Foam cutter

drcrash wrote:


There is a common myyh taht because Nichrome wire is sold as restistance wire - mainly because it has a fairly low temperature coefficient amd a fairly high specific resistance - that makes it good for a hot wire cutter.
I'd go with stainless meself, if you have it,.
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The Natural Philosopher Wrote:

I wasn't thinking it was Nichrome that Lee was talking about... figured it was nickel chrome steel. (Mostly iron, not mostly nickel). I may well be wrong about that.
My measurements of stainless wire don't show it to be more resistiv than nichrome---it's less resistive, but not a lot less. (Very roughl a quarter less, but much more resistive than plain steel.)
BTW I do use stainless myself
-- drcras ----------------------------------------------------------------------- drcrash's Profile: http://www.rcgroups.com/forums/member.php?u 693 View this thread: http://www.rcgroups.com/forums/showthread.php?tb949
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If I find a better wire I will use it.
I tried about 20 different wires before recommending the 20# fishin wire. I had to find something that everyone could get. I had to fin something that was reliable and strong. I have five foam cutters an use the same wire on all of them.
EPP wants a hotter wire and almost all of my cuts now are EPP. Th 1.9# EPP wants a hotter wire than the 1.3# EPP foam. I used to cut lot of the Dow blue foam. The cutter needs to work on all of thes foams and still cut at a comfortable and controllable speed.
I still have rolls of all kinds of stuff around here that I don't use. One of the best cutting wires was a multi strand fishing wire but i liked to run with higher amps. I have stainless steel wire, nichrom wire and even have wire I got that is recommended by Burt Rutan. have tried anything I could get my hands on.
I found a good match with the transformer and the 40" bow. The trut is that the bow and wire are so well matched that the cutter will wor without the dimmer in line and still stay within the 4 amp range.
I say the proof of the pudding is in the 400 planes we have cut and th low error rate of the cutting which is probably around 5-10% and ha been at 0% at times on certain cuts of up to 100 wing halves.
Some wires that are strong have too much overcut. Some require to many amps to get hot enough. Other wires break or stretch too easil when they get up to temperature.
Finding a wire that will work is one of the most important parts o building a foam cutter.
How long is your wire? How many amps are you using? How many cuts will a wire make before it breaks? Where do you buy your wire and what are it's specifications?
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I am building a tear drop trailer. I will be using pink house insulation from home depot, and because of the curved roof surface, I will glue the foam in and want to hot wire it using the trailer sides as a guide. I will build a 6 foot wide bow as the trailer is 5 feet wide. I want to have the foam contour the same as the sides to help support the outside skin of the trailer...
My question is, being all new to this, is if I use the wire you recommend, how much expansion do I have to plan for in the wire?
And what would be the most accessible power source. I know someone who used 115v with a dimmer switch, but sounds a bit unsafe.... could it be done with an old train transformer if I can find one?...... this will be a one time deal, so do not want to invest in a lot of expensive equipment.
Peter ..........

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az21 Wrote:

Hot wires don't really expand much---a small fraction of an inch pe foot---but that's enough to make a very taut wire go pretty slack. Most people put some sort of spring or weight in the mechanism to tak up the slack and keep the wire taut.

