Mike , Yeh , its never espressed in seconds , as its pulses per inch.
It has to be derived from frequency however - as it has to be dependent on speed of travel.
What Im wondering is .. we all realise that headspeed is never really constant ..do the machines that quote PPI actually vary freq continually to compensate for this?
In the case of a machine that allows you to set freq alone , the pulses per inch using that freq would be related to head speed. So a user would have to know head speed to set the correct PPI to avoid or minimize charring.
At the end of it all , getting back to the original question , the obvious answer is that the low powered laser is the wrong tool for cutting thicker wood if you want clean edges.
Rodney Gold, Toker Bros trophies, Cape Town , South Africa :
Roland 2300 rotary . 3 x ISEL's ..1m x 500mm CnC .
Tekcel 1200x2400 router , 900 x 600 60w Shenui laser , 1200 x 800 80w Reci tube Shenhui Laser
6 x longtai lasers 400x600 60w , 1 x longtai 20w fiber
2x Gravo manual engravers , Roland 540 large format printer/cutter. CLTT setup
1600mm hot and cold laminator , 3x Dopag resin dispensers , sandblasting setup, acid etcher