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CO2 laser tube lifespan - time used vs age of tube?

Discussion in 'LEDs and Optoelectronics' started by Ian, Mar 22, 2018.

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  1. Ian

    Ian Administrator

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    A few weeks ago I was looking for some advice on a CO2 laser cutter, but in the end I had to reel in my expectations after realising how much import duty and other fees would be. However, I have now found a really affordable K40 laser cutter based in the UK, which means it'll give me something small to start to learn from.

    There will be a lot of the time that the laser cutter is left idle, as it'll mainly just be me using it for random projects (or friends that want to borrow it). I read that CO2 tubes have a limited lifespan, but I can't tell if this is based on hours used or the age of the tube (i.e. degradation over time due to chemical interactions or gas exchange). I know that there's an element of them both at play, but I would hate to think that I may only use a tube for 40 hours one year and it's already expired.

    Any thoughts on if this may be the case, especially bearing in mind that I'll be getting a cheap-as-chips no-name Chinese CO2 laser that may or may not be 40W (rather than one of those nice tubes with a datasheet!). Getting a better tube would cost more than the entire K40 laser cutter kit ;).
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Tubes vary in quality and this can affect many operational areas, tube life being one.

    Your tube may be rated for 1000 hours of life. If you overdrive it, you'll get more power, but tube life will be greatly reduced. If you under drive it, the tube life will be extended significantly.

    Heat is the great killer. How is this tube cooled?

    Another thing to beware of is the HT arcing through the tube. If you ever see any arcing (often between the anode and ground) stop and investigate. We've had all manner of insulation failures. One reason is smoke deposits creating a conductive path. If allowed to go on too long, you'll get carbonised tracks and you'll have to replace the insulation.
     
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  3. Ian

    Ian Administrator

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    I suspect the tube included in this bundle will be as low-end a 40W tube as possible. It is water cooled using an aquarium pump cycling water via a bucket reservoir and I'll likely aim for no more than 75% max power, as I'd wager it's really not a 40W tube!

    Do you think that not using the tube would degrade it's life significantly? For example, if I didn't use the laser cutter for 6 months, should it still operate at approximately the same performance as previously (assuming it was stored appropriately).
     
  4. Hopup

    Hopup

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    Don't think it matters at all, its sealed glass tube having gas molecules inside so it should last quite long time.
     
  5. Ian

    Ian Administrator

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    Thanks for the feedback folks :). I've pulled the trigger on buying a K40 cheapo laser cutter :D.
     
  6. hevans1944

    hevans1944 Hop - AC8NS

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    Please let us know how that works out for you, Ian.

    Hop
     
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  7. Ian

    Ian Administrator

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    Will do :D

    I'll take plenty of photos whilst documenting the modifications and tweaks, as I'm already deciding how I'm going to pull bits off and improve it (at least that's the aim!) ;).
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    I also recommend you get yourself a set of safety glasses for IR radiation (they are essentially clear acrylic). Even diffuse reflection from a 40W tube can destroy your eyesight.
     
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  9. hevans1944

    hevans1944 Hop - AC8NS

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    Strongly recommend you take precautions to safely enclose a 40 watt 10.6 μm infrared laser beam. MUST prevent specular reflections from metallic surfaces, or at least absorb such reflections. Back in the day, when I was actively "playing" with small desktop Sylvania five-watt CO2 lasers, we co-axially aligned a HeNe, visible red 632.8 nm, laser beam with the 10.6 μm CO2 laser beam, using a kinematically mounted mirror placed in front of the CO2 laser output aperture to introduce the visible beam. This was only to assist in alignment of the invisible CO2 beam, and the mirror was removed when the infrared beam was in play. Of course, this was only effective with reflective optics in the infrared beam path, although if on-axis lenses and ZnSe window material transparent in the visible were introduced into the beam path, we could at least get some idea of where the infrared energy was going. And we had "paddles" coated with a substance that fluoresced under ulttra-violet light. The fluoresence was sensitive to heat and vanished when the IR beam heated a spot on the paddle. The dark spot thus represented the beam, surrounded by the cooler fluorescing coating.

    Pretty crude tools we had back in the 1960s, but we managed to not set ourselves on fire... well, most of us did. A certain engineer I knew visited a much larger laser at one of the Wright-Patterson AFB labs, leaned over the beam line while it was in operation, and neatly sliced his tie off. He proudly displayed the tie to us, as if this were some sort of achievement.

    High-energy lasers were still new and in their infancy back then. There were high hopes that laser weapon systems could be developed that could shoot down enemy aircraft and missiles. I spent several years working on that program for the AIr Force, but decided rather early on that it had a huge problem: unlike kinetic energy weapons, laser beams have virtually no momentum. This prevents laser beams from depositing large amounts of destructive energy on target, especially if countermeasures such as reflective surfaces are employed.