Train transformers aren't good---they can't really do the amps. Mos new ones will cut out on you, and old ones may burn out. (And for long bow, they probably can't put out enough voltage, either.)
One general rule is that for a longer wire, you need more volts to ge the amps you need for that type of wire. So a transformer and wir that work for up to (say) a 40-inch bow will not work for a six-foo bow. You'd need almost twice the voltage to maintain the same amps fo the longer wire, and dissipate the same number of watts per inch to cu the foam.
Another general rule is that for a thicker wire of a given type, yo need fewer volts but more amps to heat it up. (The wattage is th product of the voltage and the amperage.)
You're going to need a certain number of watts to make a given lengt cut---probably a watt or two per inch for insulation foam for a prett thin wire. So you need something like 72 to 144 watts for a six-foo bow.
You can get those watts by using a thinner wire, which has highe resistance, but then you need more voltage to push the current throug such a long, high-resistance wire.
Or you can get them with a thicker wire which has less resistance, an doesn't require as high a voltage, but in that case you need more amp to get the wattage.
To get 144 watts at 24 volts, you'd need 6 amps of current---24 volt times 6 amps is 144 watts. Your wire should have a resistance of ohms total---24 volts divided by 4 ohms is 6 amps---or two thirds of a ohm per foot.
Using dimmed wall current without a transformer is dangerous. Th transformer isolates the bow from the wall current, so that you can' just short from the (electrically) "hot" wire to any old ground, suc as your garage floor.
To get enough amps for a long bow with thickish wire, you can gang tw reasonably-high-amperage transformers of the same make and mode together in parallel. So, for example, if you go with the 5-am 24-volt transformers from All Electronics, you could make a 10-am 24-volt power supply.
Another possibility is using as 2x step up/step down transforme designed for foreign power conversion. Instead of using it to ste 120 V up to 240 V, use it backwards to step 120 V down to 60 V. If yo do that, you need the kind of voltage converter that's a rea transformer, not a solid-state switching device, and it should be rate for the amps you need, not just the watts. (You'll be running it a half the voltage and therefore twice the amps per watt, so you only ge half the rated wattage before exceeding the amp capacity.)
Unfortunately, hefty transformers of either kind aren't particularly cheap unless you find them surplus on eBay or something. (Al Electronics has a 500-watt foreign voltage transformer for about $50 which you could use as a 4.17-amp, 250-watt 60V step-down transformer. They also have a 300-watt one which you could use as a 2.5 amp, 150-wat 60V, for about $38; that would probably work with a fairly thin high-resistance wire, since you have 60V to work with, and that wouldn't require a lot of amps.)
--
drcrash
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Thank you... I'm not entirely confused, but would like some specific direction....
I don't mind spending 50 dollars on it, even if it is a one time deal.... Mainly because it would make my life a lot easier, and give me a better finished product...
So, I use #16 or #20 leader wire... I can build the bow with a spring on it without any trouble....
It only thing I'm missing is a specific transformer name/size.... I've read the below, but don't want to build anything. So, can you give me a site/name/model, or at least a name and amp/watt size I can Google to find a source....?
Thanks for any help. I'm trying to keep this as simple for me as possible because I still have the trailer to build, which is my main project.
Peter ..........

. So you need something like 72 to 144 watts for a six-foot

So, for example, if you go with the 5-amp

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az21 Wrote:

Actually, I just did some calculating, and 6 feet isn't as hard as was thinking, at least not for pink insulation foam.
I think you should be able to do it with a wire that has a resistanc of a bit under 1 ohm per foot, and a transformer that puts out 24 volt at up to 5 amps, like the one from All Electronics. That will give yo up a bit under 2 watts per inch, which I think should be enough fo pink foam. (24 volts x 5 amps = 120 watts, and 120 watts / 72 inches 1.666... watts per inch.)
(I guess I was misremembering things about 8-foot wires... whic require either more than 24 volts, or more than 5 amps to do more tha about a watt per inch.)
I'm not sure exactly what wire to recommend, but I think the 24V 5 transformer should work. I may be able to validate that (with specific wire) when I get a chance to mess around with this stuff. (O maybe Lee or somebody can say whether 1 2/3 watts/inch is enough fo pink foam.... ?
-- drcras ----------------------------------------------------------------------- drcrash's Profile: http://www.rcgroups.com/forums/member.php?u 693 View this thread: http://www.rcgroups.com/forums/showthread.php?tb949
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Lee Wrote:

Hmmm... maybe the resistance of your wire is higher than I thought.
If it's pulling 4 amps at 24 volts, that's 6 ohms total resistance. (24 volts divided by 6 ohms would be four amps.) And 6 ohms divided b 3 1/3 feet is about 1.8 ohms/ft. (Does that sound right?)
It's also dissipating 96 watts over 40 inches. (4 amps times 24 volt would be 96 watts). So about 2.4 watts per inch... and that's good fo EPP? Interesting. I've had a rough ballpark estimate of "one watt t a few watts per inch" for cutting foam, and it's good to have anothe data point.

