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Walt Langhans
02-08-2013, 3:48 PM
Hi all

I've been cutting / engraving MDF now for about 4 days straight 7-8 hours per day. Every night I clean my lens then this happened today. Any ideas?
253778

And BTW where can I get a replacement with out having to order from China?

Thanks

Lance Mills
02-08-2013, 4:42 PM
Hi Walt, I had exactly the same thing happen to mine a year back, cutting MDF too. You need to clean the lens more often than once a day to keep residues from building up on the lens. mine cracked through the centre and although it still cut it was a bad cut. I eventually contacted the Chinese manufacturer and they were very helpful and sent me my 2 replacement lenses I ordered very promptly,which is more than the local agent could manage. Good luck.

Rich Harman
02-08-2013, 5:02 PM
A strong air assist that is external to the nozzle (like what Epilog uses) can prevent this.

Richard Rumancik
02-08-2013, 8:59 PM
Which side of the lens burned? If it is the input (source) side then chances are you got some water or other contamination (oil?) on the lens which caused it to heat up and burn the coating. If it is the other side then probably due to an accumulation of smoke or tar from the wood.

Walt Langhans
02-08-2013, 10:44 PM
A strong air assist that is external to the nozzle (like what Epilog uses) can prevent this.

That was the first modification I made to my machine when it arrived. I was pretty sure this was my fault. I wasn't paying close enough attention and the junk from the MDF clogged up the nozzle and there was a big flare up (which got my attention for sure). I immediately shut down the machine (yeah for those big red buttons) cleaned the nozzle and restarted the machine, but I guess the damage was done. Rookie mistake.

@ Richard it was the output side, but that's good to know about which side for what causes. Thanks for the info.

@ Lance, yeah this MDF put's out a lot of junk, middle of the day and end of day lens cleaning might just be the way to go.

Vicki Rivrud
02-08-2013, 11:26 PM
Hi Walt,
Give Marco a shout over at LightObject . . . he carries lots of supplies for Chinese Lasers. He's in Calif and is quick to ship.

Vicki

Mike Null
02-09-2013, 8:11 AM
Walt

When you're cutting dirty stuff like mdf an inspection of the lens every hour or two will save you a lot of grief. Keep in mind that when the lens/mirrors are dirty the efficiency of the machine drops way off.

Rodne Gold
02-09-2013, 8:47 AM
You tried acetone to clean it?

David Rust
02-09-2013, 9:55 AM
HI guys, If I assume the laser beam is in the center of the burn't residue then why is the laser beam not centered on the lens? I always thought the most efficient part of a lens is the center...

Richard Rumancik
02-09-2013, 11:23 AM
HI guys, If I assume the laser beam is in the center of the burn't residue then why is the laser beam not centered on the lens? I always thought the most efficient part of a lens is the center...

You are suggesting (I think) that since the burn mark is non-centered on the lens that the laser beam must not have been centered, and are wondering why . . .

The laser "beam" is not as tiny in diameter as you are imagining. If you look at this crude schematic of a plano-convex lens you will see that the beam diameter on the "input" side is actually quite large. In fact you need it to be large. The whole point of the lens is to take the large zone of parallel rays and focus them to converge on a small spot (your workpiece). Only a small fraction of the rays actually pass through the lens center area. In fact, the center could have some damage and the lens work fine.

Contamination could occur anywhere on the incident side or exit side of the lens and that little area would heat up and damage the lens. Debris could build up anywhere due to the pecularities of the airflow.

Edit: Also note that if the beam alignment of the laser is off, there will not be perfect symmetry of the rays. But the lens will still tend to work even if it is not an optimum condition.

Kay Bengtson
02-09-2013, 12:06 PM
This is a more to scale version.
253855In my ULS 25ER, the laser beam power is mostly in a beam of 1/8th inch in diameter. This can be seen by the burn mark on tape during alignment tests with no lens in place. You can see that the narrowing of the beam results in an effective cutting area of 1/4" deep with a 2" focal length lens.

Kay

Richard Rumancik
02-09-2013, 12:39 PM
I understand what you are saying Kay. The incident energy doesn't always "fill the lens". I just grabbed a pic from Google images to help explain what I meant.

However I don't know if the burn mark will tell you the actual diameter of the incident laser beam. I suspect that there is a significant amount of energy in the beam outside the .125 diameter zone.

People sometimes use beam expanders to increase the size of the incident beam as this will result in a smaller spot size. All those rays hitting the outside of the lens do add up.

Walt Langhans
02-09-2013, 1:16 PM
You tried acetone to clean it?

I picked up some camera lens cleaner and towel and have been using it. I looked on the bottle and it doesn't say what it actually is. Aldo when I run my nail across the lens it feels like there is a coating or something that is missing in the center of it, so I'm guessing it's pretty much fried. But I do still have it so if you think the acetone will work/help I'll go get some.

Rodne Gold
02-09-2013, 2:38 PM
Camera lens cleaner wont really clean any burnt resin etc off the lens , for that you need a solvent. Maybe the SO has some nail polisher remover which is acetone and normally some lanolin (so that the skin doesn't dry). Most pharmacies/hardware shops will have pure acetone.
Shenui sell their ZnSe lenses for round $30 or so , buy a few spare , you might as well get some other focal lengths while you at it , their stock F length they supply is 38.1mm , a little short , which makes depth of focus pretty small ..try 50mm or 70mm , they wont take long ex china , they small and can fit into a small packing envelope.

David Rust
02-09-2013, 3:04 PM
Rich, Kay,

Do you agree that the laser is passing through the center of the discoloration on the original picture posted? It looks like residue collected on the lens but not where the beam passed through. It looks like the beam is quite a bit off center on the lens.



Do
I understand what you are saying Kay. The incident energy doesn't always "fill the lens". I just grabbed a pic from Google images to help explain what I meant.

However I don't know if the burn mark will tell you the actual diameter of the incident laser beam. I suspect that there is a significant amount of energy in the beam outside the .125 diameter zone.

People sometimes use beam expanders to increase the size of the incident beam as this will result in a smaller spot size. All those rays hitting the outside of the lens do add up.

Kevin Berkshire
02-09-2013, 3:11 PM
Did you try LaserBits for a new lens?
Kevin

Richard Rumancik
02-09-2013, 6:20 PM
. . .Do you agree that the laser is passing through the center of the discoloration on the original picture posted? It looks like residue collected on the lens but not where the beam passed through. It looks like the beam is quite a bit off center on the lens.

I can't speak for Kay but what I was trying to say is no, not necessarily. What I meant by the picture I supplied is that the laser "beam" is comprised of millions of light rays distributed over the face of the lens. The exact diameter enclosing the bundle of coherent rays is hard to say - could be 4mm or 6mm or 8mm. (Kay was thinking on the smaller side like 3mm.) The lens will bend the rays and they will shoot out the flat side of the lens, but they will still be distributed all over the active lens surface. Any contamination on either side of the lens will cause a hot spot and the ZnSe coating will disintegrate. It doesn't matter where on the lens the contamination lands within the beam diamter; it will create a local hot spot as it heats up instead of passing the light rays.

They often put the anti-reflective coating on both sides although I don't know offhand why it is needed on the "exit" side.