# hot wire cutter theory

Now i know this topic has been covered a fair bit but unfortunately i am in a wee bit of a pickle of sorts.I have made up my first hot wire cutter using nichrome 80 28 guage (0.32 mm) wire using a 12v battery charger.but upon giving it a test run my wire only got to a pleasant warm temp which was not going to cut a thing.So under a little bit of an investigation I came up with the following results

using around 1.5 - 2 meters of wire i have a measured resistance of 26 ohms

with a 12 v battery charger its only pulling(due to V=I/R) 12/26= 460mA which is obviously not enough current to get my wirecutter up to temp.So obviously i will need more voltage and am going to have to buy a alternative source.This is where i run into trouble.Due to wanting to spend more money on surfboard materials than an expensive power source for my wire cutter i would much rather do it on the cheap.It just so happens that i have a fan speed controller ready for the job but due to it only reducing the voltage to a certain value(which i havent measured) and not all the way to zero the transformer i buy is going to need to be around the right range for the job.which therefore means i am going to need to know what are the values need for the job itself.

unfortunately the only data that i have been able to pull off the net on nichrome wire that is of any help to me, is for nichrome 60 and is a “current temperature characteristics of nichrome 60 wire” table.

on this table a temp of 208 degC requires around 2 amps for 25 gauge wire.Now i was pulling about a quarter of that current and due to the fact that my wire was warm but i was still able to hold it i would guess that the temp would have been around the 40 - 50 deg C mark so one would assume that nichrome 60 and nichrome 80 characteristics are similar.If that were the case for 200 degC i would need (V=I/R) 226 = 52 v on the secondary, and due to P=VI a min of 522 vA rating on my transformer.

so my question is this.

what is the optimal temperature required for cutting eps foam?

can anyone confirm that the “current temperature characteristics of nichrome 60 wire” are similar to nichrome 80?

does anyone have a copy of “current temperature characteristics of nichrome 80 wire” table

and finally

If you are using nichrome 80 wire with a length of around 1.5-2.0 meter range at 28 gauge(0.32mm) what voltage are you having success with?

well i hope this hasn’t put anyone to sleep, and if you took the time to read all of this thanks for putting up with the geek in me.

any input welcome

cheers guys

James

You could do your own tests by using the wire you have, but a shorter length. Try cutting foam with shorter lengths until you get a length that cuts at the speed you want with your 12v supply, then work out how much more voltage you would need. The relationships are all linear so it’s an easy calculation.

By the way, slow and cool cutting gives easier, more accurate results than fast hot cutting. The downside is obviously that it takes longer.

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…… does anyone have a copy of “current temperature characteristics of nichrome 80 wire” table? ……

I don’t have a table for Nichrome 80, but here’s a link for Nichrome 60:

http://www.heatersplus.com/nichrome.htm

Gidday mate

You’ll snap a lot of wires using 28 gauge. Got it from Dick Smiths eh?

call these guys and get the right stuff- Sandvik (09)273 5888

I bought 22 gauge and it’s over-kill. I bought 10 metres and havn’t even snapped one wire yet!

24 gauge should be good.

You’ll obviously need more power for the thicker wires.

I couldn’t find any decent battery chargers in NZ they all have short-circuit protection, but I didn’t test them.

I ended up going to JayCar electronics and bought a kitset and soldered up my own thing.

It’s a motor speed controller and it works off my car battery.

You really want something that has variable power.

Are you using metre long wire!? you’ll never keep that tightly strung! go for a bow about 600mm

Kit

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Are you using metre long wire!? you’ll never keep that tightly strung! go for a bow about 600mm

maybe I am taking the wrong approach then, my block of foam is 3.6 x 1.2 x 0.6 .I was planning on cutting across the 1.2 meter span giving me two blanks out of one cut.I do have bass guitar tuners one each side enabling me to keep the string tight but maybe i should go the other way and just do more cuts if this is going to be a problem.

first off you need dc power for safety

i use a dicksmith lab supply with variable voltage and amps

i find the settings i need vary for the width or speed of my cut

if i didnt have the lab supply id try and make my own (there would be kits as kit says)

basically im using about 7 amps at 13 odd volts and i can cut a side in a few minutes single handed

yeah and i use wire at about .7 to .8 of a millimeter so it doesnt break when its reallly hot

i wouldnt use any less then .5 to .6 mm thick wire

a broken wire will stuff the blank

i think your pretty keen cutting a span that big

but it should be possible if you have a helper

but you should have asked me first lol

maybe you should send the block back and have it recut to 600

Get yourself a 24v battery charger, that will sort out the heat problems

Try a guitar string on a 600mm bow, and a spring to keep the tension , as it seems you have too much resistance in the wire to get the current up.

turn your block on its side and cut the 600 side you’ll still get as many blanks out of it.

