Cracked lens, why?

[daniel coyle]

I have been getting a lot of great feedback on another thread I started in January about my experience buying a new weike laser. This question is a little more specific than a review of the experience with buying the machine.

The feedback I got included figuring out how to bypass using my autofocus (it caught some material and popped off) and learn to manually focus the lens. While searching threads for instuctions on how to do this I was lead to discover that my lens was cracked! I had only used it a total of one week so I was surprised. I realized that being a noob I had not really appreciated how much inspection/cleaning I needed to do. I had been cutting cork and poron foam (urethane). the lens was still working but not awesome

cracked 38mm lens.jpg

So, aside from my guess that the grime that had built up might have caused the crack, I was wondering what other things cause this? Is it more common when operating the laser at higher power? Is it more common if the laser is not perfectly aligned (mine was not, off by a little)? I also ran the machine without air blowing when I was engraving (but always when I cut, which is what I did 80% of the time). How much does that impact the chance the lens would crack? I saw in one post that someone recommended checking ones lens every hour or two while cutting (that seems really frequent. Is it more necessary with certain types of materials?

Finally, I was trying to cut .25 inch cork with this 38 mm lens (with my 100W machine I was running at 70-80% power). Its my understanding that longer lenses are better for cutting and so I have installed a 100mm lens that I plan to try. I did not realize that the machine has come set up with the smaller focal length lens just out of ignorance. The first lens was ok with cutting this stuff but not amazing. Before now I thought it would be a matter of me learning more about the machine and optimizing things (focus alignment, speed, power, air blowing etc). I also have a 50mm and 63.5mm lens.

A little gun shy about running this new lens again before getting some feedback here. Don't want to crack it for a reason that I haven't guessed at here. I have spent a few hours surfing other posts on this subject that have been really helpful but after a while there became diminishing returns in trying to sleuth out everything I needed to figure out.

[Lee DeRaud]

A short-focal-length lens may build up residue somewhat faster since it's closer to the source, but I doubt that's a problem in a week's worth of use. My machine is 8 years old, does a ton of MDF cutting, gets cleaned only when I feel like it or notice a fall-off in power, has no air-assist, and is still on its original 50mm lens. This sounds more like a defective part, hopefully a one-off.

[Gary Hair]

Dirt is almost always going to be the culprit when you crack a lens. The frequency of cleaning will depend on the material you are cutting but it is also affected by the length of the lens, air assist, exhaust system, and the design of the machine. I laser a lot of anodized aluminum so I don't have to clean my lens more than every couple of days and it's mainly dust that I'm concerned about. One recent job I had was lasering some wood that produced a lot of smoke - I cleaned the lens about every 30 minutes. It was a pain to clean it that often but a lot less painful than replacing the lens every hour or so...

I don't think anyone can give you cut and dried answers to "how often to clean my lens", the best answer is "it depends". If you can't see your lens without removing it then I would suggest buying a little mirror that you can use to inspect the lens, that should give you a better idea just how dirty it is getting.

Gary

[Joe Hillmann]

What type of air assist do you have? If it is the cone type there was a thread earlier this month that explained if you run the laser with the cone on and the air assist off your lens will get fouled with smoke in minutes. I don't own a laser with a cone type exhaust so I didn't pay much attention to it but you should be able to find it a couple pages back.

[Joe Hillmann]

Here is a link to the thread I told you about.

http://www.sawmillcreek.org/showthre...h-Cracked-Lens

[Mike Null]

Depending on what I am cutting I may check my lens every hour to once a day but i do check it once a day. Surprisingly I find that Cermark is a dirty substance and requires lens cleaning more frequently than one might think.

[daniel coyle]

Thanks. I do have the cone type head so I will probably keep the air assist on all the time now given what was said. On a few occasions, while engraving, I didn't turn the exhaust on due to the noise and my inexperienced sense that not much smoke was being created. That said, I should have taken a photo of the 'dirtiness' of the lens before I cleaned it. I used Everclear/190proof alcohol and it came off pretty easy dipping a Q-Tip in the alcohol and then wiping.

Joe, I checked out that link. Helpful information there, not sure how I missed that thread. but no one ever answered the question about whether one could just go ahead and take the nose cone off. would be great as checking the lens often with that cone on is 'not in my wheelhouse' if you know what i mean. guess I will have to start training!

I guess since no one has responded to my comment about the lens size I was right to assume what I did about the longer 100mm lens being better for cutting than the 38 I just cracked.

