Power supply for hot wire foam cutter

[kinoav8r]

I made a search here and couldn't find anything. I'm looking for the plans/schematic for the power supply used in the EAA video "How to Make a Hot-Wire Bow." It's not explained in the video.

http://eaavideo.org/video.aspx?v=3793197808001

Anyone have any idea how to find this?

TIA,

[Bunkie]

It's not too hard to figure out using Ohm's law and some target values. For example, using a nichrome wire that is 26 gauge, the resistance per foot is 2.67 ohms. Assuming a wire length of 2 feet, you get a total resistance of 5.34 ohms. Let's assume the use of 18VAC using a step-down transformer (this is a common transformer voltage). This combination would draw 3.37 amps and dissipate 60 watts. The circuit is really pretty simple: an in-line fuse (on the 18 volt side of the transformer) of, say 5 amps and a switch. I would also add 1 one-amp fuse on the 117VAC side of the transformer.

The math is pretty simple: Current is Voltage/Resistance and calculating the power dissipated is (current*current) * resistance. This wire will get as hot as a 60-watt incandescent light bulb. The transformer would have to be able to deliver about 4 amps and a bit bigger might not be a bad idea.

You would have to determine if 60 watts is enough for the purpose. It might very well be too much. Never having done hot-wire, I can't say.

By the way, if you are uncomfortable with AC circuits, this will also work with a DC battery, but it would have to be a lead-acid type to be able to deliver the needed current while the unit is on and you will have to re-figure the math to get the right power dissipation.

[martymayes]

By googling "hot wire saw plans" I got about 25,000 hits. I liked this one:

http://www.laureanno.com/RC/foam_cutting_power_supply.htm

[wrightflyin55]

12 volt battery charger, safety wire, saw out u shape ply wood frame run wire between open ends of bow add a spring to keep wire tight use clamps from the charger clamp one lead on each end of the u, plug in or turn on the charger wire will heat up cut foam, practice on scrap first .

[Martyfeehan]

I use the power supply out of an old computer. Each of the connectors for hard drives and such put out about 12 volts. Now the fun part, I plug the power supply into a box with a plug and a dimmer switch. Turn on power supply, turn down dimmer switch until the wire you are using cuts the foam nicely and presto, your cutting foam. Not many $$$ invested in this at all. Crude but ...... it works.
Marty

[kinoav8r]

Thanks for the suggestions, but I was looking to see if anyone knew where to find the plans for the one in the video.

[Tlb67]

I am working on a simple plan layout for a hot wire cutter very similar to the one used in the video. Look for that in a future Hints for Homebuilders section of the Sport Aviation magazine.
Going over the designs that I know we use in the Sport Air Workshops and the ones that have been posted online, I see many different frame and wire attachment techniques.
All achieve the same goal. its is so dependent on the shape of the part you want to cut and the quantity. In working for the SportAir Workshops we use large quantities of the same shaped foam and are solidly fixed frame hot-wire cutter reflex that type of cutting. We run the foam through to get a certain dimension and have the hot-wire easily adjustable to change to a different cutting height dimension and run that same piece through to get the block size we need. The hot-wire setup in the video is used for the template cutting and works great for that application. Large size cutters, for example cutting the wing panels on a Long EZ, will take a larger transformer to keep that wire hot enough to cut the foam straight. I had looked through the EAA archive to see if we had any examples of drawings of a hot-wire setup with no luck. Another option is to look through some of the canard forums and composite builders web sites.

[kinoav8r]

Thanks. Any idea when that will show up in the magazine?

[Robert Dingus]

i built my foam cutter out of a 25 watt Radio Shack power supply, and a standard house hold, dial type light switch, a dimmer.
some .020 stainless steel safety wire, a wood bow, and some screw eyelets. this lets me make the bow as big or as small as needed, plus the output power is adjustable, to get the cut speed and heat just right.

Robert