This is as simple as it gets. We take an broken E-string (the small one) off an electric guitar and make a hot wire foam cutter out of it. 

The details are flexible. A hot wire cutter is simple. Take a wire and pump some electrons through it so that it heats up and bingo, it'll cut foam.

What we need:
A wire (the guitar string)
A frame (to hold the wire and put tension on it)
Moving electrons (the power supply)

Step 1: The Wire & Electrons

Before you build the frame, you should test your wire. It doesn't need to be taunt to test its heating capability. In this case, I hooked the wire up and attached to it a power supply. 

Lets talk about the power source:
Ideally you might have a variable DC power supply. In my case pumping 11.1 Volts @ 1.7 Amps did the trick. Varying the voltage will change the current. (V=I*R) The power (P=I*V) dictates the temperature of the wire. The more current, the more electrons are moving and smashing into the wire (resistance) and thereby heat it up.

If you don't have a power supply you should get one, they are very useful. But really, if you don't have one you can get buy with other power sources. Here are a few ideas:

1. A wall wart.
 These are cheap and prevalent, but it will be hard to find one that can put out a couple amps. That is the problem with any power supply you try to use. The power supply that I have can put out 5A @ 18V which is pretty good. Usually, wall warts can do 0.5A which probably isn't going to cut it.
2. A hacked computer power supply
This is a simple hack. PC power supplies are all over the place. You can make a nice 5V/12V supply for next to nothing. Just search for instructables related to that.

Fixed vs. Variable.
Because we are using some random wire the voltage needed to drive the right ammount of current through the wire will vary. This is because the resistance of the wires will vary. So, ideally, we want a variable power source. This will also allow us to adjust the voltage and therefore the temperature of the wire if we'd like.

If your power source is fixed, that is no problem. We have another option. We can still adjust the current draw of the wire by changing the resistance of the system.

<p>I knew I would have a use for all my old strings!</p>
Hi! <br>I have a mains to 12V Electronic Transformer(used for 12V halogen lights) It can be used for 50-160W, but since it is electronic, it is not DC, but AC at about 20K Hz. Most Transformers that are electronic(not Iron core) are AC with High frequency. <br>It has a variator /slide dimmer on it. <br> <br>Do you think this could be used?
I assume you are talking about SMPS (Switch Mode Power Supply), like laptop adapters. The output IS DC, except that it is pulsed hence the high frequency. <br> <br>Since your output is 12V and minimum power is 50W, the minimum output current is I=P/V = 50/12 = 4.16A which should be more than enough (too much in my opinion, but usable provided you use a properly sized wire). <br> <br> Hope this helps :)
Nice instructable. I was going to suggest that it's not too hard to get a small resistance like 0.53 Ohms. You can do so with a number of resistors in parallel. For instance two 1 Ohm resistors in parallel get you pretty close. This is also a good way to accomodate more current; each resistor might not be able to handle 1.7 amps, but they might be able to handle half of that. You can also throw in potentiometers in the mix for variability,
hey, a friend told me that a 12 volt battery will work for a hotwire cutter? I have a 12 volt 7ah lead acid battery, and if possible I would rather use that so I don't have to get a power supply
Yes, it will probably work. You might have to add a small high wattage resistor in line since you can't control the voltage. For example, I was running at 11.1V and that heated nicely. At 12V or more (batteries vary in voltage) the current would be higher, possible frying the wire. So, you'll have to keep that in mind.
Hey, I just want to add to this math by simplifying it with a <a href="https://www.instructables.com/id/Hot-Wire-Foam-Cutter-Calculator/" rel="nofollow">hot wire foam calculator</a>. The idea is the same - power source + guitar string = hot wire foam cutter. Thanks for your instructable, this is how I made mine!
I dont have much electric knowledge ,but i have to make home made electric foam cutter for my kid ( he has to make some school project ) by cutting / slicing foam . i have pen type soldering iron / room heater etc. shall i made foam cutter from it , can i make room heater coil directly connect to steel wire to make it hot.<br>please guide me ,it is very urgent .
I would not recommend working with electricity if you do not know what you are doing. This hot wire cutter works by passing low voltage DC through a resistive wire, thereby heating up. <br><br>I cannot speak to what you are planning to do. Connecting a steel wire to mains electricity is a bad idea.
one suggestion maybe you could use an old saw frame maybe a jewelry saw frame since those hold thing pretty tight.
Very good work. I have two suggest:<br><br>1) when the wire is heated, it expands a lot. The device would improve by adding a spring at one end, and a small pulley for the wire to slide on it. Although threaded rods have some flexibility, spring is the best solution. In your design, it could be also in the rear tensor. <br><br>2) As Phil B says, a rheostat enhances the utility of the device. I was successful in making one with the simple appeal to attach a piece of electric cooker/heater resistance over a piece of floor pottery, using silicone sealant. I made the slider with a snap clothespin, one of whose halves have a contact in bronze. IMPORTANT: silica should be dried thoroughly before circulating current. It should be left to dry for at least twelve hours.
Thats a great thing, I have wanted to make a foam cutter before for various projects. I am into R/C planes too and this will come in handy, one thing you can do to help get straight cuts is to make a pivot point where you can mount it to the table and let gravity pull the heated wire at just the right speed and this will also give you a straighter cut by taking the human factor out of it. Just like a bandsaw going through a piece of metal stock.<br><br>Nice work!
<br> I appreciate your current calculations using Ohm's Law.&nbsp; The guitar string for a cutter wire is a great idea.&nbsp; I improvised a hot wire cutter in step 2 of this <a href="https://www.instructables.com/id/Make-your-own-foam-grips-for-drop-handlebars/">Instructable</a>.&nbsp; Although I attempted to use Ohm's Law, in the end I resorted to trial and error using some stainless steel wire and a homemade 12 volt battery charger presented in <a href="https://www.instructables.com/id/Auto-battery-charger-for-6-or-12-volt-sytems/">another Instructable</a>.&nbsp; As you can see from the first Instructable I linked, I added length to the cutter wire and wrapped it around a piece of wood to make a crude wire wound resistor so I could get the right heat to the portion of the wire I was using as a cutter.&nbsp; Thank you for your Instructable.<br>

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