Find your kerf
Cut one small card, slide your wood into the slot that fits, and you are done. No calipers, no maths, no guessing why the tabs are loose.
Why slots come out loose
A laser does not cut along a line. It burns a groovealong it, roughly a hair's width wide. That groove is thekerf, and it is the reason a slot drawn at exactly 3mm comes out wider than 3mm — the beam eats a little off each side as it goes round.
So a slot that holds 3mm plywood snugly has to bedrawn narrower than 3mm. How much narrower depends on your machine, your lens, your speed and power, and the sheet in front of you. That is why nobody can put the number in the file for you.
The beam is centred on the line, so the opening ends up one whole kerf wider than drawn. Turn that around and you get the only number you actually need.
1 Make your test card
Pick the stock you want to dial in. The card is sized to cut in about a minute on a scrap offcut.
Whatever it is sold as. "3mm" ply is rarely 3mm and that is fine — the test measures the real thing.
Fine tuning
The card wraps onto another row rather than outgrow your machine.
SVG for LightBurn, Glowforge, Inkscape, xTool. DXF for RDWorks, CorelDRAW and older CO2 controllers. Same card either way.
2 Cut it
- Use the same material you will build with. Same sheet if you can. Kerf changes with material, and plywood thickness changes between sheets of the same box.
- Use the settings you normally cut with. Speed and power change the kerf. If you dial this in at 20mm/s, cut your files at 20mm/s.
- Red is cut through, black is engrave — the same convention as every file on this site.
- Make sure it is in focus. An out-of-focus beam is a fatter beam, and you will be measuring the mistake instead of the machine.
Keep the card. Write the material on it in pencil and it will still be true in a year.
3 Find the slot that fits
Slide a piece of your stock into each slot. You want the one that goes in with firm thumb pressure and stays put when you let go — snug, not hammered, not falling out. Tap its number.
Your number
—mm
Draw every slot this wide and your stock will fit it snugly, on this machine, in this material.
Only if you own calipers. It splits the number into thickness and kerf, which you need for tab widths and finger joints.
Your kerf: —
Your materials
Kept in this browser only — nothing is uploaded, and nothing here identifies you.
A few honest limits
- Kerf is not one number forever. It changes with material, thickness, speed, power, focus, lens and even how clean the mirrors are. Re-run the card when any of those change.
- Kerf is slightly tapered. A CO2 beam is a cone, so the slot is a hair wider on the top face than the bottom. On thin stock it does not matter. On 12mm it can.
- Snug is a feel, not a measurement. If you are gluing, aim one slot looser. If it is a press fit that must hold without glue, aim one tighter.
The one thing worth remembering
Slot width = stock thickness − kerf.
Tab width = drawn width − kerf.
Everything on this page is that, rearranged. If you ever lose the number, cut another card — it takes a minute.