Foam cutting hot wire

I want to make a hot wire for cutting polystyrene foams. I've built one from plans I found on the web, but it would not heat the wire quite hot enough. It used a doorbell
transformer and a home lighting dimmer switch. A local hobby shop wanted to sell me a $90 power supply. I'd like to keep the costs more reasonable. Does anyone have any plans, references or suggestions? Thanks in advance.
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I built a hot-wire foam cutter years ago for about ten bucks, and it is powered by a car battery. Here's what you need to get from the hardware store:
piece of 1/2" metal conduit; about 6" of dowel just big enough to stick into the conduit; pair of 1/2" #6 wood screws; pair of 1" to 1.5" 4-40 or 6-32 thumbscrews (the kind with a pinched head you turn with your fingers) plus nuts and washers; about 20 feet of lamp cord; round wire terminals for the lamp cord; 5 or 6 feet of metal fishing leader wire; electrical tape or cable ties; battery clips big enough to grab car battery posts; two short dowel pieces, 1/4 to 1/2 inch diameter by 1" long.
Bend the metal conduit smoothly (no kinks) to form a bow about a foot or two deep and long enough to accommodate the wings you want to cut, which will probably never be more than 4 feet. If you make it 5 feet, get 6 feet of fishing leader wire. If you want to make a more common 4 foot bow, 5 feet of wire will be enough.
Cut two pieces of your big dowel about 3 inches long. Drill a hole through each piece to accommodate the thumbscrews. Drill a small hole through the head of each screw for the wire to pass through. Put the screws in each hole, and put the round wire terminal, a washer and a nut on the other end of each screw, but don't tighten them all the way down. Leave the nuts out toward the ends of the screws.
Put the dowel pieces into the ends of the conduit, with the screw heads pointing toward each other. Drill a hole through the conduit into the wood and secure each dowel piece with a wood screw.
String the fishing wire between the two screw heads. Pass a few inches of it through the hole in one screw and twist it securely in place. Drill 1/16" holes lengthwise through the small pieces of small dowel, and string these onto your cutting wire like beads, then pull the wire through the other screw and twist that end to secure it in place. Don't go crazy with the twisting because you don't want to accidentally put kinks in the wire, and the length of the twisted part subtracts from your usable cutting length. Just make sure that it's quite secure. Slide the little dowel pieces that you strung onto the hot wire like beads to either end. These are used for cleaning melted goo off the wire between cuts.
Solder the battery clips on one end of your lamp cord. Solder the other end of one conductor to the wire terminal under one screw, then peel the two conductors apart until you have enough of the already attached wire to strap to the bow with tape or cable ties, all the way to the other end. Trim the excess length off the other conductor and solder it to the terminal under the other screw. If that description sounded too vague, what you want is a lamp cord that comes to one end of the bow. One wire attaches at this end, and the other wire goes all the way around to the other end of the cutting wire.
Tighten the nuts until the cutting wire is taut.
Bring your car battery in the house. Get your foam and templates set up. When you're ready to cut, clip the terminals onto the battery, and your wire will come up to temperature in about 15 seconds. I have found that with the general dimensions I gave you, the cutting temperature on my hot wire is just about perfect. You should get little hairs on your wire as it leaves the foam block when it's just right. If you're getting too much or not enough heat, adjust your dimensions accordingly.
I don't know if you've read much about cutting wing cores, but you want to mark your templates with index numbers 1/4 to 1/2 inch apart. While you and your helper are cutting, one of you should read the numbers so that both of you can stay together. This is especially helpful on tapered wings.
BCRandy wrote:

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Robert Reynolds wrote:

Do a goggle search on hobby electronics. There are many sites where you can purchase a power supply for 13 volts with a variable switch. Many kits are very reasonable and great instructions. Its a way to practice your soldering techniques at the same time. It takes up less space than a 12volt battery and you don't have to recharge it. Very simple for anyone to do. Goggle search on how to solder and you will find sites with many fine photo's and tips. Doc Ferguson
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BCRandy a ιcrit :

Some circuits here: http://www.uoguelph.ca/~antoon/gadgets/gadgets.htm .
Daniel
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Here's a nice design by a pattern pilot / electrical engineer:
http://nsrca.org/technical/tip_tricks/foam_cutter/foam_cutting_power_supply.htm
                Marty
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Martin X. Moleski, SJ wrote:

