When I was building the sheet configuration screen for Lapas, kerf was the setting that generated the most support questions in the first few months. People would upload a DXF, run a nest, send the file to their laser or plasma table, and the parts would come out 0.2 to 3 mm undersized. Sometimes two adjacent parts would overlap on the cut path and both end up wrong.

The fix is always the same: set your kerf width correctly. But “correctly” requires knowing what kerf actually is, how your machine produces it, and what the nesting software does with the value you give it. This guide covers all three.

What is kerf?

Kerf is the width of material removed by the cutting tool during a cut. The word originally described the slot left by a saw, but it applies to any cutting process:

Process	Typical kerf
Fiber laser, thin steel (≤ 3 mm)	0.1–0.3 mm
CO₂ laser, wood or acrylic	0.2–0.5 mm
Plasma, mild steel 6–20 mm	1.5–4 mm
Waterjet	0.8–1.5 mm
CNC router (bit diameter)	3–12 mm
Bandsaw	1–3 mm

Plasma kerf figures are consistent with Hypertherm Powermax series cut charts (65A–105A range on mild steel 6–25 mm). Fiber laser figures are based on TRUMPF and Bystronic published specifications for 1–6 kW machines.

The cut beam, arc, or bit removes material from the cut path. That material is gone, turned into heat, slag, dust, or sawdust. The space it occupied is now air, and any part whose edge passed through that space is narrower than designed.

Why kerf causes dimension errors

Imagine you’re cutting a 100 × 100 mm square on a plasma cutter with a 3 mm kerf. The CNC follows the programmed path exactly. But the path sits at the centre of the cutting arc, so material is removed 1.5 mm on either side of the path.

If you programmed the outer edge of the square, the actual cut edge will be 1.5 mm inside your intended line, making the part 97 × 97 mm instead of 100 × 100 mm.

This is called kerf offset error, and for precision parts it is a significant problem.

There are two places to address it:

CAM software (toolpath offset) — The CAM tool offsets the entire cut path inward or outward by half the kerf width before sending it to the machine. This is called cutter compensation or kerf offset. Most CNC controllers support G41/G42 compensation codes for this.
Nesting software (part spacing) — Even if toolpath compensation is perfect, parts on the sheet still need a minimum gap between them equal to the full kerf width. Otherwise, a shared cut edge will remove material from both adjacent parts simultaneously.

Nesting software handles item 2. CAM handles item 1. They are separate but complementary.

How nesting software handles kerf

When you specify a kerf width in your nesting software, the optimizer ensures no two parts are placed closer than that distance. This prevents:

Parts being undersized because they shared a cut path
The cutting head burning or melting into an adjacent part
Thermal distortion from adjacent cuts running too close together

In Lapas, the kerf setting is on the sheet configuration screen. Set it to your actual measured kerf width for the material and process you’re using.

Part spacing vs. kerf: what’s the difference?

You’ll often see two settings: kerf and minimum part spacing. Here’s how they relate:

Kerf is the physical width of the cut. Required spacing between parts is at minimum equal to the kerf.
Minimum part spacing is an additional gap you can add on top of the kerf — for heat distortion clearance, material fixturing, or process requirements.

For most laser jobs: set kerf to your beam width, minimum spacing to 0 (or 0.5–1 mm if you want a safety margin).

For plasma: set kerf to 1.5–3 mm depending on your machine and material thickness, and consider adding 1–2 mm of extra spacing for heat-affected zone clearance.

How to measure your actual kerf

Don’t guess at kerf width — measure it. The steps:

Cut a test square of known size (e.g., 100 × 100 mm) from the same material and thickness you’ll be running
Measure the actual cut part with calipers
Calculate: kerf = (programmed dimension − actual dimension) × 2

Example: programmed 100 mm, measured 98.4 mm → kerf = (100 − 98.4) × 2 = 3.2 mm

Measure in both X and Y directions; kerf is often not perfectly symmetric.

