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Laser-cut joinery: finger joints, dados and rabbets that actually fit
Almost every bad laser joint is the same bug: the design was drawn at nominal size and the machine removed a strip of material the designer never accounted for. That strip is the kerf, and once you measure yours, box joints stop being a lottery.
Kerf is the whole problem
Your laser does not cut along a line. It burns a slot with real width - the kerf - centred on that line. Draw two parts that share an edge and cut them out, and each one comes back half a kerf smaller than you drew it. A finger joint drawn at exactly 6.00 mm in 6 mm ply ends up around 5.85 mm wide in a 6.15 mm slot. It rattles.
Kerf is not one number. It changes with material, thickness, power, speed, focus height and lens. The only kerf that matters is the one your machine produces on the sheet in front of you today.
Measuring it in two minutes
- Draw a 50 mm square and cut it out of your actual stock.
- Measure the square with calipers. Call it M.
- Kerf = 50 - M. If the square measures 49.80 mm, your kerf is 0.20 mm.
Cut five squares across the bed, not one. Kerf drifts if your bed is not level or the lens is dirty, and a corner that cuts 0.05 mm wider than the centre will show up as one loose joint in an otherwise tight box. Our kerf calculator turns those measurements into the offset to apply.
| Material | Typical kerf, diode | Typical kerf, CO2 |
|---|---|---|
| 3 mm plywood | 0.15-0.25 mm | 0.15-0.20 mm |
| 6 mm plywood | 0.20-0.35 mm | 0.20-0.30 mm |
| 3 mm MDF | 0.15-0.25 mm | 0.15-0.20 mm |
| 3 mm acrylic | n/a | 0.10-0.20 mm |
Treat these as a sanity check on your own measurement, not a substitute for it.
Finger joints (box joints)
The classic laser joint: alternating tabs and slots on two mating edges. Three rules make them work.
Compensate in the right direction
Grow the tabs by one kerf and shrink the slots by one kerf - or, more simply, offset every cut path outward by half a kerf on the part you want to keep. Most CAD packages and every box generator can do this for you; the mistake is applying it to only one of the two mating parts.
Size the tabs sensibly
Tab width should be roughly 1.5 to 3 times material thickness. Narrower and the tabs snap off in plywood; wider and the joint looks clumsy and loses glue area. For 3 mm ply, 6-9 mm tabs are a good default. Always use an odd number of fingers per edge so the corners mirror correctly.
Aim for a press fit, not a hammer fit
The joint should go together with firm hand pressure and stay together dry. If you need a mallet, you are one kerf over-tight and plywood will delaminate along the tab. If it falls apart under its own weight, add 0.05 mm and re-cut a test corner rather than the whole box.
Cut a single test corner - two small L pieces with three fingers each - before committing a full sheet. It costs 30 seconds and saves a sheet.
Dados and rabbets: raster, don't cut
A dado is a flat-bottomed groove across a panel; a rabbet is the same thing at an edge. Neither is a through-cut, so you do not cut them - you raster a pocket to a controlled depth.
- Depth comes from passes, not power. Multiple light passes give a flat floor. One heavy pass gives a scorched, dished bottom and a lot of smoke.
- Width is drawn, not kerfed. Because you are rastering an area rather than tracing a line, draw the groove at the width you actually want the mating panel to be - plus a hair for glue.
- The floor will never be perfectly flat. A laser removes material in a V, so a laser dado has slightly rounded shoulders. For anything structural, plan on glue filling that gap.
Depth control is genuinely fiddly. Run a stepped test - the same pocket at 1, 2, 3 and 4 passes - and measure each with a depth gauge. Write the result on the offcut and keep it; you will want it again.
One caveat worth stating: a laser is a poor tool for deep dados. Past about a third of material thickness in plywood, you are burning a lot of material into smoke, the walls taper noticeably, and a router or table saw would do it better and faster. Lasers win on fiddly and small, not on deep.
Joints that come apart again
For flat-pack pieces, a captive nut joint beats glue. Cut a slot sized for an M3 or M4 nut perpendicular to a clearance hole, drop the nut in, and drive a bolt through the mating panel. It disassembles indefinitely and survives being posted.
The dimension people get wrong is the nut slot: measure across the flats of your actual nuts, add 0.1-0.2 mm, and cut a slot that deep plus a millimetre so the nut sits fully home. A nut that is proud of the surface holds the joint open.
Getting it onto the sheet
Joinery designs generate a lot of small parts, and small parts waste sheet fast if you place them by eye. Once the joints are right, run the flat pattern through the nesting tool to see how many complete boxes actually fit on a sheet - and the sheet yield calculator to price the material properly before you quote the job.
FAQ
How do I measure laser kerf?
Cut a 50 mm square from your actual stock and measure it with calipers. The difference between 50 mm and what you measure is your kerf. Cut several across the bed rather than one, because kerf drifts with bed level and lens cleanliness.
How wide should finger joint tabs be?
Roughly 1.5 to 3 times material thickness. For 3 mm plywood that means 6-9 mm tabs. Narrower tabs snap off in plywood, wider ones lose glue area and look clumsy. Use an odd number of fingers per edge so corners mirror correctly.
Why do my box joints fall apart?
The design almost certainly did not account for kerf. The laser removes a slot of real width, so parts come back smaller than drawn. Grow tabs and shrink slots by one kerf, and apply the compensation to both mating parts, not just one.
Can a laser cut a dado or a groove?
Yes, but by rastering a pocket rather than cutting a line. Control depth with multiple light passes instead of one heavy pass, and expect slightly rounded shoulders because a laser removes material in a V. Past about a third of material thickness, a router does the job better.