In the near future I will attempt to cast aluminum using the lost foam method. In order to do this, I needed some way to cut foam easily, without leaving a ragged edge. I decided to make a hand-held hot wire cutter to solve this problem, and it works well. I may eventually build a larger bandsaw-style cutter, if I need to cut a large piece of foam or make a straighter edge.
Construction was fairly simple. The jaws of the cutter are made from some scrap steel rod, with one end turned to a smaller diameter to fit inside a ring terminal. The rod was bent by notching it with a hacksaw and a file, then bending it in a vise. The other end was notched with a hacksaw to give the wire a place to sit. Both jaws were fed through a piece of PVC pipe, into an end cap with two holes drilled in it. A small strip of PVC sits inside the pipe to act as a spacer between the two rods, as well as to add tension to the wire. The wire itself is 32 AWG Kanthal wire, attached to the jaws by wrapping and twisting. Everything is press-fit, no adhesive was used. The power supply is a 12 volt DC "wall-wart" type, that I had sitting in my parts box. I cut off the original plug and connected spade terminals to the wires.
The cutter performs well, and cuts very clean edges on pieces of styrofoam. To make more accurate parts, it can be clamped to the edge of a table, and a guide can be used to slide foam through the cutter in a straight line.