What Am I Missing? Laser 5mm and 3mm above Workpiece

I need to cut 1/2" (13mm) poplar and pine/fir boards for my bride’s craft projects on the BeamBox Pro (BBP). By accident I learned that setting the wood cutting parameters to 50% power x .25 in/sec x 3 executions appears to always cut fir and usually cut poplar cleanly using those parameters IF I start the cut with the bottom of the lens housing about 3-5mm above the workpiece instead of the normal 12mm height from workpiece to housing. What is the problem, if any, in doing this? I don’t appear to get any additional dirt/fogging on the laser lens. I didn’t do this cavalierly. It started because one pass would cut approximately 50% of the way through. Given the distance should be about 12 mm from bottom of housing to workpiece I would lower the lens housing to 5mm +/- from the workpiece. For the third cut I would lower the lens an additional 2 mm making the bottom of the lens housing about 3mm above the workpiece. Then I forgot I did that an ran my project again… this time all three passes at about 3mm from the workpiece. And it worked better than when I had adjusted each pass to be about 12 mm from where it should “cut”. The ONLY discernible difference I find at this point is that the width of the cut (width of removed material) is perhaps 50% wider than when I run the cut at the prescribed difference. HOWEVER, when I would have to run it 3 and 4 times the variation in the separate runs would give me roughly the same width of cut (or material removed). It appears that the near first cut (with the bottom of the lens housing only 3mm above the workpiece) doesn’t cut any worse than if I started 12mm above the workpiece. Thoughts?

Seems like it should be fine. I’d think the air assist would still do a good job of clearing smoke and debris out of the way. Mostly, I’d think that 3mm is a very small clearance and I’d be careful about crashing the laser head into anything if the board was warped by 3mm, or there were any hold-downs protruding above the piece.
In terms of the actual behavior of the cut. I tend to picture the beam coming out of the lens as somewhat an hourglass shape. At the point where it’s perfectly focused it narrows down to a pinpoint where it’s smallest and hottest and cuts best. Anything above and below is wider, more disperse and cooler. I’d imagine that with multiple passes out of focus you’re simply passing the wider part of the beam over the top portion of the board 3 times, instead of once. This probably hogs out more material (hence the wider cut), but you’re obviously using sufficiently high power levels that it burns through just fine even though the beam is a bit out of focus.
What you’re doing is probably not ideal, but as long as you don’t crash the head into anything, and maybe clean the mirrors a bit more often, then I can’t argue with success.

You may not be up for a redesign, but have you considered doing multiple cuts in thinner material and then gluing up the pieces? I made some pretty cool race medals by cutting 3 sheets of 3mm baltic birch plywood and then gluing together. The middle sheet was painted so you got some nice color and depth effects between the three sheets.

Thanks Alex… your concerns were also mine. How, I check z- height in 5 places and use quality materials… 3mm is sufficient although I don’t see much difference between 3 & 5 mm. Thanks for the engagement.