You cut a tab and a slot, both drawn at 3 mm. The tab drops straight through and the shelf wobbles. Or it won't go in at all, and you're sanding at midnight.
Two things cause it. Your "3 mm" sheet isn't 3 mm. And the laser burns away a thin line, called the kerf, so every slot comes out a little wider than you drew it.
Fix both and you get a joint that pushes together and holds. No glue, no clamps.
What a good press-fit feels like
You push the tab in with your thumb, maybe with a light tap. It stays put when you pick the part up upside down. It comes apart again if you pull hard.
Wobbles? Too loose. Needs a hammer, or the wood splits? Too tight.
The numbers to draw
Five numbers. Get these right before you think about the laser.
- Slot width = the real thickness of the sheet. Measure it with calipers. Don't trust the label.
- Slot length = the tab width.
- Tab width: about 2 to 4 times the thickness. On 3 mm plywood, 8 to 12 mm works well.
- Tab length = the thickness of the panel it goes through, so it sits flush. Make it longer if you want it to poke out.
- Edge distance: keep a slot at least one sheet thickness from the edge, or the thin strip snaps.
Where the gap comes from
The laser can't change how thick your sheet is. But it does eat into the slot. With a kerf of 0.15 mm, a slot drawn at 3.1 mm cuts at about 3.25 mm. Your 3.1 mm sheet slides in with room to spare.
The other direction is worse. The slot gets longer and the tab gets narrower, so you lose the kerf twice.
So you have to take the kerf back out. Either draw the slot smaller by hand, or let your software offset the cut lines.
The four-slot test
Don't guess. Cut this once for every new stack of sheets. It takes five minutes.
One strip, one tab. The slot that grips is your number.
- Measure your sheet in three places and take the average. Say 3.1 mm.
- Draw a strip with four slots, all 10 mm long: 0.2 mm under, 0.1 mm under, exact, and 0.1 mm over.
- Draw one test tab, 10 mm wide.
- Cut both and push the tab into each slot.
- Keep the slot that grips without a hammer. That's your slot width for this sheet.
Want the kerf as a number too? Here's how to measure your laser's kerf.
Plywood, MDF and acrylic
Plywood squashes a little, so it forgives a tight fit. It's the easiest to press-fit.
MDF works too, but the edges crumble if you force it. Go a hair looser than plywood.
Acrylic doesn't squash at all. A tight press-fit can crack it. Aim for snug, not tight.
Doing it without redrawing
Here's the catch. Your next sheet is 2.9 mm, and every slot in the file is wrong again.
In FlexCut3D you build the parts in 3D, so the slots come from the parts themselves:
- Set each panel's Thickness to your measured sheet, or link all the panels to one Thickness Slider.
- Stand the upright panel where its tab goes through the base. Select the base, open Auto Holes and pick the upright panel. The slot is cut where they cross, even at an angle.
- New sheet? Change the thickness and every slot follows. Nothing to redraw.
- In the 2D panel, set your Kerf Offset. Parts grow and holes shrink by that amount, so they cut at the size you drew. Too loose? Raise it a little. It isn't saved with the project, so write your number down.
Then press Auto Layout and Export SVG, and open the file in your laser software.
What to try next
Building a box rather than a stand? Finger joints for laser cutting covers the sizes for corners. Or open FlexCut3D and cut your first press-fit joint tonight.