Laser mirror holder and nose getting hot

[Michelle Walker]

Good afternoon,
We have problems!
We have a 150 Watt Chinese red and black laser. The tube is not brand new, but appears to be working. We have replaced the mirrors with copper mirrors and are still having this problem. After about ten minutes of cutting (we cut primarily 1/4 inch mdf) the cuts start to not go all the way through. This is also when the carriage and nose piece are get hotter than they should.

Things we have already tried (multiple times)
* We have aligned all three mirrors (replacing the 2nd and third with copper, the first by the tube is a Mo mirror) we have them dead center.
* We replaced the painters tape with some thermal label paper on top of the foil tape and noticed it gets hot (leaves a mark) all around the opening and spreads to the carriage.
*Our industrial water chiller is working just fine, keeping it cool at 18 Celcius.
*we checked and cleaned the lens (we do regularly) and it is clean and appears unbroken.

Problem suspected- is this a split beam, how would we check and how would we fix it....?
Could it be something else?

Thanks for any and all advice!

[Ian Stewart-Koster]

If the nozzle is getting hot, the beam is hitting it.
Either it's not landing in the centre of the nozzle hole, or the hole is too small for the beam's focused point- maybe your lens is not good?
Have you tried a ramp test?

Sometimes the head gets bumped and stays crooked - a simple bump back can remedy this.
I've only ever used the mirrors that came with each of our machines- and they're shiny silvery looking metal. I've had spares but never changed them in 5+ years - BUT.... MDF is horrid stuff and I avoid it as much as I am able - the gunk that comes out in the smoke is ghastly.

As for why yours cuts then starts to fail - that usually suggests a tube that may be dying - what colour is the tube when running: pink or white?

Your split beam idea may be right.
But you can sort-of check for that by looking at the shape of the dot on Mirror #1, when you put card over it and do a test pulse. Is it a single circular/oval shape or a Dee shape or two dees ?
?

[Michelle Walker]

If the nozzle is getting hot, the beam is hitting it.
Either it's not landing in the centre of the nozzle hole, or the hole is too small for the beam's focused point- maybe your lens is not good?
Have you tried a ramp test?

Sometimes the head gets bumped and stays crooked - a simple bump back can remedy this.
I've only ever used the mirrors that came with each of our machines- and they're shiny silvery looking metal. I've had spares but never changed them in 5+ years - BUT.... MDF is horrid stuff and I avoid it as much as I am able - the gunk that comes out in the smoke is ghastly.

As for why yours cuts then starts to fail - that usually suggests a tube that may be dying - what colour is the tube when running: pink or white?

Your split beam idea may be right.
But you can sort-of check for that by looking at the shape of the dot on Mirror #1, when you put card over it and do a test pulse. Is it a single circular/oval shape or a Dee shape or two dees ?
?

Thanks for the response. The tube started changing colors after I posted yesterday So we've ordered another tube, then hope to proceed with more tests if the head is still getting hot. Will post once the tube comes.

[Michelle Walker]

We replaced the laser tube with a brand new Reci 130-160 watt tube. We can cut but are having the same problem with the head/nozzle overheating. One thought is that we have had had this problem since converting over to a new, larger tube. We had changed the power supply, tube holders, mirrors (cu), air system, and industrial chiller to accommodate for this change. We currently have a 20mm lens. Do we need to convert the laser head over to a larger lens? Would 25mm work? Is there anything else we should consider for this conversion to work? Also any ideas on how to mount that thing? I'm not finding much information online so anything anyone knows would be greatly appreciated.

[Bill George]

Did it work ok before you changed things? Never heard of copper mirrors, maybe others have. Just top of the line mirrors, all aligned correctly should be all you need. Like other have said here, your nozzle cone is getting hot because the beam is hitting the side(s).

[John Lifer]

The head may be getting hot from the copper mirror inside the head. It could be getting hot over time and if really copper, it could easily transfer heat to the head. Is it hotter near the mirror or at the lower nozzle?
if near mirror, that would be my guess. Or it could be splitting the beam.... I looked at cloudray and they show MO mirrors for over 100watt. The only copper mirror for a co2 laser I saw was WATER COOLED

[Bill George]

John I have never heard of a copper mirror, here is where I get my stuff from > https://lightobject.com/lens-mirrors/mirrors/

Ok I found gold mirrors on a copper plate laser mirror on Amazon.

[Glen Monaghan]

You mention that the nose piece is getting hot. As others have said, that could be caused by the beam hitting the cone. One way that happens is that the mirrors (especially the final mirror) are mispositioned and/or misaligned, causing the beam to be off-vertical enough to partially hit the inside of the nozzle. Another way that it happens is that the exit hole is too small and/or the nozzle is too short for the focal length of the lens (even when properly aligned, the beam isn't getting small enough to completely pass through the exit hole without hitting the edges).

