Flames below the work piece when cutting and the relation to wood, air assist and smoke extraction

Hello all,

I was hoping to start a little discussion regarding flames below the work piece, what causes them and what is a good way of getting rid of them.

What I understood is things that can cause flames is using no air assist, which I am not doing of course, a build up of flammable gasses because we are burning wood and/or excessive use of glue or other chemicals in the wood - usually with lower quality (ply)wood.

The background is that I recently rebuild my DIY machine with a lot of upgrades, including a 90W tube instead of 40W. I did a lot of testing and the cutting is done with just enough power to cut through. So far I am not using the 90W at all. I am using low power to cut 4 and 6 mm plywood.

The smoke extraction is new and very beefy. Could it be that I am sucking too much fresh air into the enclosure? Is that even possible?

I am also using our trusty air assist fish tank air pump. Could it be that the air assist pump is not strong enough? I hear people upgraded to a shop compressor and getting great results.

Are the flames caused by the (lower quality?) wood? Could be. Still using the same plywood from the same supplier. Nothing fancy, but it is plywood, so you never know what they mix into it.

Have you guys experienced this and if so, what did you do?

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Ditch the fish tank pump. It’s fine for acrylic but not really for wood.
I have mine hooked to shop air (with a 8 gallon tank) and run acrylic at ~3psi but wood at ~40psi
Not only will that keep the flames down but you will cut significantly faster through wood.
I have a line running from the main compressor to the laser where I have another regulator so I can switch pressure. Just note you don’t want to cut acrylic at pressures that high because it’ll have the opposite effect. Cools it too fast and it won’t cut well at all.
I highly recommend one of the California Air Tools aluminum tank units.


Yep, except I didn’t fork over the extra $60 for an aluminum tank like Adam did.


Thank you for the prompt reply!

I was already afraid you would say that I will need to use an air compressor…
Will have a look tomorrow at my current shop air compressor. It is a cheap and mostly loud thing, but maybe it is worth a try.

This is the sort of fish tank thing I was talking about:

Come to think of it, I think it might have to do with the fact that I am not using a honeycomb bed at the moment. Maybe it has always happened and I have never seen it.

Are the flames solely because of the air assist not being powerful enough?

Not solely but air pressure is a BIG part of the equation. I get why most lasers come with the little compressors - they are cheap and then people think they have what they need. And if only cutting acrylic, that’s true. But not for wood.
I would also check what type of wood you are cutting - if it’s ply, they type of glue used makes a big difference.

Regarding using shop air:

  • You MUST have an inline moisture/oil trap.
  • A bigger tank is of better but even that won’t do much if the compressor itself cannot keep up. You may have to limit job runtime when running at 30-40PSI for wood to give the compressor time to cool and top off the tank.
  • A cheap 6-10 gallon auxiliary tank connected inline can help with above. Possibly getting you through a longer job before the compressor kicks in.

this would also work :slight_smile:

Thanks again Adam!

I have just looked at my current shop compressor and it’s a 6.3 gallon tank (24 liters).
It is a little less than yours, but for now this should be fine. It is also a cheap and far less fancy one than yours but for now I can live with it. Luckily my compressor has two outputs, so I can have a dedicated line for the laser.

I have ordered some parts of off Amazon to build the conversion, including a moisture and oil filter. Thanks for the tip. I should get the parts around Tuesday/Wednesday.

Can’t wait to do some testing with it and see the difference. Thanks a lot for your detailed information!

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I wanted to post a follow up if that is ok.

All the parts have arrived and have been installed. I made it such that there is a valve for shutting down the whole air assist (machine off -> no air in the system), then a moisture and oil filter and then followed by a ultimate air assist setup.

I must say, you were not kidding. The difference is night and day. The cut is cleaner, and most important: no more flames :grinning:

The only issue I am having now is my cheap air compressor. I already have it enclosed below the laser, but man that thing is loud! The tank is also too small, which makes it kick on a few times on larger products - which was expected. I really do not want to bother my neighbors with my air compressor.

I had a look at the low noise ones but they cost quite some money. I think I will have to see if I can make my current compressor a little less noisy. I have learned that the air intake filter is causing a lot of the noise, it is of very low quality.

Maybe you have some tips regarding this as well?

Thanks again, I am really happy I invested in this upgrade!

So what is your solution to control the air pressure that comes out the nozzle?

Like said I have a main valve (on/off) which takes care of closing the air line when the laser is off / not in a job (NC valve). This to make sure there is no air pressure present in the system and to make sure the compressor tank does not drain overnight.

Next in line there is a moisture and oil seperator. That one has a regulator valve with which I can regulate the main pressure coming from the compressor (this pressure is used for cutting).

Lastly there is a ultimate air assist setup to switch between high pressure (cutting) and low pressure (engraving). To be able to control the engraving pressure as well, I have another regulator valve on the ultimate air assist.