Two parts that fitted perfectly last month rattle this month, in the same design, on the same machine. Nothing is broken. The sheet changed, and so did the number the file should have used.
Every laser removes material as it cuts. That removed width is the kerf, and it is what stands between a slot that grips and a slot that wobbles. It is tempting to look kerf up in a forum post and hard-code 0.15 mm forever, but it does not work that way. Kerf moves with power, speed, focus, lens and the material itself.
Kerf is not the only number that moves
"6 mm plywood" is almost never 6.00 mm. Sheets from three suppliers can measure 5.6, 5.8 and 6.2, and a single sheet varies end to end wherever the core has voids or the sander went deeper. If the file says 6.00 because the label said 6 mm, you have lost before the beam fires.
So there are two numbers to find, and one test finds both at once.
The kerf comb
One strip, four slot widths, each engraved with its own number.
What you need
An offcut of the exact sheet you are about to build from, not a scrap of something else and not last month's sheet. Digital calipers. Five minutes of machine time.
Step 1: measure the sheet in three places
Caliper the offcut near two opposite edges and once in the middle. Write down the largest reading. If the three spread by more than about 0.2 mm, that sheet will fight you, and it is worth picking a different one for anything with tight joints.
Step 2: cut a comb around that measurement
Draw one strip with four slots: your measured thickness minus 0.4, minus 0.2, exactly, and plus 0.2. A sheet measuring 5.9 gives slots at 5.5, 5.7, 5.9 and 6.1. Engrave each slot with its number, or you will be measuring the comb afterwards to work out which is which.
Cut it at the settings you actually use for that material. A comb cut at different power and speed is a comb for a different kerf.
Step 3: push the offcut in
Take the same offcut you measured and try it in each slot in turn.
Falls straight through: the slot is wider than the material, so the file took out more than the beam did. Too much kerf compensation.
Slides in under thumb pressure and stays put: that is your number.
Will not enter: too narrow. Add kerf. Do not sand the tab, because sanding one joint to fit is how a box ends up loose everywhere else.
Step 4: do the subtraction once
Your kerf is the measured thickness minus the slot width that gripped. Sheet measured 5.9, slot that held was 5.7, kerf is 0.2 mm. That number is good for this machine, this material and these settings, and it stays good until one of the three changes.
Keep the comb with that batch of plywood. When the batch runs out, cut a new comb.
What this means for the file
The number that belongs in a slot is not the nominal thickness off the label. It is the measured thickness minus your kerf. A drawing that hard-codes 6.00 mm is a drawing that fits one sheet you no longer own.
This is the part FlexCut3D takes over. You give the model a material-thickness slider and a kerf value once, and every slot, tab and finger is expressed in terms of them instead of as a typed number. Move the thickness to 5.9 and every joint in the file recalculates. Cut the same design next month in 3 mm and it still assembles, because nothing in it was ever a fixed 6.00.
Open FlexCut3D in the browser, with nothing to install, and try it with the numbers you just measured.
What to try next
Once the comb has given you a kerf, the next thing worth getting right is the joint itself: how wide a finger should be, how many of them, and how deep they go. That is worked through in Finger Joints for Laser Cutting: Sizes That Actually Fit.