Kerf Compensation: The Complete Guide

Kerf compensation is one of the most important concepts in CNC foam cutting. Getting it right means your parts come out at exactly the dimensions you designed. Getting it wrong means every part is slightly too small—and that adds up fast on production runs.

What is Kerf?

When a hot wire passes through foam, it melts and vaporizes a thin strip of material. This strip is called the kerf. The kerf width depends on several factors:

Wire temperature: Hotter wire = wider kerf
Feed rate: Slower movement = wider kerf (more heat exposure per unit length)
Wire diameter: Thicker wire = wider kerf
Foam type: Different foams melt at different rates
Foam density: Denser foam generally means narrower kerf

Typical kerf widths for hot-wire foam cutting range from 0.5mm to 2mm (0.02" to 0.08").

Why Kerf Compensation Matters

Without compensation, the cutting path follows your design exactly. But because the wire has width, it removes material on both sides of the path. The result: your finished part is smaller than designed by approximately one kerf width on all sides.

For a part designed at 100mm x 100mm with a 1mm kerf, the actual result would be approximately 99mm x 99mm. For packaging inserts where fit matters, this can mean parts don't seat correctly.

How Kerf Compensation Works

CAM software offsets the cutting path outward by half the kerf width. For an external contour (cutting around a shape), the offset pushes the path outward. For an internal contour (cutting a hole), the offset pushes the path inward.

This ensures the edge of the kerf aligns with your design boundary, producing parts at the correct dimensions.

Measuring Your Kerf

The best way to determine your kerf width:

Design a simple square (e.g., 50mm x 50mm)
Cut it with kerf compensation set to zero
Measure the finished part with calipers
The difference between designed and actual size is your kerf

For example, if you designed 50mm and measured 49.2mm, your kerf is approximately 0.8mm per side, or 1.6mm total. Set your kerf compensation to 0.8mm.

Setting Kerf in FoamCut

In FoamCut, enter your kerf value in the settings panel. The software automatically determines which contours are external (offset outward) and which are holes (offset inward) using its island detection algorithm.

Tips for Accurate Kerf

Measure regularly: Kerf changes as wire condition and temperature vary
Test per material: Each foam type has a different kerf
Create a reference chart: Log kerf values for your common setups (foam type + temperature + feed rate)
Account for direction: Kerf may differ slightly between axes if wire tension varies