Cutting Foam with a Hot Wire

We have another great build article from Charlie Fite!


Using a Hot Wire to Build RC Planes

By Charlie Fite...

Since balsa has become both scarce and expensive, those of us that like to build our airframes from scratch have turned to foam as our primary building material. It is plentiful, and for the most part, inexpensive. It’s also available from big box hardware stores. The two most popular types of commercially available foam board are Formular by Owens-Corning (Pink) and Dupont XPS (Blue). Both are closed cell extruded polystyrene. There are other types available: EPS (Polystyrene), EPP (Polypropylene), EPO (Polyolefin), and PU (Polyurethane) to name a few. All of them can be expanded or extruded, and each has their application. They are available in all kinds of thicknesses and sheet dimensions.

My personal favorite is the Owens-Corning (Pink) insulation board and I use it for 99% of my airframes. It can be cut with a knife, saw, or a hot wire and it can be milled, shaped, and sanded. For the purpose of this article, I’ll address how I cut it with a hot wire.

I started cutting foam with a hot wire almost seven years ago when I began building twin engine EDF jets. I tried all kinds of power supplies, wire, bows, and frames, and I learned something different on every cut. It took me almost two years to settle on all the equipment and the technique to successfully cut a shape out of foam.

Let’s start with the bow.

I learned very quickly that the bow had to have enough length and enough throat to be able to cut larger pieces. This one has an 18” wire and a 7” throat, and it will cut 99% of the shapes I need. The frame is aluminum channel with oak dowels as stiffeners, and pine dowels to isolate the wiring from the frame. The most important part of this bow is that spring at the top. When you heat wire, it expands, and that expansion has to be accounted for. Otherwise, the wire will sag and you won’t get a nice, straight cut.

Here’s a few examples of what can be done with some rigid insulation and or packaging material:

When I first started cutting foam, I tried to hold the foam blank on the table and manipulate the bow with the other hand…..Needless to say I made a lot of scrap and cotton candy. So now I clamp the bow to my bench and then pull the blank across the wire with both hands.

The bow can be almost any shape or proportion that you want. But no matter the shape, the cutting wire has to remain tight through the entire temperature range.

I use26 g Nichrome 80 wire to cut with, but you can use the B” string from a guitar.

The power supply is probably the most important piece of the hot wire puzzle. I tried numerous fixed voltage/fixed amperage power supplies (the little black cubes) with very little success. This is the one I have: Eventek KPS305D, 0-30 volts and 0-10 amps. About $60 on Amazon There are commercially available bows with variable power supplies, but they’re usually in the neighborhood of $120 to $250. My setup cost less than $75….and besides, I like to build as many of my tools and fixtures as I can.

So now, we have a bow and a power supply. The next questions are: “What do I want to cut?”, and “How do I cut it?”.

For the purpose of this article, I’m cutting the EDF nacelles for the CF-100 Canuck. But the procedure is the same for any straight-line shape.

I’ve cut wing cores, fuselages, EDF nacelles, canopies, and nose blocks, all using the same technique. I determine the cross section of each end of the piece, and the length. Then I cut the blanks to length, and ~ 1/2” oversize for the height and width. Then I locate and mark the centerline of the piece on the top and both ends.

The next step is to apply the appropriate template to the ends of each blank using a general purpose spray adhesive.

I stay away from the high strength sprays because they can and will melt the foam. A light spray using a general purpose adhesive will work just fine. Spray the template, not the foam.

Finally, I pull the piece across the wire, usually starting with the inside of the profile, and cut half the piece each way. Remember to maintain template pressure against the wire with a smooth and steady feed rate. My power supply was set at 9.0 Volts and 4.5 Amps. I find that to be the “Goldilocks” setting for pink foam. Each type of foam is different, and only a test cut will tell you where to set the power.

I don’t worry much about chatter or hesitation marks because I’ve settled on covering all my pink foam with brown paper. But I do sand the pieces smooth with 200 or 300 grit sandpaper and fill any “divots” with lightweight spackling. I’ll also sand off all the “angel hair”, both inside and out.

Repeat each of the above steps for each shape, and in no time got (2) EDF nacelles.

I hope this will help any of you that decide to try to cut foam. You can do some amazing things with a straight wire, a power supply and some bits of plywood…..

Y’all keep your nose up in the turns.

Last edited by Jim T. Graham; Jul 16, 2021 at 08:25 AM..
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Jul 18, 2021, 11:10 PM
world's slowest builder
Air-Jon's Avatar
thanks for the nice write up on hot wire cutters! I've put off making a cutter for about 30 years, but i'm on the cusp now!
Last edited by Air-Jon; Jul 19, 2021 at 09:14 AM.
Jul 19, 2021, 07:24 AM
Registered User


Nice write up. Lots of good information to point me in the right direction. Thank you
Jul 20, 2021, 12:44 AM
Registered User

Cnc foam cutting

Great article and some nice planes you've built!
Jul 20, 2021, 05:08 AM
Registered User
Wow, I'm very impressed! That's an RCAF CF-100 model without wingtip tanks right?! That pic is what brought me to the article. I've only ever done a flitetest kit I couldn't imagine the time and patience required here but I love the looks of that model. I bet it's an excellent flyer and a good paint scheme for visibility. It's an edf I assume and wonder about the size and guts you used?
Jul 20, 2021, 05:46 AM
Ad eos qui nesciunt crepitus
Old_Pilot's Avatar

CF-100 Canuck - Assembly


Correct, it's a Canuck..(2) 12 blade 64mm fans, 80a ESC's, Lemon 6-channel recvr, 17gr MG digital servos, 1/2" pink foam fuse and nacelles, 3/16" Elmer's board wings and feathers...4000 mah 60C 4S battery

Here's the assembly drawing.

It's probably no more time consuming than building a similarly sized balsa version of the "Clunk"'s just a lot less expensive. I tend to simplify my foamie builds at the expense of extreme scale....besides, at 50ft and 70mph, who can tell.

Thank you for the kind words

Keep your nose up in the turns

Latest blog entry: Playing chase again
Jul 20, 2021, 08:26 PM
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Kevin Cox's Avatar
Nice work!
Jul 21, 2021, 04:52 AM
Registered User

Hot wire

What csd you using?
Or just cutting plans for templates
Jul 21, 2021, 05:33 AM
Registered User

Hot wire

What cad you using?
Or just cutting plans for templates
Yesterday, 05:20 AM
Ad eos qui nesciunt crepitus
Old_Pilot's Avatar

CAD program

Originally Posted by RAPPTORS
What cad you using?
Or just cutting plans for templates
I use Draftsight 2021 Professional as my CAD program

I use Easel and EstlCAM as my CAM programs

If I understand your second question....actually I use my CAD program to produce the DXF files I use to generate the
NC code that drives my CNC router. I cut balsa, ply, foam board, rigid foam....... basically everything I need to build an airframe
Last edited by Old_Pilot; Yesterday at 05:39 AM.

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