Air assist range and benefits

[Keith Downing]

I'm reading/hearing different information on the air assist use and psi when it is on.

I have been running air assist on, with a 60w boss laser. Seems to blow a little bit of debris when rastering, but masking solves that issue when it arises.

What is a major issue, is cutting 1/4" birch plywood in complex shapes/curves I'm having all kinds of problems with flair ups. Literally can't get through even slightly complex words even soaking the wood with water without it just burning through letters.

I've read on here in numerous threads that air assist actually helps "blow out" flair ups. I'm seeing the opposite effect though. At least that is how it appears to the naked eye. It looks like when the laser passes back over spots that have been cut nearby (like a close cursive L) it starts to burn and the air just fans the flame before I can stop the damage.

I tried cutting once tonight with air assist off and it worked perfectly, BUT I just read all about how that can have negative affects inside the cone and even crack the lens.

So what's the answer? What do I try next? Been using the Chinese air pump (best guess is about 30 psi coming out), but have the air compressor plumbed in now so I can start playing with different air settings via a direct regulator.

Could I just need a little less air? Or WAY more?

[Allan Longson]

If you remove the cone you mitigate the risk of damage by heat to the lens but the lens will get dirty much quicker.
I cut 3/4/5mm thick veneered mdf, I used to have a lot of problems with flare ups even using the honeycomb table with air assist. Masking both sides helped but what really solved the problem was building a frame that is 40mm high and laying it on a non burning base eg plaster of paris or sand or glass (I use a mdf sheet that has been scorched so many times it is now basically just a layer of charcoal on top of it). No flare ups, no burning or scorch marks on the back or front of the work, the only time I mask now is if the work I am cutting will not fit on the frame (ie cut lines pass over the frame supports).
It helps that I cut a lot of the same shapes repeatedly so have built a number of frames for my work, if you are doing custom shapes all the time then you might consider using pins to hold the work up, as long as there is nothing under the cut lines (or at least enough distance from the material getting cut to the base) for the beam to hit and encourage the flare up then you should see a big improvement.

[Keith Winter]

That's a tough one. Air assist with a cone is your best bet cutting on a honeycomb table. Is it possible you are going a) too slow causing too much dwell time or b) too fast not cutting all the way through on a single pass? Finally try a different plywood if all else fails, not all plywood is equal. The cheap stuff can be quite aweful to cut. You can probably find different grades local, but if not, and if you really want some clean stuff rowmark makes laser ready plywood now too. It's expensive but nice.

[Keith Downing]

If you remove the cone you mitigate the risk of damage by heat to the lens but the lens will get dirty much quicker.
I cut 3/4/5mm thick veneered mdf, I used to have a lot of problems with flare ups even using the honeycomb table with air assist. Masking both sides helped but what really solved the problem was building a frame that is 40mm high and laying it on a non burning base eg plaster of paris or sand or glass (I use a mdf sheet that has been scorched so many times it is now basically just a layer of charcoal on top of it). No flare ups, no burning or scorch marks on the back or front of the work, the only time I mask now is if the work I am cutting will not fit on the frame (ie cut lines pass over the frame supports).
It helps that I cut a lot of the same shapes repeatedly so have built a number of frames for my work, if you are doing custom shapes all the time then you might consider using pins to hold the work up, as long as there is nothing under the cut lines (or at least enough distance from the material getting cut to the base) for the beam to hit and encourage the flare up then you should see a big improvement.

This makes a lot of sense and will probably be the next thing I try. Even though the honeycomb tables has decent sized holes; I still see a lot of energy dispersed when the beam hits the metal. That's got to add to the problem.

[Keith Downing]

That's a tough one. Air assist with a cone is your best bet cutting on a honeycomb table. Is it possible you are going a) too slow causing too much dwell time or b) too fast not cutting all the way through on a single pass? Finally try a different plywood if all else fails, not all plywood is equal. The cheap stuff can be quite aweful to cut. You can probably find different grades local, but if not, and if you really want some clean stuff rowmark makes laser ready plywood now too. It's expensive but nice.

Unfortunately with the 60 watt, there's a pretty small window of what seems to cut all the way through. It's speed between 10 mm/s and 20 mm/s and 95-100% power. So I'm kind of trapped there. I will try some other types of wood (just ordered some thin baltic burch sheets online), but I'm using cabinet grade stuff now from the big box stores (about $30 a sheet). So it's not like I went ultra cheap. LOL

[Kev Williams]

First, I don't do a lot of cutting-- but what cutting I HAVE done over the years, I've found only one benefit to air assist, and that's simply to blow out flames.

The very first thing I did after getting my 80w Triumph up and running was to remove the cone. I haven't missed it one bit. I HAVE put it back on a couple of times just for fun, and each time I found a noticeable shortfall in power-- meaning, part of my beam is hitting the cone. It's especially noticeable with a 3" or longer lens. According to a piece of tape, my beam is hitting the mirror, lens and cone pretty much in the center, but not all of it according to my tests... Anyway, no cone for me. FWIW, none of my 3 metal lasers ever used a cone...

So how do I use the air in my Triumph with no cone? I inserted a piece of piece of smaller air hose inside the Triumph's hose, to add some length mostly. I bent up a paperclip that holds the air line out of the way when I'm not using it (most of the time),
335191

And when I DO need it---
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Yup, paperclips and rubber bands. Hey, I don't need "pretty", all I need is "functional". And it's been like this since a week after I got it. I'm just using the supplied pump, all the air it'll push. As you can see it's not aimed anywhere in particular, just blows a ton of air straight down, and blows out any flareups. That's all I want it to do, and it works great.