Its hard enough keeping the wire on the guides at 600mm wide, let alone 1200mm.

Regards

Daren

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maybe you should send the block back and have it recut to 600

yeah it is 600 deep so its no problem flipping it over and cutting it that way i will just have to make a new bow. and thats not a problem either as i still have a plenty of board.

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first off you need dc power for safety

i use a dicksmith lab supply with variable voltage and amps

if i use the fan speed controller i will be using it in conjunction with an isolating transformer so there will be no fault path to earth so should be reasonably safe.The only problem with the fan controller is it does not reduce the whole span of the voltage supply.I just tried it out with a low power electronics transformer that i have 230/15v and i can get down to around 8.5 volts with that controller which means i need to be careful when picking my transformer so that the voltages required are in that top half of the transformers output span.I would love to spend 250 or so on a variable dc power supply but as my wife has all ready pointed out, this is getting to be an expensive hobby. so i took that to mean “find a cheaper option” :).

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i think your pretty keen cutting a span that big

but it should be possible if you have a helper

yeah my wife is happy to work one side of the cutter and that was my plan.

Y’know, I see lots of discussion here about how to futz with the voltage, but myself, I have found that if you don’t have enough amperage in the first place the ability to futz with voltage doesn’t matter.

Most of the variable power sources put out X watts at a variable voltage…which is fine, but when you have

V=IR

Then if your power supply can’t melt foam at its top setting it really doesn’t matter if ya crank the voltage down some.

Now, have a look at http://www.heatersplus.com/nichrome.htm and http://en.wikipedia.org/wiki/Polystyrene

You will see that polystyrene melts at 240°C , which means that you want your wire temp up to something like 300°C, being as if it’s much lower the wire will drag and mess up your straight lines if the temp is just barely at melting point. Besides which, the melting of the foam itself transfers some heat, no?

Okay, to CURRENT TEMPERATURE CHARACTERISTICS OF NICHROME 60 STRAIGHT WIRE - nothing listed for #26 wire, but plenty for #25, and that’s close enough, gives you a fudge factor.

So, to crank a #25 nichrome 60 wire up to 316°C ( close enough ) , that takes just under 2.5 amps…per foot. Call it two feet, near enough to 600mm. Which gives you five amps total that you need. Figure more than that, you’ll have losses of one kind or another. Note that the table gives numbers for ‘Applying only to straight wires stretched horizontally in free air.’ - again, the actual foam melting reduces the heat, as do other things.

Okay, does your 12V battery charger have a six amp setting? Use that, and you can always put in an inline rheostat of some kind to lower it down and fine-tune your power and heat.

Oh, one more thing - rather than the guitar tensioner, I would use a light spring. See, when the wire heats up it both gets longer and softer. Longer is okay, providing it isn’t a lot longer. 'Cos it also gets softer, and when there is too much tension on the stuff it stretches, and when it stretches it’s like pulling taffy or making Chinese noodles the traditional way, the wire gets thinner someplace which means that for that particular spot the resistance goes up, so it gets even hotter right there, which means it gets softer and stretches more in that spot, which means it gets thinner, and hotter, and softer and stretches more and son of a gun, it breaks right there in that hot spot.

I’ll even bet you can guess how I found that out. Wanna buy some short pieces of nichrome wire?

But if you use a light spring and just pre-tension it a little, well, the tension on the wire varies with how much the spring is stretched, so as the wire stretches the tension on it gets less, and thus you’re less likely to bust a wire.

hope thats of use

doc…

I made my own for under AU\$20.

Works a treat.

yeah Hunty,

I also thought about cutting two blanks at once, but flagged it.

The wire will most likely wander too much, it wanders a little even at 600mm.

