Hot Wire Cutting on the Cheap
Transcript of the narration below
PDF files of the patterns for the Big Mouth, Jag and Baby Bug, and for the Mama Bug .
PDF files for A4 (European size paper) Jagwing and Baby Bug A4, and the Mama Bug A4 thanks to Wilfred van Norel of The Netherlands
Link to Jacobs-Online (ni-chrome wire source)
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Building a Simple Hot Wire Cutter and Getting Nickel Chromium Hot Wire (small quantities inexpensively)
If you decide to cut your own sheets of foam, the size I use for Baby Bugs and Jagwings is about 153mm by 65mm or 6" by 2 1/2". The thickness is .5mm to .6mm, but you might be able to go thinner with a high-quality EPS denser packaging foam. Cutting the large (223mm by 96mm and 1.2 to 1.5mm thick or 8 3/4" by 3 3/4") sheets for the Mama bug is more difficult and it's hard to find foam pieces big enough. Start small.
Copper wire does not work for hot wire cutting. It has low resistance, so it creates a short circuit, ruining batteries and other power sources. It gets weak when hot. Nickel Chromium wire, on the other hand, is the opposite in every way, perfect for electrical resistance hot wire cutting. Made with the elements nickel and chromium you can think of it as stainless steel wire.
Nickel chromium wire might seem exotic and expensive. It's not! As I write this, 10 feet (about 3 meters) of 40 gage ni-chrome wire is only $2.25 US, no additional charge for postage or handling, and it gets much cheaper when you order larger quantities!
Earlier, you had to buy large quantities of ni-chrome wire. Presently, I know of only one source of Nickel Chromium hot wire in small, reasonably-priced quantities. It is http://jacobs-online.biz/nichrome_wire.htm. run by a person named Gary from the state of Washington. I get the impression that he started out supplying materials for model rocket people (including igniters). Then model airplane people cutting their own foam wings quickly flocked to him when they saw he was filling the small-order niche.
If anyone knows of somebody selling small quantities of ni-chrome wire reasonably in the European Union/Asia/Africa/Australia please let me know and I'll list them. Otherwise you can still buy from Gary but you will have to contact him for destinations outside the US.
Wire diameter in the U.S. is still measured by an bewildering, archaic method of "gauge" or "gage." Counterintuitively, the smaller the gage, the bigger the wire! There are even two different gage systems: one for ferrous (iron containing, steel) wire, and another system for every other kind of wire (copper, brass, etc). Stainless steel is non ferrous because--despite the name--it's usually made mostly of non ferrous metals. Confused?
I use thin-as-hair 40 gauge (sometimes 38 gage too) ni-chrome wire. Typical hot wire cutters use much thicker wire, from 26 to 20 gage. I use the thin stuff because it works with small batteries and I think it makes a smoother cut on very thin foam. Just like saws, how wires make a slot, known as a "kerf." I want as small a kerf as possible because that melted plastic has to go somewhere. Thin wire is also very cheap.
Here is some additional information about making the more conventional (and more expensive) hot wire cutter,--including variable power supply--that would be good for cutting up big blocks of foam.
Air Surfing Part 6 Video Transcript.
THIN-SLICING YOUR OWN SCRAP FOAM FOR GLIDERS
Sciencetoymaker.org has always been about do-it-yourself. I do make sliced sheets available because it can get to be a little more do-it-yourself than a lot of people want get into. But let's say you played around the ni-chrome wire and you really want to make a useful slicer for recycling your scrap packaging white foam. Here is the simplest way I know to slice foam for gliders You’ll need a piece of plywood 10cm by 20cm or 4 by 8 inches, smooth on one side. Luan is usually the cheapest and many building centers will make the cut for a nominal fee.
The plywood needs to be up off the table more than an inch, or 25mm, but we can just use some books. By a happy coincidence, thin cardboard, that breakfast cereal and all sorts food products are packaged in, makes a perfect spacer between the plywood bottom and the hot wire, so it slices foam to just the right thickness.
