Hot Wire Foam Cutter

Cheap ($30+-) and easy to build hot wire foam cutter made from commonly available parts. Cuts styrofoam for surfboards, model plane wings, sculpture, model train or tabletop wargame terrain, 3D sign letters, mold models for lost foam casting, etc.

5/17/2007, Important note! Please be sure to read the comments that other users have left below. There's a LOT of good info there. Check out the links that folks have posted and study what they've said here and you will be able to build a cutter thats suited to your budget and the materials you have access to. There are a lot of alternatives listed or linked below for the frame, the transformer, the wire, the enclosure, and the heat control. Don't underestimate the power of comments! :)

The parts should be pretty easy to find.

1. 12 foot, 16 guage extension cord, about $2 at Home Depot

2. 2 wooden yard sticks from the Home Depot paint department, $.97 each

3. 4 #10-24 x 1.25" machine screws with nuts, $1 at Home Depot

4. 10 #10 washers, $1 at Home Depot

5. About 12 feet of strong, low-stretch string. I used 200 lb dacron kite line.

6. A single-pole dimmer switch. About $10 at Home Depot.

7. A 25 volt, 2 amp transformer, $10.49 from Radio Shack.

8. An electric guitar string, about .10 - .16 size. I think around $1? You can get these individually at a music store or you can use either of the 2 smallest strings from a packaged set. You should keep a spare handy because they can burn out or break from too much tension.

9. A length of two conductor electrical wire with a regular plug on the end. I salvaged mine, but you could use another extention cord if you like.

10. A piece of wooden dowel or stiff plastic rod about one foot long (not shown). I used a bamboo skewer.

11. Optional: 4 regular-thickness CD cases. These are for the box that holds the transformer and dimmer switch, but you would be much better off with something like a "project box" from Radio Shack.

You might be able to do the whole project with just a knife, a drill and some tape, but it would be better to have the following:

1. Utility knife

2. Small wood saw

3. Drill with a bit slightly bigger than the #10 screws

4. Screwdriver

5. A couple of cable ties or twist ties

6. Electrical tape

7. Wide packaging tape

8. Nibbler

9. Multitester (you don't NEED one but it's a good safety check)

10. Wrench to match the nuts (I didn't have my SAE wrenches handy so I used a 9mm)

11. Solder and soldering iron, if you like

Cut one of the yardsticks in half. In each half, drill a hole in the middle and one about 1/2 inch from each end. One end of one half will already have a big hole in it, so you won't have to drill that end.

One the remaining, uncut yardstick, drill a hole about 6 inches from the pre-existing hole (see photo), and another about 1 inch from the other end (not shown in this photo, but visible in later steps).

Make a sort of a big H shape out of your pieces by bolting them loosely together with the machine screws. Don't tighten the nuts down yet, just get 'em on there kinda loose.

Now we're going to attach the wires that carry the 25V current to the cutting wire. We'll use two machine screws as terminal posts.

First, cut off each end of your 12 foot extension cord. Save the plug and outlets for future projects, if you like. Strip the insulation off the last inch of one end of one wire of the cord.

Insert a machine screw in the top of the right leg of your "big H" as shown in the photo. This is the leg that is on the other side of the H from the handle. Use one washer on the screw head side and two on the nut side, as shown in the second picture below. Put the nut on, but don't tighten it down at all. You need room between the washers so you can put your wire in there.

Bend the bare wire of the extension cord (the part you just strippeed) into a U shape, and hook it around the screw between the two washers. Now you can tighten down the nut. You can see what the final hook-up looks like close-up in the photos for step 7.

Starting the the end you just hooked up, pull the two conductors of the cord apart so that it's split for about 3 1/2 feet. Cut the unmounted side of the split to about one foot. Strip the end of the one foot section and mount it to the other leg of the H the same way you hooked up the first wire. You'll see what I'm talking about here if you look at the photo.

Use cable ties, twist ties or string to keep the wire close to the frame so it won't get in the way when you're using the tool.

Take a piece of string about 6 feet long and thread it through each hole on the other ends of the legs, as shown. Tie it into a loop so that when you pull the legs apart the string keeps the legs fairly parallel. So, the length of the loop when taut should be about the same as the distance between the bolts.

While you are hooking up your wire, try to avoid making any kinks it.

Your guitar string should have a sort of a bead (for lack of the proper name) on one end. Make a loop by feeding the other end through this bead. Hook the loop over one terminal and sinch it up. Keep the loop pretty close to the nut or there will be too much twisting force on the leg when the wire is tensioned. But also make sure it's not touching the wood.

To hook up the other end of the wire, pull the frame legs towards each other so your string tensioning loop on the other end is taut. Wrap the end of the wire around the other terminal screw and twist it off. See the second photo. It might help to use pliers to keep good tension, but be careful not to pull too hard and break the wire.

If the wire is not super tight at this point, don't worry. It can be kind of floppy when plucked but should be pretty much straight when at rest. We'll add more tension later.