Kerf varies with material and thickness

The same machine will produce different kerf on different materials and thicknesses. Factors that increase kerf:

Thicker material (longer beam path)
Higher cutting speed (rougher cut)
Worn or contaminated cutting nozzle/lens
Faster feed rate (less focused energy)
Higher power settings on some machines

Best practice: create a kerf reference table for each material/thickness combination you run regularly.

Kerf in practice: laser vs. plasma vs. router

Laser cutting

Fiber laser kerf is typically 0.1–0.3 mm on thin steel, acrylic, and aluminium. This is small enough that for many jobs you can set it to 0.2 mm and get acceptable results without test cuts.

For precision work (tight tolerance parts, assemblies that press-fit together), measure kerf on every new material batch. Lens condition, gas pressure, and focal distance all affect it.

Plasma cutting

Plasma kerf varies more than laser kerf, typically 1.5–4 mm. It also varies with:

Cutting speed — faster cutting produces narrower (but rougher) kerf
Amperage — higher amperage produces wider kerf
Material thickness — thicker steel typically widens the kerf
Gas type — oxygen plasma produces different kerf than air plasma

For plasma nesting, use a kerf value from your machine’s cut charts (Hypertherm, ESAB, Lincoln publish these) or measure directly. Errors of 1–2 mm in kerf setting are common and will compound on jobs with many closely-packed parts.

CNC router

CNC router kerf is simply the diameter of the cutting bit. A 6 mm end mill has 6 mm kerf. A 3 mm spiral bit has 3 mm kerf.

For woodworking and MDF cutting, this is the most intuitive kerf setting. Note that for nested CNC routing, you often need larger part spacing than just the kerf, since the router path needs room to start and finish cuts without running into adjacent parts.

Common kerf mistakes

Setting kerf to zero — This tells the nesting software parts can touch. In practice, two adjacent parts with zero gap will share a cut path, making one or both undersized, and risks damaging the cutting head.

Using nominal kerf instead of measured kerf — Machine specs say 0.2 mm but your worn lens is actually cutting 0.35 mm. Always measure on your actual machine.

Using the same kerf for all materials — Steel, aluminium, acrylic, and wood all have different kerf on the same machine. Use material-specific values.

Confusing nesting kerf with CAM kerf compensation — These are different. Nesting software handles spacing between parts. CAM handles the toolpath offset so the cut lands on the right edge. Both are needed for precision work.

Setting kerf in Lapas

In Lapas, kerf is set per job on the sheet configuration screen:

Open a new job
On the sheet settings panel, find the Kerf width field
Enter your measured kerf in millimetres
Optionally, set Minimum part spacing for additional clearance

The optimizer will enforce this gap between all adjacent parts. You’ll see the gap reflected in the live nesting preview as parts are placed.

For jobs where some parts require more spacing (e.g., thin parts prone to thermal distortion next to heavy plasma cuts), Lapas Pro allows per-part spacing overrides.

FAQ

Do I need to set kerf if I’m using G41/G42 compensation in my CAM software?

Yes. G41/G42 handles the toolpath offset so the part comes out the right size, but does nothing about spacing between adjacent parts on the sheet. You still need the kerf setting in your nesting software to ensure parts don’t share cut paths.

What if I nest without kerf and all my parts come out slightly small?

This happens when parts are nested with zero gap and they end up sharing a cut path. The cut removes material from both sides simultaneously. Set the kerf to your measured value and re-run the nest.

Should I set kerf or minimum spacing, or both?

Set kerf to your actual beam/arc/bit width. Use minimum spacing as an additional buffer on top of kerf, useful for plasma (heat distortion) or routing (bit entry/exit clearance). For laser cutting thin materials, kerf alone is usually sufficient.

Does kerf affect utilization significantly?

At laser scales (0.2 mm), barely. At plasma scales (3 mm), yes, especially on jobs with many small, closely-packed parts. A 3 mm kerf on a job with 200 parts, each needing 3 mm clearance on four sides, can add up to 5–8% of sheet area consumed by cut paths alone.

Kerf Compensation in Nesting Software — What It Is and How to Set It