Hot-Wire Foam Cutting Nunchucks

If you've ever tried cutting foam with knives, it can be a frustrating ordeal. With a material that absorbs a lot of shock, you can't apply sufficient pressure to cut through it. The solution? BURN through it, with your very own hot-wire foam cutter/ nunchucks!

With all the parts parasitized from old, and unused everyday items, you can build this foam cutting contraption for less than five dollars! We did it for $3.50, INCREDIBLE! So let us begin our amazing journey to the world of ninja foam cutting....


WARNING
This project uses high current and hot wires, so don't burn/ electrocute yourself. Also, beware of toxic fumes, and work in a well ventilated area.

Step 1 Materials Needed...

MATERIALS

1. (1) Old hairdryer
2. (1) AC power adaptor, at least 1 amp
3. (2) Pieces of 1/2" PVC, about 8" long
4. Electrical tape
5. Silver bearing solder

TOOLS

1. Soldering iron
2. Means of cutting PVC pipe
3. Ninja
4. Pliers

Comments

[nzlemming]

You can rough-cut foam and polystyrene with a serrated edge (bread-knives are idea, and then finish them off with heated wire.

[Broom]

Why bother cutting twice? If you have the hot wire, why not just cut with it?

[nzlemming]

"Can" not "must". I do it this way because I can quickly get a shape I want, and then add detail (or subtract, really). I use poly foam for sculpture. I find that rough-cutting with hotwire usually means I take too much off.

Plus, may be just me, I feel more in control with a knife in one hand. Used properly, you can get very smooth surfaces. What I'd *really* like is a heated breadknife :-)

You do it what ever way works for you. I'm just offering something that works for me.

[teaton]

What you really want is an electric carving knife/turkey knife... I use them when making stage weapons from polyurethene foam... They work a treat and they don't cost too much... You will eventually have to replace/sharpen the blades though. I didn't believe it when I heard it, but I couldn't believe my eyes and now i'd be lost without mine :)

[arhillbilly]

You might try a hot capping knife like they use for decapping honey comb. Look for apiary supply then hot knife.

[Broom]

Thanks. I haven't worked with a hot wire, so I assumed you'd have more control there.

[ac-dc]

Because it's usually more precise to trim off the bulk of the *material* in one pass then come back a second time and make the final precise cut.

As for "bother", because you "bother" to do the project at all.

[kelseymh]

Very nice! A few, very minor, "safety" suggestions (mostly for other users who might build this).

• Run the wire up through the PVC, rather than along the sides.
• You could use pipe caps to close off the ends with the wire threaded through.
• Finger shields: Little disks at the end of the PVC (think sword hilts) to keep your fingers from slipping onto the hot wire. Something like a cap for a lamp.

[ac-dc]

No. It is no more safe inside, this is not a dangerous current. It is safer to have it outside so you can see if it is getting hot enough to melt the PVC, and that it cools at a more rapid rate outside. However, the whole idea of using PVC is pretty terrible. Far better would be to use regular copper pipe, this way it's quite heat resistant and it slightly heatsinks the ends of the nichrome wire which is desirable, preserves solder joints too. However, it should not use solder joints at all actually. The proper method of construction with nichrome wire is a mechanical one where it's crimped, screwed, or riveted onto the holder. Finger shields seem pretty pointless. If your hand slips so easily it is advised to wear thick heat resistant (like leather) work gloves.

[chamunks]

I would probably highly recommend against using any form of conductive pipe at all. The idea of something a bit nicer than pvc is probably a bit un-needed at 12vdc. I mean I guess if you maybe coated the pipe so it would no longer be conductive that it might work but just be careful.

[chamunks]

During my quick review I would highly suggest against the copper pipe rout if your using 1 amp dc adaptors or more you only need about .85 amps or so to stop your heart under certain conditions.

[ac-dc]

This is incorrect. Far far less than 0.85 amps can be deadly BUT it is not current that kills (per se), it is voltage. Even a few dozen mA is deadly but won't flow without the voltage to push it. You are plenty safe holding some copper pipes, BUT you don't necessarily need to hold onto the bare copper, might as well wrap some tape around them or wearing gloves is even better so your hands have the most reasonable protection possible. The far more reasonable risk to be concerned about is accidentally burning yourself when nichrome wire hot enough to melt foam comes in contact with a material like PVC that melts, and/or gets brittle after exposure to this much heat. At least if it melts you have a more gradual warning but if it gets brittle then the next time tension is placed on it, it could snap apart and the heated wire flies towards whichever arm is holding the end that didn't break. There are several ways the nichrome wire could be well enough insulated, and/or a holder resistant to heat like metal pipe could be used, OR you could do both, use a heat resistant pipe AND both thermal and electrical insulation from that pipe. We need to keep something else in mind though, others have compared licking a battery but not what is happening when you have a nichrome element between two copper pipes. In such a situation the nichrome wire is a load, it is substantially reducing the voltage below 12V. You could probably jump in a pool and be dripping wet, lick one pole while putting the other under your armpit, and never feel a thing with the nichrome wire pulling the voltage down to almost nothing, it would probably read under 3V.

[pbwingman11]

Then how do you explain 100KV stun guns? Or electric fences? Amperage is the "push" in the equation, not voltage.