[Paul Phillips]

Daniel, you should always use the vacuum/exhaust no matter how little you're cutting so that you don't get any dirt/build-up on the lens and as for the nose-cone, you can remove it for engraving/rastering but must keep the air-assist off, if cutting or running with the nose-cone on, you should always have the air-assist on, if you run it with nose-cone off and air-assist on it is as bad as nose-cone on, air-assist off. (to qualify that statement let me say that, that has been my experience on my machine, yours may act differently but I would assume that the same principles apply).
As for lens length, there are many others that are more qualified to answer but from what I've learned, the longer focal length should only be used to cut thicker materials because it has a larger spot size which means it will take you longer to cut through a given material (someone correct me if I'm wrong here) ideally you want the smallest spot size you can get on whatever you're cutting because the smaller the spot the more energy is being focused to vaporize the material, for example if you're cutting .125" Acrylic, a shorter lens will cut faster than a longer one because the beam density will be greater and therefor cut it faster. The reason for a longer lens is when you need to cut thicker material. (hope this makes sense and again feel free to correct me if I'm off at all).
Paul

[Lee DeRaud]

It's been my suspicion for awhile that a sizeable percentage of cracked lenses are due to air-assist compressors with missing or inadequate water traps. As I've said, I don't use air-assist, but the water trap on my airbrush compressor, while good enough for that application, will still allow enough moisture through to condense on a cold surface...probably a fatal amount if deposited on a laser lens.

[Rich Harman]

I have found the 63.5mm (2.5") lens to be best for cutting.

I 3d printed a bracket to hold a length of 1/8" aluminum tubing bent so that it aims the air assist directly at the cutting point 1/2" off the surface. I have a Gast compressor that supplies up to about 25psi. The original air assist connection to the nozzle is now open since the connector is removed.

The combination of longer lens, powerful (external to the nozzle) air assist and open hole on the side of the nozzle has had the result that I rarely need to clean the lens. I need to clean it so infrequently that it is difficult to estimate how many hours of cutting elapses between cleanings, the second mirror ends up needing to be cleaned much more often, about once a week if used daily. Most of what I cut is either 1/4" plywood or acrylic.

[daniel coyle]

Paul, Rich

Thanks for info. Paul, I was intrigued by but a little lost in your articulation of focal length/cutting. Not sure what you mean by "spot size". Having higher beam density makes sense to me where cutting is involved but I had thought that the longer focal lengths would create a more concentrated/thinner beam at their focal point.

Rich, your description of your system and why it works makes total sense to me. couldn't one just use the existing compressor to do what you did after creating a bracket to hold an improvised 'nozzle'? you opted for a bigger one, the Gast?

I just walked in from trying to cut the .25 inch cork with the 100mm lens. I should say I was trying to figure out how to manually focus the lens. to the degree I was figuring that out it seemed that cutting with that lens on that material might be a no go. Since my 38 is cracked I will try the 50 or the 63 tomorrow. I have seen on several occasions that .25 inch cork is more difficult to cut than .25 inch wood (of any type). wish me luck.

[Rich Harman]

Longer focal length = larger spot size, lower energy density and greater depth of field.

The difference between the Gast 71R and the glorified aquarium pump that came with the laser cannot be overstated. They are not even close to being similar.

When cutting out smaller parts using a pin table there is a loud bang each time one of the pieces is blasted free and onto the table beneath.

[Albert Nix]

Plus 1 on what Lee said....I found the coating on my 2.5" lens has flaked off a little on one side and I am blaming it on water. It started on the outside next to the holder not in the center. Number 4 mirror also has a haze on it that will not clean off that looks like a stain. I am thinking water here also. I have an inline filter but it is not going to get all of it. These small compressors we use for air assist get hot and that causes a lot of condensation in the lines. That reminds me I need to change my filter lol.

[Paul Phillips]

Daniel, I was assuming you knew that the diameter of the laser beam increases with the length of focus i.e. a 1.5" lens has approximately .003" beam diameter and a 4" lens would have .007"-.008" beam diameter, the smaller the diameter the higher the beam density but also the shorter length. Think of it as using a magnifying glass to burn something when you were a kid, the laser lens focuses and extends the depth of that energy.
I'm sure this article from Epilog will explain it better than I could.
http://www.epiloglaser.com/tl_focus_lens_101.htm

[Craig Matheny]

Daniel I am not sure if this was brought up as I did not read every response but a few things come to mind....

1. with a cone type system you must run your air all the time.
2. with a cone system your air needs to be perfectly dry. If you do not have a dryer on the line and your unit is creating any moisture when it hits the lens it will crack every time. Personally I believe this is your problem not dirt.