2.8W jTech laser vs. 8 months of saved Amazon boxes…
2 passes @ 200 mm / min: I almost set the top ablaze, but only got a few singe marks on the bottom. It wasn’t what I’d call a cut - just some burn marks.
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5 passes @ 500 mm / min: The lines were a lot cleaner and I got a few very tiny cuts on the bottom, mostly around the sharp corners.
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What settings would you guys suggest?
I think air assist would help. Keeps it from flaring up.
A fan and shroud are a necessity for cutting cardboard cleanly and without flareups. In the following video, 3mm cardboard is being cut in one pass at full power and 100 mm/min with a Banggood 3 watt laser… no air assist used here (though provision is made for it), but the shroud directs a steady stream of air onto the cut area to clear smoke and suppress flames…
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The procedure I use to determine feed and speed is described in the following post…
https://www.v1engineering.com/forum/topic/mpcnc-inspired-corexy-laser-engraver/#post-36423
– David
Thanks guys! I’m not quite dialed in where I want it to be, but that helps a bunch.
I have a shroud and fan assembly. The fan blows across the laser’s heat-sink and then narrows down to about a 8mm hole near the work surface. My idea was to create positive pressure inside the shroud and enough of a breeze coming out the end to clear the smoke near the work surface.
Here it is without the laser shielding in place.
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I think I just had it too close to be in focus, and in one spot it started to drag on the cardboard because of cupping.
Here’s the results after raising it up a few mm.
Ryan has already supplied exactly what is needed… his laser focus script
https://www.v1engineering.com/forum/topic/laser-focus-script/
Easy to use for any material thickness… intentionally lower Z to “too close” and run the script. Ten lines are drawn 1mm apart, each 2mm higher than the one before. Count lines back to best focus, multiply by 2mm, and lower Z back down by that amount. Here, call it 6 lines back… lower Z by 12mm and you are ready to run your job 
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My focus test… 7 lines down, so I lowered it by 14mm.
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Underside of my speed test
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Top of my piece, cut at 73mm/min… It almost caught fire.
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Bottom… It didn’t cut through completely
The test piece and the actual cut are from the same cardboard box, so it’s not a variation in the materials.
I’m stumped. I suspect I need to do multiple, faster passes to avoid burning, and lower the laser a scootch with each pass.
Your focus pattern doesn’t look right… you should see 10 lines and a very distinct difference near focus. Just like in my photo…
A quick way to see if you are in the 20mm of range for the script…
I send M106 S2 (you may need more depending on your laser’s threshold) from the sender program to turn on a very dim beam… barely on. Move the Z-axis down very close to the material and then use the manual Z controls to raise the laser… watching the beam closely. It should get smaller/tighter until the absolute sharpest/smallest dot is attained… that should get you close. Then, lower Z from there 10mm and rerun the script. Your pattern should look very much like the one in my photo… with the sharpest lines near the middle.
When you get a properly focused beam at the top of the material, your 2.8 watt laser should cleanly cut completely through in one pass. I’ve had at least 4 or 5 different lasers in the 2.8 - 3.5 watt range and all have successfully cut through 3mm cardboard in a single pass at 100 mm/min and full power. Sharp focus is crucial, however… it’s an energy “thing”.
This very issue was discussed – and solved for at least 2 other people – in my lengthy thread over on the FliteTest forum, if you’re still unconvinced and care to wade through it…
The discussion, between me and 2 others, is scattered over the next 3 or 4 pages and the bottom line was:
It’s power, feed, fan, and… focus, Focus, FOCUS!
– David
Thanks David.
The 10th line is the squiggly one, because I was dragging the laser & shroud across the cardboard.
My Z holds without the steppers engaged, so I can turn it by hand. I’ll give that a shot.
You should be able to twist the lens to change the focal distance. You need a bit of room between the work and your shroud or no air will move. So lasers list the “ideal” focal length somewhere in the range of 2"-5". The laser should have no issues getting through cardboard. You seem to have way too short of a focal distance so your beam is diverging too fast and not making it through the backside. Really the longer the better, and easier to fine tune with the script.
Thanks, Ryan. I should have mentioned that I initially set up the focus for all my lasers to about 55mm or so… for smallest spot by adjusting the lens barrel. As received, they all focused much closer and there would not have been sufficient room for a shroud. I suspect that is indeed the case here…
Since the shroud suffered some minor damage during a collision, I printed a new one with the laser closer to the tip of the shroud…
I was also prepared to twist the lens, but figured I’d see where I was at first.
You were correct, David. The focus point was up inside the old shroud.
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Testing image upload…
nope!
I almost have that picture issue figured out. If you switch browsers it should work. There are only two of you that seem to be having the issue. Maybe a plug in or something.
We at least know the issue now. The first thing that really needs to be done with these lasers is to adjust the focal length… just as Ryan explained. I’m sorry I didn’t mention that up front.
The easiest, and least dangerous, way to set the focal length IMO is to raise the Z-axis so that the lens is 2-1/2" or so above the material. Turn on the laser at absolute MINIMUM power (M106 S2 does it for mine) so that you see the beam on the material and then screw the lens barrel out until you get the smallest dot you can… the laser won’t hurt/burn/cut you, at that power, as you mess with the lens barrel. Once that’s accomplished, that becomes your laser’s new focal length and you should now also have room for your existing shroud. In use, my shroud is only 5mm-10mm above the top of the material… it doesn’t need to be too much and insures good strong airflow.
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New shroud… [attachment file=68961]
My laser is 1" closer to the tip of this shroud, and best focus on the test pattern was around 14 to 16mm, so that means the focus point was around 10mm up inside the old shroud.
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So, I’m pretty sure I’ve got a good focus now. However, I’ve still got a problem cutting through cardboard before it burns.
At my new height (Z15), the almost-burning-down-the-house line is 50mm / min and 25mm/min increments with each line over.
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On the back, you can see 50mm / min cut through and 75mm / min scorched the bottom layer - but both of these are terribly burnt on top.
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This test was with the windows off the shroud, so I had a gentle breeze at best. I’ll try again later today with the windows in place so the fan blows through the tip on to the cardboard.
I think the next test needs to be power from the driver to the laser. Something is still off.
With the windows back on, so some air blowing… at 16mm above the top of the cardboard
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50mm / min - complete cut, some burning
75mm / min - almost cutting, some burning
125 mm /min - not cutting, no burning
mm/s?
