Seeking Advice on 101.6mm (4") Lens Focusing Issue for 90W CO2 Laser Cutter

Hello everyone,

About two years ago, I successfully designed and built a 90W CO2 laser cutter, which has been running exceptionally well. I’ve primarily used a ZnSe Meniscus Focus Lens (20mm / 63.5mm or 2.5") for crisp engravings and cutting materials up to 5mm thick.

Recently, I’ve been exploring the use of thicker materials and noticed the limitations in the focal area of my current lens. For experimentation, I decided to try the 101.6mm (4") ZnSe lens, which I had ordered along with the 63.5mm lens. However, I’m encountering difficulties in focusing this lens properly. Despite numerous ramp tests and adjustments at various distances, I haven’t achieved a clean focus. The best result was a kerf of about 1.5mm, surrounded by at least 3mm of burnt area, far from a clean cut.

Am I missing something here? Is there another parameter, apart from the distance to the material, that I should consider?

The machine works perfectly fine with the 63.5mm lens, but I’m curious about what’s causing the issue with the 101.6mm lens. Also, it’s been a while since I ordered these lenses – are there any new lens types on the market, other than ZnSe, that I should consider?

Any advice or suggestions from fellow enthusiasts would be greatly appreciated!

Thanks in advance!

That seems a bit excessive.

To confirm, you’re mounting the lens convex side up?

How are you measuring focus distance?

Can you post some photos of the resulting burn?

Initially, I positioned the lens with the convex side facing upwards. However, after encountering issues with the focus, I began to question this orientation and decided to experiment by flipping the lens to test both sides. Interestingly, this change in orientation didn’t result in any significant difference in the focusing outcome.

For the ramp test, I covered an extensive range of distances, deliberately exceeding the expected focusing distance for the 4" lens. This was to ensure that no potential focal point was missed within a reasonable range.

Currently, I’m away from my maker space, but I understand the importance of visual details in addressing this issue. I plan to capture and share a picture of my setup as soon as I return, which should help in diagnosing the problem more effectively.

This doesn’t entirely surprise me which is why there’s still some controversy about correct orientation. Convex side up is still your best chance of getting to nominal expected performance.

You may not want to rely exclusively on the ramp test to determine cutting performance. While related, the tightest line on the ramp test does not always translate to best cutting performance. When you get a chance try an actual cut test at various focus heights below and above the 4" focal length.

This will be helpful. Looking forward to it.

  1. properly working tube
  2. clean and aligned optics
  3. proper focus

If you have these three, they work.

Your statement seems to indicate a number 2 issue. :poop:

The 4" will have a larger dot than the 2" lens. From this on-line calculator.

Lens length mm (inches) Spot size (mm) Depth of Focus (mm)
50.8 (2) 0.170 1.9
101.6 (4) 0.342 7.59

This is a 2" lens cutting 5mm sub flooring… standard aquarium pump, lid up.

You can see it’s not right. It has a depth of field of < 2mm, leaving at least 3mm of material not within the focal range. You can see dark areas when it’s cut out. A rag wiped the debris off.

A photo of your results would be nice…

I’d suggest you track down the focusing this lens properly. Focus is what makes these work. If you can’t get a focus, it will never be up to par…

  1. lose or not level lens.
    a. this is pretty common. If it doesn’t sit level the beam will emerge out of alignment. Dirt, objects not letting is seat correctly.
  2. elaborate a bit more to us about what/how it’s not properly focused, such as vertical alignment, mirrors …

You can ensure it’s coming out of the nozzle with this Russ Sadler tool. In this good alignment video … at 32:31 he uses this tool.

Beam Checker 2.lbrn2 (12.2 KB)

Good luck


So, after your comments I stepped away from the ramp test and actually measured my distance from the material and set it to exactly 4" from the lens. It turns out this gives a pretty decent result.

The kerf is a bit wide for my opinion (0.9 ~ 1.1 mm), but it does cut smoothly trough 9mm plywood with 5mm/s.

It’s an interesting result. It might be fun to try a 1" or 1.5" lens next.

Thanks all, for your help!

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Much better result but that kerf does seem quite wide, but I don’t have experience with that particular lens.

@jkwilborn raises an important point about dot size. You may want to confirm that the beam is exiting the center of the nozzle and is indeed perpendicular to the bed.

After a good cut, I can wipe a white Kleenex over the cut and there is little to no mark…

Sounds like you’re getting a handle on it…

I assume you are focusing to the center of the material…? have to ask.