Need Advice on Building Rope Cutter using specific items

Thread Starter

Triple9Nickel

Joined May 31, 2020
28
Hi, I'm not an engineer. I'm writing a script and one of our characters is an electric engineer. In one scene, she is kidnapped and must burn through ropes using available items in the middle of nowhere.

I've researched foam cutters enough to know that you need 3 things: a power source, conducting wires, and the "blade" (usually a piece of steel wire). I've tried duplicating this at home using a D battery, two silver conducting wires (because a silver necklace is feasible here), and a steel wire blade. It sort of works, but it's very slow to heat up, and I'm also trying to figure out how you can hold this thing without burning your hand. Can anyone suggest a way to improve this design? I need a reasonably quick way to burn/melt rope.

If there were a 12V or a car battery around, would that work better, or is it too much power/too dangerous? I'm happy to clarify any details. Thank you.
 

Analog Ground

Joined Apr 24, 2019
397
Is it reasonable to have a car with jumper cables available? The car battery provides the power source and the jumper cables are your "conducting wires". The "blade" is a thin piece of wire or metal. Hair pin? Pen casing? Paper clip? Hold one end of the "blade" in the jaws of one cable. Then, momentarily touch the other cable to the other end of the "blade". Vary the touching and releasing to get the blade material red hot but not melt. Use nylon rope. Easier to melt than burn. Holding the jumper cables is safe. They will not get hot. BTW, there will be lots of good sparks for effect.

Edit: Mcgyver?
 

MrChips

Joined Oct 2, 2009
21,123
On the technical side neither D-cell or 12V car battery would work. They are on opposite sides of available power.
A D-cell, as you say, sort of works but it does not have enough staying power.
On the other hand a 12V car battery has a lot of available power. Almost any piece of electrical wire placed across the battery terminals will glow and burn up instantly as a fuse wire would.
 

WBahn

Joined Mar 31, 2012
25,760
My first (very jaded) thought is that it doesn't matter whether the script bears any resemblance to plausible reality. Virtually no big-budget film cares about it at all (beyond all of their self-congratulatory commentaries and special features where they proclaim how well researched and accurate everything is -- as they show some scene that is so far around the bend from reality that the two will never see each other). If the big boys don't care -- why should you?

But, assuming that you want to buck the trend, then good on you.

A 12 V car battery is definitely better for something like this than a D cell battery.

A silver necklace isn't likely to work too well because of all the poor connections from one link to another.

If your heroine has access to a fairly long piece of steel wire, then one thing she could do (assuming her hands are free enough) is fold the wire from the ends so that there are many wires in parallel on each end separated by a single wire in the middle. This gives the low resistance that you want going from the battery to the cutting wire and the high resistance you want from the cutting wire itself. The trick now is to determine what range of diameters of wire will work and then find some plausible reason why some of that wire is sitting around.

Getting back to my jaded view, I actually had a very interesting conversation with a gal on an airplane who was a producer for indy films and I asked her why the same glaring, stereotypical mistakes are still made over and over again -- such as someone standing there with a six-shot revolver and watching them shoot it a dozen or more times without the camera moving off of them, plus a handful of other oft-repeated mistakes. Her answer was rather surprising -- I was expecting something along the lines it being difficult to get all of that right at the same time you are editing the available footage to tell the story that you want. While she confirmed that it is a pretty low priority because few audiences even notice the most glaring mistakes like that, she said that the body of films over time have effectively created their own universe and what audiences will buy as "real" depends not on how real or unreal it is in actuality, but on how well it conforms to the expectations of what's "realistic" in that universe. Audiences have come to expect the ability to take a grainy, low-light security camera image taken from several blocks away and be able to "image enhance" it to read the tiny barcode off the missing kid's bicycle registration tag that is reflected in the car window as it goes racing by (I forget which movie I saw that absurdity in). They expect someone that gets shot by a large caliber handgun to go flying back through the air from the impact. If you show these, or any of a litany of other things, realistically then most audiences will feel that the film is, in fact, completely unrealistic.
 

MrChips

Joined Oct 2, 2009
21,123
Is the script setting placed in modern era?

In a pinch, stripping a strand of wire out of a piece of electrical cable and using power from your smart phone, flash light/power pack charger might do it.

DO NOT TRY IS AT HOME WITH YOUR ELECTRONIC DEVICES.
 

MrChips

Joined Oct 2, 2009
21,123
@WBahn Reminds me of the Woody Allen movie when they pulled out what was supposed to be a futuristic looking hypodermic syringe which just happened to be a Soldavac desoldering tool (solder sucker) painted white.
 

Thread Starter

Triple9Nickel

Joined May 31, 2020
28
If the big boys don't care -- why should you?

