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Old Dec 12, 2005, 10:54 PM
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"Gilligan's Island" hotwire foam cutter; easy yet full-featured

Hi everyone,

I thought I'd share this super simple and easy design for a hotwire foam cutter. It's made of bamboo, steel wire, lamp cord and alligator clips. That is all! Yet it provides these powerful features: 1) variable heat control, and 2) easy power requirement: 12V RC power or car battery!

It's composed of two integrated sections, the bow for cutting, and the "variable resistor", where the same wire is just wrapped around the form for extra length, i.e. additional resistance. By clipping the alligator clip to different parts of this "variable resistor", the total resistance can be adjusted to provide the correct cutting temperature.

This design can be adapted easily, such as making the variable resistor section separate, and adding an ON/OFF switch such as a household light switch.

Enjoy!

-Vince
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Old Dec 12, 2005, 11:03 PM
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A few details...

The cutting wire is 24-gauge steel wire. The size is a good tradeoff between tensile strength and high enough resistance for heating. It was available here in the Philippines in the flower market in large bundles. Folks report very good results with guitar string wire also, but is more expensive and shorter.

The wood doesn't burn because the temperature to melt the plastic is much lower than the kindling temperature of wood.

The alligator clips need to be small so there is no gap in the teeth and clamps the heating wire well.

The connecting wire is 16-gauge lamp cord. 18 or even 20-gauge should probably work, but check the wire if it overheats.

Make sure the cutting wire is steel. Steel has 7 times more resistance than copper and you need this resistance for correct heating.

The power supply will need to provide around 6 Amps. Inrush current is a bit higher since the resistance is lower when the wire is cold.

FYI, this came about because I forgot to bring some wings for a homebuilt airplane and I now have to hotwire a new set of wings while I'm overseas. I feel like the Professor in Gilligan's Island.

I figure this will be very useful for those guys in the sticks who have limited access to stuff, or just want a super simple design with hardware store equipment.

Enjoy!

-Vince
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Old Dec 12, 2005, 11:25 PM
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Ahh.. now that is prettttty nifty..
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Old Dec 12, 2005, 11:52 PM
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I wonder if you could use a light dimmer in place of the vr you created? They seem to hold up to 600 watts for the better dimmers, which should be plenty enough at 6A, 12vdc. Not only could you "dial in" the heat setting you'd like, but also use the switch to turn on and off the cutter.

Jus' me thinking out loud....
Chris
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Old Dec 12, 2005, 11:58 PM
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Light dimmers work.. Lots of people do it.
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Old Dec 13, 2005, 12:09 AM
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As you can see, I'm new to the hobby, sorta! Sorry for that post!!!


Chris
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Old Dec 13, 2005, 12:35 AM
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the only trouble is that light dimmers don't work for switching power supplies, eg old computer supplies, they only work for inductance transformers.

this is because the switching supply automatically adjust the duty cycle to make the output 12V, no matter what the input.
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Old Dec 13, 2005, 01:22 AM
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flyinhawaiian - no sweat.. Your post was fine..

minifly - I was just about to ask if a PSU could be used..

What would you reccomend instead of a light dimmer, if one wanted a "knob."
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Old Dec 13, 2005, 03:08 AM
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if you want a 'knob', i would suggest you get some sort of '12V regulator' that goes between the power supply and your wire, just make sure it can handle tha amps you will be putting through it.

come to think of it, an old brushed esc would work great, you would just need some sort of servo tester with a knob on it. even a cheap 10A esc would be fine...
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Old Dec 13, 2005, 03:49 PM
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Hi Chris,

Yes, guys have used lamp dimmers successfully, but I would think you'd have to use it on the line side of the transformer.

They handle high voltage lower current so a rheostat type of dimmer would have very poor adjustment range at 12V. An electronic (triac) type dimmer cannot switch DC and must be used on the primary side.

-Vince

Quote:
Originally Posted by flyinhawaiian968
I wonder if you could use a light dimmer in place of the vr you created? They seem to hold up to 600 watts for the better dimmers, which should be plenty enough at 6A, 12vdc. Not only could you "dial in" the heat setting you'd like, but also use the switch to turn on and off the cutter.

Jus' me thinking out loud....
Chris
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Old Dec 13, 2005, 03:52 PM
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Yes, I thought about this too, and I think it would work great.

-Vince

Quote:
Originally Posted by minifly
if you want a 'knob', i would suggest you get some sort of '12V regulator' that goes between the power supply and your wire, just make sure it can handle tha amps you will be putting through it.

come to think of it, an old brushed esc would work great, you would just need some sort of servo tester with a knob on it. even a cheap 10A esc would be fine...
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Old Dec 13, 2005, 03:58 PM
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It's supposed to be simple

A lot of great ideas, guys. But I think you're missing the point. If I have access to anything, I can build this a gazillion different ways.

But where I'm at, there are no hobby shops, no Radio Shacks, no electronic supply shops, not even a big Home Depot. When I ask for a transformer, they show me, I kid you not, a dozen different sizes of 220-to-110V transformers, but look puzzled when I ask for 6V at 6 Amps.

This was the simplest design I could come up with which still provides adjustable heat and powered by a universally available power source (car battery).

-Vince
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Old Dec 13, 2005, 08:15 PM
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brilliant
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Old Dec 13, 2005, 08:53 PM
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I use an automobile battery charger to go from 110vac to around 12vdc. The battery charger is plugged into a variable transformer on my setup, but you could also adjust the length of the wire as vince has.

- Brad
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Old Dec 13, 2005, 09:58 PM
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Quote:
Originally Posted by vpatron
This was the simplest design I could come up with which still provides adjustable heat and powered by a universally available power source (car battery).

-Vince
its so simple, its cool. no moving parts = good. nothing will break or be a pain in the ass.

nice idea
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