Beginner's Guide to CNC Foam Cutting with Hotwire for Hobbyists
CNC foam cutting is a brilliant way for model makers and RC enthusiasts to produce accurate wing cores, fuselage plugs and display models with minimal waste and a small workshop footprint. This guide focuses on hotwire CNC systems because they are affordable, relatively simple to set up and ideally suited to cutting airfoils and lightweight structures. I will cover the essentials you need to get started, how to work with templates and airfoil data, and practical tips for finishing and assembly so you can move from first cut to flying model in a few sessions.
At its simplest a hotwire CNC machine consists of a heated wire mounted between two carriages that move on a gantry while foam is advanced through the wire or the wire slices across a foam block. The wire is heated by DC current so it melts or vapourises the foam cleanly, and the two axes are driven by stepper motors with a controller that follows G-code. Hobby machines vary from desktop two-axis cutters to larger portal-style tables, and many hobbyists build their own frames from aluminium extrusion or timber combined with affordable controller electronics such as GRBL or TinyG.
Choosing the right foam matters for strength, finish and fumes produced during cutting. Expanded polystyrene (EPS) is cheap and cuts well, but it can produce unpleasant smoke so a well ventilated workshop is essential, and you should consider a respirator with organic vapour filters. Extruded polystyrene (XPS) gives a smoother surface and easier finish for glassing, while expanded polypropylene (EPP) is more flexible and forgiving for impact-prone models. Always test a small scrap to dial in temperature and feed rate, and use sacrificial boards and sacrificial skins where you need to glue or laminate.
Airfoils and wing cores are the most common projects for hotwire CNC, and a little understanding of the geometry will save hours of rework. Airfoil shapes are defined by coordinate files that specify the top and bottom profiles along the chord, and you generate a wing by stacking scaled slices or extruding a single profile along the span with twist and taper applied. Plan internal features such as spars, shear webs and servo bays before cutting so you can leave voids or cut slots in the same programme. If you want ready-made templates or inspiration, check the WatDaFeck site for downloadable patterns and project notes that work well with hobby CNC setups.
Software and templates form the bridge between design and cut. Many hobbyists use airfoil libraries from XFLR5 or Profili to choose profiles and then export coordinates into CAD for scaling and arranging slices. A common workflow is to generate a stack of 2D slices according to your wing plan and save each slice as a DXF or SVG, then convert those to G-code using CAM software or a slicing script that accounts for wire angle and offset. Keep template outlines simple for hand-finishing, and always include datum holes or locating notches so you can align skins and joiners when assembling the finished core.
Practical workshop tips will improve results rapidly and reduce frustration when you start making wing cores and foam templates. Use a voltmeter to adjust wire temperature rather than relying on current alone, and keep spare lengths of nichrome or specialised hotwire on hand because the wire will eventually fail. Clamp your foam solidly and cut shallow passes if you see drag or rough edges, and sand gently with coarse grit before applying epoxy or glass fibre cloth. For structural wings, use a hardwood or carbon tube joiner and pre-cut holes or pockets into the core using the same CNC job to ensure perfect alignment during final assembly.
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