Hot Wire Foam Cutter


Intro: 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! :)

STEP 1: Parts

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.

STEP 2: Tools and Supplies

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

STEP 3: Making the Frame Pieces

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).

STEP 4: Bolting the Frame Together

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.

STEP 5: Attaching the Lead Wires to the Frame

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.

STEP 6: Making the Tensioning Loop

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.

STEP 7: Attaching the Hot Wire

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.

STEP 8: Cross Strings

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.

STEP 9: Applying Tension

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.

STEP 10: Wiring Up the Transformer and Dimmer Switch

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.

STEP 11: My Crappy Project Box

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?

STEP 12: Turning It On

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.

STEP 13: Quick and Dirty Sample Cuts

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.

STEP 14: Alternatives and Expansion Ideas

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:

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.

STEP 15: Acknowledgements

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

Both pages have more good info. Check em out.


I don't understand electricity. Why don't the wires that deliver the current to the guitar string heat up and melt their insulation?
Didn't need to make precise cuts. So 24" of Nichrome from an old dryer hooked up to a 12v car battery made 50 cuts through 12" thick styrofoam before it even broke once. Still have a few yards to spare. Don't bother without the Nichrome. Steel wire broke broke right away.

Another site suggested using a PC power supply instead of a dimmer switch, etc. I went that route and connected one of the 12V outputs of the power supply to a length of nichrome wire salvaged from an electric heater. It worked quite well. The wire did not turn red, but was quite hot enough to make a nice cut through 6 inch foam blocks.

Now I like this idea. The uses for computer power supplies are nearly endless.

Here's my experience:

The idea of a dimmer and transformer didn't make sense to me and didn't work for a lot of people, so I went with a car battery charger with a 2A current limit setting. This worked, but the guitar string broke after about 30 seconds. I tried 3 different strings of different gauge and composition with the same result. I finally got some 22 gauge nichrome wire. At 15 inches it is 1.1 ohms cold. At 2A this would be around 4.4 watts (W=I^2*R), which doesn't seem like enough, but actually it gets plenty hot and works fine. I vaguely remember that light bulb resistance goes up by around a factor of 10 when it heats up. Maybe that's what's going on - a 44 watt light bulb would be plenty hot. Probably a much lighter gauge would work fine as well, but may not be as strong. The wire does get loose with heat. Instead of the ingenious tensioning method described, I used a guitar tuner, which I can tighten when the wire heats up and loosen after using. This thing cuts through styrofoam like butter.

I'm thinking about making a contraption that will run between the chuck and the base hole of my drill press (properly insulated!) so that I can get more accurate right angle cuts by guiding the styrofoam through it.

I deleted a comment or reply I made about the resistance measurement I took on the ''E'' of my guitar. I needed the information you provided before making a proper comment. Now, you've made it make sense. I thank you, Sir.
I read this article in order to learn what kind of power source I needed to make a hot wire foam cutter. Im not sure how long the 25v transformer will last but it is cheap enough to give a try. To make a tensioner for the hot wire I would buy a machine head from the guitar shop/ music store. designed exactly for tensioning guitar strings it makes tightening the hot wire quick and easy. Instead of looping the wire around the bolt simply drill a hole thru the bolt and feed the wire through it. I have not read the other comments yet but I know the guitar string will limit the length of wing blank you can cut. I also believe that there is a NACA database you can use to find airfoil shapes from which you can create masonite templates to use as guides for your hot wire for smooth accurate wings. Now to check the closet for my used guitar strings.

I want to cut a lead block by using the hot wire process. The melting point of lead is 330 degrees celcius.
Can this be attained by using this process.

No. Lead conducts electricity so will short out the wire as soon as it touches. This will mean the wire quickly cools and won’t get further than leaving a line in the surface. Even if it would work you’d struggle to get sufficient power to melt such a dense material so quickly.