MEASURING THIN SHEETS
When we get into very thin measurements it can be difficult to measure with a ruler. There are special tools like this micrometer that will do it if you know somebody with that kind of thing. But there is a cool workaround if you have a metric ruler. What if you increased the sample by an order of magnitude? So if we take 10 pieces of cardboard stacked together and it measures about 6mm, then just one piece of cardboard must be about .6mm, or 6 tenths of a millimeter. We get that because we’re measuring 10 pieces, so we divide by 10 to get one piece, and with metric we just more the decimal point to the left. It’s more complicated to try it with inch fractions, but that’s how you’ll be able to measure your thin slices of foam armed only with a metric ruler.
So cut out two straight strips of food packaging cardboard, 6mm or ¼’ inch wide and about 8cm or 3” long. Tape them to the edge with following things in mind: centered lengthwise, positioned exactly on the edge—the cardboard should not stick out at all-- held on with tape that will not go any further into the board, and stretch as you tape the second end down. I’m exaggerating, but you don’t want something like this.
The nickel chromium wire would eventually burn into the cardboard, so we protect it with a single layer of aluminum foil. Just the regular, cheap stuff, not heavy duty, which is thicker. Make the strips at least 2.5cm or and inch wide. It should cover the cardboard and also be taped on so no tape goes further into the middle of the board, and the second end under a little bit of pull tension. Most of the aluminum will hang over the edge.
You need some thin rubber bands to always keep the ni-chrome wire under some spring tension for two reasons. First, the wire will expand when heated, becoming loose. Second, a little pull might help flatten the cardboard spacers if they bump up. Cut the circles and make little loops in the ends of each.
Cut about 20cm or 8” of wire. Try not to kink the wire like this because it weakens it and this thin wire will break. Fortunately nickel chromium hot wire is available in small lengths of a few meters, it’s really cheap—pennies per meter--and cheap to mail.
Stick about 12mm or ½” of the end of the ni-chrome into a rubber band loop and roll the rubber so the wire twists a lot. That’ll hold it in. Do to both ends. Tape down the rubber band ends with an equal amount of wire sticking out over each side, and under some spring tension.
This'll be some pretty crude wiring, but I said simple. I stick the battery out of the way under the board. I’m taping one stripped wire to the far side aluminum and my switch is just a keeping the other wire touching like this. The hot wire makes electrical contact with the aluminum but it can still stretch and float. With only one battery you’ll cut very slowly. I wiggle the foam a little because you’ll melt a gouge in if you stop. You can also hook a second battery in series to speed up the cut. If you don’t leave the wire connected you can get a lot of cuts per battery. If you do a lot of this you’ll get an adjustable voltage supply, like a train transformer. Never use the first slice from a block.
Types of Foam
My students sometimes express disappointment that they can’t cut pencils or…fingers with the hot wire cutter—only foam. But not all foams seem to work well for superlight gliders. I had high hopes for thin slices of these very smooth foams—either for below grade building insulation or produce packaging for grocery stores. But even when I cut this foam so thin I can almost see through it, it seems much heavier so I need to use more front weight and it flies much faster.
I use the white stuff in which you can clearly see the little spheres it’s made of. It’s called EPS, or expanded polystyrene. And even that’s not all the same. In North America it’s classified by pound density. One cubic foot of it weighs a pound, or 1 ½ pounds, or 2. I use one pound density EPS foam because it’s very light, but it has some little holes that annoy some people. The packaging from computers and other electronics packaging is usually higher density. It’s beautiful stuff. Smooth and no holes. It's likely denser, so a little heavier, which means it might fly a bit faster and require more front weight.
Unfortunately, what looks like a thick chunk of foam might actually have channels and voids molded in. Foam boxes that perishable foods are shipped in are something of an ecological disaster, but they yield lots of flat, convenient pieces.
You can use a hacksaw blade—outside—to cut pieces to size.
You can experiment with more control of the thickness. You could use several pieces of heavy paper instead of the packaging cardboard holding the ni-chrome wire up. Or you could—in effect—raise the table by taping something in the middle, on the wood. Paper can burn. Aluminum doesn’t burn, but you shouldn’t let it touch the ni-chrome wire or it could electrically short it.
Remember, you can accurately keep track of foam thickness by putting 10 together and dividing.
If you don’t have thin copper wire for the front weight, recycle some twisty ties, maybe pull off the paper or plastic to get to the wire.
If you’re slicing out your own foam, you’re likely doing some interesting experimenting. Stay in touch. With your permission I’ll link to your video or web pages. It's a very exciting time to be be alive, what with all the innovations and branches that people are forming.
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