Lens diagnosis wanted

[daniel coyle]

The nightmare that is my summer laser machine experience is, thanks larglely to sawmill creek folks, slowly winding down. Nonetheless, there are still some things for losing sleep over. I am using an 18mm diameter 80mm focal length lens to cut 6mm cork. unfortunately i don't have an 80mm nozzle so i rigged up a tube air assist.
IMG_20130827_203823.jpg
this was a huge improvement over using the coaxial nozzle that was more of a 40 mm nozzle but still not perfect. apparently(?) this didn't prevent enough smoke from getting into the lens AND the cuts were inconsistent/not awesome. they did not usually cleanly cut all the way through and they often had rough edges. this is what the lens looks like now AFTER being cleaned with grain alcohol and cotton swabs.
IMG_20130827_191735.jpgIMG_20130827_191904.jpg
any thoughts?
dan

[Walt Langhans]

WOW That's crazy that you lens get that messed up at that distance with the air assist. How much smoke is the cork producing? A video of the laser cutting it would be good to see. I also have an air assist set up like you did but you still need to have air going through the nozzle as well. The beam passing through the nozzle creates heat which results in different air pressures between the nozzle and the air outside the nozzle and will create a vortex effect and bring smoke into the nozzle and thus messing up your lens.

But yeah that lens looks cooked to me... sorry.

[Dan Hintz]

any thoughts?

Yep. You need more airflow across your cutting table (the air nozzle only keeps down fires at the laser focal point, it doesn't keep off of your lens). I would also highly suggest plumbing a bit of your air across the lens itself to cut down on contaminants that reach it.

[Mike Null]

Besides the other answers how about a little housekeeping.

[Joe Hillmann]

I am not familiar with your machine but it looks like you still have the cone on for the old air assist. If that is the case are you still running air through it as well as through the new tube air assist? If not that could be your problem. Also how much vacuum do you have on your exhaust. You should have enough air flowing through your exhaust so that you either can not see any smoke inside the machine (that requires a lot of air movement on things that smoke a lot) or if you do see smoke it shouldn't have a chance to rise more than 1/4" to 1/2" off the surface of the materiel you are cutting before it gets sucked out. As Walt said, a video may be helpful if you can post it.

Edit: Also how much cork are you cutting? If you are cutting 8 hours a day 5 days a week all summer long then maybe your problem isn't that bad, Just plan on replacing the lens on occasion. If you are just dialing it in and haven't cut much then it is something you need to fix.

How often do you stop and clean your lens? When I am doing wood cutting all day I will check the lens after every part or two (using a mirror so I don't have to take anything out) and clean the last lens every hour or so. And clean the other lenses and mirrors twice a day. That is with air assist and a very strong exhaust. I find that it is best to clean the lens before it gets dirty. I usually when I clean them the look clean to the eye but when I wipe them with a q tip and lens cleaner the q tip ends up with a bit of yellow/ brown on it. If you let the smoke build up too much on the lens eventually it will start absorbing heat from the beam, taking away cutting power and baking the smoke to the lens. If you let it go long enough that way it will ruin your lens.

[Frank Corker]

Wow you've burned off all of the coating on your lens. Not seen one that looks quite so bad. Air assist is going to be a definite to look at. The resins in some woods just coats itself onto the lens' and for some that has allowed their lens to crack with heat. I have to say I don't know the whole purpose of the lens coatings, but in the condition yours are in, I would just polish the hell out of them and give them a go, most likely it will still be useable, but left as it is I think it would distort the beam

[Gary Hair]

By the looks of that first picture I'd say you need to work on cleaning your machine. If there is that much "gack" on the rest of the laser then you are asking for problems. My guess regarding the lens is that you need to clean it more often - how often? You should be cleaning it before you get any buildup at all on the lens, by that time it's too late and you end up with what we see in the next two pictures. I think I would run two dust collectors in series and see if you can reduce the amount of smoke/soot being deposited everywhere, that would help tremendously. I run two harbor freight dc's in my sandblasting booth and it's amazing how much more you get with two than one, not double but close enough.

[daniel coyle]

Thanks.

The mess on the laser head is because in my experimentation I duct taped the air assist tube you all see to the head. I had taken the tape off before taking the picture and what was left was the gunky adhesive. In my newbie-ness I had interpreted prior advice as "either/or" with the air assist so I set it up so that it was dedicated to the tube and there was none in the nozzle. Just one more thing to learn.
So, I took the 80 mm lens out (the one you all saw was already cleaned so, I am guessing it is toast, but it was still cutting.) My old machine didn't require me to clean the lens for several days so I don't really know how long it took to mess up that lens but without the nozzle air i am guessing it was pretty quick.
I do feel like the exhaust fan is weak and I feel like the air assist could be stronger.
I set the machine up with a 45mm lens and just did some cutting at 80% power (100 Watt machine, 15 speed) using 6mm cork. It did cut the cork but you will see the awful burning that happened on the surface.
IMG_20130828_141933.jpg

Seems clearly helpful to increase the exhaust wind, the air assist power and even get the tube back on their working simultaneously with the nozzle air. Any thoughts or other ideas?

