5/17/2007, Important note! Please be sure to read the comments that other users have left below. There's a LOT of good info there. Check out the links that folks have posted and study what they've said here and you will be able to build a cutter thats suited to your budget and the materials you have access to. There are a lot of alternatives listed or linked below for the frame, the transformer, the wire, the enclosure, and the heat control. Don't underestimate the power of comments! :)
Step 1: Parts
1. 12 foot, 16 guage extension cord, about $2 at Home Depot
2. 2 wooden yard sticks from the Home Depot paint department, $.97 each
3. 4 #10-24 x 1.25" machine screws with nuts, $1 at Home Depot
4. 10 #10 washers, $1 at Home Depot
5. About 12 feet of strong, low-stretch string. I used 200 lb dacron kite line.
6. A single-pole dimmer switch. About $10 at Home Depot.
7. A 25 volt, 2 amp transformer, $10.49 from Radio Shack.
8. An electric guitar string, about .10 - .16 size. I think around $1? You can get these individually at a music store or you can use either of the 2 smallest strings from a packaged set. You should keep a spare handy because they can burn out or break from too much tension.
9. A length of two conductor electrical wire with a regular plug on the end. I salvaged mine, but you could use another extention cord if you like.
10. A piece of wooden dowel or stiff plastic rod about one foot long (not shown). I used a bamboo skewer.
11. Optional: 4 regular-thickness CD cases. These are for the box that holds the transformer and dimmer switch, but you would be much better off with something like a "project box" from Radio Shack.
Step 2: Tools and supplies
1. Utility knife
2. Small wood saw
3. Drill with a bit slightly bigger than the #10 screws
5. A couple of cable ties or twist ties
6. Electrical tape
7. Wide packaging tape
9. Multitester (you don't NEED one but it's a good safety check)
10. Wrench to match the nuts (I didn't have my SAE wrenches handy so I used a 9mm)
11. Solder and soldering iron, if you like
Step 3: Making the frame pieces
One the remaining, uncut yardstick, drill a hole about 6 inches from the pre-existing hole (see photo), and another about 1 inch from the other end (not shown in this photo, but visible in later steps).
Step 4: Bolting the frame together
Step 5: Attaching the lead wires to the frame
First, cut off each end of your 12 foot extension cord. Save the plug and outlets for future projects, if you like. Strip the insulation off the last inch of one end of one wire of the cord.
Insert a machine screw in the top of the right leg of your "big H" as shown in the photo. This is the leg that is on the other side of the H from the handle. Use one washer on the screw head side and two on the nut side, as shown in the second picture below. Put the nut on, but don't tighten it down at all. You need room between the washers so you can put your wire in there.
Bend the bare wire of the extension cord (the part you just strippeed) into a U shape, and hook it around the screw between the two washers. Now you can tighten down the nut. You can see what the final hook-up looks like close-up in the photos for step 7.
Starting the the end you just hooked up, pull the two conductors of the cord apart so that it's split for about 3 1/2 feet. Cut the unmounted side of the split to about one foot. Strip the end of the one foot section and mount it to the other leg of the H the same way you hooked up the first wire. You'll see what I'm talking about here if you look at the photo.
Use cable ties, twist ties or string to keep the wire close to the frame so it won't get in the way when you're using the tool.
Step 6: Making the tensioning loop
Step 7: Attaching the hot wire
Your guitar string should have a sort of a bead (for lack of the proper name) on one end. Make a loop by feeding the other end through this bead. Hook the loop over one terminal and sinch it up. Keep the loop pretty close to the nut or there will be too much twisting force on the leg when the wire is tensioned. But also make sure it's not touching the wood.
To hook up the other end of the wire, pull the frame legs towards each other so your string tensioning loop on the other end is taut. Wrap the end of the wire around the other terminal screw and twist it off. See the second photo. It might help to use pliers to keep good tension, but be careful not to pull too hard and break the wire.
If the wire is not super tight at this point, don't worry. It can be kind of floppy when plucked but should be pretty much straight when at rest. We'll add more tension later.
Step 8: Cross strings
Tie a loop in one end of a 3 foot piece of string and hook it under the washer of one of the middle screws. See the photos. Thread the other end of the string though the opposite leg's hole. Square up the frame and tie off the string. Repeat with another string for the other side, but when you tie off the string at the top of the leg this time, make sure you have some tension. Now both of the criss-cross trings should be pretty tight, and there should be some slight tension on the wire too. It's ok if the tensioning loop is a little floppy at this point. Now you can tighten down both of the middle screws with the wrench. No need to chrush anything, just make 'em tight enough to hold everything together.
Step 9: Applying tension
You'll have to readjust the tension later, after the wire gets hot for the first time. Or maybe every time.
Step 10: Wiring up the transformer and dimmer switch
This photo is just to show what connects where. You should of course use the wire nuts that came with the dimmer switch (esp on the 120V connections) and/or tape to ensure that no bare wires touch each other, or you, or your pet. Be careful not to electrocute yourself or start a fire.
Step 11: My crappy project box
If you drop the box, the transformer will shatter it. Have I mentioned that you should use a different design for your enclosure?
I think this kind of box might have some other uses. Maybe you could make a version without a top and bottom, put some nice photos in it, and set a small plant pot inside with the plant sticking out?
Step 12: Turning it on
Turn the dimmer all the way down (counterclockwise).
With your fire extinguisher handy and your body away from the device, plug it in. Are the lights still on? Is the hotwire still whole? Sweet.
You can use your multimeter to see if there's any current between the terminals. There shouldn't be yet. SLOWLY (like 5 degrees per second) turn the dimmer up (clockwise) til the wire starts to quietly hum. With the wire I used that's about 1/4 or 1/3 of the full rotation. If the wire doesn't hum or heat up by the time the dimmer is halfway up, turn the dimmer all the way down, push in til it clicks, and start again.
If you turn up the dimmer too fast, your wire may burn out before you realize that it's even hot.
Once you are sure everything's all set, grab the frame and try laying the wire on some scrap styrofoam. It should slice smoothly into the foam. You shouldn't have to push very hard. Try playing with the dimmer switch setting to get the best cut. I've read that cutting slower and cooler makes a smoother cut.
The wire heats up and cools down within a second or two.
Next time you use the foam cutter, make sure nothing meltable or flammable is touching the wire when you plug it in, just in case the dimmer isn't off.
Step 13: Quick and dirty sample cuts
Step 14: Alternatives and expansion ideas
The enclosure for the transformer and dimmer needs to be more rugged, and it should incorporate a fuse, maybe a power indicator, and maybe a modular connector to facilitate attachment of larger or smaller cutters.
Instructables user Moofie suggested rigidifying the connection to one of the legs, perhaps simply by using a second bolt where the yardsticks overlap, to eliminate the need for the criss-cross strings.
Step 15: Acknowledgements
Both pages have more good info. Check em out.