But, assuming that you want to buck the trend, then good on you.

A 12 V car battery is definitely better for something like this than a D cell battery.

A silver necklace isn't likely to work too well because of all the poor connections from one link to another.

If your heroine has access to a fairly long piece of steel wire, then one thing she could do (assuming her hands are free enough) is fold the wire from the ends so that there are many wires in parallel on each end separated by a single wire in the middle. This gives the low resistance that you want going from the battery to the cutting wire and the high resistance you want from the cutting wire itself. The trick now is to determine what range of diameters of wire will work and then find some plausible reason why some of that wire is sitting around.
I do care, thanks for asking. I grew up on MacGyver and I'd like to similarly showcase the power of science here, especially to inspire younger viewers.

One of the reasons I like the silver wires idea is that she can be wearing a necklace. I understand what you mean about the chain links being weak, but I did find continuous silver wire chains twisted or braided that worked quite well as a conductor for my D-battery test: https://ae01.alicdn.com/kf/HTB1RPAFbdfvK1RjSspoq6zfNpXaQ/2-2-5-3-4-5-6-7mm-Women-Chain-Necklace-316L-Stainless-Steel-Silver-Twisted.jpg

As for the "blade", I can use a hoop earring or a hairpin as suggested. The battery is where I'm stuck. Would a 9V be a middle compromise? @MrChips says the car battery will melt the wire, so what's in between a D-battery and a car battery?

Thank you.
 

Thread Starter

Triple9Nickel

Joined May 31, 2020
28
Is it reasonable to have a car with jumper cables available? The car battery provides the power source and the jumper cables are your "conducting wires". The "blade" is a thin piece of wire or metal. Hair pin? Pen casing? Paper clip? Hold one end of the "blade" in the jaws of one cable. Then, momentarily touch the other cable to the other end of the "blade". Vary the touching and releasing to get the blade material red hot but not melt. Use nylon rope. Easier to melt than burn. Holding the jumper cables is safe. They will not get hot. BTW, there will be lots of good sparks for effect.
Thanks for the suggestions, I like the idea of touching the blade to the jumper grips, though hand mobility may be an option. What are some simple ways to insulate your hand from touching a hot conducting wire?

Also, since I don't understand how different conductors work, why is it that the silver wire only gets warm, but the "blade" gets red-hot enough to burn? Does the silver have a property that prevents it from getting very hot? If so, maybe she can just hold it and it will get annoyingly hot but not scalding like the blade would.
 

MrChips

Joined Oct 2, 2009
21,123
None of the regular off-the-shelf batteries, AAA, AA, C, D, 9V batteries will generate enough energy to do what you want.

A 12V car battery has enough energy to burn a house down. You don't need to electrically insulate your hand from a 12V battery, except for the fact that the wire will get hot, however, very very hot. Touching any wire to the car battery will be very dramatic. A small piece of wire will explode instantly. There will be no opportunity to make and break a circuit with a piece of wire. Any large metal object will instantly melt and fuse itself to the jumper wire. Either the metal will glow red hot and disintegrate or the battery will explode.

Cell phones have enough energy stored in batteries to cause a serious fire. Samsung found out the hard way.

DO NOT ACTUALLY TRY ANY OF THE ABOVE.
 

Thread Starter

Triple9Nickel

Joined May 31, 2020
28
@MrChips I appreciate the suggestions, but I'm not sure that no off-the-shelf battery will do the trick. As I mentioned before, I was able to slowly cut through a rope earlier today with a D battery, two silver conducting wires, and a thin steel wire blade. My assumption is that so long as the wire is sufficiently thin, this should somehow be feasible.

If I car battery is far too powerful, surely there is some option in between. What about one of those large 6V flashlight batteries?

If it really is not possible, then is it better to electrify larger metal like a buckle with a car battery? I feel like this might be far more hazardous than the previous option.
 

MrChips

Joined Oct 2, 2009
21,123
In technical terms, what you need is resistance.

The silver necklace and a metal blade both have low resistance. They behave differently because the two situations are different.
In the case with the silver necklace the battery does not have enough power to warm the necklace.
A rope cutting blade is designed with the proper amount of resistance to work with the given voltage from the power supply.

A full length 1st string steel wire off a guitar has about 1.5-ohm resistance. Connected to a 12V car battery it will conduct about 8 amps and will get very hot, dissipating about 100 watts of power. You only need about 20W to cut poly rope.

You cannot cut rope in this manner if the rope is made from natural fibre, such as hemp, sisal, manila.
 

Thread Starter

Triple9Nickel

Joined May 31, 2020
28
@MrChips Okay, thank you for the education. As I mentioned, I'm learning as I go.