[Scott Shepherd]

I agree with Joe. That's caused by leaving the nose cone on and the air not going through the nose cone. That's a VERY bad thing to do. Our rule is "Nose Cone on- Air On, Nose Cone Off- Air Off". If you don't follow that, you'll have a lens that looks like yours in no time flat. I mean minutes, not hours.

[Dave Sheldrake]

Hiya Dan mate

Your nozzle needs to be a LOT closer to the job to get benefit from co-axial air, Cole CNC in China do an extended nozzle that will fit, just be sure to tell Hui that you have an 18mm lens. About 40 bucks from memory. Looking at the last pic you seem to be getting beam scatter, probably from the beam hitting the side of the aircone (the exit hole is too small) that burn back is unusual in a machine that is focussed with no scatter.

I have to say I don't know the whole purpose of the lens coatings

AR coatings prevent backscatter coming back through the optical train (from hitting stuff like reflective metals) and damaging stuff in other lens's they can be used to form a hard wearing surface on the lens that is more resistant to scratching.

cheers

Dave

ps: Got your message Dan I'll get some documents from the collection that may help and send them over.

[Joe Hillmann]

I am kind of getting off track but hopefully this will help you.

I had a piece of 5mm cork laying around so I threw it in the laser. Using a 50 watt laser that needs a new tube(I don't know how many watts it is actually putting out) I was able to cut the cork with no charring and almost no residue on the front surface. The back surface had smoke damage in some areas about 1/16" from the cut. The residue on the back could be gotten rid of by cutting on a honey comb table or lifting it off the base. The sides of the cut had charred residue on them. I was able to wipe the sides on a dry towel to remove it and then the edges were a dark golden brown but didn't leave marks when rubbed against something.

I was cutting a 1oo% power and 8% speed with my homemade tube type air assist on. When cutting there should be no visible smoke or flame. The air assist should prevent flaming and the exhaust should be moving air fast enough that the smoke shouldn't be visible. I don't know what brand of cork I was using it was from 12" x12" tiles meant for cork boards. If you can't get good results with the cork you are using it may be due to whatever binders they are using, it may be worth it to try other brands. I doubt that an extra mm would cause such a drastic difference in cut quality.

[daniel coyle]

Thanks again guys.

Thanks for doing the parallel diagnostic Joe. Wondering what type (focal length) of lens and nozzle you were using. air assist in nozzle AND homemade tube?
I have never been able to (or outsourced to anyone else who could) cut 6mm cork with a 60 watt machine or less so it is encouraging to hear you were able to do what you did. I don't ever move that slowly either. Usually takes me 2.5 hours to cut a 3'x2' sheet as it is so I guess i am always trying to optimize speed. the type of cork might matter some. almost all agglomerated cork, i believe, uses a polyurethane binder. but there are exceptions AND, more common, they vary in the granule size that the agglomerates are made from. Was yours very fine or coarse cork granules?
Dave, I will check on the scatter thing more but I had already aligned the beam through the nozzle so I am not hopeful that I will figure that out. The machine is used and the nozzle has a small imperfection/damage on the opening so that its not perfectly round. Could the air be funneled through it irregularly so that it affects the beam?
I am pretty clear that the "air on in nozzle" rule is necessary. do people think it is useful to run a secondary assist as well in the form of my homemade tube?

[Joe Hillmann]

The lens has a 2 inch focal length.

Here is a picture of my air assist.
DSCF4410.jpg
It is a wooden box that fits over the head of the laser with needle for inflating basketballs and some clear tubing. It blows a very small fast moving stream of air at the beam of the laser. There is no other air assist on the laser.

I was focused about halfway into the material.

The largest "grains" in on the surface of the cork were about 1/4 inch but most are closer to .1 inches or smaller. I don't know if that is fine or course, I have nothing to compare it to.

[Joe Hillmann]

I also came across this. I never tried it on cork before but I have used it on wood and it is a lot of extra work but it does prevent smoke damage.

[daniel coyle]

I also came across this. I never tried it on cork before but I have used it on wood and it is a lot of extra work but it does prevent smoke damage.

I think you forgot the link to what you wanted to show??
Thanks for sending the pic, Joe. So that is all the air you run? No air in the nozzle (there was no image of the head in the pic.) Seems like everyone else is saying that one must have air in/at the head/lens.