Film/TV question: How to build Foam Cutter from only household items

Question

I'm writing a scene for television and I need to make a foam/rope cutter from household items. I've watched several videos, including this one: https://www.youtube.com/watch?v=cM0lP2WCzY0, and determined that I need two sticks (can be anything), two copper wires (for electrical conductivity), a battery to power them, and a "blade" to heat.

Since nichrome wire is not readily found, I'm thinking of using a sharp-edge belt buckle or coiled earring. What material could be a reasonable substitute?

Also, since this cutter is homemade (to use when there are no other options), the copper wires would not have sheath coverings. Would the user be able to handle the device and related shocks, or would this be unrealistic to handle unprotected?

Thank you for your feedback. This is my first post, and we may be looking to work with science advisers on future gadgets, I am happy to discuss further privately.

EDIT: I wasn't expecting so many great suggestions, and I am aware that my question wasn't precise. I will correct in the future. Thank you all.

Answer 1

"Cut the foam! Cut the foam!", she cried as she wriggled and squirmed in the styrofoam dishwasher packaging she got stuck in.

"With what?", cried Bob as he scanned the kitchen for something suitable.

"With a hot-wire styrofoam cutter, of coarse! You'll need about 50 cm and a couple of hundred watts.", said Alice.

"There isn't one in the cupboard.", said Bob. "But wait - there's a toaster!"

Whipping out his Swiss Army knife he unscrewed a couple of Philips screws on the bottom, ripped off the covers and bent out one of the element panels. Deftly cutting through the panel's insulation he was able to pull out about 50 cm of wire. He had his hot-wire foam cutter. Now he just needed to avoid electrocuting both of them.

He put the disassembled toaster on the floor, grabbed the silicone oven mitts and two forks to hold each end of the wire. Stretching it as taut as he could he depressed the toaster lever with his foot.

"Don't move!", he said as the element quickly turned red hot. The radio which had been on all the time bizarrely started to play Buffalo Springfield's Down to the wire. He pressed the hot wire against the foam and it started to cut right through. Bob had forgotten about the smoke and the smell but carried on until he was within an inch of her skin. Kicking the toaster he turned off the power and breathed a sigh of relief. He grabbed Alice's hand and yanked her out of the packaging breaking the remaining bits.

"That's the last time I'm playing hide and seek with you.", he said as he wiped the sweat from his brow.

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Maybe I'll stick to electrical engineering. And maybe this isn't a good "how-to" for a TV show.

Answer 2

It is difficult to imagine that someone would need to cut a rope rather than untie it if they have sufficient mobility and use of their hands to construct an apparatus like the one in the YouTube video.

However if we set aside all other alternatives and imagine that something equivalent to the YouTube device is essential and must be made with materials and tools found in the average home, it might be possible.

Copper wire for connections, tape and some type of small battery should be easy to find. Shock will not be a problem at 12 volts and less. Burns and fire from the copper wire should not be a problem with small batteries and not too much current used for the cutting wire. An automotive battery could be dangerous if not used very carefully. It could supply enough current to make any household-sized copper wire very hot.

For a heating element, you could try the soft iron wire in a twist tie. When heated, it could oxidize and break quite quickly, but it is worth trying. Another alternative is the spring in a ball-point pen. That is probably soft enough that it can be straightened without breaking. A paper clip is worth trying also, but it may be too thick. Remember that the copper wires will get too hot if the cutting wire takes too much current. Modern picture frames often have loops or notches on the back for hanging, but many are still hung using a steel cable across the back. That is not likely to be stainless, but it might work. Stainless cables are used to suspend some light fixtures from the ceiling and between the posts of stair railings etc. Cables would need to be cut and unraveled. Your average Swiss Army Knife might not do that, but the best SwissTool and Leatherman might.

Only the thinest wire will get hot enough with the current that a 9-volt battery can put out. D-cells may be the best. Start with just one. If one will not push enough current through the steel wire, try more connected in series. You can probably tape several in series to something that will keep them lined up. It may be difficult to keep them connected together securely. Perhaps there is a flashlight around that you can hack.

Test of Twist-tie Wire

The wire from the twist tie was about 4 inches long and 0.015 inches in diameter. The resistance was about 0.75 ohms. Three D-cells were able to push about 4 amps through the wire. The internal resistance of the battery, the resistance of the connecting wires and the temperature of the twist-tie wire affected the results.

The wire got hot enough to cut through a 1/4-inch nylon rope.

Another Test

Here is a test with one of those battery holders that holds three AAA cells in the little LED flashlights that are commonly sold for little more than the price of a small package of AAA cells.

The left end of the wire is captured by the spring that contacts the negative end of the holder. The right end is held against the positive end of the holder with my thumb with a little piece of cardboard to prevent burning my thumb.

The wire didn't get quite hot enough to cut the rope. The AAA cells apparently have too much internal resistance / not enough voltage. It might have worked with fresh batteries. Something like this might work with AA or C cells, but making the series cell connection and the connection to the steel wire might be quite awkward.

Success!

The secret is not to distribute the battery power over too much wire. I wrapped all but an inch of the steel twist-tie wire around the ends of short pieces of copper wire. I believe it is AWG 18. A single AA cell got the steel wire red hot as shown below.

