Hot wire

Aaron N

What is the start up cost for cutting Foam!

Thanks Aaron.

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> Aaron N
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> What is the start up cost for cutting Foam!
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> Thanks Aaron.
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Now that's a tough one. You can buy an automatic cutter for a hundred dollars on up, or you can make a simple bow for little or nothing.

For a power supply you can use something elaborate like a variac, or you can use something simple like a model train transformer (We have used both HO DC models and Lionel AC models) or even a light switch dimmer (This is, perhaps, the riskiest, because you are hooked to the electric circuit, albeit through a current limiting source).

And then you need to buy foam and make your patterns. How elaborate do you want to go? How many of these things are you going to be making?

Well to start with LE molds and Top Block molds.
I would like to try to make one.
Leonard could you make list of parts needed?

Thanks Aaron

This is what I have made and it works very well. Didn't cost a thing cause everything was scrounged from old projects. I made my bow so I could cut a 48" wing panel, but the size is up to you. Be careful! You won't electocute yourself but you will get a nasty burn. I have suspended mine from the cieling with some strips of rubber, that way I don't have to lay it down between cuts.

I use an even simpler bow. 1x2 lumber. One long piece. One short piece (about 4") nailed to one end. Another piece, attached to the other end, that's twice as long (8" or so) that pivots in the middle. Wire is attached to the tops of the cross pieces with a small nail. On the other end of the pivoting piece is a larger nail, used to attach some big rubber bands to provide tension. No need for springs, insulators, alligator clips, etc.

Cutting wire is a steel guitar string. .015" control line wire works well, too.

27" wire needs about 13-14 volts. I can stretch the wire so tight it sings when you pluck it. That way, no sagging wires. I've cut hundreds of wings using this type of bow.

I saw this thing on eBay. http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=3149106292&category=19164

Just what you need, something complicated.

In addition to what the top gentlemen suggest, this website might help too.
http://www.public.iastate.edu/~orman/air/cutter/hotwire.html

if you use a dimmer switch it puts 110v on the wire. Very dangerous.(Don't ask me how I know!)
If you use a dimmer put an isolation transformer, 20-30 volts, between the wall socket and the dimmer. That limits the voltage on the wire, but all dimmers may not work properly at the lowered voltage.

If you use a variable transformer make sure it is grounded properly. One side should be at earth ground, the other end of the transformer is the working voltage- which can go up to 130 volts or so. Again, be careful, especially working on concrete floors.

Safest setup is a short bow, 24 inches or so. Run it off a car battery and adjust the length and diameter of the wire to get the right heat.

"If you use a dimmer put an isolation transformer, 20-30 volts, between the wall socket and the dimmer."

The light dimmer should go in the primary (110 volt) side of the
transformer. The cutting wire goes in the low voltage side. I have used this system for more than 15 years. Be sure to get a light dimmer rated for inductive loads. There are details on wiring one up here:

http://www.charlesriverrc.org/articles/tools/joedydruliafoamcutterps.htm

For templates 1/16 plywood works fine. I usually use 1/16" G10 printed circuit board material.

I'm glad you described the SAFER way, with a dimmer switch and an isolation transformer, before someone gets killed on line voltage. Radio Shack has a 24V filament transformer which works well for this.

I add a 1A fastblow fuse and a switch in series on the hot (black) side in the primary circuit.

A variac is "smoother", you CAN use it on the secondary of a 24V isolation transformer (recommended to reduce maximum voltage and provide maximum adjustability). The variac is actually an adjustable transformer, so its output is (smooth) sine wave voltage, while a dimmer switch is a solid state switch which is "chopping" the full line voltage, turning it on and off, to reduce power.

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"If a man writes a book, let him set down only what he knows. I have guesses enough of my own." -Johann Wolfgang von Goethe

"I'm glad you described the SAFER way, with a dimmer switch and an isolation transformer, before someone gets killed on line voltage. Radio Shack has a 24V filament transformer which works well for this."

On that topic, I've heard about using an auto battery-charger (i.e. the thing meant for boosting or recharging a car battery), with a dimmer switch for controlling the voltage. Would that set-up be safe? I think the charger converts the house current to 12VDC? I happen to have a charger, and was thinking of using this idea...

>"I'm glad you described the SAFER way, with a dimmer
>switch and an isolation transformer, before someone gets
>killed on line voltage. Radio Shack has a 24V filament
>transformer which works well for this."
>
>On that topic, I've heard about using an auto
>battery-charger (i.e. the thing meant for boosting or
>recharging a car battery), with a dimmer switch for
>controlling the voltage. Would that set-up be safe? I
>think the charger converts the house current to 12VDC? I
>happen to have a charger, and was thinking of using this
>idea...

A dimmer switch is NOT a DC device, so it would have to drive the AC power to the battery charger. A (cheap) battery charger is probably just a transformer with a full wave rectifier (no special voltage regulator circuit) on its output, so it may work fairly well. It should be safe, because its output would be a low voltage, and it is isolated. Try it.

