Model airplane hobbyists often make smooth cuts in polystyrene foam. To the neophyte, cutting such a soft material would seem to be a simple matter. The reality is not so simple, though. A sharp knife will cut it, but it does not leave a smooth surface. The answer is a hot foam cutter, which uses a heated wire to slice through the foam. These cutters are expensive to buy, but you can build your own inexpensive hot foam cutter that uses an automotive battery charger as a power supply.
Make the Bow
Cut the perforated steel bar in the center with sheet metal shears. File the cut ends and all corners with a metal file so they cannot scratch or cut you.
Drill a hole through each end of the stick, 11/16ths of an inch from the ends. Fasten the center of a metal bar to one end of the stick using the screws, washers and nuts. Fasten the other metal bar to the other end of the stick in the same fashion. Leave the nuts just loose enough so the bars can pivot.
Bend the nichrome wire three inches from each end. Insert the ends through a hole at one end of each metal bar and twist it around the standing wire to form a loop. Hook the bungee cord through a hole at the other end of each bar so it holds the wire taut, like a bow.
Insert the open end of the appliance cord through the wire clamp and into the outlet box. Connect the green wire to the ground (green) screw of the dimmer switch, and the black and white wires to the brass and silver screws respectively of the line side of the dimmer switch.
Connect the Romex wire from the dimmer switch output to the duplex electrical outlet. Fasten the dimmer switch and outlet to the outlet box, and attach the cover plates.
Cut the battery terminal clips off the charger output wires. Strip one inch of insulation from each wire and twist the bare end of one wire around one end of the nichrome wire. Connect the other wire likewise to the other end of the nichrome wire.
Using the Cutter
Turn the dimmer switch all the way down. Plug the charger into the outlet.
Plug the appliance cord into a live wall outlet. Slowly advance the dimmer switch until the wire becomes hot enough to melt the foam.
Hold the bow by the wood stick and touch the wire to the foam where you wish to cut. Adjust the dimmer switch for the most efficient cutting speed.
The best charger to use is a simple 12-volt, 10-amp manual charger. An automatic charger may not work properly.
Never touch the wire while the cutter is plugged into the wall outlet. Unplug the cutter when you are finished, and wait for the wire to cool before you touch it or lay it down. Always wear gloves and eye protection when using the cutter.