Laser kerf: how to set it in LightBurn, xTool and RDWorks

Every laser file you download from Moxie has the exact sizes. A 3 mm slot is 3 mm wide, a 20 mm finger is 20 mm long. No kerf compensation baked in. That surprises people, because the first box they cut usually comes out a little loose, and the obvious guess is that the file is wrong.

It isn't. The beam just burns away a thin strip of material on every cut, and how thin depends on your machine, not on the file. So the fix belongs in your laser software, and it takes one number.

What kerf actually is

Kerf is the width of the line the laser burns out. On 3 mm plywood a diode laser typically takes something like 0.1 to 0.2 mm, a CO2 laser a bit more or less depending on the lens and the power. Half of that comes off each side of the line. So a piece you drew 20 mm wide comes out a hair under 20, and a 20 mm hole comes out a hair over. On a plain shape you will never notice. On finger joints and slots, where two parts have to grip each other, those tenths are the difference between a snug fit and a wobbly one.

Measure your kerf once

Cut a 20 mm square out of the material you actually use. Measure the square that falls out and the hole it leaves behind, with calipers if you have them. The hole minus the square, divided by two, is your kerf. If the hole is 20.18 and the square 19.82, the kerf is 0.18 mm.

Write it down per material and thickness. Plywood, MDF and acrylic all burn differently, and so does the same plywood at a different speed.

Where to set it in your laser software

LightBurn. Open the layer's Cut Settings and look for Kerf Offset (Line mode only). Enter half your kerf, so 0.09 for the example above. LightBurn works out on its own which shapes are holes and which are outlines, and moves each cut the right way: outlines grow, holes shrink. That is exactly why one file can work on any laser. If you can't see the field, Beginner Mode is hiding it; switch it off in the settings. LightBurn also has a built-in test pattern for measuring kerf, worth a try.

xTool Creative Space. Turn on Kerf Offset in the processing settings. It takes values from -1 to 1 mm, positive moves the cut outward. xTool suggests 0.1 as a starting point; test it on your material.

RDWorks (most CO2 lasers with a Ruida controller). The setting sits in the layer parameters and is called Sew compensation. Its sign doesn't behave like LightBurn's, so cut a small test piece before you trust it.

Glowforge. As far as I know the Glowforge app has no kerf setting. The usual workaround is to offset the design before uploading it, for example in Inkscape with Path, Outset or a linked offset by half the kerf.

Why I keep it out of the files

Because the second compensation is the one that bites. If a file already has kerf baked in and you also set it in LightBurn, the parts shrink twice and the joints get too tight to close. People hit this with files from all sorts of generators, and it's maddening to debug, because both the file and the software look right on their own.

So the rule on Moxie is simple: exact sizes in the file, kerf in one place only, your laser software. Set it once per material and every generator here fits your machine.

The honest catch

It is one more thing to set up before your first cut, and kerf isn't perfectly constant either. A slightly dirty lens, a different focus, a faster speed, a warped sheet: all of them change it a little. If a joint is a touch too tight, lower the offset by 0.01 to 0.02 mm and cut a test corner instead of the whole box.

Try it on a box

Finger joints show it best. Measure your kerf, set it in LightBurn, then cut a box with a lid or a crate and see how the corners close.

Once it's set, you won't think about it again until you change material.