Answer 3

How safe does this have to be? How long does it need to work for?

One possible solution is to disassemble a hair dryer. Leave all of the wires connected as much as possible. Remove the heating element from the mica insulator that the coils are wound on and straighten out a length of the element coil.

Turn the hair dryer onto its lowest power setting. The heating element will get hot.

The heating element may self-destruct unless you use the hair dryer at the lowest possible power setting.

Note that this is dangerous. Be certain the outlet the hair dryer is powered from is protected with a GFCI / RCD. Be careful not to touch any exposed connections or bare conductors.

Be especially careful that the heating element that is doing the cutting does not touch any person or come into contact with any metal object.

Another, safer possibility is to strip a hair dryer for parts. Remove the heating element and remove the coiled wire from the mica insulator.

Stretch the coiled wire straight and measure the total length. Assuming that your Mains voltage supply is 120 Vac, cut off about 1/10 of the total length, plus one inch or so. I'm going to call that cut piece of heating element "Resistance Wire" from now on.

Cut the plug off the end of the AC Mains power cord, cut the cord free from the remains of the hair dryer. Use a pair of nuts & bolts to connect the cut length of resistance wire to one end of the power cord. Insulate the connections so they can't short together.

Use the other end of the power cord to connect to a 12V automotive battery. You can do this in the vehicle or remove the battery from a vehicle to use as a portable cutter.

The resistance wire will eventually burn out but you still have 8 or 9 other pieces of wire remaining from the original heating element.

Depending on how much time your character has available, this 2nd approach is much safer that what I first proposed.

Answer 4

Stainless steel wire works and is better than nichrome in some aspects. It is cheaper and stronger than nichrome, and stretches less. So you you can put it under more tension for cleaner cuts and and tension springs are not as important. It is not as high resistance as nichrome but is still higher than copper so you do not need unreasonably low voltages to not overheat. It oxidizes more easily than nichrome but that doesn't matter for occasional use.

Do not use copper wire. Expensive, weak, oxidizes, and too low resistance,

Buy stainless steel wire from fishing or hardware stores. Do NOT use the power from the mains without running it through a power supply or transformer of some kind to step down the voltage. You want adjustable too or else it will run way way too hot.

Using a wall wart might be most convenient. Will need adapter clips to convert the barrel connector to something you can clip to other stuff.

Answer 5

This setup is unsafe, but assuming it's an emergency.

A hairdryer will have two heating coils in it remove one of them and use some of it use it to construct the cutting wiret use an equal length to construt a shunt element, wire the shunt and the cutting element in parallel and connect this parallel combination in series with the hair dryer

The cutting wire element will get about half its normal current, but because it will get no wind it will still get very hot. plenty hot enough to cut foam do not touch the element or the conductors that feed it. the best that can happen is you blow an ELCB the worst is electrocution.

You probably want to get your art department to fake this using a safe DC power supply instead of exposing actors to this potentially unsafe setup. although if ypu power it from an isolating transformer that would significantly reduce the risk.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 6

Err on an E string

If your household has a guitar about, the strings may be a suitable cutter-wire. You'll need approximately 12V at 2A DC.

^{From https://www.instructables.com/id/Guitar-String-Hot-Wire-Cutter/}

There's a heap more documentation at the linked website https://www.instructables.com/id/Guitar-String-Hot-Wire-Cutter/ but you need either an electric guitar or a steel-strung guitar. If there's nylon strings then this won't work. Also the thinner strings will have higher resistance and will heat up better - so don't use a thicker bassier string.

(leaning a bit on worldbuilding) if there's urgency in the moment, one might cut the other five strings, then stamp on the neck to shatter the guitar, connect the supply to the remaining strings, and have a ~1 metre long cutter-wire with some handles that won't get hot. Plausibly finding 12V at 2A in a hurry might be explained by other guitar equipment.

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Hypothetical follow-on, what size string would it take to cut foam if it were connected to mains voltage AC without blowing the circuit's fuse ?

Answer 7

If you take the wires suggested in the other answers attached to an old analog model railway transformer you'd have a plausable scenario.

A few inches of toaster wire will probably heat up when using a low voltage variac (which is what those transformer are). And it is safe to touch.

Much safer to show than using a live toaster, you know... copycats. Even though the story of Transistor is nice.
It is plausable, but may not be very usable. Does it have to be?

Answer 8

That video has very thin wire, much thinner than is used in common heating appliances like toasters and hair dryers.

You might find the right gauge of wire in an electric blanket

Answer 9

I use an old soldering iron (low power) for cutting foam. The cut is wider than a wire, but it is easy to guide as it requiere some force to cut thru material.

It is an household item? Well, yes. I use them for minor repairs, like toys and remotes (battery wires getting unsoldered from PCB).

Answer 10

I have in the past (in the past few weeks, honestly) used an old computer power supply and the thinnest steel fishing wire I could find at the hardware store. It's not exactly a household item, but it's quite possible that it's laying around. As mentioned elsewhere, any stainless wire should do.

Wrap one copper wire around the barrel of the plug, and put the other one inside (balled up if needed to keep it there), wire nuts for the other end.

The wire needs to be sufficiently long, as modern power supplies with cut out if they see too much current. 2-3 feet is usually sufficient.