I have recently bought a K40 laser cutter and came across something a bit

I have recently bought a K40 laser cutter and came across something a bit strange. When adjusting the mirror alignment I also did a final spot test with the masking tape over the final aperture (before focal lens) and I found that the beam was always offset from centre no matter what I did.

I took the laser head off, stripped it down, measured all the components and made a drawing. This what I discovered. To get the beam central on the focus lens, I need to be 1.45mm above the entry aperture centre.

Am I correct in this conclusion?

You may be. I found mine had the same problem. So I added some washers to shim it up. However, I was replacing the stock lens housing with the air assist LightObject one at the time so…

The K40 alignment is challenging with the poor mechanical design they have.

may very well be true. Although it doesn’t really matter after the beam entered the laser head since it focuses to a fixed point anyway.

I had shim my head up as well.

This could explain why I have barely been able to cut through 1mm cardboard on 90% power. I haven’t had time to troubleshoot that yet. The other thing I noticed with mine is that the large hole where the beam enters the cutting head is off-center. Specifically, the center axis of the hole does not intersect the center axis of the vertical cylinder–not even close. I dismissed it as not being much of a problem, as long as the beam hits the center of the mirror. I will reexamine that latter assumption when I look into the beam height issue noted here. Thanks!
EDIT: this may explain how the beam wanted to burn through my 3D printed air assist nozzle before I could cut with it.

@SirGeekALot James, cardboard at 90% means you have a more of a problem than I had. Tip: when I was going through the mirror alignment procedure and had got them as good as I could (I was using masking tape) I too couldn’t cut cardboard, most frustrating … and then I cleaned the mirrors and focal lens with acetone and bingo, straight through 3mm acrylic at about 10mA and 10mm/sec. (38%/40%) on my dial. The glue from the masking tape had deposited on the mirrors and diffused the laser beam.

@Duncan_Caine I might have to try that. So acetone is safe on the lens and mirrors?

I wouldn’t. I don’t know about anyone else but I use alcohol and distilled water 50/50 and good cotton balls. softly dragging the soaked ball over the surface but never pushing down on it .

One wipe around get another clean cotton ball do it again one wipe only. As many as you need until clean. You know it’s right when it dries clean.