What do you mean by "overcut"? Are you talking about the kerf o melted foam around the wire?
Several people have told me that they think thin nichrome wire is goo for CNC cutting, where there's essentially no force on the wire becaus the wire moves at a very steady speed and vaporizes the foam withou touching it... but that thicker stainless is better for hand-cuttin and gravity-fed cutting, where the wire drags in the foam. Th stainless is physically stronger, and you can tighten it up more t keep the cut straight, without breaking it.

I have some 304 stainless wire from Small Parts, Inc (www.smallparts.com, not to be confused with smallpartsinc.com).
They sell 30-foot lengths for around two or three dollars in the siz range we're talking about:
http://www.smallparts.com/products/descriptions/swx.cfm
I got the .011, the .016, and the .020. With shipping, it came t thirteen dollars or so.
I haven't used it much, and I haven't tried cutting EPP yet. (I don' have any yet.)
I roughly measured the resistance (with a cheap meter), and got
.011 3.7 ohms/ft .016 2 ohms/ft .020 1.3 ohms/ft
I've been meaning to do some controlled experiments and more precis measurements, but haven't gotten around to it yet. I figure I can us my car battery charger (which has an ammeter in it) for th experiments.
(If you want to experiment with it, I could send you 5 foot pieces o each of those sizes.)
BTW, if 4 amps is enough, you can wire two 2-amp transformers i parallel, if they're the same make and model. (I did that with th Radio Shack 24-volt transformers when I realized I wanted more than amps.) See my big summary for details on getting the polarity right. http://www.rcgroups.com/forums/showpost.php?pg69158&postcount=2
-- drcras ----------------------------------------------------------------------- drcrash's Profile: http://www.rcgroups.com/forums/member.php?u 693 View this thread: http://www.rcgroups.com/forums/showthread.php?tb949
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Here is my power supply set up.
http://www.utahflyers.org/FCSetup.php
I actually have 5 amp 24 volt transformers now not just the 3 amp show in the picture. The 3 amp seemed to work as well as the 5 amp in thi application but I was over the suggested amp limit.
I had so many questions from the local flyers that I poste instructions on the web. Maybe it will be helpful to some of you.
I started with a 72 inch cutter and used a rheostat and long befor that I cut with 120 volts and lived. I never did get shocked but realize death was just a mistake away. The transformer will heat th 72 inc wire enough to cut but very slowly. To make this long of a cu you need a different wire or preferably more voltage.
I had to experiment to find what worked. I could easy get the heat bu I was using too many amps for the transformer.
I tried two different train transformers as a power supply with variety of cutting wires and neither worked well. I tried my ca battery charger but it hummed so bad I thought I would melt it down.
The set up as shown is the best transformer set up I tried
-- Le ----------------------------------------------------------------------- Lee's Profile: http://www.rcgroups.com/forums/member.php?u694 View this thread: http://www.rcgroups.com/forums/showthread.php?tb949
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Thank you both for the information.. I've printed it all off and have bookmarked the link.. will study it all tomorrow... when I'm more awake..
--

Peter
..........
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I have actually caught a belt off the LOPT on a TV at 21,000 volts but very low current i.e about 1.5 amps, it certainly makes your eyes water but it isnt the voltage that zaps as long as you are healthy with a good heart but the current behind it. In the UK our houses have three main power supplies with lighting being on a 220/240 volt 5 amp ring and power sockets on a 220/240 volt 13 amp system although the feed currecnt is something like 30 amps which is what is the killer, the most powerful is the 240 volt 45 or 60 amp ring that powers showers and cookers etc and this is a definite no-no, you dont play with it unless you like a good smoke. The lighting ring is only 5 amps so no problem and I have caught many a belt of it when wiring in lighting etc and for me all it did was turn the air blue, still not to be trifled with though as is any electrical current,
regards, Terry
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Actually, a zap of only .5 amp can kill you, if it travels across you chest, and your heart.
The big trick of getting zapped, is to make sure it does not travel across your chest, as in from one arm to the other arm.
--
Jim in NC