    CO2 lasers are effective for cutting materials because the beam is highly focused and close to the target material. This is much more difficult if the target is a few kilometers away and the beam is propagating through the atmosphere. So even with adaptive mirror optics (which work really well) weaponized lasers remain a laboratory curiosity, not a battlefield deployable option.

    Fast forward to the 21st Century. Diode and fiber lasers now do most of the things that optical cavity lasers did in the 20th Century, and less expensively too. I am looking to get my hands on an inexpensive diode laser that can perform mild steel-foil cutting when mounted to a 3D printer-like, CNC controlled, x-y-z platform. I will keep this forum posted if I find something that I can afford that actually works.

    Hop
     
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  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    We've been looking at purchasing some beam combiners for the two laser cutters we have at my local hackerspace. Obviously we'd use a semiconductor laser these days :)

    One problem is that we have switched to a lens that is opaque to visible light, the other is that the beam combiners are not particularly cheap.

    Actually, they're getting cheaper...
     
  11. Ian

    Ian Administrator

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    Lots of useful info here - thank you!

    Am I correct in thinking that this wavelength is going to be absorbed by pretty much anything I'd wear? I've got lots of acrylic safety glasses in the workshop, which I assume would be good enough for a diffuse beam or specular reflections at 10.6 μm? The laser tube and cutting area are all enclosed and the first thing I would be adding is a lid cut-out switch for the laser (you'd think it would be fitted already, for the tiny cost!). I don't want to skip on doing things safely, as I've only got one pair of eyes!

    Here's the model I went for:

    https://www.aliexpress.com/item/UK-...-Engraver-Cutter-woodworking/32810421426.html

    I'll get a project log started once some of the ancillary parts have arrived. It looks like I need to use distilled water for the water cooling loop, which is surprisingly expensive and hard to obtain in the UK. De-ionised water is readily available and cheap, but that isn't recommended. Rather than keep ordering water online, I'm going to buy a cheap water distiller, which should pay for itself after 4 loop refills. I was going to just use filtered tap water until I read about the effect it has on tube life. It looks like this will arrive much later than the laser cutter, so my patience will be tested ;).

    I had considered building a unit using a laser diode (they seem to be readily available up to ~6W, with some over-driven up to 15W for short bursts). However, it looks like they'd need considerable work to cut something like 6W plywood like I'm after. I'm sure with some clever development the motion platform could perform multiple passes and use the Z-axis for moving the focus point after each pass - but by the time I'd built it all it would cost a lot more than this CO2 unit.

    I'll be really interested to see how it works if you do build one! I saw some reasonably priced (questionable quality) kits on Aliexpress that have a simple XY gantry built using aluminium extrusion. It's all powered via Arduino, so I think it may be easy enough to re-flash to accept GCODE:

    https://www.aliexpress.com/item/50-...-Desktop-Wood-Cutter-Printer/32847761533.html

    I'm sure building one from scratch would be far superior, but perhaps one of these kits may be fun for someone to tinker with.

    You've been involved with some fascinating projects @hevans1944 - I really enjoy reading about the things you've worked on :).
     
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  12. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    I would get proper safety glasses. You only get one set of eyes.
     
  13. Ian

    Ian Administrator

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    I had a further read and found this article, which does some pretty interesting testing (firing a CO2 laser at pig eyes behind different types of glasses):

    http://www.williamosman.com/2017/05/safety-glasses-vs-laser-glasses.html

    CO2 laser Safety glasses are being ordered as I type :). Edit: Hmmm, a lot of the glasses I've found seem to be a bit questionable in terms of promised specs - so I'm going to do some further reading and see if I can buy them from a more reputable supplier.
     
    Last edited: Mar 23, 2018
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  14. hevans1944

    hevans1944 Hop - AC8NS

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    IMO, you don't really need (and should not want) a beam combiner. As you noted, many (actually, most) infrared transparent materials are optically opaque in the visible wavelengths, so a combined infrared and visible beam would not pass through these materials. As long as the path from the infrared laser is unobstructed, or is intercepted only by specularly reflective components, a visible collimated beam from, say, a diode laser or an inexpensive HeNe laser, will be sufficient to co-axially illuminate the infrared beam path for initial alignment purposes... but it is not necessary to do this simultaneously with the infrared beam.

    Sure, a polished yellow-orange, visibly transparent, zinc selenide window can be introduced as a beam combiner, the infrared beam passing through it while the visible beam is reflected off the polished output surface, but we found little advantage in doing that back in the day (when polished ZnSe was expensive). Instead, we built a removable kinematic mount for an ordinary first-surface mirror that had alt-az or pan/tilt mirror adjustments and plopped this assembly down in front of the infrared laser exit window. A small HeNe laser was aimed at the center of the mirror, which was oriented at 45 degrees to the infrared beam path. We set up paper "burn targets" in the path of the infrared beam and made burn marks with the mirror removed. Then we placed the mirror on the kinematic mount and adjusted it so the visible beam from the HeNe laser was centered on the burn marks. For greater alignment accuracy, you can use two widely separated burn targets to make sure the visible beam goes through burn holes on both targets.

