Why Your Laser-Cut Box Doesn't Fit Together (Kerf, and How to Fix It)

You designed a box. The tabs and the slots are the same width, it looks perfect on screen, you cut it, and the pieces either rattle around loose or refuse to go together at all. Nothing is wrong with your design. You are just meeting kerf, the single most common reason laser-cut joints do not fit, and once you understand it the fix takes about thirty seconds.

This is a practical guide: what kerf actually is, why it makes a tab-and-slot joint loose by exactly twice the beam width, how to measure your own kerf in one cut, and the two ways to compensate so parts press together with a satisfying friction fit.

What kerf is

A laser does not cut along a line like a pen draws. It burns away a channel of material, and that channel has width. That width is the kerf. On a diode or small CO2 laser cutting plywood or acrylic it is usually somewhere between 0.1mm and 0.25mm, depending on the machine, the lens, the focus, the material, and the speed.

The important part is where that channel sits relative to your line: the beam is centered on the path, so it removes roughly half the kerf from each side of the line. That one fact explains every fit problem you have ever had.

Why a tab and a slot drawn the same size end up loose

Follow the material, not the line.

When you cut a shape OUT of a sheet (a tab, the outer edge of a part), the beam eats half the kerf off the outside of your line all the way around. The part comes out a little smaller than you drew it, by about half a kerf on each edge.

When you cut a hole or a slot (you are removing the material inside the line), the beam eats half the kerf off the inside of the line. The opening comes out a little bigger than you drew it, again by about half a kerf on each edge.

Now put them together. A tab drawn at width W comes out at about W minus one kerf. A slot drawn at the same width W comes out at about W plus one kerf. The gap between them is the difference:

This is not a mistake in your file. It is physics. The fix is to account for it on purpose.

Measure your own kerf in one cut

Published kerf numbers are a starting point, not gospel. Yours depends on your exact setup, so measure it. The cleanest way needs one small cut and a pair of calipers:

1. Draw a square, say 20mm by 20mm, and cut it out as a single closed shape.
2. You now have two pieces: the hole in the sheet, and the little square that dropped out.
3. Measure the inside of the hole and the outside of the dropout square.
4. The hole will be slightly larger than 20mm and the dropout slightly smaller. Your kerf is the difference between them, divided by two.

For example if the hole measures 20.18mm and the dropout measures 19.82mm, the difference is 0.36mm, so your kerf is about 0.18mm. Write it on the wall above your laser. It will not change much unless you change material, lens, or focus.

If you would rather dial in clean cutting first, our free material test card generator finds the lowest power and highest speed that still cuts through, which is also the setting that gives you the most consistent kerf.

Two ways to compensate

Once you know your kerf, you have two honest options.

Option 1: kerf offset in your laser software. This is the easy one and most people should start here. LightBurn has a kerf offset setting on each cut layer. Set it to half your kerf (the beam radius, so about 0.09mm for a 0.18mm kerf) and it shifts the actual cut path outward on parts and inward on holes, so the finished geometry matches your drawing exactly. Tabs and slots drawn the same size then fit with a light friction press. Nothing in your file changes, you just tell the machine to account for the beam.

Option 2: design the compensation into the file. If your software has no kerf offset, or you are sharing a file that has to fit anywhere, bake it in: make slots narrower by about one kerf, or make tabs wider by about one kerf, so the joint comes out with a slight interference. This is more work and it is specific to one material and one machine, which is why the layer-offset approach is usually better. But it travels with the file.

A good rule for a firm press-fit in plywood: aim for the joint to be about one kerf tight before cutting, then let the kerf bring it to a snug zero. For a slip fit (glued joints, or parts that need to slide), leave it at nominal and let the natural two-kerf clearance do the work.

The thing that has to be true first

None of this works if the geometry itself is broken, and converted or downloaded files often are. Kerf compensation offsets a closed loop by a fixed amount. If your slot is actually an open path, or two paths stacked on top of each other, or a loop with a tiny gap that looks closed on screen, the offset has nothing clean to work from and the cut wanders or double-burns regardless of how carefully you set the kerf.

So before you fight the fit, make sure the file is sound:

• Every cut shape is a closed loop. Open paths cannot be kerf-compensated sensibly and will not release cleanly.
• No stacked duplicate lines. Two slots on top of each other double-cut and widen unpredictably, which destroys any fit you calculated.
• No micro-gaps. A loop that looks closed but is not throws off both the cut and the offset.

You can check all three in a few seconds with our free Cut File Checker before anything goes near the machine. It flags open paths, double lines, and gaps, no login, nothing uploaded. We built it after laser users told us their files were breaking in exactly these ways, which is the same reason we wrote up the laser file problems we actually fixed.

Where this leaves you

Kerf is not a bug in your design or your machine. It is a known, measurable amount of material that the beam removes, and once you know your number you can either let your software offset it or design around it. Measure it once, pick one of the two methods, and your joints go from rattling to clicking together.

If you want a starting point for the geometry itself, you can convert any image or sketch to a clean, closed, laser-ready file for free and see the result before anything else. And if there is a fit or kerf workflow you wish a tool handled for you, tell us at contact@vectosolve.com. The features we are proudest of all started as one cutter telling us what was getting in their way.