I worked this out. Prob was the metal was getting too hot. At high temps, where the metal starts getting plastic, the galvo cutting process will fail.
The metal will absorb the energy, but doesn’t create the tiny explosion that throws off a metal particle as a spark. It will just get hotter.
It will not cut or engrave very effectively in this state. The cut’s brilliance will fade even though it’s in focus, giving the appearance that perhaps the laser is losing its power output. But that’s not the case.
It is MUCH more prominent of an issue with stainless. The metal stock thickness and its overall size matter a lot. A cooling fan is of some help. I think I’m going to try a mister like you use for CNC cooling.
I have a JPT M7 300W, I don’t know anyone else running one this big. This surely made the effect more noticeable for me. The 60W or 100W that everyone else has would have a hard time overheating.
A stainless coin blank can overheat really fast if you use the cutoff freq, but it depends on how much of the layer has to be rastered. It’ll warp, too.
I think the best way to limit it is not to reduce power, but slow down the scan rate, pick a freq that makes 1-2 pulses for the focal spot size, and the pick the longest q-pulse that you have that stays below the cutoff window