Also, it’s easier to get the block home in my car if it’s cut in half!

Man, you should have seen me driving down the motorway with the massive block on the roof, it’s bending in the wind, i’ve got my hand out the window holding the straps to stop it blowing away… I thought it was gunna break in half.

Now I just get them to cut it in half for me.

I found it really hard to find a second hand bench power supply (school power pack), even living in Auckland.

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Okay, does your 12V battery charger have a six amp setting? Use that, and you can always put in an inline rheostat of some kind to lower it down and fine-tune your power and heat.

thats what im getting at, no matter what amount of power my charger can deliver, at that 26 ohm resistance 12v is only going to push half an amp through the circuit.My resistance or load is constant so i need more voltage to be able to push more current through.

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You will see that polystyrene melts at 240°C , which means that you want your wire temp up to something like 300°C, being as if it’s much lower the wire will drag and mess up your straight lines if the temp is just barely at melting point. Besides which, the melting of the foam itself transfers some heat, no?

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So, to crank a #25 nichrome 60 wire up to 316°C ( close enough ) , that takes just under 2.5 amps…per foot. Call it two feet, near enough to 600mm. Which gives you five amps total that you need. Figure more than that, you’ll have losses of one kind or another. Note that the table gives numbers for ‘Applying only to straight wires stretched horizontally in free air.’ - again, the actual foam melting reduces the heat, as do other things.

Now these are the two golden nuggets of information that i’ve been looking for(or missing), one thing it looks like i hadn’t been paying attention to is the “per foot” part in that table (which turns on the light bulb now for me being confused as to why silly said he was using 7 amps in his setup). so the longer i go with the wire, im going to need even more current and therefore voltage(most likely increasing the transformer cost) .So cutting @ 600 wide is going to be more efficient cost wise and physically as well, so it looks like i’ll be making a new bow.

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Y’know, I see lots of discussion here about how to futz with the voltage, but myself, I have found that if you don’t have enough amperage in the first place the ability to futz with voltage doesn’t matter.

Most of the variable power sources put out X watts at a variable voltage…which is fine, but when you have

V=IR

Then if your power supply can’t melt foam at its top setting it really doesn’t matter if ya crank the voltage down some.

I can see what your saying here, but given that i’m trying to do it on the cheap by using a transformer it is unlikely that i will get one that has the exact right output voltage to give me the correct current at my circuit resistance.So its most likely i’ll have to go up to the next voltage level transformer and reduce it to the voltage needed, or alternatively control the output current with some kind of current limiter.But like i say i all ready have a fan speed controller so its most likely going to be voltage.

thanks heaps for the info doc, i think thats probably most of the puzzle peices that i’ve been looking for now.

Epaiana ahau!!

James

look on ebay for an old model train transformer, that’s what I’ve used in the past and you can tailor the voltage to just where you need it. Also, to make a really good bow, go to your local hobby store and pick up the biggest steel wire (piano wire it’s usually called, my place had 1/4 as the largest) and get around 3-4 ft of it. Make sure it’s the spring steel type stuff. Now, get a piece of hardwood and drill holes angled out at around 30-40 degrees or so and put equal lengths of your wire in there, and file notches on the outside of each wire. Now make your cutting wire slightly smaller then the width between the ends of the spring wires, and squeeze them together and slip it on. THis way, no matter how the wire stretches when your cutting, it’s always tight. This is how the pro machines do it, backyard style.

Nathan

nichrome is far better

it retains its tensile strength at high temperatures

do some searching…check out this one…

http://www.swaylocks.com/forum/gforum.cgi?post=235104;search_string=hot%20wire;#235104

Search some more…

Using cheap wire with 8 volts I can cut all day without a problem. Just cuts slow. Lots of fumes. have a fan handy!!!

We used a battery charger with giutar strings at Surf4fins house no problem…6 volts…

Nichrome works better.

With the 24 gauge Nichrome wire we could set the battery charger at 12 volts and get good results

Wear eye protection…

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Okay, does your 12V battery charger have a six amp setting? Use that, and you can always put in an inline rheostat of some kind to lower it down and fine-tune your power and heat.

thats what im getting at, no matter what amount of power my charger can deliver, at that 26 ohm resistance 12v is only going to push half an amp through the circuit.My resistance or load is constant so i need more voltage to be able to push more current through.

