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Angus Hines
01-23-2008, 3:21 PM
Was sitting at the laser today and noticed smoke coming out of the top of the lens. Stopped the laser and found this. Mirrors clean lens was clean not sure what happened??? Anybody got any ideas ???

http://i111.photobucket.com/albums/n127/Ubrut_3/lenz002.jpg

http://i111.photobucket.com/albums/n127/Ubrut_3/lenz010.jpg

Joe Pelonio
01-23-2008, 5:00 PM
That happened to a guy that had an older laser that I know a few years ago, when cutting wood. He thought debris hit it and the beam ignited the debris
burning off the gold coating. I suppose it could just be a worn out coating though. I once noticed some bare spots on mine so I replaced it.

Angus Hines
01-23-2008, 5:10 PM
I had just cleaned it and was cutting acrylic so I don't think it was debris, and I didn't even think to look for bare spots. and more than just burn the coating it actually lasered a hole through the entire lens which you can't see in the pic but when I look with a magnifying glass its there a big ol'hole.

Frank Corker
01-23-2008, 5:24 PM
That looks like major pitting. Like Joe I have had a few small but tiny pitt marks on my main lens (not in need of replacement yet though). You may have had something under there which has reflected the laser beam back up and engraved it.

Scott Challoner
01-23-2008, 5:25 PM
Angus

In the picture, it looks like the hole is off center of the lens. Since you had just cleaned it, is it possible that you cross threaded the lens when you put it back on? How much product did you run after you cleaned it?

Sandra Force
01-23-2008, 5:33 PM
From the looks of the lens you are lucky that you didn't loose a mirror also. Usually when I have a lens go that bad I have also lost a mirror. I probably loose more lens than most because of the enviorment here ( Industrial setting and lots of airborne particulates.) From my experience though if you were doing acrylic you might well have hit an air pocket in the acrylic that burst and blew a piece of hot acrylic onto the bottom of the lens and presto - bye bye lens. Double check all of your mirrors and if you have a window between the laser tube and engraving compartment check it for haze and discoloration from heat before running again. Hope you keep spares on hand.

Angus Hines
01-23-2008, 5:47 PM
I had only run a half sheet of 32x20 when it happened the tube it screws to is aluminum and would be hard to xthread without noticeing. But I have on previous occasions noticed when cleaning it the dirty spot always looked a little off center.

Could there be an alignment issue that is small enough to cause this, but, not really show up in the final work???


Angus

In the picture, it looks like the hole is off center of the lens. Since you had just cleaned it, is it possible that you cross threaded the lens when you put it back on? How much product did you run after you cleaned it?

Sandra Force
01-23-2008, 6:23 PM
I would realign your mirrors and lens. But that is probably not what caused the lens burn. It could be as simple as your exhaust missed a beat and allowed smoke on the lens, or any one of a number of things.:cool:

Joe Pelonio
01-23-2008, 6:41 PM
The gold coating prevents the laser from etching the glass on the lens. If that is breached at the point where the beam hits it seems like it would etch the glass, constantly over several seconds or minutes, heating it to the point where it would break and burn through as yours has. When cleaning, I always examine the mirrors and lenses to see how the coating looks. I often wonder whether the constant cleaning could be causing wear on that coating.

When I replaced them I kept the old ones to use as temporary spares. Their coating problems were not in the center.

Gary Hair
01-24-2008, 2:59 PM
I have on previous occasions noticed when cleaning it the dirty spot always looked a little off center.

I don't think being off center would cause this problem, but you do need to correct it. The lens focuses best at the center, that's how it is designed. It's possible you are losing power by not sending the beam through the center of the lens. It's not too difficult to do, but it is a bit time consuming, depending on where the alignment is off. I had to align mine from the laser on up and it was an "interesting" experience...

Gary

Rob Bosworth
01-28-2008, 12:03 PM
Angus, a lens usually does what your's did when smoke and/ or debris gets on the optic. It is a exponential effect. First you get a little build up on the optic. Then instead being 98% transmissive, the dirt on the optic starts to absorb energy. As the energy is being absorbed, it converts to heat. The warmer it gets, the less efficient it becomes. The wrmer it gets, the more stuff builds up on it. Pretty soon you have a very hot optic, then bang, you get a hole in the optic. It can happen pretty fast. Miht start out with a bit of smoke, or a spot of molten material, and just keeps getting worse. Scratches on the optic's coating can also absorb energy.

It can also happen if your alignment happens to be clipping the metal mount. I do not think your hole was caused by the beam hitting the mount, but you should check your alignment before you get your new lens in place. It does not have to be off by much. All it has to do is warm up the assy. The warmer it gets, the less efficient it becomes. Thermal dynamics will cause the alignment to go further out of wack.

You might have had an optic up beam from the lens that is going bad or is misaligned. As the corner mirror heats up, thermal expansion will make the beam alignment change. The more it changes, the less efficient everything becomes, and blown optics are a result.

Start with making sure your machine's movement system is square. Then start checking alignment all the way through the optic path. If your machine movement has gotten out of square, a slipped tooth on a belt, a colisiojn with something being run, and the beam will no longer be going down the center of your optics.

Frank Corker
01-28-2008, 2:55 PM
I think you've hit it on the nail there in your first paragraph Rob.

Kevin Huffman
01-28-2008, 3:13 PM
Hey guys/gals,
We think we got this one all worked out.
1. We replaced the lens. (under warranty, just in case it was a manufacturer defect)
2. We did a beam alignment (just in case, it never hurts to be in alignment)
3. We are going to up the inspection of the optics. We suggest once every 8 hour work day, he is going to be taking a look at them every 4 hours. If they are clean, no harm no foul. If they are dirty it will save both of us some money.