Can you help me understand one thing that's confusing me? The reason I chose a silver necklace was because on every electrical forum I visit, silver seems to be listed as "the ideal" conductor. But in your previous statement you said it has low resistance. So is there a better material for the necklace that would make these battery options perform better?

I'll grab a 6V lantern battery tomorrow and do some tests. I've got my goggles and rubber gloves.
 

MrChips

Joined Oct 2, 2009
21,123
Silver, gold, copper, iron, tin, brass, aluminum, zinc, etc. are good electrical conductors. They all have low resistance (or what we would call resistivity).

What you want is just a few more ohms of resistance to reduce the current, not increase the current.
The resistance of the object will depend on the type of metal, the length and cross-sectional area of the object. Hence for any metallic object you want it to be long and skinny in order for it to have some resistance.

A piece of steel guitar wire of 1-ohm would conduct 6A from a 6V battery. This will dissipate 36W which would be just right for cutting poly rope.

You didn't say the year or era of your scene setting and what articles could possibly be at hand.
 

Thread Starter

Triple9Nickel

Joined May 31, 2020
28
You didn't say the year or era of your scene setting and what articles could possible be at hand.
The era is modern. I'm kind of married to the idea of using some form of jewelry because this is a female character and I'd like to put a different spin on it.

As it is currently written, they are trapped in an old wooden shack in the middle of nowhere (or the desert, etc). So I figured the jewelry is available, and I could rationalize an old lantern lying on the ground. Basically, if I can't make this work, then I'll have to change direction and do some other escape, like burning rope with magnifying glass but that's a whole other set of problems.

Thanks for your thoughts.
 

WBahn

Joined Mar 31, 2012
25,760
I do care, thanks for asking. I grew up on MacGyver and I'd like to similarly showcase the power of science here, especially to inspire younger viewers.
Oh, well if you just want to showcase science like MacGyver did, just have her make a battery out of a gum wrapper, salt, and spit and use the pull chain from a light socket for the cutting wire.

MacGyver was just about the epitome of piss poor science and technobabble. It was all hype. I well remember all the promotion the show got before it premiered and how they went on and on about how it was all science based and completely researched and true -- oh, except they always changed one little thing so that people couldn't get hurt by trying to do it themselves. That's what really revealed that it was all publicity hype and not any kind of serious stories because the articles in TV Guide, the Sunday Rocky Mountain News, and Parade magazine all made the same claim nearly word for word. But I guess they were telling the truth -- the one thing they changed was the laws of physics.

But I was young foolish enough to buy into into the hype and awaited the show eagerly; I missed the first half of the first season because I was in basic training and tech school and then getting settled in at my first duty station, so the first episode I saw shortly after I turned 21 and was the one where he is on a ship and there are two sophisticated bombs. He and a buddy have to defuse them and they are sealed in a vacuum chamber. His buddy tries to feed a metal rod coated with Vaseline through a seal to sample the mysterious powder and it leaks air into it and explodes and kills him, thus revealing not only that the situation is really serious, but also that the powder that is on a pan inside the bomb is red phosphorous and they must not, at any cost, let air get to it. So what does MacGyver do? Takes a fluorescent light tube, scores it and breaks it open, shoves a cork into the end of it, and then proceeds to shove this through the seal on the other bomb without any problem at all. But the really cool "science" was how he explained that by knocking off the cork on the edge of the pan, as the gas in the tube comes out it must be replaced by something and so it will suck up all of the red phosphorous powder from the pan -- and, of course, this is exactly what happens. My sixth grade daughter has learned enough science to recognize the absurdity in that premise. It was almost as bad as the coyote and road runner they way they played fast and loose with the physics. Every one of the handful of episodes I ever saw was the same way -- pure technobabble written by writers that clearly never passed anything resembling a high school physics or chemistry class.

As for the "blade", I can use a hoop earring or a hairpin as suggested. The battery is where I'm stuck. Would a 9V be a middle compromise? @MrChips says the car battery will melt the wire, so what's in between a D-battery and a car battery?
A 9 V battery won't do it, the internal resistance is too high. The car battery is a good choice, you just need to find the right size wire. Can your heroine manage to find an old toaster lying around that she can rip apart? They have pretty much just the wire you're looking for.

What kind of rope is this? How thick? If it's too thick you might have to worry about it being able to carry the heat away from the contact site as fast is it is being supplied.
 

