Kerf: why your laser-cut parts do not fit together

You draw a 50 mm square, cut it, measure it, and it comes out at 49.8 mm. You draw a tab exactly as wide as its slot, and the finished joint is loose. This is kerf, and once you know the number for your machine and material you can design it away completely.

What kerf is

A laser does not cut along a line the way a pen draws one. It removes a strip of material with a measurable width. That strip is the kerf.

The beam follows the line you drew, so the kerf is removed centred on your line, roughly half from each side. That has two consequences:

• Parts come out smaller than drawn, by one kerf width in total (half off each side).
• Holes come out larger than drawn, by one kerf width in total.

This is why a tab-and-slot joint drawn to exact dimensions is always loose: the tab lost half a kerf on each side and the slot gained half a kerf on each side. The gap you end up with is twice the kerf width, which is why the joint feels much looser than the kerf number alone would suggest.

How wide is it?

On a CO₂ laser cutting non-metals, kerf is typically 0.1 to 0.3 mm. That range is wide because kerf is not a fixed property of the machine. It changes with:

Factor	Effect on kerf
Lens focal length	A shorter focal length focuses to a smaller spot and cuts a narrower kerf. A long lens cuts wider.
Focus height	The kerf is narrowest at true focus and widens as focus drifts either way.
Power and speed	More power, or slower travel, puts more energy in and widens the kerf.
Material	Acrylic, plywood, MDF, leather and felt all behave differently, and thickness matters too.
Material thickness	The cut is slightly V-shaped, so kerf at the top face is a little wider than at the bottom.

Because of that list, there is no useful universal number. Measure your own, for the combination you actually use.

Kerf is not constant through the thickness

The beam is a cone, so the cut is very slightly wedge-shaped, wider at the top than the bottom. On thin material this is irrelevant. On thick acrylic it is not, and it is why a part that press-fits in 3 mm can be too tight in 10 mm. If you need precision in thick stock, measure the kerf at the face that matters for the joint.

Measuring your kerf in one cut

There is a ready-made gauge for this, by Nameek and Elplatt, and it is the one we use at BRM Lasers: kerf gauge on Thingiverse.

If Thingiverse is unavailable

We link the original because the creators deserve the credit. If you cannot reach it, we host a copy: kerf gauge (SVG).

1. Import the file into your laser software and apply the same settings you would use for the real job.
2. Cut the design.
3. Slide the small square-holed piece along the gauge. It stops at the point where the slot is no longer wider than the cut. Read the value at the top.

That reading is the full kerf width. Halve it, and that half-kerf is the figure the software offset expects, because the beam takes half a kerf from each side of your line.

Write it down

Keep a small table of kerf values per material, thickness and lens, and reuse the figure whenever that combination comes round again.

Applying the compensation

Option 1: let the software offset the cut path

In LightBurn the setting sits on the layer. Open the settings for a line layer, go to Advanced, and fill in Offset. A positive value compensates on the outside of the shape, a negative value on the inside.

Option 2: design the compensation into the drawing

For joints specifically, building the allowance into the CAD rather than relying on a software offset is explicit: it travels with the file, and it survives being sent to a different machine.

If you are drawing a tab and a slot to press-fit, make the tab wider than the slot by one full kerf. The tab loses half a kerf per side and the slot gains half a kerf per side, so they meet exactly.

Many box and finger-joint generators ask for your kerf up front and do this arithmetic for you. Give them the number you measured.

Do not use both at once

If you design the compensation into the drawing and set a kerf offset in the software, you will apply it twice and the parts will not go together. Pick one method and be consistent.

Designing joints that actually work

Kerf compensation only matters once the joint geometry is sound. A few rules that save material:

• Finger length ≈ material thickness, or a small multiple of it. A joint with fingers much shorter than the stock is weak, and one with very long fingers is fragile to cut.
• Test before you commit. Cut a single joint (one tab, one slot) before cutting a whole box.
• Aim for a light friction fit, not a hammer fit. Acrylic in particular will craze or crack if a joint is forced, sometimes days later.
• Decide beforehand if you want to glue. If the joint will be glued, a slightly looser fit is better, because a press fit leaves nowhere for the glue to go.
• Remember the sheet is not exactly its nominal thickness. “3 mm” plywood is routinely 2.7 to 3.2 mm. If your slots are drawn for 3.0 mm and the sheet measures 2.7 mm, the joint is already 0.3 mm loose before kerf comes into it. Measure the sheet rather than trusting the label.

On a BRM machine

Two things to be aware of specifically:

• Your kerf depends on which lens is fitted. BRM machines can be fitted with different focal lengths, and they do not produce the same kerf. If you change lens, re-measure. See different lenses and lens compatibility by machine model.
• Kerf is a good early warning. If a kerf measurement that has been stable for months suddenly comes out much wider, that is worth investigating rather than compensating for; a widening kerf usually means the focus has drifted or the optics need cleaning. See my laser no longer cuts all the way through.