Build A Tiny Hot Wire Foam Cutter

Let’s face it: cutting foam with a knife, even a serrated plastic knife meant for the job, is a messy pain in the ass.  This is as true for insulation board as it is for the ubiquitous expanded polystyrene kind of foam used for everything from coffee cups to packaging material.

Those stick-type hot wire cutters from the craft store that plug into the wall aren’t much better than a knife. The actual cleaving of foam is easier, but dragging a long, hot flexible wand through rigid foam just right, without making burn marks, is pretty frustrating. It’s not like you can hold the other end to keep it steady. A foam cutter built like a coping saw but held parallel to the wire would offer much better control.

[Techgenie]’s handheld hot wire foam cutter is a simple build based on a single 18650 and a piece of nichrome wire. While this is probably not the most Earth-shattering hack you’ll see today, it’s a useful tool that can be made in minutes with items on hand. Laptop chargers are full of 18650s, and nichrome wire can be sourced from old toasters, hair dryers, or space heaters.

You shouldn’t use just any old wire for this, though, or the battery will get hot and potentially explode. Nichrome wire has a high resistance, and that’s exactly what you want in a tool that essentially shorts a battery to make heat. [Techgenie] used a momentary button instead of a switch, which is a good way to stay safe while using it. It wouldn’t hurt to add some protection circuitry and take the battery out when you’re done. Burn past the break to watch him build it and cut a few tight turns with ease.

If you have bigger, more complicated foam-cutting jobs in mind, why not build a CNC version out of e-waste?

34 thoughts on “Build A Tiny Hot Wire Foam Cutter

  1. You have to be careful what diameter wire you pick so you don’t blow out the battery. Nichrome wire has a resistance of about 1 – 2 ohms per foot for a 0.6 – 0.4mm wire, so that piece there may easily draw 10 amps out of the battery and not all cells are designed for that.

    You also have to watch out for undervolting the cell to not destroy it, because there’s no safety circuits here. Also the tiny switch may weld short because it’s not designed to switch that sort of current.

        1. Or you just get a protected 18650. They’re hardly more expensive and you don’t get to accidentally discharge them enough to kill them, the integrated circuit cuts off before you do.The downside is the capacity is slightly less but that’s a small price to pay for ease of mind on a quick hack like this.

  2. Yeah, way super simple design. This is the best portable design I’ve seen for sure.

    Never thought about just using an 18650 battery. I have been thinking about making an RC aircraft of some sort using foam instead of balsa wood like back when I last left off on hobby craft work. I’m even thinking about making a larger water craft tri-maran jet boat. Interesting seeing what the kiddos are producing online as I used to get pyro with my action small scale models “play.” Foam will be more gross to combust though.

    Interesting how the scale of the models changes over time; for me at least… from innate vision, inanimate prototype, to functional small scale system, to actually implemented in production full scale and now back to hobby like R&D with more cost effective, albeit cheaper quality “tools” that for prototyping are just great. Not only hacking other devices components for parts as this can be made from an old laptop battery pack, toaster oven wire/guitar string/piano wire and other salvaged components or scavenged like tree limbs carved.

    Reminds me of a portable version of this that I think I am going to move forward on contruction:

  3. Well now try it with some hard 18650. It can delivery pretty lot of the current.
    Also the micro switch will melt quickly in my opinion (on the other hand it has second function – fuse :D).
    And these holders are also very bad if you want higher current.

    So design and simplicity is very nice, but I wouldn’t recommend it for people which doesn’t know anything about how much current to draw etc., mainly when they use some sh** batteries like UltraFire BRC, which can even blow up!

    1. Right, high discharge rate battery and something more like the INR design or at least the fault/overload protection if you don’t notice your hand warming up from the button or battery I’m thinking is ideal. Thinking this article goes well with the “How to Make a Shunt Current Sense Resistor:” Maybe someone will make a meter on their cutter. These can work with plastic also… fyi… stinks though… however, if you’ve tried plastic welding… well?

    2. I wonder if this is legally tricky for instructables.
      With all the items itemized with purchase links it’s too specific to say any unhappy result would the fault of the choices of the reader like it would be if you used more general terms like ‘a suitable momentary switch’ and such generalized instructions.

  4. I just had the thought of an electrocautery pen application also. Reminds me of the article ( ) post where I had the same thought from a dermatological perspective as a medical device… though from this articles perspective if you think about using like the battery powered portable designed systems where you can search online for the different style tips:

  5. I have a couple of larger (40mm and 0.5m -ish) ones like this, and found that a great place to source the wire from is those smoke shops that carry vaping supplies. I don’t care for vaping and all that, but it was surprisingly hard to find the appropriate wire nearby, but lo-and-behold, they had a spool for dirt cheap at a smoke shop a 5-min walk away. Mine are also mains-powered by a couple of old laptop power supplies from the local second-hand thrift store.

    These are super cheap and unbelievably useful, I used mine to make a 2.5m tall octopus sculpture. Just make sure you wear a respirator with the proper rating when cutting. I use 3M Respirator Cartridges, Manufacturer Part Number 60926. You should also wear glasses, because the tension of the wire can sometimes flick molten poly when it emerges from the part, and that shit stings!

    1. Some laptop chargers do in fact contain 18650s… like laptops without user-replaceable battery packs (of the 18650 cell construction or the poly-pack constructions).

      Laptop power supplies…. they certainly don’t contain 18650 cells ;)

  6. Hmm, back in the day, I built several hot wire cutters. Found out that the optimum level was around 1 W/inch of wire. Go much above that and you wind up burning the foam and breaking the wire. Go much below that and it does not cut very well at all. Spring loading the wire also helps with keeping the wire taut, as it heats up it expands. Again, not a good thing if you want to have a good cut. As an experiment, I once used electric fence wire and a car battery. Again, 1W/inch, which meant maybe 20-30 ft of wire lying on the floor, getting very hot.

    A hot wire cutter design got me a job working at a small company making educational toys. Neat stuff. Made the owner wealthy enough to retire (isn’t that the objective though?).

  7. Clever design! I like it. A friend of mine built a hot wire CNC cutter to make model airplane parts with and he used a piece of MIG wire, probably .023 non flux cored, with a big spring to maintain tension and an old 12V car battery charger. That worked well for him.

  8. Daiso (A Japanese 100yen store that runs a sub-chain of $1.50 stores in the US) sells a foam cutter that operates by rapidly draining a C-cell. I’ve used it for carving port holes in makeshift plastic enclosures.

    1. Love Daiso – can be a surprisingly good source of maker supplies.

      Never seen that item in the Daiso’s is my area – they seem to rotate items pretty quickly, and have wildly differing inventories at their different locations.

    1. Metals normally have a positive temperature coefficient. So you want a constant voltage source (more heat -> more resistance -> less power) a constant current source would increase the power with temperature. This is not what you want.

  9. This battery holders are a bad idea in high current applications. I used them already for a charger and at 2A the springs slowly melt out of the plastic. Similar that small push buttons, they are normally only rated for 50 or 100mA.

    I started a similar project with a 10A buck converter and did some tests in a table saw like fashion with a vertical cutting wire, but then decided to just use the jigsaw to cut the insulation boards. Although it cuts only about 6cm of 8cm, the rest was easy to break. While I would have finished the cutter I already installed several square meters of insulation board.

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