CO2 laser questions

[nophead00]

Hi,
I am planning to build a 40W DC CO2 laser cutter from components and have a couple of questions: I understand the normal layout is to have the laser tube horizontal and use one fixed mirror and two moving mirrors and a lens.

I have an existing very accurate moving table machine, so the easiest arrangement for me would simply mount the tube vertically pointing straight down through a lens mounted on my z-axis to allow focusing. Does anybody know if the tube will like being mounted vertically. I.e. will the gas discharge be affected by gravity?

If it needs to be horizontal then I will need one mirror. I have seen that the mirror mounting brackets are usually metal. Do the mirrors get hot, or need heat sinking, or can I use plastic brackets? I ask because I have a home-made FDM machine so plastic is easiest for me.

[zax15uk]

Hi, well yours is a common question so you'll find some threads covering this type of conversion in the archives.

I actually started with a CNC router and added a laser but quickly found the solution was not ideal. Of course I wanted to keep the router function too. It actually works ok (not great) with a diode laser but not a CO2 tube.

The CO2 tube will work at any angle, no problem there but it's fragile so best mounted in a fixed position. If you have a water cooled or flowing gas laser then I would definately go with a fixed installation.

The other problem is that a router XY table is designed to have the torque necessary to carry a load (the Z-axis and spindle) and drive the cutter into a material (causing some side load). This usually means lead/ball screws and heavier motors, which in turn have lower acceleration and reduced top speed. A flying optics laser on the other hand is light (only a small mirror / lens assembly is required on the head) so benefits from a lighter setup using belt drive and high speed low inertia motors etc. If raster engraving this is even more important but for cutting you'll also find the router table restricts your maximum power output.

If the mirrors are clean, they should not get hot but any fumes or particles on the mirror will generate heat and potentially crack or damage the mirror. The same is true of the lens which is why most systems use air to shield the mirrors and lens from fumes. The need for such a system depends on what you intend to cut. Air assist is also a requirement for most cutting, where the air comes from a cone mounted to the bottom of the focus lens and helps remove the vapourized gases from the cut path (to prevent restriction of the laser which results in a deeper and cleaner cut).

Zax.

[nophead00]

Zax,
Thanks for the info. Yes I am planning to fix the tube to the frame and just focus the lens with the z-axis. It will be sealed gas, water cooled.

My table has ball screws and can move at 40mm/s. I am planning to cut acrylic from 2 to 8mm thick. I enquired about a 40W commercial cutter and got speeds of 16mm/s for 3mm acrylic and 10 mm/s for 6mm. I can accelerate to 16mm/s in 0.1mm, so I figured the laser would be the limiting factor as far as speed is concerned. Also I have 6um resolution which is less than the laser wavelength! So the spot size also be the limiting aspect of the accuracy as well. I may progress to flying optics to get a larger build area in the future though so I will get a set of three mirrors and a lens.

Yes I was planning to have a plastic cone under the lens and a small fan blowing air down it. Again I can make that very easily with FDM.

I looked at laser diodes, for example this beast. But the beam seems to be a wide line rather than a spot. Is there an easy way to focus it to a spot? Do the lenses designed for CO2 work with the higher frequency?

[zax15uk]

The specs on your XY table sound ok for cutting applications. I typically use >100mm/s for engraving and am still not power limited by the laser.

I would probably re-think the air assist as I'm not sure you'll get sufficient pressure but it may work ok.

The bar diodes are difficult to integrate and optics are expensive so I would avoid them.

It sounds like you have a good plan, and well thought out.

Zax.

[nophead00]

What sort of laser diode did you use?

[zax15uk]

On the CNC retrofit I used a 300mW DVD-burner diode. It worked ok for cutting thin veneers and marking, but was slow and had limited capability. I then purchased a separate 40W CO2 machine, and find the router and laser complement each other well.

Zax.

[bdring]

I am actually doing roughly the same thing. I have a big strong router, but decided to build a separate machine. I wanted to primarily do cutting, but some engraving. Engraving requires one axis to move very quickly (or it will take forever). Moving the tube at those speed will put a lot of stress on it.

I am almost done with mine, I only need to install the tube. I had two delivered in tiny sharp pieces, and the third is on the way.

See my build log here...

http://www.buildlog.net/cnc_laser/index.html

[zax15uk]

I've said it before, but I'll say it again... great build log with lots of useful resources and experience for others to benefit from.

Your machine is looking good, and I hope the tube is 3rd time lucky!

Those others were packaged to be broken, they do know they're glass - right?

Zax.

[bdring]

I finally got a good tube. If anyone is curious how these things are shipped, I documented the un-boxing (un-tubing..actually) on my build log.


http://www.buildlog.net/cnc_laser/unboxing.html