Very Strange Issue With Beam Deflection Through Lens

Hi. I’ve built a 50W CO2 laser cutter to my own design after using a converted K40 for a few years. The K40 is great but the small bed size was frustrating, hence the custom build. The new machine has a usable area of 650mm x 450mm and a Z travel of 100mm. Everything works great apart from an issue with the laser beam being deflected by the lens by about 4 degrees to the left, so any cuts I do have a 4 degree slope on the edges. It’s hard to explain what’s happening but I’ll give it a go. I’ve spent a lot of time aligning the raw beam from the tube. The beam hits the centre of every mirror at all points on the bed and runs perpendicular down the head tube to the lens. If I remove the lens and test fire at Z0 and then move the bed down 100mm and test fire again the laser spot is in exactly the same place and exactly in the centre of the head tube. If I then put the lens back in, still at Z100, and test fire, the spot is 7mm to the left of where the raw beam hit. As I move the bed back up the two spots get closer together. My first thought was that the lens was faulty but I’ve tried a new lens and the results are the same. I am absolutely stumped. Can anyone help?

I my experience there are only a couple things that can do this.

If everything is as you state, it should work, so I question all of the data.

I think you were right to check the lens, did you check that it’s not in there crooked? Something holding up the lens so it doesn’t sit flat. You could replace the lens and still have it ‘tilted’ because of debris or seal issue…

The other one is the bed is not flat, tube is not perpendicular to the bed.

I’ve aligned a few machines, but I have never seen one where the beam ended up in the center of m3, it usually took some kind of Y axes adjustment on the head to get it to go down the center of the tube…

What type of head do you have?

I use this acrylic jig on the end of my down tube for vertical alignment, maybe something similar you could try…?

I think I’d really inspect the tube/lens and how the lens is seating…


One thing that I would also check, is make sure that the Lens Tube is Perpendicular to the base plate. That “could” explain why the beam is ok without the lens, and then NOT OK with the lens. Think about the geometry a bit. You said all the mirrors were aligned, good… but that last few inches is also governed by how perpendicular the lens tube is. I’ve knocked mine out of alignment enough to speak about those concerns!

Either your beam is not hitting the lens dead center, or the whole carriage is tilted in that direction

Did you post a photo of your carriage? Is it custom or stock k4 style? The mirror #3 can be exactly 45 degrees but be rotated about the vertical axis which will give an angle to the beam.