So after my last test I wanted to revisit the idea of how spot size affects the cut depth.  I can't think of a reliable way to measure spot size, let alone adjust it with any reliability.  However I can raise and lower the laser head while cutting to take it out of focus without actually knowing the spot size. That should give me some sense of what is going on.  I had tried this previously but it was a failure.  The air assist I was using is attached to the laser head so it moved up and down with the head and affected the cut depth.  I also had tried turning off the air assist but that just resulted in a lot of burn damage.

This time I removed my existing nozzle and used a wire attached to the Y carriage to keep the air focused at the cut. This is not ideal, it was sitting at a fairly steep angle, but it seems to have worked.  I positioned the laser head as close to the work as I could, then ran a 30 mm long line while raising the head up 75 mm. The line was run at 200 mm/min at 40% power in a 12 mm deep piece of basswood with my NEJE A40640 laser module.

It is difficult to capture with a photo, but this is really far out of focus at the end of travel.

I need to repeat this with some more carful measurements but I think I have marked the position in orange where the laser would just be focusing on the surface of the material. You can see that is also roughly the point where we get the deepest cut.  This plays well with my previous experiments that seem to show the laser cuts deepest when focused at the surface, and not several mm below the surface as some manufacturers recommend.

The green line shows that there appears to be a linear relationship between focus height and depth.  I would need to do some math to see how height affects spot size.  I suspect the tapering off near the orange line has more to do with the waist of the laser resulting in basically no change in spot size across that whole range of focus.

Anyway this is not enough data to really tell if depth = 1/spotSize or not, but it is an interesting first pass at the question.  The most interesting to me is how broad of a range of focus you can use and still get a reasonable depth of cut.

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I put up another project to summarize all my depth of cut experiments into one single post.

https://hackaday.io/project/184785-lasermathics