But that doesn't seem like it would explain the carriage getting hot unless the beam is also hitting there. As I understand it, typical laser beams exiting the laser tube are pretty wide, not the tiny spots that most people think about when they think of laser beams. A laser engraver has to have an optical path (including mirrors and focal lens) with large enough diameter to pass the entire "pretty wide" (that's a technical term ;^) beam without any of the beam spilling over and hitting anything along the optical path other than the faces of the mirrors and working area of the lens. I'm wondering if you "new, larger tube" is producing a much larger beam than the previous one, larger than the diameter of your current mirrors or (more likely) the lens. If so, the outer edges of the beam would be "spilling over" and heating up the surrounding structure. Have you checked for heating at the mirror mounts?

Even if the lens and mirrors are the same size, the lens is probably the main constraint because the mirrors are typically mounted from the back which makes the entire mirrored front surface available, while the lenses are typically mounted in a holder that blocks the outer edge of the lens which makes the lens' effective diameter smaller. If the new beam is wider than the aperture of the lens holder, the edges of the beam would be hitting the holder and heating more than just the nozzle. Or perhaps the beam is wide enough to hit the edges of the hole where the beam enters the carriage before the final mirror.

Just my WAG... Wonder if you could use a thermal infrared camera to get a better sense of exactly where you are getting localized heating?

[Bill George]

Glen on the lasers I have had the dot size is about 3mm or so and needs to hit dead center on the final mirror before its directed down to the lens where its focused to a . size . My guess as others its perhaps hitting the side of the cone before the lens. If the cone end can be removed while leaving the lens in place would help narrow it down.

So the problem must be solved.

[Kev Williams]

My Gravograph LS900, after the tube rebuild, part of the incident beam is hitting the edge of the last mirror's frame even tho the center of the beam during a tape test doesn't seem that far off center; it's up and right of center a bit but doesn't seem all that far off. Never HAS been dead center. How I know the beam is hitting the mirror's frame is because I have this bad habit of laying my arms across the opening with the lid up while watching it work, and when running cermark or cutting at 100% power, I can feel heat on my right arm from from beam reflection! I made a tweak and it did help but I can still feel the heat at times. Part of MY problem may be one of the internal mirrors within the laser tube-I didn't even know tubes had mirrors until Jean at Evergreen said my laser's mirrors were in horrible condition when they rebuilt my tube. But they didn't get changed out because at the time their mirror supplier was sending them 'junk' and she advised against it. The recharged tube fires great, but it's a 40w Synrad, and after the recharge all it can muster is 31 watts. It's a GOOD 31 watts and isn't really all that noticeably different than before, but even with the beam hitting all the mirrors "fairly" centered it's hitting something around the final mirror.

Do glass tubes have internal mirrors? thinking.gif

As to the cone, just get rid of it. The beam can't hit the side of it if it ain't there!
I fixed my air line like this--
airline.jpg
--it's a piece of .093"OD/.031"ID flexible air hose (used on my Gravograph machines) shoved inside the machine's original air line a few inches, then I shoved a piece of copper tubing over THAT, and bent it some so the shape cooperates. I have a homemade metal ring that goes around the lens tube (just some trophy aluminum) that I use normally to aim the hose but sometimes I just use rubber bands, easier to mess with. And with the rubber bands I can move the hose easier so it's not blowing directly at the cut to blow the smoke away from the lens when engraving wood, that way the lens stays clean and air pressure doesn't fuse the tree sap goo to the wood so bad around the engraving, much easier to clean up after.

Could those 'copper' mirrors simply be typical gold-colored mirrors and not to do with actual copper? And do gold mirrors have anything to do with actual gold? Just wondering...

Some thermal fax or adding machine paper may work to help find where the errant beam is hitting? - Very little power would darken it fast, and very little power is hard to do with a 150 glass tube!

[Glen Monaghan]

Bill, it occurs to me that I was originally thinking about a laser system with a beam expander, which is used to produce a smaller focal diameter than normally achieved (focal diameter is proportional to f/D where f is lens focal length and D is beam diameter before the lens, so a shorter focal length and a wider beam both make for smaller focal diameter). Systems with a beam expander might have a beam diameter of 1/2 - 3/4 inch, so the possibility of a centered beam hitting the edges of a mirror or lens is greater than with a typical, non-expanded beam. IIRC, a typical CO2 laser tube produces a beam that is about pencil thick, ~1/4-1/3 inch diameter, or ~6-8 mm. 3mm diameter sounds a bit small to me but I'm certainly no expert on that, but I would think that would tend to be found on lower power laser tubes.

[Bill George]

Since the OEM mirrors are 20 mm as mine were, my 3 mm dot size fit nicely. The OP never came back so I am assuming its fixed anyway I am done with this.

[Michelle Walker]

After much trouble shooting the end diagnosis of our laser was simply that we didn't have large enough mirrors for the tube we installed. Our solution was trying 25mm mirrors. If you've ever tried to install 25mm mirrors in a red Chinese laser you would learn that they don't fit...however we machined them to work. We had to adjust so the larger laser head didn't bump into everything, but at last we have a happy working laser. We also traded out the copper mirrors for gold plated copper mirrors. The laser head is lightly warm to touch, however, not at all hot like before. Likely if we switched back to MO mirrors we would have a cooler head, however, cutting the MDF we do, we need mirrors that can hold up to some abuse.
Lesson learned, just because the Chinese laser can fit a larger and more powerful tube, upgrading is a whole lot more complicated than initially expected.
Many thanks, for the suggestions.