Wiki › Foam & the Science

Kerf, corners & cut quality

What kerf is, why it matters for dimensional accuracy, how to compensate for it, and how to get crisp corners and clean surfaces from a hot-wire cut.

Kerf is the width of foam the wire removes. Because the hot zone is a little wider than the wire, your finished part is slightly smaller than the path you cut, and any hole is slightly larger. For rough work this does not matter; for fitted parts it does.

How big is the kerf?

Typically the kerf is the wire diameter plus a fraction of a millimetre of melt on each side — often somewhere around 0.5–1.5 mm total depending on wire size, temperature and feed. Hotter and slower = wider kerf. Measure it: cut a slot in scrap and compare the slot width to your programmed width.

Kerf compensation

To get an exact outside dimension, offset the cut path outward by half the kerf; for an exact hole, offset inward. Some workflows bake this offset into the design; cncfoam.com keeps the wire diameter in mind in its previews, and kerf-compensation is on the roadmap. For now, the simple approach is to draw your profile slightly oversize by half a kerf where dimensions are critical.

Crisp corners

Sharp corners are the enemy of a bowing wire. Strategies:

  • Slow into corners. Reducing feed at acute angles lets the wire catch up so it does not round the corner.
  • Add a tiny dwell at a true point if your controller supports it, so the wire reaches the corner before turning.
  • Design gentle corners where you can — a small radius cuts far cleaner than a knife-edge point.
  • Keep tension high — a taut wire bows less and corners better.
cncfoam.com flags a cut-speed mismatch when one side of the wire must travel much farther than the other; that side risks melting. Reshape the profiles or raise the feed if you see the warning.

Surface finish

A good hot-wire cut leaves a smooth, slightly glazed skin. To improve it:

  • Use XPS rather than coarse EPS for the finest finish.
  • Keep the feed steady — speed changes leave visible lines.
  • Avoid stop/start in the middle of a visible face; lead in from an edge.
  • Keep the wire clean; baked-on residue drags and marks the surface.

Lead-in and lead-out

Start and finish cuts from the edge of the block or a scrap area, not in the middle of a show surface, so the entry/exit mark lands where it will not be seen. cncfoam.com automatically picks a lead-in point close to the origin to avoid undercutting generated shapes.

← PreviousWire, temperature & feedrate Next →Safety & ventilation