Hot wire foam cutting

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This is a useful technique for making long, acurate cuts through blocks of foam. It's ideally suited to slicing sheets or making solid foam wings to acurate aerofoil shapes.

Components

All hot wire cutters have the following components:

Wire

The wire should be steel, not copper (which isn't as strong and has much lower resistance). There are many flavours of steel wire, some may be easier to find than others.

  • Nicrome wire is ideal, as it doesn't rust, and is fairly high resitance.
  • Stainless steel fishing line can be easy to find.
  • Unwound steel guitar strings are easy to get and can be had in different sizes. .010" (0.25mm) works ok.
  • MIG Welding wire is OK, but 0.6mm is the smallest size commonly available and this is a bit thick.

A good starting size for hand cutting appears to be around 0.4mm. Thinner wire will give a neater cut and melt less foam but is more prone to break. Thicker wire takes more power and has to melt a wider slot.

A good source is wires.co.uk

Power supply

DO NOT USE MAINS POWER! The voltage is far too high; it'll melt the wire and could kill you.

A lead-acid battery charger is an easy power supply, especially as they usually have an ammeter built in, so you can see the effect of changing the wire length. You can use other transformers or a battery, but it's a good idea to have some way of checking that you aren't drawing more power than it can supply.

Whatever your power supply, be aware that you have no idea how hot the wire will get when you first connect it. It could supply far too much power and damage the power supply or melt the wire. Start with a long length of wire, and be ready to switch off if it glows red hot. Hopefully it won't look as if it's doing anything; gradually shorten the wire (or move the contacts closer together) until it's hot enough to melt foam. The write down the combination of wire size and length and the power supply settings.

Templates

You can't make an accurate cut free hand, you need something to cut around. Templates are usually made by printing out a suitable aerofoil, sticking it to some board and accurately cutting the board to shape.

  • Cardboard can be used, it's very easy to work but isn't very smooth and the hot wire can burn it.
  • Plywood is much better, especially if you wax it before each cut.
  • Aluminium is harder to work but won't burn. 1-2mm sheet is good.
  • Formica is popular, if you can find it.

A template needs to be very smooth, because any notch or bump will stop the wire for a moment and cause it to melt more foam at that spot, resulting in a line along the foam.

There are various types of template design:

  • Male template - You can just stick the template to the foam and cut all the way around it but if it moves the shape won't be accurate.
  • Female template - the aerofoil shape is cut out and the wire moved around inside the shape. The templated can be screwed to a baseboard to stop it moving, but cutting the leading edge is difficult as the wire tends to bend slightly.
  • Separate upper and lower surface templates - This way you can let the wire cut past the leading edge on both cuts, rather than turn right around the leading edge (and you can clean it up afterwards if there is a bit if a ridge). This method needs some way to ensure that the second template is put in exactly the same place as the first one. A good way to do this is to drill holes for metal pegs in both templates and baseboard, then glue the pegs in the baseboard.

Designs

There are lots of different way to cut foam with a hot wire:

Simple hot wire cutter

If you are making a tapered wing, you can fix one end of the wire to a nail, and move the other end around the template. This has the huge advantage that you don't need to control both ends of the wire (which usually takes two templates, two people or a swing arm). It has the disadvantage that it only cuts tapered wings, so you have to cut each wing half separately, and you can't use different aerofoils at each end, or build in twist (wash out).

The wire should be roughly twice the wing span, although the exact ratio depends on how much taper you want.

You need some sort of handle on the wire so you can keep it tensioned, winding it around a bit of wood will do.

The fixed end should be fixed at half the height of the template, or the wire could break out of the foam.

Bow cutter

The next step is to mount the wire on a bow. This is just a device to keeping it under tension; usually a long H-shape of wood, with the wire between to of the prongs, and bungee cord between the other two. It's necessary to have some tension on the cord, because the wire will stretch as it heats up. You can attach one side piece of the bow to the centre piece, but the other must be free to pivot as the wire stretches.

A bow can be used by two people, each cutting around a template on their end of the foam. It's useful to mark off and number intervals around both templates and for one person to call out the numbers as that end reaches them so the other person can keep in time. It's also important to agree whether to cut the top or bottom surface first!

Swing arm

This is a device for pulling a bow at both ends. A long piece of wood is attached to the front of the desk and pivoted at one end. String or flexible wire is run from each end of the bow to the swing arm, usually via some pulleys.

Sheet cutter

This is a variant on the bow device, designed purely for cutting sheets of foam from blocks. With careful setting up, it can cut very smooth sheets quickly and repeatedly.

The wire runs along metal runners, which are carefully aligned parallel to a strong, flat surface and a few milimeters above it. The runners should be polished smooth and can be greased or you could support the wire on nylon blocks to further reduce friction.

The bow can be suspended from the ceiling, or hung under the table. If hung from the ceiling, it should be on bungee cord or elastic bands or it will lift off the runners at the ends of it's travel. If under the table it should be as light as possible to reduce friction.

The bow is pulled through the foam by string, run over pulleys to weights. You can use a weight at each end or use a few more pulleys to connect the strings to a single weight, which ensures that both ends are pulled at the same speed.

The foam should be held down to stop it moving (especially as you get to the end of the block) but avoid point loads. These will squash the foam into the wire, resulting in a hollow in the block that will get worse with each cut. A large flat bit of wood is a good weight. Use just enough weight to prevent the foam moving.

Vertical Cutter

This variant is for cutting patterns from thin foam sheet material like bluecor, protectionboard, or depron. It consist of a table with an arm holding a hotwire perpendicular to the table. The plans are printed or glued to posterboard and cut out. The templates are then taped with packing tape to the foam. The hotwire cuts through the tape as easy as through the foam. Care must be taken that the wire is still taught when it is hot. This can be done with a separate spring, or by making the arm from springy wood and bend it when stringing up the wire.

The advantage of this method over cutting out the patterns with a knife is that it is easier to cut out more patterns in one go (for replacement parts or making more planes)and the edges are always nice and square.

CNC cutter

The ultimate foam cutting machine controls the wire using stepper motors, driven from a computer. These are not beyond the capability of a hobbyist, although they can cost a few hundred dollars (up to a few thousand if you buy everything).

Foam cutting links