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