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Here are the ones that fell out on their own.
New top for two 12v 0.18A squirrel cage fans:
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It’ll be coming off the printer in about 3 hours.
I’ll be connecting them to a 12V 30A power supply. Should I wire them in series or parallel?
With that sized PS parallel is fine. Are you cutting on the diamond plate? I used heavy aluminum mesh/screen on things I wanted to cut through so air had somewhere to go. There are fancy honycomb plates but I grabbed some stuff at home depot for a few dollars…I have most of a full sheet left.
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Yes. This last time, I cut on the diamond plate. I swapped it in at the same time as the bigger fan, so I can’t say how much of the change was due to the diamond plate and how much was the fan.
It seemed worse at slower speeds on the bottom, but better on the top. The slower speeds were closer to where I taped down the edge, so maybe that has something to do with it.
I had no idea the laser would be this sensitive the air flow. This is nuts!
2.5 hours left on the print.
I would put some spacers or something under it, 1.5" outta do it. Air needs somewhere to go. And it can bounce back making the charring worse on the bottom.
Now you see why I ditched my 40w laser…
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I’m tempted to rig up something to blow air in to the cardboard from the side, between the top and bottom layers. It won’t be nearly as versatile as a top-down air assist or fan approach, but for this specific case, maybe…
Good idea! I’ve got a few stacks of magnets all the same thickness
I’m getting there, slowly. Every change and tweak gets me a little closer.
Thanks, Barry, for your wise words and explanation of what’s going on here… 
Lazier than Jason, my first inclination was to go out on Thingiverse and see what might be available… and lookeee here! “Pawpawpaw85” (Jonas Hansson) has already created just what I need… https://www.thingiverse.com/thing:2832127.
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I already had a couple of squirrel cage fans on hand (don’t remember why I ordered them…), so I just need to wire it up and give it a go. But it’s looking good so far…
– David
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Running again with the v3 shroud and 50mm regular case fan, but with 2mm neodynium magnets as standoffs and more magnets on top instead of tape.
The faint thud of cardboard falling on to diamond plate is very satisfying.
(The plate isn’t magnetic though, so magnets on top, in the bottom, and between the cardboard and plate)
With magnet standoffs, it’s a lot better, but still some burning on the bottom at single pass speeds
Further off the surface and stick a fan down there. 1.5" or more.
Look at how the 40w lasers do it. They have a down/cross draft table.
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When you start engraving you even need to pay attention to the direction of air flow to get the best picture. The smoke wants to stick to the wood so you start at the fan and move away so you burn any smoke stained wood.
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Okay! Hooked up the fans in parallel… went back to the original, small, quiet, 40mm x 40mm fan for module cooling and added in the squirrel cage fan. Reran the same tests as before… this time got clean cut-through in single pass at 100 mm/min. The nice focusing barrel allowed simple re-adjustment of focal length… probably a little more spacing between material and shroud than necessary now but it can be easily be brought back down to get closer to the material and a bit stronger air flow.
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I use the diamond plate primarily to protect my table top from inadvertent engraving. On cut-throughs I usually use a chunk of heatsink that came off some ancient piece of electronic equipment… you do need some spacing underneath the material for air flow.
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Alright! There’s little doubt in my mind that there is great improvement in air-flow using the squirrel cage fan. I repeated the previous test and was able to eventually do the profile cut at 200 mm/min in a single pass. I also adjusted the focal length to where I have only 6mm or so between the shroud and top of material when in focus, using Ryan’s focus script. The cutout on top of the stack was the one cut at 200 mm/min. It’s been a good day of discovery and I’m a happy camper 
Thanks, Barry! – David
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Nice, this sounds like I need to update the laser page.
I tried this morning to fix all the air leaks – tape around the laser heatsink and squirrel cage fan nozzle… not a great change in performance though it does seem a bit louder now, so I think there may be a slight bit more air-flow. It does cut ~3mm cardboard nicely with little/no burning/charring… engrave at 800 mm/min (13.3 mm/s), through-cuts at 200 mm/min (3.33 mm/s).
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I’m calling it good for now and gonna get back on the FoamRipper and vacuum pad. Daughter and family coming in a couple of days and I need to clear out of the dining area
– David
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Those cuts are crazy clean. I feel like I need to see if I can get that to happen with mine…and I should get some fans for the shop.
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Edit: can’t get my images to upload
Sorry. I still can’t find the issue. as long as I manually clear all the caches at least every other day images are fine. Should work now.
I think I might have finally fixed it, changed some settings. I can see a list of image errors, but it doesn’t say why, although I can see the picture you uploaded I can not “release” it or anything.