Hot Wire Bow for Foam Cutting

The hot wire bow is my preferred way to tension the wire for CNC foam cutting. The picture below shows my bow which I use on my CNC foam cutter.

Advantages of using a bow

The bow doesn’t exert tension between the left and right carriages but the spring method does. When foam cutting wings without too much sweep back the spring between the carriages works well. But if the left and right carriages move at different speeds the spring can become very extended. In the diagram below the right carriage is much further forward. I did once have the hot wire snap cutting a large sweep angle

sweep angle — Hotwire attached to the carriages can become very extended

Disadvantages of using a bow

The main problem with a bow is that you need to support it on the machine while still keeping in good contact with the carriages to guide the wire. The picture below shows how I support mine using a 3d printed bracket. The wire sits in some V-grooves with a slight downward pressure adjusted by the bracket

Advantages of the spring method

The spring attached to the carriages is a simple set-up and works well providing the spring isn’t extended too far by a swept wing design. I attached the spring with a strong fishing line and secured it on the vertical drawer slide.

Disadvantages of the spring method

The spring tension can put a load on the carriages if it’s too much and in the extreme case, the wire can break. The power feed wires to the hot wire need careful management to stop them from fouling the foam as the carriages move. With the bow, the power feed wires are attached to it so they don’t get in the way of the foam

Making a bow for Hot Wire Cutting

This is quite an easy job and can be made with various materials. I used a piece of softwood timber about 25mm x 13mm x 1000mm and two aluminium right angles 20mm x 20mm. But anything similar will do. You can use wood instead of aluminium.

To keep the tension I used an expansion spring at one end connected to the other end by some wire. I had some Mig welding wire but any stiff wire will do. If you use an aluminium angle like I have make sure to add an insulator at one end of the spring otherwise you will get a short circuit. I used a cable tie.

For the wiring, I used some twin core 32 gauge wire two crocodile grips and a Deans connector. That allows me to remove it from the machine.

The hot wire I use is nichrome around 0.4mm in diameter this works well for me. I power the wire from the CNC controller. I have used a car battery charger and my iCharger 206B charger with a foam-cut setting.