Ahmm- okay, lets look at this a different way. See, current through the whole circuit you don’t really care about.

Huh? Right. Think about it a bit. Consider, oh, household current stuff. You got yer five watt night light bulb, your 100 watt reading light, two horsepower electric motor, your 15 amp electric fire, all kinds of stuff, running on the same voltage. How can that be? Amperage supplied to 'em.

Go back to the basic equation, V=IR

And ( doing a little algebra) V/R = I or I=V/R

So at 12 volts ( though prolly closer to 14.5 for the max voltage that comes out of your basic car battery charger ) volts the most you are gonna get through a nichrome wire with a resistance of 26 ohms is a half an amp.

Yeah? So what? You don’t care about that. Or shouldn’t. Because the term above is through the nichrome. Lets draw a circuit here;

Let’s say you supply 4 amps to the circuit, and lets consider the lead wires to have an effective resistance of zero. And let’s measure current across interval A…and we get four amps.

Okay, that was fun, how about across interval B? You measure there, it’ll be more like half an amp. That’s your half amp through the circuit. So, where did the other three and a half amps go?

Well, what happens when you push amperage over a resistance? The resistant thing heats up. Sonofagun, just what you wanted. You lose power ( current and or voltage) across the nichrome segment, which is fine, and that becomes heat. Basicly, you’re burning electricity. The current goes away and becomes heat.

So, you don’t care about current through the whole circuit, what you care about is current delivered to the nichrome. If you have just about zero current downstream from that, nifty. It doesn’t matter. Higher voltage is all well and good, but low voltage will work just fine.

Now- you had half an amp of current in your circuit after the nichrome, max. Anything more than that delivered to the nichrome got turned into heat. But it only heated up to what, 75°C? Not enough amperage: divide that amperage by six or so ( the rough length of your 26 ohm resistance #26 wire nichrome segment in feet ) and compare it to the values on http://www.heatersplus.com/nichrome.htm - my guess is you’ll find something like half an amp per foot, if that.

Shorten the length of your nichrome, your resistance goes down and the current that can go through it goes up. But, for the same power source, the current per foot available for heating the wire goes up too.

The above diagram also tells you where to put the rheostat in the circuit, no? A rheostat is essentially a variable resistor that controls the current that can go through it. In a DC circuit, power flows from the negative to the positive, so you’d put the rheostat in interval A.

Where does this V=IR stuff come in handy?

Consider, oh, long distance power transmission lines. Smaller wires have greater resistance than larger wires. Let’s take that as a given. And some really good materials for long distance wires ( good tensile strength and low cost) have a higher resistance than, say, copper. So to deliver the most current over a long distance, given the above, we do a little algebra: I=V/R , and to maximise I given that R is a large number, they jack up V quite high. Thousands of volts.

Same deal goes with extension cords. Use too light an extension cord ( with too high a resistance ) for the current load and the voltage at the far end is lowered to where the tool can’t get the voltage to turn over and it burns out. One of my pet rants is about people I have worked with who use a \$500 tool with a \$9.95 extension cord…and burn out their tool.

Anyhow- executive summary is jack up your amperage supplied to the nichrome, sufficient to heat it up to your desired temperature. If you don’t, it won’t.

hope that’s of use

doc…

A good option is a laptop powerpack. I use one with a 600mm hotwire. It produces about 70 watts which is just enough - 90 to 100 watts might be best. You can get them off Trademe for \$30 to \$40. And I use a cheap battery charger producing about 20 watts for a 200mm hotwire for cutting the profile. I use the 28 nichrome and as long as you’re careful it doesn’t snap too often.

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…… Ahmm- okay, lets look at this a different way. See, current through the circuit you don’t really care about. ……

Yeah? So what? You don’t care about that. Or shouldn’t. Because the term above is through the circuit. Lets draw a circuit here;

Let’s say you supply 4 amps to the circuit, and lets consider the lead wires to have an effective resistance of zero. And let’s measure current across interval A…and we get four amps.

Okay, that was fun, how about across interval B? You measure there, it’ll be more like half an amp. That’s your half amp through the circuit. So, where did the other three and a half amps go?