[ac-dc]

Basic science. Voltage is a difference in potential, that potential is what determines how much current will flow until you reach the limit of the power source. A stun gun is an example of what I wrote, kilovolts of electricity just to make a few milliamps of current flow. Amperage is not the push, it would be like you pushing a brick across the floor and YOU are the determing factor of how far the brick is pushed up to the maximum force you can exert on it. Current is then akin to the weight of the brick, the quantity being moved. Therefore, we don't have to be concerned about the current. If you touch two electrodes with 2 volts and 20 million amps from a power source it will not hurt you. If you touch two electrodes with 2000 voltags and 200 milliamps, you better hope you let go while your body uncontrollably shakes.

[Hugo Boom]

Tell me if I'm wrong here but in your analogy wouldn't weight and quantity of bricks being moved be better compared to resistance in a wire? and so the current would be represented by the speed at which the brick would move?

[rennoc]

wow-  your smart, i don't know a thing about electricity, should i stick with 1 amp for this particular experiment?

[ac-dc]

You don't need to limit current, only voltage to the extent that you use high enough voltage to force the needed amount of current through the wire element.

For example (and I am making up numbers, not measuring a real cutting wire), suppose when the wire is hot enough to cut foam it has 5 ohms resistance, you would be (if using parts on hand) stringing the wire and applying increasing voltage until the wire becomes hot enough to cut the foam.  What voltage that needs to be depends on the specific wire you use and how long it is.

Suppose you need 30V to get it hot enough, and it is 5 ohms resistance, then 30V/5Ohm = 6A using a basic ohm's law equation, meaning your PSU would need to be capable of at least 6A, but it could be capable of more than that current and be even better because then it isn't running at full load which could be stressful to the PSU.

[chamunks]

gg

[radioape]

Probably should mention that I built this at my friends house in under an hour, using largely what we had lying around. I would have used steel screw eyes, and PVC end caps to secure the wire, but we didn't have those on hand. As for the copper pipe, I think that would conduct too much heat away from the wire, and it wouldn't cut very well. Feel free to improvise off of the design, this project was very unrefined.

[astrong0]

Wow... is there anything you don't know? jk.

[rennoc]

im not           lol

[ac-dc]

If there is significant heat that would be conducted to the copper pipe, that is probably too much heat for the PVC (gets very bittle subjected to heat, if it doesn't outright melt), and for the solder joints. I don't know the resistance, how hot the particular nichrome wire you use will get with a mere 12V/1A supply, but generally speaking since the entire length of the wire would get hot, 'sinking away heat at the ends could be a positive thing since a thin wire by itself doesn't conduct very fast and you would be cutting somewhere a distance from that spot. The copper pipe seems ideal also because you're not trying to solder wires to the nichrome, you can tap a threaded hole into the copper and solder the power lead to the copper further down on it (use a blowtorch to heat the copper pipe in a vise with some flux and solder on it and the wire will solder on from that residual heat stored in the copper pipe). Copper cuts fine. Use a hacksaw or regular pipe cutter, then a piece of sandpaper or a file. Pipe finishes quite well with fine grit sandpaper or even metal polish if you want it mirror-like though of course being copper it will eventually dull in color from exposure to air, but actually it finishes better than PVC due to being able to bend, sand or polish it. We are doing what we always do though, suggesting things that go against your goal which was getting the job done with parts on hand. Given only the parts you had it is proof enough that it worked... I just don't know how many times PVC would hold up to being heated by the wire and when suggesting the copper pipe it was just one alternative, some people might have spare copper pipe instead of spare PVC lying around.

[chamunks]

You could on the other hand still use pvc if you had some sort of heat resistant insulating medium (porcelin is a good choice) at the connection between the hotwire and the cords and where it meets the PVC handles. You wouldnt have to worry too much about the heat ruining the PVC with this method. I believe that you probably would have plenty of warning as well if the pvc was degrading too heavily due to heat damage and would be able to give it some cooldown time. I normally wouldnt go into detail like this but I just dont want instructables to make the next darwin awards.

[knotmuch]

damn beat me to them. I had the same thoughts
=)

[QwertyuioLP]

Could u make this battery powered? just asking otherwise kool instructable :Þ

[radioape]

Yes, but you would need to use enough batteries to get the right amount of current. I tried batteries first but I couldn't get them to work.

[QwertyuioLP]

How many batteries are we talking about? 5,10 more?

[snowluck2345]

depends on the battery

[T_T_]

would a AC 6.2V 1.24A transformer work?

[xylemsci]

I didn't have much luck with the 12V 1A transformer. Burned it out within a couple minutes of use. What did work really well was an old ATX power supply from a discarded computer. You have to short the Power-On lead and then you have a really nice source of 5V and 12V current. I used the 5V and it works wonders!

[hybridracers]

how do you determine what resistance is needed across the wire to not blow up the adapter?

[AACRJimbo]

Don't have a ninja, can I substitute a pirate?

[nick222]

A shaolin monk would do LOL

[ProjectZro]

iv got a monkey named ninja, does that count?

[Haidir]

very nice!! thats good idea..

[heathbar64]

Sweet! I saw anothe hotwire machine which called for a transformer, dimmer switch etc to power it. I wondered then if I could use one of my many wall warts for a simpler tool. I've got a bunch of wire from an electric dryer, I'm thinking this might be ttoo thick to work with a low powered unit like this. Am I correct?

[natethegreat88]

if you had a longer wire you could add a couple more amps and then choke people but at the same time electrocute them. (It's a geeky ninja's idea... so deal with it!)

[gormly]

I think you'd be burning them, not electrocuting them.. Neat idea though..

[chamunks]

Might electrocute if they were grounded, if I'm not confused.

[natethegreat88]

Well it would still do something to them not just your ordinary choke.