I just did a couple calibration patterns on a piece of paper that isn’t perfectly flat. The engraving starts at the bottom and travels up.
less flat:
more flat:
It should be flat across the bottom, and it’s not.
But, even though the paper isn’t flat, the tops are straight across.
I think the conclusion I’m coming to is the laser can be on the verge of engraving, but just not quite enough to get the burn started (not enough power, focus just slightly out, slightly too much speed). It takes a bit for it to actually get enough heat to start the burn. But once it’s started, it has no problem continuing and won’t drop out - there’s a lot of hysteresis and would require a large change.