Well, what happens when you push amperage over a resistance? The resistant thing heats up. Sonofagun, just what you wanted. You lose power ( current and or voltage) across the nichrome segment, which is fine, and that becomes heat. Basicly, you’re burning electricity. The current goes away and becomes heat.

So, you don’t care about current through the whole circuit, what you care about is current delivered to the nichrome. If you have just about zero current downstream from that, nifty. It doesn’t matter. Higher voltage is all well and good, but low voltage will work just fine.

Sorry doc, but current is of prime importance since it is the current (and wire guage) that determines the wire temperature; wire length (and wire guage) determine the resistance (where the ohms/ft come into consideration); the current and the resistance determine the voltage required. So your power supply has to be capable of providing both the desired current (wire temperature) and the voltage required to drive that current through the resistance of your circuit (the latter being dependent on wire length [and wire temperature]).

In your circuit example, the same current flows through the entire circuit (i.e. Interval A and Interval B have the same current flowing through them). Current per foot is meaningless. What changes is the voltage, which drops as one moves along the length of the wire. Think of water flowing through a pipe…the same flow (“current”) occurs everywhere along the pipe, even if the diameter of the pipe changes from section to section. It is the pressure (“voltage”) that changes along the length.

FWIW, I incorporate an ammeter in my hot wire system. Then all I have to do in order to always reproduce the same wire temperature (neglecting the differences between the portion of the wire in air and the portion moving through the foam) is set the current the same–no matter whether the wire length is 1 ft or 4 ft (the voltage required, of course, changes with the change in wire length).

mtb

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Huh? Right. Think about it a bit. Consider, oh, household current stuff. You got yer five watt night light bulb, your 100 watt reading light, two horsepower electric motor, your 15 amp electric fire, all kinds of stuff, running on the same voltage. How can that be? Amperage supplied to 'em.

Go back to the basic equation, V=IR

And ( doing a little algebra) V/R = I or I=V/R

The current in the circuit is a result of your load and supply voltage using the formula you have given.Current(amperage) is more drawn from the supply rather than supplied to the load.Think of it using the water analogy, more pressure(voltage can be thought of as pressure) = more water flow(current or amperage),less pressure = less flow.But pressure determines the current for a given load.

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Let's say you supply 4 amps to the circuit, and lets consider the lead wires to have an effective resistance of zero. And let's measure current across interval A...and we get four amps.

Okay, that was fun, how about across interval B? You measure there, it’ll be more like half an amp. That’s your half amp through the circuit. So, where did the other three and a half amps go?

Well, what happens when you push amperage over a resistance? The resistant thing heats up. Sonofagun, just what you wanted. You lose power ( current and or voltage) across the nichrome segment, which is fine, and that becomes heat. Basicly, you’re burning electricity. The current goes away and becomes heat.

So, you don’t care about current through the whole circuit, what you care about is current delivered to the nichrome.

the circuit you have drawn is a series circuit, in a series circuit the current is the same in that whole circuit.Current does not vanish out of the system as heat, it does do work producing the heat though.Maybe a good way to explain it is give your self a chinese burn with your finger.your finger will not vanish as heat, but work will be done creating friction and heat.So to create the amount of heat we want, we need the required amount of current(flow) and since we have a constant load(the wire) we need the correct voltage(pressure) to gain that current(flow).

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Consider, oh, long distance power transmission lines. Smaller wires have greater resistance than larger wires. Let's take that as a given. And some really good materials for long distance wires ( good tensile strength and low cost) have a higher resistance than, say, copper. So to deliver the most current over a long distance, given the above, we do a little algebra: I=V/R , and to maximise I given that R is a large number, they jack up V quite high. Thousands of volts.

they are not trying to deliver the most current over a long distance they are trying to deliver the most power with the least amount of losses in the supply network(circuit).The higher the current is in a given resistive network(circuit) the more power will be lost as heat in that network, which means less

power delivered to the consumer.Power(in a purely resistive circuit) is a product of voltage x current P=VI so to get less current for a given amount of power(and therefore less losses)you need to increase the voltage(which is why long distance transmission lines use such high voltages).

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Anyhow- executive summary is jack up your amperage supplied to the nichrome, sufficient to heat it up to your desired temperature. If you don't, it won't.

this is very true, only trouble is i need more voltage to do so.