Saturday 4 July 2015

Hot Wire foam cutting rig

A couple of things prompted me to try this traditional foam wing cutting method.

1. Thick boards of XPS insulation foam appeared in Bunnings. Either I have been unobservant or they have just recently added this stuff to the product line. 30mm and 50mm thick 1200mm x 600mm boards are available for $12 and $20.

2. There was an old PC power supply that was about to be trashed at work and I already had some 0.6mm MIG welding wire for the resistive wire.

I followed some online videos about getting 12V DC from the PC power supply. First time I turned it on the house safety switch tripped so it went in the bin.

Instead of a mains supplied power source I decided to try using a 3S LiPo, and it worked beautifully.

I'm using a watt meter to keep an eye on battery level, current and consumed mAh

The pine frame stretches the resistive wire across a 750mm span at the front. Tension is maintained by stretchy shock cord at the back.

Voltage is applied to heat the wire via alligator clips and I have an ON OFF switch on the frame. The LiPo connects via an XT60 plug.

Airfoil images and plots can be downloaded from The University of Illinois Airfoil Data Site. To make templates the airfoils need to be resized, printed then cut out of a rigid heat resisting material. I'm using 3mm MDF board but G10, formica and carbon fibre are also used.

The MIG wire broke mid cut a couple of times so I changed to 40kg stainless steel fishing wire and it's working well. It's 7 strand and nylon coated so the coating needs to be stripped off. I burned it off but it can also be easily peeled off using a knife.

Here's the "How to" video including a test flight.

As these wing cores are solid foam there is nowhere for the spar to fit. 
This video shows three ways to form a spar channel.

UPDATE Jan 2016

I am now using a 4S Lipo for more heat in the wire, enabling a faster and smoother cut for 850mm wing halves.

I have changed to thin aluminium sheet (I think it's window flashing) for the templates. Can be cut out with scissors.

4S power and Aluminium template in use


  1. Where are your new models??? i´m wainting impatiently.

    1. Haha. Sorry to keep you waiting but I'm working on better cutting techniques and a fuselage. See progress here -

  2. I am very much pleased with the post. I want to thank you for this great article. I enjoyed every little bit part of it and I will be waiting for the new updates.

  3. What adhesive do you use for the boards of XPS insulation foam?

    1. Hello Al. I use hot glue or epoxy to join the wing halves, and medium foam safe CA to glue in strip style spars.

  4. Thanks for the ideas you gave me for building my own hotwire-bow. It looks very much like yours, uses a 4S Lipo and a Guitar-String. I started even with the Clark-Y-Profile from which on my fourth try came out ok. The first 3 were more like practice-cuts with quite a lot of dents and bruises. Now my next step will be to solve the question of how to create removable wing-spars. Is the spar-drill-method still your favourite?

    1. Hi Jan. That's wonderful to hear. It is very rewarding. For spars I now prefer to glue thin strips of CF into slots top and bottom. 3mm x 0.5mm for smaller wings and 6mm x 1mm for big wings. Just cut a slot with a knife, insert the spar edgeways and drip CA glue along the slot.

  5. about cutting unsymmetrical for flying wings, like having 10.5inch root chord and 4.5inch tip chord?looks a bit harder to cut

    1. I have done that on the mini arrow wings, much more difficult but do-able

  6. Hi Andrew, could you please tell me wat's the density of the foam board you use for your wings?