[Keith Winter]

Unfortunately with the 60 watt, there's a pretty small window of what seems to cut all the way through. It's speed between 10 mm/s and 20 mm/s and 95-100% power. So I'm kind of trapped there. I will try some other types of wood (just ordered some thin baltic burch sheets online), but I'm using cabinet grade stuff now from the big box stores (about $30 a sheet). So it's not like I went ultra cheap. LOL

Keith I use speed 16mm/s on my 130w when I cut 1/4" plywood. Either you're not doing a true 1/4" plywood (big box stores tend to skimp on thickness, it's probably .19"-.20") or you're going too fast. With a 60w I'd expect you to be in the 7-10mm/s range.

[Rich Harman]

...BUT I just read all about how that can have negative affects inside the cone and even crack the lens.

I disconnected the air from the cone and routed it outside the nozzle. The only air that flows over the lens now is from whatever comes in through the little hole on the side where the air used to be. My lens stays cleaner now than it did before. That was years ago, no negative effects for me.

[Keith Downing]

I disconnected the air from the cone and routed it outside the nozzle. The only air that flows over the lens now is from whatever comes in through the little hole on the side where the air used to be. My lens stays cleaner now than it did before. That was years ago, no negative effects for me.

Thanks for the input guys. Always so much to consider.

This is especially interesting Rich. I'm assuming it is debris buildup/burn on the lens (not actual heat from the beam) that causes cracking. Do you think the external air still keeps enough debris out of your cone? Or do you simply think that a few fluke cracked lenses caused the entire industry to rethink things.

[Keith Downing]

Also, I will add that while I was told the pump that came with the laser was putting out 25-30 psi, after hooking up the regulator to the big air compressor and running it at a true 30 psi, I can tell you the small pump was probably only putting out about 12-15 psi.

So maybe I just need to up it to a true 30 psi to hit the "sweat spot" where it's blowing out flames not igniting the embers?

I'll do some more testing tonight.

[Glen Monaghan]

Thanks for the input guys. Always so much to consider.

I'm assuming it is debris buildup/burn on the lens (not actual heat from the beam) that causes cracking.

The laser beam has no "heat" in and of itself. You get heat when the laser beam hits (and is at least partially absorbed by) matter and causes the atoms/molecules of that matter to vibrate more rapidly. Ideally (and with a good, clean lens, for most practical purposes), the beam doesn't interact with the lens in that way so the lens doesn't heat up significantly due to the beam's passage. However, get a little dirt, smoke, grease, and/or other debris on the lens surface and that crud WILL absorb a portion of the beam. The crud on the lens will heat up because of that, the resulting heat will be conducted to the lens, and the lens may crack or pit as a result.

[Rich Harman]

This is especially interesting Rich. I'm assuming it is debris buildup/burn on the lens (not actual heat from the beam) that causes cracking.

That is how I understand it.


Do you think the external air still keeps enough debris out of your cone? Or do you simply think that a few fluke cracked lenses caused the entire industry to rethink things.

I think mainly gravity keeps debris out of the cone. Once in a great while something will get on top of the lens by entering the top near the mirror. I think the best solution would be a low flow of air through the cone, with the cone reaching within a mm of the material. Perhaps in addition to an external air assist. That would keep it cooler and the positive pressure would keep debris out. However, it is far more valuable to me to have the cone clear the work piece so that it can pass over the magnets that I use to hold things down.

[Bert Kemp]

Keith I cut 1/4" baltic from woodworkers source on my 60 watt at 7mmps and 75% pwr 1 pass, slightly charred edges but not to bad.

[Keith Winter]

Keith I cut 1/4" baltic from woodworkers source on my 60 watt at 7mmps and 75% pwr 1 pass, slightly charred edges but not to bad.

Yeah that's right in the range I was expecting he's cutting at double that speed with 60w.

[Keith Downing]

I'm sure the "quarter inch" plywood is actually slightly less. But I can easily cut through it at 10 mm/s 100% power and usually barely get through at 20 mm/s 100% power. The tube is new though, and well aligned so maybe that's also helping me out?

[Allan Longson]

I don't know how one gets away with having the cone on and no air assist going into it. I had the compressor fail on me last year half way thru a job whilst I was out of the room, came back 5 minutes later to find the lens had destroyed itself. Whilst it hadn't cracked, gunk had built up on the lens and after removing this, the surface of the lens was permanently marked, an expensive failure of an expensive lens.
I'm pretty anal now about running air assist, we used to operate without it and the cone but we found one has to keep cleaning the optics every few days and repeated cleaning eventually destroys the optical surfaces, regardless of how well you do it. I run the compressor at 50psi (which is about the limit I can run it at without moisture issues in the line - and this includes 2 inline filters and a 100 feet of copper tube in a chiller) which converts to around 20psi into the machine. I have tried higher pressures when the air was dry (up to 35 psi into the machine) but found I started to get small marks in the mirror and back of the lens so I immediately backed it off again.
My next project is to modify the bottom of the machine by cutting 2 holes in it, plumbing in a Y tube and attaching a decent sized dust extractor to it, remove the table and replace it with a fine stainless steel mesh in a frame. I want to try and draw as much smoke and junk as possible from the job downwards and therefore keeping smoke etc out of the top of the machine where the lenses, mirrors, beam, belts etc are. Whilst it won't exactly be a vacuum table, it should be close.