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.
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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.
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any thoughts?
dan

[Walt Langhans]

WOW :eek: 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.
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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.
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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.

[walter hofmann]

hi your air assist is to close to the surface this is what mess up your lens, the distance from the air assist to the surface should be around 5mm and the exhaust should be around 500CFM. I use a 6 inch inline fan from hydroponics ( around $ 80 on ebay) reduce the 6 inch ports to 4 inch with duct reducer. greetings waltfl

[Dave Sheldrake]

There are a number of reasons for running co-axial air Dan and not all of them relate to cutting. The air also cools the lens (hot lens's have abberation problems) It prevents backsplash (detritus being ejected from the cut onto the underside of the lens) CO2 build up in the cut (CO2 gas absorbs the wavelength we use like a sponge) Kerf extension (the same way a metal cutting laser works without the oxygen)

Have a look at page 10 of this doc from BOC

http://www.boconline.co.uk/internet.lg.lg.gbr/en/images/laser-cutting410_39553.pdf

even a small damage point on the final exit hole can cause problem, even if the beam is central it can still be getting scattered by the cone (longer focal lengths are wider for more of the distance)

cheers

Dave

[Joe Hillmann]

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.


Yes I forgot the link. The link described putting tissue paper over the cork then misting it with water. Although that only works for short runs, on long runs the paper dries out and becomes a fire hazard.

The only air on mine comes out that small tube. My machine doesn't have any other type of nozzle on it. The reason everyone is saying you need to also be running air through your nozzle is because if you don't it becomes an area of low pressure and actually sucks the smoke into it when the exhaust fan is on. So that means you need to have air blowing through it or remove it. If you remove it then it isn't a problem.

[daniel coyle]

Walter
I am running the nozzle right around 5mm off the field and have switched to a 45mm lens that is compatible with the nozzle size. I ran this with a shop vac blowing over the field while still using the nozzle air of course. It stopped the flaming but didn't stop the charring. It just seemed to blow the char to one side.
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I am certainly learning a lot but this is definitely a drag. There is definitely very little in the way of idiot proofing with this thing. I will probably go out and buy a more powerful air compressor and possibly hook up another tube assist in addition to the nozzle assist in order to try and get past the latest obstacle, the charring.

Dave, do you still think this is most likely to be from beam scatter?

[Joe Hillmann]

I would put the tube air assist back on and aim it so it is blowing exactly where the laser is cutting. Also maybe turn down your power or up your speed until it just barley cuts all the way through. If you are cutting on a solid aluminum table you could reduce the flash back by cutting on a honey comb table or put a piece of mdf on top of your table to be used as a sacrificial table. That way it will absorb the extra laser power rather than bouncing it back up into the cut again.

[Dave Sheldrake]

Dave, do you still think this is most likely to be from beam scatter?

I'm pretty much sure it is Dan, the heat distribution is way too wide for a normal lens to be causing that. It could be an out of focus situation but scatter would be where I would look.


cutting on a honey comb table or put a piece of mdf on top of your table to be used as a sacrificial table

That can cause a whole new series of problems Joe if gas (specially flammable gas) gets liberated between the two sheets.

cheers

Dave

[daniel coyle]

Alright. I did some tuning and found a place where there was some air leaking. I imagined the problem being fixed! Not so fast. I AM cutting on a honeycomb table right now, don't have a solid table just this and a cross bar type. Anyway, the surface still flames/chars up but cuts ok. I checked for beam scatter a second time in the only way I know how. Tape under the nozzle and then look for the position of the laser burn. it never reaches very close to the nozzle end and is pretty consistently placed near the middle at all four corners. besides that i was thinking it would be hard to cut as well as i am if there was beam scatter (thoughts?). Really perplexed about this and the charring effect associated (?) with it. Haven't added extra air through a tube yet but adding extra air via shop vac has had limited effect.

The end of my nozzle is slightly damaged/disfigured (as i had mentioned before.) i wonder if this could have a big effect.

thanks again guys

[Dave Sheldrake]

besides that i was thinking it would be hard to cut as well as i am if there was beam scatter

Not always Dan, you still have a dot that is in focus but you also have a lot of scattered radiation around that spot. A damaged nozzle will do it for sure.

cheers

Dave

[daniel coyle]

thanks dave for the expertise on the subject. can't tell you how much that saves me from myself! gonna do what i can to get the nozzle replaced. then. There was not so much of this effect when I was using an 80mm focal length lens but of course i was using the same nozzle (the only one i have) so it was a good 40 mm or more above the field and I was using my homemade system to do air assist and not running any air through the nozzle. I am guessing it is the turbulence in the air flow caused by the damaged nozzle that causes the scatter? In any event, thanks again and again for mentoring me through this!