No. Even if you could get the wire that would stay strong enough at the appropriate temperature, the lead would act as a big heat sink and wick heat away fromwhere you want your cut. Even if you could get the wire to cut, the lead would simply fuse together again once the wire had moved on. Foam cutting works because the foam shrinks where the wire cuts it (the bubbles in the foam collapse), so even though the melted foam re-hardens, a gap is left where the wire passed through.

Because this post helped me, I'm adding photos of my setup.
I didn't use a dimmer, rather a transformer that has 8 volt settings. 1 settings being 0v and the rest starting from 9v working up to 30v.

15v worked best with a 70cm gap or length of nichrome wire.

I know this is some old commentary, but I wanted to correct some mis-information. The old-fashioned heavy type of transformer is the kind you can use with a dimmer to control the output. Buy the High-frequency ,switching type of transformer is NOT compatible with messing with the input voltage.

I have used professional ones and they have a pre measured nichrome wire with loops on the ends. They attach over pins on the arm and base. The arm is a semi rigid spring steel 90 degree bend that bends down to accept the wire and rebounds to apply tension. This makes it quick to disconnect the wire and run it through a closed hole in the workpiece to make interior cuts. I cut about 60 "Coors Light" cursive letter signs for a contract with this rig and never burned a wire. 600F is optimal cutting temperature. They also rectified the AC to DC to make it a lot safer for the end user as I was touching the wire more than once.

Made one, but burned out my transformer? For some tips on the dimmer circuit, I made mine but had no variable out of the dimmer. I looked at schematic again and realized even with AC, you need the BLACK wire out of the black and white AC plug cord wires to go to the black wire of dimmer (I used a 15A fuse in line from a car stereo) and then red out to the transformer Primary INPUT, (green to transformer case) then the white AC side goes into the other Primary side input. if you do it backwards, you wont have any adjustment. Also, measure your resistance of the cutting wire. I used stainless wire from a 1/16 cable strand bought for .17/foot at Home depot. You get like 16 strands in there of some pretty small gauge wire. single 40AWG strand broke under stress so we upped it to 3 strands. The resistance of the 1 strand was 2.2 Ohms. and we were using the 24V output of a multi tap transformer out of a cassette deck. Variable 1-24V worked great but it died after about 5 minutes on 12V. The transformer was cooking when I felt it! I should have used the 11V (it had 11, 24 and 44V) tap and varied that or just use the proper transformer. Even a 6V lantern battery straight wired to cutter generated enough heat to slice the foam.

Also for the budget oriented people, you can salvage like 10m worth of Nichrome wire out of a broken hair dryer (just take it apart, its the silvery wire around the edge of the airflow tube, just uncrinkle in with a bolt thread, there is usually 1-3 different gauges of it inside and its designed to take the heat. also there is usually lots of mica heat board in there too , good to act as insulated guides for the wire as well

I made one, with kanthal heating wire I had for vaping and a 10v DC power pack, but I'm having an issue. The foam melts/shrinks too far making it impossible to cut to accurate dimensions. I increased the length of the wire so that it is no longer glowing red, but I have the same issue.

Your wire is still too hot, look up nichrome transformer calculator online to figure out the right amperage for the length, like for a 12 inch wire of 30ish gauge wire you shouldn't be putting more than 1.6 amps through it. For your 10V DC power whats the Amps (it should be 16 watts, if its more the wire is too hot)

Hi! I just saw this video tutorial about how to make a simple and handy foam cutter, and they only use 2x AA 1,5V batteries. It seems a lot cheaper and easier, why? What is the difference if we are using a transformer or not? Is it due to the type of string they use (nichrome wire)? Thanks!

Constantan... in my opinion bad idea, you want normal wire that has more resistance as it gets hotter, that way it helps keep temp more consistent.

Power source... old 12 volt lead acid battery with battery charger is often already available.

Ohm meter is your friend, measure resistance of wire, you take the voltage, divide by the resistance and you have the amps of power that will flow through wire. Make sure your power source/transformer (and fuse) is rated for higher than that to avoid wrecking transformers.

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