Foam cutting & sanding

The best way to smoothly cut foam is using a 'hot wire cutter', a length of wire which is stretched tight inside a frame and heated as a resistive element in an electric circuit. Smaller cuts can be made with cold knives, including the quite common task of hollowing out a cavity in a flat face of foam, e.g. toplace a servo. Other techniques can be used for things like placing a reinforcement rod.

The ease with which foam can be sanded varies with the type of foam, but it is optimal to use rough grit, low pressure, and a fast moving surface like a belt sander.

If heating foam, always do it with adequate ventilation.

[edit] Cold Knife

The most basic way of cutting foam, and the most common way, is with precision tools like the X-Acto Knife, and wood/clay working wire tools. Skills in this area are going to be required for hollowing out compartments.

[edit] Manual Hot Wire

A frame is created, probably using a pair of threaded rods and wood blocks with wing nuts like in a large clamp. Nichrome wire is stretched between the wooden blocks to keep it taut, and a current is run to heat it up.

[edit] Freehand & Extruded

Freehand with a hot wire cutter can somewhat replace sanding, but a better way of doing things is to create and secure rib end-blocks, that the wire can be rested on, which are shaped like the desired wing or half-wing core. Drag the wire over the rib, and eventually one can make an extruded form in the same shape. Sharp curves need to be painstakingly aligned to avoid slicing through, and this is somewhat slow if done by hand.

[edit] 2D Layer Cutting

A thin, flat sheet of foam is cut on the vertical axis... then another layer is laid on top and cut to a slightly different form... and on until a 3D wing has been produced. This has the advantage that hollow spaces can be easily created, but the disadvantage that lots of glue and sanding is required. If you want to make a KFM airfoil, this is the smoothest way. Note: Doing the exact same thing with a knife may be easier, depending on the type of foam.

Can also be made into a CNC.

[edit] Hot Wire CNC

Airfoils are precision shapes, and a certain degree of computer control gives a repeatable, automatic workflow for experimenting with different shapes. Most hot-wire-cut foam cores are produced using a computer controlled robotic hot wire cutter.