[Dave Sheldrake]

I am guessing it is the turbulence in the air flow caused by the damaged nozzle that causes the scatter?

Surprisingly the air won't affect it, scatter is usually where a portion of the beam hits a semi reflective surface on it's way through the nozzle causing all the nice tightly aligned photons to fly round in all directions scorching everything they hit.

Just out of interest, what method are you using to set the focus height?

cheers

Dave

[Joe Hillmann]

Can you run the laser with the nozzle removed?

[daniel coyle]

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Thanks again Dave, Joe

The pics i attached are of the damaged nozzle opening. Like I said before, I set up everything so that there was apparently a nice centered beam going through the opening but.....? So, Joe, i have not tried running it without the nozzle though I am open to trying that. I'm currently down to my last good lenses so I probably won't do any more "new to me" things til I get some more backups. Is there anything I should worry about it I did this? I would do it with the 80mm lens so the head would be pretty far removed from the field.
Dave, as far as focusing goes I do a "newbie" trial and error style where i just keep moving the table up and down until i can get the thinnest line from moving the laser. its not quick and efficient so if you have tips i am more than open to them.

I am trying to source and order replacement nozzles for the head. one for the 45mm and one for the 80mm lens.

right now I went back to the 80 mm lens on the 45mm nozzle (the messed up one in the pic, only one i have) and plumbed the air lines so that there is an air assist tube on the field of cutting and one going into the nozzle. Its sitting waiting for a go right now. wish me luck.

[Dan Hintz]

It is unwise to use a longer FL lens on a shorter FL cone... the cone's opening is typically not wide enough and part of the hourglass-shaped beam is blocked by the cone. This heats up the cone (as well as the lens), which causes gunk to build up... it also means your beam is not as strong as it could be at the focal point.

[Dave Sheldrake]

Focus, a definitive way (from the Master that is Tweakie)

"If you ever have occasion to change a lens then one way of measuring the correct focal distance or checking the existing gauge is to set up a small piece of Ply as shown in the diagram below then laser a line across the piece.
The correct focal distance can be measured between the point at which the line is thinnest and a suitable point on the lens assembly. If a new acrylic gauge is now made to this dimension it can then be used to set the correct focal distance for any thickness of work material."

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The absolute focal point of a lens is from the back of the curved surface to the surface of the material HOWEVER it is never an exact figure in real world terms, the ramp test is a much more accurate way.

Dan H is spot on, where the beam interrupts the cone the photons will scatter causing burning around the cut line.It can also cause heating of the lens holder leading to abberations and broken lens's :(

That cone is pretty much shot Dan, I'd change it out for a new one, everything in the optical train should be as clean as possible, even small imperfections in the way of the beam as it is focussed will cause scatter and problems.

cheers

Dave

[daniel coyle]

thanks guys. til i get replacements i will make do with just the 45mm. with the two air assists it is now cutting a little better but probably still some scattering. thanks Dave for the focus info. i recall seeing that before in the past when i REALLY didn't know left from right and felt too overwhelmed to set it up.

as feared there are, as of yet, no US suppliers of 18mm nozzles. rabbit laser can do a retro fit to 20mm or machine a new nozzle. so. i'll first work on getting replacements from china.

thanks again

dan

[Rodne Gold]

I have found that coaxial air on my chinese machines is rather useless , apart from the air itself if not sufficiently moisture/oil trapped contaminating the lens (and you just need one trap to fail to mess up a lens) , the dross generated from the lasering operation tends to spread out in a radial pattern around the engraving or cut. I only use an obliquely directed nozzle for all my machines. Chinese or not

I found with my Shenui machines that the nozzle supplied (looks like one most chinese machines use) actually created some vortex within and the lens got gunked up quicker with using air thru it than without. If you don't use air thru the nozzle , you can enlarge the nozzle exit hole a little quite safely if you feel the beam is hitting it.. I doubt the slightly larger hole is doing tocause a problem

At any rate , we consider lenses cheap consumables , I buy pretty good ZnSE ones from Shenhui and it takes 3-4 days to get here , in fact I just got 7x 18mm dia 38.5mm ones and 3 x 15mm dia 50mm one (for my spirits) last week for $360 including DHL courier fees. $36 a lens...