set. My bench power supply wasn’t up to the job, but I had an oldish PC power supply kicking around. Now, these often need some kind of a load on the rails for them to work, so we plugged it into an old motherboard – which had the extra benefit of a nicely labelled header for the power on switch. That’s easily shorted with a screwdriver to fire up the PSU, and an old CDROM plugged into one of the hard disk power connectors showed it was working.
Then all we needed was to strip a red/black (5 volt supply) pair from another hard disk connector and connect it to the end of the nichrome wire. We used normal screw terminal blocks to do this as it makes it easy to change!
The wire needs supporting vertically. I had an old coping saw frame which was never going to take a blade again, so we screwed that to a piece of board and attached the wire to one end. The wire then passed up through the board to the top part of the coping saw. Now, this end has to be kept electrically isolated from the saw frame, as otherwise all the current will just flow through the heavy metalwork, rather than the 1ohm or so resistance of the nichrome. We achieved this by wrapping the top of the saw with self-amalgamating tape (although ordinary insulation tape would likely do) pushing a short length of copper pipe over that (making sure it’s shorter than the insulation!) and then more tape over the top of that. The tape will melt a little when it gets hot, so the copper tube stops it melting through to the saw frame.
After that it’s just a matter of firing up the power supply, and doing some cutting. Do it outside though – as the fumes from melting polystyrene are horrid!