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Here goes with the two big questions (with lots of sub-questions!) that are still bugging me:
a) Laser "Kerf" and how to deal with it?
I made a very simple picture (a square, triangle and circle on top of each other) as a test piece. I vector cut the veneer pieces easily enough (one sheet of veneer at a time), but when I put them together to make the picture, the laser "kerf" (gap between the pieces) seems much larger than I was led to believe it would be.
I ran another test job with a bunch of vector cut lines, each one mapped (via color) to a different speed setting on the laser. I tested cuts from 20% speed in steps of 10% up to 100% (power was 100% for all cuts) and found that there is quite a bit of difference in the kerf width. As expected, the 100% speed / 100% power cut gives the narrowest kerf and easily cuts the thin veneer material. I measured it at about 5 mil on the top of the veneer and about 3 mil on the bottom. Why is it thicker on the top?
Ok, there are two effects in play here. As others have noted, if the focus of the beam is at the top surface, the beam diverges between there and the bottom surface. But, as you say, that's backwards of what you normally see, and in any case is a very minor effect on stock that thin. The beam width is actually much less (at the focus point) than the kerf width: the kerf is mostly material that has been vaporized by the heat. The beam loses power as it works through the wood, both through absorbsion and divergence, with the result that the kerf is
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I had ran the initial test piece using 30% speed / 100% power, so the kerf was about 15 mil per the second test - no wonder it was "sloppy" marquetry! I re-ran the square/circle/triange job using 100% speed / 100% power, but even with the narrowest possible kerf, the gap is really too big for good quality marquetry.
You'll want to reduce the power for cutting veneer. You've done your experiment with power fixed, now start at 100% speed and step the power down until the veneer is just