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True Jim, if your going to take the chance make sure your free hand is not touching anything and well clear of metalwork or whatever that way any juice goes down to ground and if your standing on a good thick carpet with rubber boots on so much the better, when I caught that belter off the Line Output Transformer on a TV it burned a little round hole in my finger which took weeks to heal up because the damage wasnt on the surface like a normal burn but deep in the tissue where it had cooked the meat,
regards, Terry
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I've been hit by 120 volts so many times, I couldn't count them. Not a big deal, so far. I have gotten across some 240 a couple times, but that was not as fun!
I've never been hit so hard that I got my meat cooked from the inside out. That sounds like it would really "suck."
I have a very healthy respect for high output high voltage devices, like TV's. I'm sure you have even more respect, now! <g>
--
Jim in NC


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A good spark will leave a little round burn on the skin like a white spot but the damage is inside and this is why it takes so long to heal as blood vessels and nerve endings are destroyed, all I remember is a loud crack and a sharp pain and next thing I know I am standing 6 feet away from the TV with a funny smell in the air a bit like Ozone, my friend said I just lifted off the floor and moved backwards without actually moving a muscle so it must have been quite a belt. Few years earlier I was working on a job inspecting power feeds for electric vehicle battery chargers which hadnt been used in ten years and was supposed to be dead, 11000 volts and 120 Amps melted 2 inches of my screwdriver when I shorted out a couple of terminals and left me almost blind and half deaf for hours after, thing is do not take chances no matter how safe it may look and dont take other peoples word for it - they may be wrong and it wont hurt them but its your life on the line and you only got one of em, I was lucky but someone else may well have been dead,
regards, Terry
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The biggest danger is when the cutting wire breaks. All of the powe
apparently goes off and it would be easy to forget the cutter is stil charged and waiting like a rat trap to zap you.
When a cutting wire breaks I turn everything off and unplug the powe supply too just to make sure. The light I use on my power suppl reminds me of the power status of the power supply.
On one of the automatic cutters years ago I used insulated wire to pul the bow through the foam because I was having trouble with stretching o the strings to the weight. If you watch the foam cutting video on ou web page you can see the cutter. Some how these wires shorted ou with use and I had live power to the front of the automatic mechanis that I didn't know about. the wires got so hot they melted th insulation. My first indication of a problem was a melting plasti smell. There is always something to watch out for.
The only injury I have had is when hot plastic driped off the cuttin wire onto the back of my hand.
When I cut with the 120 volts I wore welding gloves and rubber sole shoes and made sure I didn't stand on bare concrete. I am extremel safety conscious and made sure that my life insurance is double fo accidental death to take care of my family when I am gone
-- Le ----------------------------------------------------------------------- Lee's Profile: http://www.rcgroups.com/forums/member.php?u694 View this thread: http://www.rcgroups.com/forums/showthread.php?tb949
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Hi Lee, thing to remember is water you can see and gas you can smell, but electricity is silent and you dont know its there until it bites, if there is enough of it you have one chance and thats your lot so dont take it for granted or become too familiar with it to the point you lose respect for it, sure as hell it wont have any respect for you if you get things wrong,
regards, Terry
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Back to foam cutting.
All cutters have a learning curve. There are a bunch of small problem that pop up that make you keep thinking. The CNC cutter have their ow set of problems because of more moving parts, more programing, les ability to react during the cut, more parts to fail. CNCs are also ar bigger to store.
Don't get me wrong I think CNC cutters are the way to go in some case but if you are a weekend builder and want a replacement wing for a ki or want to design your own plane you can get great cuts withou spending all of the money. CNC cutters at their best when they can b fine tuned to cut the same cut over and over
-- Le ----------------------------------------------------------------------- Lee's Profile: http://www.rcgroups.com/forums/member.php?u694 View this thread: http://www.rcgroups.com/forums/showthread.php?tb949
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