Janis59

Joined Aug 21, 2017
1,179
12V is fit only for giant cutters like those esed in foam-polyplastics production, where out of the foam-forms fall 12 m long and 2x2 or 3x3 m thick "brick" what must be nicely cuted in slices. Your case the wire seems be short, thus the resistance will be small. First important note - resistance wire at warm status ir importantly longer and soft, thus the any even best pre-straightening will be lost. Always. Thus the cut will happen inaccurate and skewed waveshapely. Solution is to straighten wire with the spring in end of wire, let the current is not flowing via the spring. Spring hardness is sth to adjust in the process. For case of rope cutting - try to take ANY small trafo and thread via just ONE turn, say 0,1...1 Volt. It would be enough for some 0,63 mm thick and 2,54 cm long Nichrome wire with 5...6 Amps to get 500-600 C high. Then 1...1,5 *E-6 Ohm/m means that pi()/4*0,63^2=0,495 ~~0,5 mm2 thus the R=2,54E-2*1.5E-6/0,5E-6=0,078 Ohm. Then V=R*i=0,08*6=0,5 Volts.
In one end use an double wire winkeled together with own let the loophole for fixing on nail will happen but all current path until the loop goes the double wire - that must not be warmed much. Other end makes the same loop, but to nail hedge it goes with current, then current is going the duplicated nichrome foot. In loop put the spring toward the hedge nail.
PS: Actually, those tho-loop is more effective, if to winkel together thre not two wires, then one loop for srew of wire with current is made by first bend, and other loop for mechanical fixing is made by second bend.

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WBahn

Joined Mar 31, 2012
25,760
Another possibility -- the specifics need to be checked out -- might be to take a light bulb and break the glass exposing the filament. The base gives you something to hold onto and hold the electrical contacts to. The filament will get very hot and burn out quickly, but you only need it to last long enough to cut the rope. You can try difference voltage/wattage bulbs to find one that works. The higher the voltage the higher the resistance for the same wattage, so if you use a 12 V battery on a 120 V bulb, it will only dissipate a fraction of the wattage. But filament resistance is highly nonlinear with temperature, so there are same curlicues involved that just have to be explored.

If you find a combination that works, consider that you did it through a lot of experimentation. How realistic is it that your heroine is going to know exactly what to do so that it works the first time? Since you want to impress the youth with science, make it at least a bit more reasonable by (for instance) finding a dusty box with a bunch of various bulbs on a shelf, including some standard indoor 60 W bulbs, some higher power outdoor floodlights, and maybe some 12 V automotive headlights or bulbs. Whoever lived there just had a box of spare bulbs -- not at all unreasonable (unless the place is off-the-grid and never had any self-generated electrical power. So have her muttering to herself as she decides which one to try and then have it not work. Have mutter about what the problem is and how she needs to change her choice of bulb (or battery). Have it work on the third attempt, or show the first two attempts and then cut to a dozen broken bulbs lying on the ground as she finally gets it to work.

Coming up with a plausible reason for finding a bunch of different batteries around the shack also isn't hard -- but depending on how long it's been sitting empty may make it difficult to explain why any of those batteries are still charged. Anything more than few years starts pushing the bounds for many battery types, anything on the order of a decade is getting pretty dicey for most. But I can attest that even a pretty old (but still in good shape) car battery can hold significant charge for at least three years. When I went into the service I parked my car in an outdoor lot and had no time at all to prep it -- so I just parked it and walked away. Three years later I went to get it out and even though two of the tires were flat I was curious and put the key in and the dome light came on, so I turned the key and the damn thing actually started -- with three year old gas, none-the-less. So I drove it around the lot once (grass lot) even with the flat tires and even the automatic tranny worked (never got out of first gear, of course). At the time I didn't realize how much damage I could be doing, since I hadn't checked the oil or the coolant or anything, but it seemed none-the-worse for the experience.
 

Janis59

Joined Aug 21, 2017
1,179
Any "Edisson" lamp called until now in russia "Lenin`s lamp" but in China after their ban in EU "the Calorific Bomb" (there are no law against to sell and import "calorific bombs" into EU :) have a VERY huge disadvantages
1) Wolframium is damn oxidative in the oxigenated atmospheres. I dont believe any human may choose to cut ropes into vacuum
2) Wolframium is mild metal only until the first warm-up. After it becomes damn brittle. One might to use some rope moving robot with microns per second to not crush the filament. I think it would not be nice.
3) The filament itsel is (dam)^3 thin object, normally having the 10-30 microns diameter of wire let the resistance pass with high voltage (Wolframium relative resistance is about the same as nichrome, steel or copper). Why You not see such in lamps? Because it is threefold bobbined. First, the factory laid it via waltzes to get the wire plane-shaped instead of round. Then the plane is swirled to make a screw-style object. This swirled plane is then spiralized in the spiral of diameter about 0,1 - 0,2 mm. And then this small spiral is spiralized again, around some 0,6 -1,2 mm matrix. Thus, let take the microscope and watch bit better. Dont believe the lamps in America is produced an another way as at our Riga Electro Lamp Fabric (let it have light sand in the cemetery after bancrupsy at 1993).
 
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