I’m trying to cut through plywood that’s at most 5mm thick. I’m using Lightburn’s # of passes / z step per pass settings to cut through.
I’m burning at 2mm/s (120mm/min), with a 0.33 mm z-step.
The problem is, I’m doing WAY more passes than I think mathematically should be enough. Here’s my scrap test piece, with # of passes and z-step listed for each test:
As you can see, I’ve done as many as 27 passes with 0.33 z-step per pass - which should be 9mm of cut depth. As I mentioned, the piece is no more than 5mm thick.
Here’s what the other side looks like - remember this is flipped horizontally from the previous picture.
My theory is that since my focal length off the material is about 24mm, by the time you get 4mm closer to the surface, a significant portion of the laser cone is being occluded by the top surface, essentially casting a “shadow” into the trench. Is this a thing?
I tried slowing the laser down at deeper depths, for example on the bottom left, I did 21 layers, with 0.33 step, at 2mm/s, then 3 layers at 0.33, with a 7mm offset, at 0.5mm/sec to try and really burn through the bottom.
I feel like either my math or my understanding of the problem is way off, very grateful for any thoughts or suggestions!
My next step is to try @chazr33gtr 's idea of increasing the focal length, but thought I’d ask first.
Best,
Aaron
You’re probably trying to cut through 1.5mm more than the SM can do.
You can try changing the focus. There’s a thread somewhere around here with @brent113 experience with that.
Your step-down is larger than you want for that thickness. Should be around .15 (thickness of material divided by number of passes)
One thing you could also try is adding a fan near work piece to get rid of smoke and increase combustion.
-S
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Awesome - thanks. So the SM is really only supposed to cut about 3, 3.5mm? If that’s the design operation limit, that might make sense.
I’ll try a lower step-down - thanks for the guidance.
I think @chazr33gtr also had a post about changing the focus, will look up @brent113 's as well. Thanks again!
That won’t help here as increasing the focus will widen the beam waist making it cut worse. And decreasing the focus has similar issues where the sides of the beam will contact the top of the plywood sapping energy, also making it not able to cut.
My experience with this type of plywood is it took me 50 passes at 1mm/s. Clearly not feasible.
Imagine the plywood having thickness along this beam shape, before convergence at the waist (thinnest part, highest energy density) the wider area will be scattered on the wood above the cut.
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Oh wow - I didn’t realize increasing the focus (I assume that means “lengthening the cone”) will widen the beam waist (which I assume means the size of the convergence point / focus point). I’m not familiar with those terms - is that the correct interpretation? Thanks for the education!
I think I’m gonna chalk this up as “material that really isn’t gonna work” and call it done. I’m really leery of changing the focal length until I know what I’m doing, and if general consensus is “this is too thick”, I’m good with that.
Source: https://www.edmundoptics.com/knowledge-center/application-notes/lasers/gaussian-beam-propagation/
Here’s a calculator you can play around with to see the effect of focal length on spot size:
https://www.ophiropt.com/laser--measurement/laser-power-energy-meters/services/focal-spot-size-calculator-for-gaussian-beams
With these initial parameters I get a spot size of .15mm (lots of assumptions, doesn’t need to be 100% accurate here to get the point across).
Increasing the focal length to 40mm results in .2mm spot size.
The reason I increased my focal length is you can exchange worse energy density for a longer depth of focus, which for my use makes engraving slightly more consistent as subtle variations in material height don’t affect the peak density in the center of the beam as much.
If you want maximum cutting power you would want the shortest focal length that doesn’t result in the beam cone intersecting the top of the work (difficult for thick workpieces given the limited laser power).
I think from the factory the focal length is pretty good for both cutting and engraving. I don’t do cutting on anything except paper or thin balsa, so I don’t need the peak power. Everything else gets CNCd
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