Cutting foam is difficult with traditional methods. The best way is with a hot wire. If you read Hackaday, it is a good bet you can figure out how to use electricity to make a wire hot without any help. However, there’s something  clever about [MrGear’s] minimal build.

As you can see in the video below, he uses a 9V battery, a clip, some popsicle sticks, and the wire from a ballpoint pen. He also used a switch, but we couldn’t help but think that was unnecessary  since you could just unclip the battery to turn the device on and off. Since he used hot glue to attach the switch to the battery, replacing the battery would be a pain.

This is a quick tool for short-term use: we imagine shorting out a 9V battery will require you to replace the battery fairly often. Of course, you could probably use a lantern battery if you wanted something that would last longer. Just don’t use a lithium ion battery since shorting them is very dangerous.

If you want something more involved, have a look at [Grant Thompson’s] build (see second video, below) that is a lot more substantial. He uses a strand of steel hanging wire for the cutting element and a spring to keep it tense. He also has a good trick for getting the waves out of the wire, that might come in handy even if you build the [MrGear] version. Looking for something between those two extremes? We’ve covered several foam cutters in the past.

A few years ago I was working with a foam cutting tool someone else had cobbled together which looked like a pallet knife who’s handle was removed and then shoved into the end of a cheap soldering iron. Probably overkill, but worked great, and you didn’t have to cut away from the edge like with this design.

I have a old toaster with mica sheets and NC ribbon wire in it, I wonder if I could shape such a knife out of those?

You can break apart low value 5 Watt ceramic resistors for a length of nicrome (resistance) wire.

Bonus information, most vape shops sell lenghts of kanthal and nichrome wire of various thicknesses for relativelty cheap, theyr’e a good source for the stuff because there’s one basically everywhere and they ship fast in my limited experience (order one day, receive the next one).

True for now, but in the USA regulations kick in on Aug 8th and DIY ecig supplies will not be allowed, as I understand. Not sure of the specifics but ecig stores probably won’t carry kanthal or nichrome wire after that.

You can also buy some on Amazon for cheap. They sell for around $5-10 for around 50 feet of wire.

Nothing wrong with using lithium batteries; you just need to be sure to respect their current limits.

The 9V option depends on the (high) internal resistance of the battery for a current limit; you could build a great wire-cutter with lithium as long as it has some deliberate current limiting because the internal resistance of the battery is so low.

In my understanding the resistance of the heater wire should be the current limit. We don’t want to waste power on an external resistance. Of course the resistance has to be matched to the power source/voltage.

Of course, that’s why the article just told you not to short lithium batteries.

Nichrome guitar “E” strings work well.

I would expect nichrome guitar strings to be a pain in the … because they would expand with temperature increases and constantly require re-tuning????

If you watch the second video, the King of Random guy describes and shows how to compensate for that by integrating a spring.

I think he means for use as a guitar string since it would lose tuning with changes in temperature. I don’t play…My guess is that the wood changes with temperature more than the string for ambient temps so the concern is mute.

The King of Random uses stainless steel hanger wire, not nichrome (3:14 in the video).

I can’t find any reference to guitar strings or piano strings being nichrome. They seem to be most often some variation of steel.

It depends, my standard strings rust, but I know you can also get coated and nickel strings, but some people are allergic to nickel, there may even be strings that have silver in them. The bottom line is that if you just grab some old string that is in a junk box it is probably steel as you have pointed out, but you can’t be sure what it is, so just buy the real thing off ebay as it is dirt cheap and comes in a range of gauges and lengths to suit different projects.

hair dryers/heat guns and toasters are also good sources of resistance wire

yes, but be careful, at least sometimes the wire is sharp like a knife!

Ok, first this guy makes something that looks like a prison lighter and then he makes a gun out of foam, are you sure this guy hasn’t done time?

Necessity is the mother of invention. Although not sure what use a foam gun would be.

John Dillinger would know what to do with a foam gun.

http://www.johnniedillinger.freeservers.com/WOODENGUN.html

I don’t know a “prison lighter” but at least you can not endanger anybody else with a gun out of foam.

All those resistance wires are either for 110 and 220. Here is one that’s universal, 12volt cigarette lighters. They’re in every car and truck but most people don’t smoke but do plug in accessory stuff that runs on 12v. Look in glove box. Uncoil the length and calculate how much less than 12 (13.8) is a single lithium. Make longer to keep hot but not glowing. Euro style terminals without the plastic are good for a connection to the hot metal. Take the metal piece out with screws out, then reinsert the screws. Hacked e-cigs are ready to go for this.

Can you build the frame to allow the wire to be longer than needed and have one power lead movable so you can treat the wire itself as a variable resistor to adjust the cutting temperature without any other component such as a dimmer?

Car cigarette lighters don’t use wire, in my experience. It’s a coiled strip of metal. A car battery can provide large amounts of current, so it’s presumably, for 12V, pretty low resistance.

A resistance wire isn’t really for any particular voltage. It depends on how long a wire you use.

Two dollars on ebay for some nichrome wire. I didn’t even bother scavenging at that price. I built a larger hotwire cutter using the temperature dimmer from my weller soldering iron. Worked well enough to cut some wing airfoils.

That is a neat idea, if you have one spare, but I wonder if a mod of this design would do the same job?

http://www.zl2pd.com/img/SolderingStation/ATtinyBasicCct.gif

from: http://www.zl2pd.com/SolderingStation.html

Removing the switch would be easy. Just apply some rubbing alcohol to the hot glue. It will then peel off really easy.

Of course you have the battery unplugged before pouring flammable alcohol on it.

According to my experience this is not even necessary. Hot glue on metal has not that good adhesion, so it peels off also without additional solvents.

If your pumping enough power into a wire to heat it, then your also creating a decent amount of resistance (thus why resistance wire is used for hot knifes), as such should the net resistance of the heating element not act as a resistive load in the circuit making it not a short? Granted I would still stick a small cap in the circuit anyways, but…

Correct, adding heat increases resistance. Last time I did the numbers, nichrome’s resistance at at temperature doesn’t necessarily increase a lot though that depends on the temperature difference. Hot enough to melt or burn polystyrene isn’t too terrible much in the grand scheme of nichrome’s usable range.

A straight piece of resistance wire will mostly look like resistance, not like it will have meaningful induction. I don’t understand what a capacitor would do for such a circuit. I suspect it would mainly serve to add to the parts count.

I found most of what I needed to know here,

http://cecs.wright.edu/balloon/images/2/22/Nichrome_Wire_Heating_Element_Design_Basics.pdf

A rough rule of thumb with the wire is 1 Watt per inch. Dissipate much more than that and the wire softens and breaks.

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