Optimal Laser Distance

I did a few experiments with my 5.5W (at least advertised) Aliexpress laser. I wanted to find the optimal focus distance, as I couldn’t find much in the forums other than “loosen the lens as far as you can”, although Ryan’s laser guide says a longer focal length is optimal.

So, I decided to do some quick experiments. It’s mostly what’s expected, but there does seem to be a sweet spot. Let me know if anyone wants me to do some more cases. This is specific to my laser, but I think it should apply to any diode laser with a screw-on lens.

The focusing tests were 75% power, 750 mm/min, 2mm Z change.
The cut tests were done at 90% power, bottom line 200 mm/min -> 150, 100, 80, 60, 50, 40, 30.
Air assist on the cuts.

At ~ 8mm focal length, there is a small window where the laser even leaves a mark. ± 4mm. The cut through 3mm (Amazon box) cardboard didn’t succeed until 40 mm/min.

At ~20mm focal length, there is a little bit more wiggle room, with the optimal focus staying consistent ± 1mm. The cut succeeded at 80 mm/min. (I ran the wrong code first, then bumped the cardboard during the cut test).

At ~45mm focal length, the optimal focus could vary by about ± 2mm. This one could cut through cleanly at 100 mm/min.

At ~85mm focal length (lens pretty much fully tightened), there was very little variance in optimal focal length, but I couldn’t get as fine of a point as at 45mm. This one also cut through cleanly at 100 mm/min.

Curious, I chose a spot between those 2:
At ~65mm focal length, the optimal focus could vary by about ± 3mm. This cut through clean at 150mm/min! (Although, I slightly adjusted my air assist angle. Would that have had that much of an effect? I don’t feel like running all the tests again hah)

So a longer focal length does in fact improve the cutting performance, especially since the focus point doesn’t vary much over a few mm in z change, which will help with multiple pass cuts and uneven surfaces. But go out too far and it seems like the spot size doesn’t get as fine, although the cutting power doesn’t seem to diminish much.

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How are you determining the focal length?

I set the laser power to 1, adjust Z until the spot is as small as I can make out, move Z down another few mm then run a focus script that burns 10 lines with +2mm Z height increase. Find the thinnest line and that’s the sweet spot. My measurements are from the bottom of the lens housing to the top of the workpiece.

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Brendan, Thank you for the detailed testing. It sounds like you have a 3 element laser lens. (Most Common). Here is a link to a LOT of detailed information about a few of the most common diode laser lens types. I don’t recommend anyone purchase from Endurance lasers for several reasons. (They sell your contact info, their lasers work well but look like they were built by a guy in a garage. etc…) But this link is full of really useful info and I encourage you to give it a good read. You might find yourself wanting to swap out lenses for different jobs.

Yeah, it seems likely it’s a 3-element. That is some good info on the different lens types. I figured I would post my findings as I’m sure many people are going with the cheap laser route, and there was a definite sweet spot to be found through testing.

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