Yeah, it's a nice little unit, but I'd rather spend less time building a foam cutter and more time cutting foam. Why am I not surprised that a pattern pilot thinks using a car battery is "dangerous"?
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Robert Reynolds wrote:

short out a car battery and you will find it is dangerous, the amount of power you can get is enough to fuse a metal object placed between the terminals and explode the battery and if you don't believe it give it a try and post back the results, ok its only 12 volts but more than capable of discharging 1000 amps over a short period
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funfly3 wrote:

A company that is very reasonable is at this site www.demandfoamcutting.com One thing about foam, have an exhaust fan or work in the open. Even heavy sanding of foam you should wear a mask. Heat, excessive heat will produce cyanide gas in small quantities but who know how long multiple exposure is? We use a lot of foam in building sceanery and we always wear a good mask. In model airplanes you will not create the amounts that I describe. The cycling type of cutter is like a low end welder. It needs to cycle Use Let cool, use and cool again. Failure to do this and you will be insured to burn out your unit prematurely. Doc Ferguson
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funfly3 wrote:

I'm sure shorting out a car battery and discharging it in a few moments would be dangerous, but I'd like to see you do it with a piece of metal fishing wire.
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Robert Reynolds wrote:

I never said you could it was a reply to "using a car battery is >>> "dangerous"?" which the poster seemed to think is a laugh,a car battery if abused is very dangerous I have seen the aftermath of a lead acid battery that exploded thankfully in a unmanned charging bay
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funfly3 wrote:

Yeah, it is a laugh. Seems like a guy who is so fond of naked whirling propellers wouldn't mind a little ole car battery.
I'm just trying to keep things in perspective. Statistically car batteries are relatively innocuous. They're all over the place, and I almost never hear about them causing problems. This doesn't need to be a big argument or anything. I just think the warning about car batteries is a little bit silly. Probably a better reason to use a transformer would be to keep you from having to reset the clock in your car.
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The electrical engineer who designed the power supply and put it on the web up for folks to use had a long, successful career in Detroit designing electrical systems for automobiles.
It is at least conceivable to me that he might be considered a reasonable authority for a warning about what can go wrong when using automobile batteries.
YMMV.
                Marty
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Martin X. Moleski, SJ wrote:

Purely philosophical question: Do we really need an authority figure to warn us about car batteries? Just to be clear, in persisting on this subject I'm not really talking about car batteries any more. I understand that they can be dangerous because apparently .0000001% of them explode. What I'm talking about is whether or not we need to be warned every five minutes by an Expert about every little risk.
In addition to RC airplanes I also build tube amplifiers for harmonica (www.fatdogamps.com, and yes, I will have photos posted within the next week or two). When I was starting out and looking for technical information, every single person I contacted, and I mean EVERY one of them, warned me about the high voltage in this type of circuitry, as if I hadn't ever heard about it before. It was as if they all had been brainwashed to issue warnings. It seems that this type of brainwashing is becoming universal in our society. Every product comes with a sticker on it telling you not to take it internally or use it as a personal flotation device. It used to be just my Grandma who thought I was going to kill myself through carelessness, but now it's everybody I meet.
Here's another example. I prefer not to wear shoes, although I wear them when I have to. Whenever people see my bare feet, they feel compelled to warn me that I am going to hurt myself, as if I hadn't been walking around for almost 40 years. What do they think is going to happen to my feet? Wouldn't it have happened already? And where did each individual get the idea that he or she bore the responsibility to save me from my own self?
In the kind of society we live in, isn't it surprising that it is still legal to operate a flying toy that looks so dangerous with its whirling propeller blades? Never mind the fact that more than 99% of RC flights happen without incident. Model airplanes seem relatively dangerous in a society that has warnings on a box of Q-Tips. If we want to keep enjoying our risky hobbies, maybe we should all lead by example, i.e. let everybody else assess their own levels of risk and stop condoning and/or participating in the endless meaningless warnings that serve only to dilute any warnings of real danger.
And yes, of course I know what the obvious rebuttal is to that statement. "We all need to be more aware than ever of potential risks and dangers because in this litigious society we are in greater danger than ever of losing the right to enjoy our hobby... blah blah" I just think it's time for the pendulum to swing back the other way, toward self reliance and individual responsibility, and I'm ready to lead by example. And that's why the battery thing struck me as being laughable.
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Great rant, Robert! We need more people who think that way!
Harlan

