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Schematic for dimmer, doorbell transformer foamcutter
Power supply
Getting the power from your 110 volt receptacle to a suitable foam cutting format presents several problems. A standard light dimmer can be successfully used to vary the heat, but it can't heat the wire in a direct circuit. With a light dimmer powered by straight line current, you are taking a great risk of electric shock. Also, a guitar string is not going to be longer than about 2-3 feet (unless you get it bulk). But then how long of a wire are you planning to cut with? If the wire is too short (like only a foot or two) the light dimmer won't be able to go low enough and may self destruct from excessive current, or it may light the wire up orange hot and snap it.
What I recommend for a power supply is a bell transformer. They are available at your local Kmart, Walmart, or Home Depot for about 15 dollars, and are intended for doorbells. They put out 24 volts nominal, and they can be operated with most light dimmers to vary the current. I haven't found a light dimmer yet that doesn't vary a transformer. This despite the warnings that you shouldn't use a dimmer with a transformer. (But don't try to vary the speed of your ac-adapter powered tape recorder with this arrangement.) The bell transformer also isolates the resistance wire voltage from line current so that if you do come in contact with the wire or it's terminals that you aren't a short circuit to ground. Also, there's less voltage there to hurt you should you happen to touch both ends of the wire at once. I don't mean to be simplistic about safety, just extra cautious, as I am not a professional, but a hobbyist.
An alternative to bell transformers: I use transformers that come out of old power supplies for radios, office equipment, etc. These can have varying voltages, allowing for longer wires for cutting, etc. If you do end up needing a higher voltage for longer cuts, just gang up two or more bell transformers in series. Or, need more current? Gang them up in parallel. Just make sure you get the polarity right. No, transformers don't have a plus or minus, but when you have two windings (four wires) they do have polarity with respect to each other.
Wire Temperature
The cutting wire does not get red hot to cut foam. It does get hot enough to smoke, however. The power supply ideally should be capable of heating up the wire well above the cutting temperature, and the reason is covered here. Getting the wire red hot isn't all bad. I find that being able to heat the wire up well beyond cutting temperature is helpful in cleaning the wire. When the wire gets gummed up with residue, it can easily be cleaned by heating up the wire till it starts to smoke, then wipe a cloth over it and it comes nice and clean. The trouble with residue is that it leaves little blobs on the finished cut. However, getting the wire red hot takes the temper out of it, and too much red heat on your cutting wire will fatigue it and cause it to fail, possibly at an important stage of a complicated cut.
Cutting Speed
The speed that your cutting wire moves through the workpiece is noteworthy. If you try to cut too fast, the wire will make a distorted cut because it is pulled into an arc in the midsection of the workpiece. If you cut too slow, the wire will cause the foam to shrink back from the heat, and also droplets of melted foam will adhere to the wire and leave hard little knots in the surface of the cut. It is important to understand what the wire is doing as it cuts foam.
Some important events take place when the wire initially heats up, when you start into the workpiece, and when you come to a corner or a detail in your stencil. One thing I have discovered is that after you turn on your power supply and dial it to the desired heat setting, the wire has already gotten too hot. But that very same setting is too cold once you've cut an inch or two and cutting goes SLOW. This is a problem that I have been thinking about for sometime, and am on the verge of an invention with.
The Bow
Taut-hot-wire foam cutting requires some kind of bow, or hoop, or frame, to keep the wire tight and straight. These can be any thing your imagination can cook up. It should meet a few simple requirements, however, to make the technique as trouble free and practical as possible.
The bow needs to allow wiring or other system of electrical conduction to the wire, to heat it, of course.
The bow needs to exert some tension on the wire to keep it straight.
The bow needs to have sufficient throat to accommodate desired depth of cuts.
The bow needs to be able to be handled, or to be fixed to a stationary surface in such a way to allow the workpiece to be moved across it.
If you do much foam cutting, you will find that soon you have an array of bows, each for a slightly different type of cut. The bow that I prefer to use for small lost foam patterns has a table that keeps the workpiece square with the cutting wire. This allows nice square cuts, perpendicular to the flat surface of the foam sheet. The next section will cover shaped wire cuts, which have their own special type of wire holder, different from the bow in that it does not exert tension to the wire, but rather holds it in a fixed position, maintaining the intended shape of the wire.
Shaped Wire Cutting
This technique was employed in the making of the step pulley patterns. Here, stiff, heavy wire was bent into a shape that was to be applied to the workpiece: V-grooves. The factors that need to be considered here are the actual shape desired from the wire shape (they're not equal), and the cutting resistance of the foam. There may be other factors, as well. Almost any hot wire cut will show signs of the foam shrinking back from the hot wire. This may mean that in order to cut a 1/4" groove, you may need to bend up a wire shape that reflects more of a 3/16" groove. Bending wire shapes to cut a predetermined shape in foam is an art, and I have not mastered it, but have certainly had a lot of fun experimenting with it. That's all for now, folks!