    Just in case everyone is not familiar with kinematic mounts, here is a link to a brief introduction. They basically consist of three ball bearings cemented to the bottom surface of a support platform to form a triangle; The balls, held by gravity, rest upon two pairs of parallel cylindrical rods, mounted at angles to each other on a base. A nearby flat surface on the base supports the third ball.

    This arrangement successively restricts the translational and rotational degrees of freedom of the kinematic mount. As the first ball bearing is rested (plopped down) on a pair of parallel rod supports (v-grooves are also used) two points of contact are made, limiting translation to motion along a direction parallel to the rods but offering no restriction to rotation. As the mount is further lowered to bring the second ball bearing into two points of contact with the second pair of parallel rods, translation in the plane of the rods is eliminated, but the mount is free to rotate about the first two ball bearings until the third ball bearing contacts the flat surface, thereby eliminating the third degree of rotational and translational freedom.

    There are other kinematic arrangements possible, but the one I have described is easy to implement and not particularly sensitive to errors in layout or precision. Some users prefer machining v-grooves, instead of installing parallel rods on the base plate, but either way works fine. The advantage of using a kinematic mirror mount on the optics table is the mirror can be removed and replaced with almost no error in the alignment of the beam reflected from the mirror. As a practical matter, most mirror mounts with adjustable pan/tilt screws also use a form of kinematic mount for the mirror to prevent interaction between the two adjustments.
     
  15. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    The visible laser is really useful when positioning the laser head. In this case it's turned on all the time, showing where the IR laser will be cutting.
     
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  16. hevans1944

    hevans1944 Hop - AC8NS

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    No argument from me on the convenience of this.The key is the inexpensive, infrared transparent, optical "beam splitter" or "combining" element. If you have one of these, there is no reason NOT to use it to create a co-axial alignment beam. A kinematic mount is unnecessary when the combiner remains in place. I now see these for sale on eBay for around thirty-five bux, one tenth, or less, of the price they sold for forty years ago.

    Be careful handling zinc selenide: it is very soft (compared to glass) and easily scratched. It is also quite brittle and easy to chip or shatter if dropped onto a hard surface. I still have a few small rounds stashed away somewhere that I was thinking of using as output windows for a DIY CO2 laser. The idea was to mount them at Brewster's angle on the plasma tube and use external cavity mirrors to resonate the laser, but I never did find my "round tuit" to complete the project. However, they would make suitable beam-splitter/combiners for use with a commercial C02 laser, which is now much less expensive to buy than to build... even if you know what you are doing.

    About CO2 laser gas: the usual mixture consists of helium, nitrogen, and CO2. There is nothing that I know of that will contain helium for extended periods of time. It's a largeish molecule, but it slips through everything eventually because it doesn't chemically react with anything. So eventually the laser gas becomes depleted of helium. Not a problem if you replenish periodically anyway, but something to keep in mind with sealed CO2 resonators. Same problem occurs with sealed HeNe lasers, but there are red-emitting diode lasers to replace those puppies.
     
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  17. Ian

    Ian Administrator

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  18. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    I didn't realise that.

    We have a large tube (almost 2m long) that got a hole in it. One of our crew are thinking about sealing off the water jacket (the hole is actually in the water jacket) and filling it with neon to make a huge neon indicator tube!
     
  19. hevans1944

    hevans1944 Hop - AC8NS

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    That would probably be a waste of a good laser tube, depending on other factors of course. Does the tube have external resonating cavity mirrors, with windows mounted to the end of the tube at Brewster's angle? If the laser tube is sealed with cavity mirrors permanently mounted... never mind. Is there plumbing to the laser tube that will allow its vacuum evacuation and re-fill with a laser-gas mixture? If not... never mind. If so, does your group have the gas handling and vacuum pump equipment necessary to service the laser? A 2m long CO2 laser should be good for about 400 watts or so with water cooling, barely adequate for thin metal cutting but fun to have around.

    I don't understand how sealing off the water jacket and back-filling it with neon will make a neon indicator. Where are the electrodes located? Is there a current-limited neon sign transformer available to "light it up?" Or was this a facetious remark by a member of your crew?
     
  20. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    It's a relatively cheap ($2k) laser tube that is fully sealed with no user serviceable parts inside :-(

    I believe there gas leaked out via the water jacket. We'd have to open that up, fix the hole, and somehow close up the glass envelope again.

    I'll have to refresh my mind about where all the stuff is inside the tube. I'll try to remember to take a few pictures and post them.
     
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