Hot Wire Foam Cutter

Homemade Hot-Wire Foam Cutter

4-Aug-09 UPDATE: Modifications to the string tensioning assembly

I originally used my dad's 14" wood-cutting band saw to make my first foam moulds for lost-foam casting, but it's a terribly dangerous machine and is pretty excessive for cutting foam. I decided that a hot-wire foam cutter would be much better suited, and would be a fun project.

In case you're not familiar with the concept of a hot wire foam cutter, it's quite simple. A wire is heated up, usually by putting electricity through it, and it is then used like a band saw or scroll saw for cutting through foam. It's not enough power to get a shock, and it's barely hot enough to burn you, so they're very safe and easy to use. I was concerned that it would smell from burning the foam, but the heat is low enough that it barely melts the foam, except for where it's cutting, so there are no noticeable fumes.

I googled "hot wire foam cutter" and got a few design ideas, but most of the designs I saw seemed really flimsy. I recognise that this doesn't need to be built terribly strongly, but the wire shouldn't move when cutting. I decided to build the arm on mine out of 1" square H.S.S., with a wall thickness of 0.100". My biggest reason for this material was that I already had it on hand, but it also welds nicely (and I'm always looking for excuses to play with my welder!)

The arm is welded to two pieces of ¼" x 1" CS flat bar, which is screwed into a piece of laminated 5/8" melamine. The wire used for cutting is 0.017" solid steel guitar wire, which is threaded through a piece of ¼" aluminum and tied to a brass screw. The screw can be tightened to put tension on the wire, which works well. I used brass and aluminum to connect to the ends of the wire as they are better conductors of electricity than steel. Number 12 copper wire provides power to each end of the wire.

The brass screw allows me to tension the guitar wire, and works really well.

4-Aug-09 UPDATE: I was about to use my foam cutter the other day and noticed that the string had broken. I'm not sure why, but it gave me the opportunity to try using a finer string. I tried using a 0.011" string, but it kept breaking as soon as I applied any tension. I went back to the 0.017" string which I had no problem tensioning.

In addition, I made some updates to the brass tensioning assembly. Originally I welded a 1/4" nut to the frame and the brass screw was threaded to that. The problem with this is that as the wire is tensioned, it gets twisted. I think this may be why I had problems with the string breaking. It occurred to me that the welded nut is unnecessary; the wire can be tensioned by adjusting the position of a nut on the screw. The screw does not rotate and it moves up/down according to the position of the nut. (See picture for a more clear description).

I also cut a slit in the side of the screw which allows me to unhook the wire and pass it through a center hole if I want to cut an inner diameter.

The modified string tensioning assembly

The Power Supply

I initially used a toy car transformer to power the wire, and controlled it using a light dimmer. This worked great, until the transformer died in a puff of smoke. Undeterred, I tried another transformer from an old printer. It lasted less than 2 minutes. I tried my junky soldering gun, which didn't have enough power to even warm the wire. Finally, my dad suggested winding my own transformer. I had a couple old microwave transformers which I'd originally planned to use on a homemade welder, per Dan's website (that guy has some great ideas!). It took a bit of trial and error to get the windings correct, but I ended up replacing the secondary windings on one of these transformers with 2 windings of number 12 stranded wire, and I now have a perfect power supply. It looks like Dan uses this same technique to power his foam cutter too.

To be sure that the transformer would hold up, I left the foam cutter on full power for about 1.5 hours to see what would happen. The primaries got to be about 80°C while the secondaries got up to 40°C; easily within the ratings on the wiring. Also, I'll usually run it at less than full power, so I don't expect this thing to burn up any time soon!

For a few evenings work, and from stuff I had laying around, I've got a pretty decent hot wire foam cutter. It was fun to build and is fun to use.

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