Cutting 6mm plywood

I am running a 15 W laser from ebay. Trying to cut through 6mm birch plywood. I am able to cut test circle, d=20mm, no problem. Hold the laser at constant height and do 24 passes @ 400mm/min. When I try to cut out bigger shapes, ie Moose head mount, with the exact same settings it does not get through the wood. I even run the same cut again over the original cut for 24 passes and drop the laser 2mm, still does not work. Laser is focused, everything is working great. It almost seems that there is a buildup of burnt wood that the laser cannot get through. Any suggestions would help. I have tried using multiple passes and dropping the laser 0.25mm per pass, I seem to get better results if I hold the laser at constant height with multiple passes. Any help suggestions would be appreciated.

Does your laser have air assist to clear smoke and crap out of the way?

I have a small centrifugal fan to clear the smoke. May need to upsize it for better clearing of all the junk.

I have a 40watt Chinese K-40 and I am having trouble with plywood also. It cuts clear acrylic in 12 passes at 50% power. Once the first 1/8 of plywood is black, it doesn’t seem to cut much deeper no matter how long I leave it. I have come to believe the energy from the laser is not making its way through the mirrors to the target effectively. Every time I go through the alignment procedure it gets better. Not saying you have an alignment problem, just that I have found that to steal power that needs to get to the target. Apparently the focus lens if pretty forgiving.


6mm plywood with a diode laser, it is a very difficult task and it amazes me a little that you have been able to cut the small circle at all. With my 5.5 watt diode laser it has been too unproductive to cut 4mm plywood, sometimes I used 25+ laps … You can buy specially “plywood with” organic "glue, suitable for laser cutting. But it is relatively expensive in the longer term. I have bought myself a K40 Co2 laser which does not cost much more than a 15 Watt laser diode, but the difference is huge!

With a diode laser like that you’d likely need to drop the focus down a bit after a few passes. Or at the very least start focused on the center of the wood… but then it probably won’t start cutting in the first place as it would be too out of focus. Acrylic will act as a wave guide so you don’t need to refocus as much but not so with wood obviously.
Also, if you are doing this with plywood from the normal hardware store you probably won’t have much luck. The glue they use is much harder to get through, and can actually gum up and become harder to get through as you go. Plus… the fumes are cancerous.

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It sounds like you’ve got some alignment issues. One of my K40’s came badly misaligned, so I had to shim the x-axis (mirror 3) up by 1.5 mm to get it aligned with the rest of the gantry. Before I did that, It engraved well, but cutting was a charred mess. Once I got the beam aligned to center, cutting became much less of a chore. I can cut through 6mm birch at 25% power in 2-3 passes. I think 30% is at around 4.7mA (I don’t have my conversion chart sitting in front of me at the moment). On that particular laser 45% is 12.1mA, which is the highest that I’ll go, as going beyond will overpower and shorten the life of the tube.

Would like to use a CO2 laser but they are way more money than my $100 15W diode. Plus I am using a MPCNC and grafting a CO2 laser to it is a huge challenge. Waiting for someone to develop a 50W diode laser.

Update: The whole laser delivery system and XY axis was in misalignment. It was severe and I was forced to disassemble most of the mechanism to effect repairs. Now I cut thin wood - veneer in 1 pass on the lowest power setting that makes a beam. Thin acrylic is 2 or three passes and 1/4” birch plywood is 6 to 8 passes depending on cleaning the lens and water temp. Will be adding an air assist and pulling out the extra sheet metal from the exhaust fan assembly.
Thanks to everyone for the advise and help.