K40 Big Ghosting issue

Hello everybody,

After some issues last week with my K40, it started to work. Yesterday I made a cutting board test for my father’s brithday. Today when I tried a second test, I’ve noticed that there’s ghosting or additional burn beside the center point. In picture you can observe that beside the expected point there’s a curved line.
My actions:
I have cleaned all the mirrors, checked if centered, and all are fine
Cleaned the focal lens (the one on the bottom) and the mirror in the laser head (the top angled one) but no cracks or debris.
What can it be? I checked all Youtube advices and followed them accordingly, I have checked Don’s forum but I cannot find a simillar issue.

I have attached pictures with tests and the actual issue at multiple focal distances but the same issue it’s appearing again and again.

I will provide additional information if necessary but at the moment I cannot understand what’s happening.

Thank you!

One suggestion: With the nozzle and lens removed, put tape on the bottom of the lens tube and mark the full circumference of the lens tube. Test fire, remove the tape, and post a good picture taken from directly above the tape showing the burn and the marked circumference.

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I did as you have suggested:
It projects a curve.

Huh, was that with the lens as well as the nozzle removed?

You’ll definitely want to work backwards to find out where two problems are introduced: the beam is not centered in the lens tube (you definitely want it to be), and the arc is of course not the beam shape you want.

That arc shape is often caused by an off-center laser beam (as you have) reflecting off the inside of the air nozzle (but you have removed your air nozzle to put the tape on the tube). The only way I know of to narrow this down is to work backwards, up the beam, one step at a time.

  1. The next step is to put tape where the beam enters the head and do one test fire there, and similarly show the shape of the beam there and the location relative to the hole.

  2. I haven’t asked whether this problem is the same no matter where on the bed the laser is firing. So… Try test firing the laser without lens and without nozzle at paper in all four corners of the bed, to test whether it is the same at every extreme.

Hello, yes the pink marker was traced around the empty thread. I have checked and I think the mirrors are not properly aligned for the far end of gantry. The first test was done close to the origin/home and it went well but further it gets from it, the worse it is. I will try and realign all the mirrors. Also i have unmounted the full head and check it out, it was machined so coarse, I will file it down a bit maybe it affecta the reflection. I will come back with the results.

I would not expect coarse machine work to affect reflection; the beam shouldn’t touch the metal of the head at all, nor the metal of any of the mirror holders before the head.

I can’t remember whether we’ve shared this with you before, so just to make sure you have this information easily to hand, from K40 initial setup and regular maintenance: Alignment

The path that the laser takes along three mirrors and the lens needs to be precisely aligned. Here are three resources that may help you understand how to do this.

If you prefer video instruction:

Ep6: Laser Beam Alignment. The DIY CO2 Laser Cutter / Engraver Build Series

If you prefer written instruction, here are two guides:

Note that if the third mirror does not align the beam with the middle of the lens, the beam will not be focused correctly and will not be aligned with the hole in the nozzle. You may wish to use a device that helps you align using a red visible laser. One kind uses a “beam combiner” that slightly reduces beam power, and the other is a tool used only while performing alignment .


Yes, these are the instructions I am using for the alignment. It’s a tiresome process to be honest, for a beginner like me but I will come back with results.
Until now: I have sanded the pieces of material that were hanging, I have aligned and centered the first two mirrors, I saw a big air bubble stuck on the beam end of the CO tube so I got rid of it (i am not sure if this might affect), I have leveled/bent just a bit the piece of metal that holds the second mirror, because it was a bit to low and it forced some strange alignment angles.
Ongoing: Alignment of the head mirror (reported to 2nd mirror), it is a huge delta between the point at the close range and far range of the X axis, in fact, on a 50 by 50mm piece of tape, the distance between them is like 40mm.
Thanks for support, topic is ongoing.


Hello again,

So, I can confirm that it was a combination of mirrors missalignment, bent brackets and lack of QC. Now the beam is centered, without artifacts and it burns/cuts as it should.

Thank you very much for the support!