I normally have a 2" lens in my 50W CO2 and can cut nicely through 6mm Oak. I needed to go a little thicker for a project so invested in a 4" lens hoping the additional focal length might help. However, there was no increase at all in the depth of cut. I did the normal focus check and increased and decreased that to position the focal point at the top and centre of the piece, but couldn’t even go any faster than cutting with the 2", let alone go deeper. The cut width just seemed wider and more chard, and possibly even not as deep as the 2". Tried the lens in the other way up just in case, but only resulted in an even wider kerf. Is a 50W laser just having to go so slowly that the chard wood is simply blocking the beam? Full air on and tried longer and shorter nozzles to get it closer to the cut. Too far away and I just get flames!
Am I expecting something not possible? what is the point of a longer long if there is no perceivable benefit?
Cutting through something requires enough energy to vaporize the material in the path of the beam. The energy deposited is (pretty much) the ratio of the tube power and the cut speed, with the tube power being constant: faster cutting → less energy in any one place.
Different focal length lenses change the beam shape, but nothing else, so, to a first approximation, the energy remains the same. Vaporizing twice as much material (say, 6 mm instead of 3 mm) requires twice as much energy, which, with the same tube power, requires reducing the speed by (about) a factor of two.
What a longer focal length produces is a somewhat longer “in focus” zone that can make the side walls of the cut look more like a straight line, but the beam must still deposit enough energy to vaporize all the material in its path.
You can’t focus at top and center at the same time. You need to determine the center of your focal point and set your focus to the center of your material. You theory is correct a 4" lens has a depth of focus of almost 11mm. This is the main reason for a longer lens, so you have a parallel cut vertically. The tradeoff with longer lenses is the spot size. A 4" has about a 410 microns or 0.410mm, so a larger kerf … a 2" lens has approximately 205 micron spot.
You can’t compute the difference in cutting by thickness alone – the lens/spot size needs to be taken into consideration. A 50W machine with a larger spot has less of an effect because that same 50W is across an area of twice the size, still 50W overall but beam density has a serious effect. We see this a lot with fiber lasers.
I cut 6mm and 10mm acrylic on my 45W, but solid wood is substantially different animal.
Speeds/feeds are also critical to get the best cut. It will cut at many different speeds/feeds, but only a very limited range of speeds/feeds will cut the best. Just like any CNC machine.
…what is a little thicker?, 10mm?
The jump from a 2" to a 4" is quite drastic, I think. I have tried 3" lenses, but the effect (compared to 2") is limited by the tube itself and therefore not so violent, at least on my machine. I always end up with a 2" lens which is probably the most universal lens for my low-power machine.
In the picture there are shapes cut out of 15mm pine wood, I could not see a wow effect with a 3" lens compared to a 2" lens. Not even in clear acrylic, which is the best visual test for effect and lens types.
The 2" is what’s in my machine over 90% of the time. I have used the compound lens for detail then changed lenses to the 4" for cutting out the design.
… it just surprises me a little that there is no bigger difference between 2 “and 3” lenses. Just have laved tests again, in 20mm pine tree, 4 “the lens is more” straight “like my 2” lens but beyond it does not cut better or more/deep. 20 mm wood is also above the limit of the machine, I will not be able to run production with it, 5mm/s at 57.5% power makes no sense to me.
I was aiming for just a couple of mm thicker. 6mm to 8mm. 10mm would have been a bonus.
I wasn’t saying I focused at the top and centre at the same time, I was saying that I tried both places one at a time to any difference.
I understand about the power/energy increase required. I was cutting 6mm at 350mm/min and reduced that to 200mm/min. I am already at 100% pwr. I was hoping it was just the beam being out of focus as it cut deeper, hence going for the 4" lens.
I now assume that trying to cut this close to my energy limits is my issue. There is no energy left to compensate the larger spot size. Plus, wood is a funny material as it burns and forms a hard crust in the kerf.
It confuses me that you use mm/min and not mm/sec. Which is more “normal” for CO2 laser machines, but you are already very far down in speed and it often deteriorates the quality, ie. The edges don’t get neat.
You may have reached the limit of your machine.
Yes, mm/min. It’s a home built that was designed for its size rather than speed. The cutting area is 1000 x 500, but it has relatively small motors and drivers. Increases in speed/accel would require bigger section beams, bigger motors, bigger belts and more powerful drivers etc. Not something I want to do at the moment. It earns a good living as part of my business, but sometimes I reach its limits!
The cutting speed comes from the power of the tube and the material: the machine must move at whatever speed deposits the correct amount of energy into the material to vaporize it.
My 60 W CO₂ machine can cut ¾ inch oak at 2 mm/s = 120 mm/min and 70% power:
That’s two pieces stacked together. Cutting that much wood (with a 2 inch lens!) isn’t something it should do on a regular basis.
Your machine may be traveling too slowly to properly use its available power, which is why it chars the sides rather than clearing the cut.
To be more exact, the area ratio between 3mm and 6mm is about 4:1 rather than double. This means a 25w laser with a 3mm dot would require 100w with a 6mm dot to have the same cutting power.
I learned the hard way my 20w diode with a larger dot size does not cut 2x as fast as my 10w diode laser.
I have tried it at various speeds and the result was pretty much the same. the cut just got shallower. May be your 65W makes all the difference to my 5 year old 50W
Come to think of it, check the beam for proper shape. Perhaps the tube is not operating in TEM00 mode and cannot produce a properly focused spot no matter what you do.
A previous discussion goes over the details (start here and read the next few posts):
If the machine doesn’t cut well and does not recover after cleaning / aligning and the test spot looks bad, then the tube has definitely failed.
I don’t think this is just tube age as for the 6mm thickness it cuts the same as it has done for years with the same settings, so performance doesn’t seem to have dropped off. I think I have just reached the limit of my system and throwing 50W at it going that slowly is just charring the wood to the point that it can’t be cut through. I will, however, try the 4" lens on some acrylic and foam to take advantage of the straighter cut.
I considered several rounds of laser cutting, in order to cut through thicker material, always as an “emergency solution” or compromise. Bringing energy twice in the same cut is rarely optimal and is used primarily when using diode lases, or CO2 lasers that are too weak for the task.
I think it is another situation when talking about deep engraving (3D) with eg fiber laser, but this process cannot be compared to cutting straight again of a material.
In my naivity, I also tried a 4" lens with my 100w CO2 and realized that the 2.5" lens produced superior results in “hard” materials (i.e. wood). It seems that only softer materials benefit from the longer focal length lenses. Even with 100W I struggle to cut materials I see others cutting with lesser machines. Could be my settings are not as optimized as I think. But, to reiterate, I was largely underwhelmed with the 4" lens in wood and immediately switched back and have not used the 4" lens since.