Now we need to install the mechanism that keeps the the legs square to the wire. Without this step your contraption will easily wobble into a parallelogram.

Tie a loop in one end of a 3 foot piece of string and hook it under the washer of one of the middle screws. See the photos. Thread the other end of the string though the opposite leg's hole. Square up the frame and tie off the string. Repeat with another string for the other side, but when you tie off the string at the top of the leg this time, make sure you have some tension. Now both of the criss-cross trings should be pretty tight, and there should be some slight tension on the wire too. It's ok if the tensioning loop is a little floppy at this point. Now you can tighten down both of the middle screws with the wrench. No need to chrush anything, just make 'em tight enough to hold everything together.

Insert a ruler or dowel in the tesioning loop and twist it until it seems to be getting a little tight. Careful not to twist too much or you'll break the wire or the frame. Pluck the wire and listen for a musical tone. If it sounds like "fwubababa" it needs more tension. If it sort of hums it should be enough to start. You can always add more if it seems too floppy when you try to make your cuts. Once you are happy with the tension, slide the ruler or dowel down so that the yardstick keeps it from unwinding (see the photo).

You'll have to readjust the tension later, after the wire gets hot for the first time. Or maybe every time.

This photo is your wiring diagram. The black two conductor wire on the left goes to the wall plug, and the brown one on the right goes to the hot wire.

This photo is just to show what connects where. You should of course use the wire nuts that came with the dimmer switch (esp on the 120V connections) and/or tape to ensure that no bare wires touch each other, or you, or your pet. Be careful not to electrocute yourself or start a fire.

I really want to make a different box for this project, or buy a project box at Radio Shack. This box was made from 4 cd cases and some packaging tape. Two CD cases are openned to right angles, then pushed together to make 4 walls. The corners join up nicely. Run a strip of packaging tape down each corner. Now take the lid off another case and tape it to the bottom of the 4 walls. The bottom will touch two opposite walls and there will be gaps under the other walls. Don't tape over the gaps, they provide ventillation. Take the lid and plastic tray off a 4th cd case and throw them out. Nibble or cut a hole for the dimmer switch and tape it in. Then connect up all the wiring according to the diagram in the previous step, tape in the transformer, and tape the top on. In the photo, the box has been turned on it's side.

If you drop the box, the transformer will shatter it. Have I mentioned that you should use a different design for your enclosure?

I think this kind of box might have some other uses. Maybe you could make a version without a top and bottom, put some nice photos in it, and set a small plant pot inside with the plant sticking out?

Set the rig up somewhere where it won't catch the house on fire or melt the carpet if something is wired wrong, or if the wire overheats and breaks. Take a moment to look over your creation and make sure all the wiring seems to make sense.

Turn the dimmer all the way down (counterclockwise).

With your fire extinguisher handy and your body away from the device, plug it in. Are the lights still on? Is the hotwire still whole? Sweet.

You can use your multimeter to see if there's any current between the terminals. There shouldn't be yet. SLOWLY (like 5 degrees per second) turn the dimmer up (clockwise) til the wire starts to quietly hum. With the wire I used that's about 1/4 or 1/3 of the full rotation. If the wire doesn't hum or heat up by the time the dimmer is halfway up, turn the dimmer all the way down, push in til it clicks, and start again.

If you turn up the dimmer too fast, your wire may burn out before you realize that it's even hot.

Once you are sure everything's all set, grab the frame and try laying the wire on some scrap styrofoam. It should slice smoothly into the foam. You shouldn't have to push very hard. Try playing with the dimmer switch setting to get the best cut. I've read that cutting slower and cooler makes a smoother cut.

The wire heats up and cools down within a second or two.

Next time you use the foam cutter, make sure nothing meltable or flammable is touching the wire when you plug it in, just in case the dimmer isn't off.

Here are some freehand and template-cut shapes to give you an idea of what the tool does. These are really basic one-cut shapes. You can of course bevel edges, etc, with a second pass.

Some people set up their cutters in sort of a bandsaw configuration, so that small pieces can be manipulated against a table for precise cutting. Those are generally not useful for cutting wings or slicing chunks off larger pieces. See this page for an example:

http://www.hhhh.org/~joeboy/resources/hotwire_foam_cutter/hotwire_foam_cutter.html

The enclosure for the transformer and dimmer needs to be more rugged, and it should incorporate a fuse, maybe a power indicator, and maybe a modular connector to facilitate attachment of larger or smaller cutters.

Instructables user Moofie suggested rigidifying the connection to one of the legs, perhaps simply by using a second bolt where the yardsticks overlap, to eliminate the need for the criss-cross strings.

I collected much of the raw info for this project from these two pages :

http://members.fortunecity.co.uk/slmohr/rcinterest2.htm

http://www.vatsaas.org/rtv/construction/hotwirecutter.aspx

Both pages have more good info. Check em out.