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Well...maybe. It's a fine rant, but it's not clearly applicable to this situation.
Most people never get very close to a car battery nowadays. Car batteries spend their time...wlll, in CARS, and in particular inside the engine compartment, where most people don't go. That makes danger from batteries relatively small. (Much like danger from bears: few people each year get killed by bears, because people and bears are in different places in general; that doesn't make bears "safe", and indeed, when you put them together, people sometimes die because of underestimating the danger.)
Back to car batteries: those of us who own boats and work on them ourselves (a fairly small group, too!) tend to see a lot more of car batteries close up. And the results are a bit scary. My friend Gerard has a nasty burn on his wrist where the metal watchband he was wearing happened to touch both the engine block and the "positive" binding post on the alternator of his engine; the resutling current did its best to melt the metal, and came close to succeeding. And I one, working on my engine, dropped a 12" crescent wrench from a greasy hand as I tried to reach up and set it on a counter next to the engine. It fell down and across the terminals of a battery, making a VERY loud "pop" before it bounced free. (I now ALWAYS keep the lids on my battery boxes unless I'm actively doing something with the battery.) If it had wedged in place, I'd probably have ended up with 6 tons of melted fiberglass and lead in the back yard. As it was, I ended up with a crescent wrench with two nasty divots in the metal with burn marks around them.
So: auto batteries are generally not a source of danger to humans, but when we're placed in close proximity to access to their un-fused current, it's quite likely that we can come to grief. Warning people of this seems just fine. You don't have to listen. You don't even have to do any warning. But you might at least let OTHERS do it if they're worried that that might feel personally responsible for the trouble caused by someone else's lack of experience with something they suggested.
--John
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This talk of car batteries calls to mind something from many years ago involving a Model A Ford. It's 6V, with the battery mounted under the floorboard on the driver's side. The car has mechanical brakes, with the brake rod to the rear wheels running next to the battery. The rod is about 5/16" dia. steel. Anyhow, the battery's hot post had got jammed against the brake rod, which heated to a yellow heat. The battery did not explode, and remained OK. That's not saying what a modern 12V battery would do under a dead short, though. Bill(oc)
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A few years ago I was filling my truck with gas and I noticed a man in the next bay had his hood up and was starting to check his battery. A cigarette was dangling from his mouth. He took the caps off the battery and bent over to check the water levels and the battery blew up in his face. You could hear him screaming in agony. They turned a hose on him to help clear the acid from his eyes. I don't know the end to this TRUE story because they took him to the hospital in an ambulance.
Take heed! ________________________________ Earl RC 'AMA' #40329

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wrote:

Amen.
One of my brothers lost a pickup truck when a commercial mechanic accidentally shorted out the battery. One day you have a nice vehicle, the next day a charred hulk.
Ed Cregger
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H Davis wrote:

Thanks, but judging by other comments we really do need to issue constant warnings against stupidity and carelessness, as if stupid and/or careless people would even listen.
Maybe I appear selfish, foolish, or whatever, but I really think that when warnings become routine and mundane, nobody even hears them any more. I'm all for warning people about unexpected things, but smoking at a gas station? Haven't we all heard that one before?
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On Fri, 22 Dec 2006 00:25:53 -0600, I said, "Pick a card, any card"

Nah. Just sit back and watch Darwinism at work. Whether you like it or not, there are still folk out there who do not have an intrinsic knowledge of some dangerous things and warnings can and do save damage to life and limb. Period. Get annoyed if you wish but every child has to be taught sometime about electricity and the dangers associated. Feel free to ignore the warnings, Robert, but don't act surprised when you get the Darwin award some day.
Authors (as in authority figures) don't have a clue who is reading their words so the ever present warnings may actually be appropriate for only a small percentage. For those reading the warnings for the first time, it could be 100% appropriate and life saving. Your comments appear to be quite selfish. Don't heed the warnings. The College of Hard Knocks has a nice cemetery out back. -- Ray
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