Introduction: Hot Wire Foam Cutter

I recently decided to try to build an RC airplane and needed a clean and effective way of cutting the foam I will be using for the plane. After some research I decided that a hot wire foam cutter would be the best solution. 

I wanted something that would be able to produce clean, even, reproducible cuts so a hand help cutter was out of the question. I ended up going with a band-saw type of design.

Step 1: Tools & Materials

Materials Used:

>2x4- x2 18" pieces & x2 15" pieces
>1x2- x3 15" piece
>Sheet of some sort of flat surface- I used 1/16th ply board

>Drywall Screws- 1-1/4" & 2-1/2"
>Adjuster Bolts- 2" (top) & 4" (bottom) + nuts
>Metal Plate Roughly 4" square + 2-1/2" Bolts (optional)
>Wire tensioner spring + Washer (Of equal outer diameter)\
>Shaft (Of inner diameter equal to above OD)
>Some sort of bead (I used a fishing lure sinker)

>Thin Steel Wire
>Power Supply


Saw (Ideally mitre saw or circular saw)
Drill (with some drill bits)

Step 2: Main Structure

The first thing to do is to create the main wooden structure. There isn't much to explain here as it is a very simple structure but I have included a sketch of the plans I made. 

The only thing I did a bit different was that I made the "mast" and overhanging beam removable for easier storage. I did this by screwing a metal plate onto the mast and then bolting the metal plate onto the base. This way I can just remove the nuts off the bolts and slide the plate off along with the arm and support.

Step 3: Adjusting Mechanisms and Tensioner

This is really the meat of the instructable. Once again nothing complicated but I tried a few revisions and this seemed to work the best. I was also pretty happy with the automatic tensioner.


I tried to square the wire the best I could while building the wooden frame but I figured some adjustment ability might be a good idea.The wire adjusters are made by running the wire over a bolt, between two nuts that can be moved to adjust the wire. The top adjuster can adjust side to side and the bottom back and front. (I temporarily substituted a screw driver for the bottom bolt adjuster as can be seen in the pic)

One of the hurdles I had to overcome was maintaining a relatively constant tension on the wire. When the wire heats up, it expands and become less taught. In order to eliminate this problem I added a spring tensioner. The spring slides into a sleeve (I found a pen case that worked perfectly for this) and is capped with a washer. The wire feeds through the entire assembly and its secured on the other side of the washer (I fed the wire through a fishing lure sinker and looped it back on itself to keep the wire from slipping).

See the diagram + picture for more details.

Finally the wire passes over both adjusters and with the spring compressed, is secured at the bottom by simply wrapping it around a pair of screws below the deck.

Step 4: Electronics

In order to heat the cutting wire, a current is passed through it. Because of the increased resistance of the steel wire (versus the conductive copper used in your leads), the steel will heat up allowing you to cut through the foam with ease. 

For my source I used an old modified computer power supply. The PC power supply was a good choice for me because they are so easy to get for free. At least where I live, I can go to a reuse-it or recycling centre and rummage through a pile of old computers for free. In addition to this, they have very low tolerances as they are used to power electrically sensitive devices so they are great for other projects as well.

I have broken out the 12V, 5V and GND lines to 4 connectors (Very useful thing to have for all kinds of electronic projects) on the case of this PC supply for ease.

Now you simply connect one lead to one end of the steel wire and the bottom to the other. Initially I tried using the 12V line but it was drawing too much current and the over-current protection was shutting things down (another useful aspect to the PC supply) however, it works fine on 5V.

Once you have everything connected, turn on the power supply, give it a few seconds and get to cutting!

Step 5: Results

Unfortunately I have misplaced the pictures of the things I've cut with it so this instructable looks a little unfinished but there you have it!