Sorry if this duplicates others work- I was curious about marking stainless steel. Saw some posts about vinegar, paper towels, mustard, etc, etc, which I questioned. So I fired up the machine. Here are my results.
Stock is 440c stainless steel, precision ground (surface finish is somewhat like a scotchbright finish). I tested speeds 300, 200, 100 at full power. 225 lpi. One pass. I also included a Sharpie square of black for photo ref. since taking pictures of metal is so vague.
I did not use any vinegar, mustard etc. As far as I can tell, it is unneeded. Maybe I’d get a better quality (darker, but sharper) at more lines per inch but faster speed?