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Jake, Sven,Thanks for the responses. I wasn't looking to engrave to any significant depth in metal, I was just wondering what was the minimum power needed to just put a readable mark on it. Since reading your post I have looked around and see that LED lasers are available up to 10W.
Nice idea. i don't have a 3D printer but I could fit it to my CNC mill. I saw mention that 500mW is not enough for metals, does that mean that it won't even mark them or just that it won't burn deep?