Also, let me emphasize here that variacs (all that I have seen) are auto transformers. This means they definitely should be used with an isolation transformer for safety. This is better anyway, as it provides a finer adjustment on the (lower transformer voltage) output. That is, a complete turn on the variac knob changes the output voltage from 0 to ~24V, versus 0 to 110V (line voltage).

One more thing. I use (braided steel) CL flying lines instead of nichrome wire for the cutter. I'm sure there's good reason to use nichrome, but I'm not.

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"Whatever you can do, or believe you can, begin it. Boldness has genius, power and magic in it." -Goethe

>
>Also, let me emphasize here that variacs (all that I have
>seen) are auto transformers. This means they definitely
>should be used with an isolation transformer for safety.
>This is better anyway, as it provides a finer adjustment on
>the (lower transformer voltage) output. That is, a complete
>turn on the variac knob changes the output voltage from 0 to
>~24V, versus 0 to 110V (line voltage).
>
>One more thing. I use (braided steel) CL flying lines
>instead of nichrome wire for the cutter. I'm sure there's
>good reason to use nichrome, but I'm not.
>
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>
>"Whatever you can do, or believe you can, begin it. Boldness
>has genius, power and magic in it." -Goethe

Many low cost batter chargers only have 1 diode in them. If you want to use a battery charger for a power supply, you are going to need one of the larger ones. You will need as much as 20 volts to cut foam. If you can find an old booster style charger that ought to do it. Nicrome takes a little less voltage than old control line cable, but nicrome streches with heat and control line cable does not.

Always always always use an Isolation transformer when using a variac. Your life is not woth the $20 you can get them for. I have my variac mounted on a board with the isolation on it so I don't get lazy. At work, I won't test anything without an isloation transformer. Also ground one leg of the secondary (low voltage side) of the transformer to be extra safe.

If you don't know the differnce between an auto transformer and a isolated one, make a friend who does to check it out before you turn it on.
Dave

TO All:

Be aware that not all "variacs" are isolation transformers. Many are autotransformes that do not provide isolation.

If you don't understand the difference, you are probably in the danger zone setting this all up.

FWIW

While we are at it, is one kind of foam better than any other to use?
I'd like to cut some foam bucks for balsa molding as well and am wondering what kind of foam to use for that purpose and where to get it.

Thank You All some great info!

I will be very careful building my HOT WIRE foam cutting
machine!

Thanks again Aaron Nottingham.

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>While we are at it, is one kind of foam better than any
>other to use?
>I'd like to cut some foam bucks for balsa molding as well
>and am wondering what kind of foam to use for that purpose
>and where to get it.
>
>Cy.

Major cities have plastic supply houses that sell large chunks of foam. An old friend who cut a lot of foam wings went to a place that would cut him any size slab he wanted. One that he got filled the back of a full-size pickup. You want "virgin" foam that has not been recycled. The home remodeling store foam may have trash in it that will foul up a cut. I would think that for molding blocks and the like, heavier foam would be fine and a little sturdier. Wings need to be made of 1 pound/cu foot foam. Isn't the white wing foam a polystyrene? BTW, scraps of Formica countertop material make wonderful patterns that can be polished to a very smooth finish (sand with 1000 grit paper or finer), so no snags while cutting. Leaving long points on the ends of the patterns allows the wire to glide in and out smoothly.

http://webpages.charter.net/rcfu/ConstGuide/FoamWing.html

God bless you!

In Christ,

If you use a Variac, you still need an isolation transformer!

All Variacs I have ever used (including some big 3 phase ones, are Auto transformers, and do not isolate you from the load (wall plug)

Using a variac alone is dangerous, not mater how it is grounded.

You can get an isolation transformer from lost of places are they are not expensive.

Radio shack has a 24 volt unit that is OK, but it is rated at one amp. A foam cutter pulls about 1-2 amps, but a foam cutter is never on for more than a few minutes so it should not overheat. Better sources are 24 volt control transformers used in Furnaces, and HVAC systems ect. look for one that is rated for 2 amps on the secondary at 14volts, about 30VA or so. You can get them from electrical supply houses. An old boat anchor battery charger is a great choice for a isolation transformer.

Absolutely...

I replied to this effect before I saw your caution.

Good Catch!

A few things.

1) Nichrome isn't strong enough. Stretch it moderately, and it breaks. Steel is better. The full size homebuilders use aircraft safety wire, but CL flying wire works pretty well (.015 or .018).

2) Battery chargers put out 12V, 14V max. This really isn't enough unless your wire's pretty short.

I made my own hot wire supply using a circuit I found online, and a pair of 12V transformers meant for halogen lights. These can handle something like 4 amps, no problem, and were CHEAP CHEAP CHEAP. Check with your local lighting contractor. Otherwise, look for cheap transformers at surplus places like America Science and Surplus, or All Electronics.

Here are the circuits I used, using a dimmer or a variac.

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With 30" of steel wire on the bow, the dimmer has to be turned up almost all the way. The variac has the ability to put out more than 110V, so it can deliver more power.

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"On the outskirts of every agony sits some observant fellow who points." -Virginia Woolf