How to Make a Hot Wire Knife

There are so many uses for cut polystyrene foam that it often pays to get large sheets of it and cut it to the size you need as you need it. This will save you a lot of money rather than having the polystyrene foam cut to size where you buy it. The problem arises when you start to cut. If you do not have the proper tool, you will waste a lot of the foam. Here is a home-made hot wire knife that will cut through polystyrene foam the way a butter knife goes through brie.

Things You'll Need

• 100 watt soft soldering gun
• Mains wire intended to carry 40 amps with 7 internal wires
• Wire cutters
• Copper wire
• Yardstick
• Marker

Instructions

1. Cut about 16 inches of mains wire. Mains wire is electrical wire used at home for low-voltage installations. Use the wire cutter to cut about 1 inch of the rubber or plastic coating from the mains wire. Strip each end so that you can see the actual copper wires. These will be attached to the soldering gun.

2. Measure 8 inches from the bottom of the mains wire. Put a mark where the measure is 8 inches. Measure down 2 inches on each side. Place a mark on each 2-inch point. This should give you 4 inches right in the center of the wire.

3. Strip the rubber or plastic insulation from the wire carefully being sure not to cut through the copper wires. Separate and cut the six wires surrounding the center wire. Do this right in the middle of the wires. Make sure that you do not cut through the center wire and that there are six strands cut straight through the middle of the wire. The center wire should run intact from one end of the wire to the other.

4. Bend the wire so that it resembles a goalpost with the two arms extending up and the bottom wires closer together. Snip the six extra wires sticking out from the sides as close to the insulation as possible without cutting through the center wire.

5. Stick the bottom of the wires with the inch of insulation removed into the solder gun terminal posts. Make sure the wires are in securely. Pull on the insulation to make a straight cutting edge of the remaining center wire.

6. Plug in the soldering gun and test the “blade” on a small scrap piece of polystyrene after allowing it to heat up for about two minutes.

Tips & Warnings

• Use longer wire if you need to cut bigger pieces of polystyrene. Just make sure that both prongs are the same size.

• Only use to cut polystyrene foam because polyurethane foam will give off poisonous fumes.

References

• Club Cycom: Hot Wire Cutter for Polystyrene