I’d hate to reply to this discussion further about marking on metal under the post for plywood knots; however, I can’t resist responding since there has been so much talk about it. When I had access to a laser cutter a few years ago, I purchased some [dry moly lube] (http://www.mscdirect.com/product/details/00155416) instead of purchasing Cermark, etc. I cleaned a piece of aluminum with IPA (isopropyl alcohol), waited for it to dry, and sprayed a coat of the dry moly lube all over the aluminum 6" diameter disk (1/4" thick of 6061-T651 aluminum…the T-651 is the heat treatment applied which may/may not affect the “marking” results). I waited for the moly lube to dry (10 to 20 minutes) and threw it in the laser cutter (40W Epilog). I put the power on 100%, and the speed as slow as it could go (I don’t remember what number value that was or what real world value that has…I also think the frequency was set to 5000 or 500…the Epilog has the ability to change what frequency the laser pulses at (not 100% sure that is what the frequency does but it’s my working theory)…I set the frequency once real low so it made a dashed line along a vector engraving path so one could fold a piece of paper back-and-forth and tear it off…like a perforated line).
Sorry, that was a rather long tangent. Getting back to the topic at hand, the metal marking was performed with raster scanning (not vector “cutting”) and the best results were obtained with multiple passes over the same area to be marked (I just kept the part on the laser bed and re-printed two or three times). I marked a graphic that was about 1" x 1" (the job took about 15 minutes each time…that’s how slow I went). To finish the job, I took IPA to clean up the rest of the moly lube. When you do this, the marking stays behind, but if one scrubs to hard, the mark tends to “go away” and lighten (and with a slight amount of elbow grease, the mark can be rubbed off completely). I would imagine a better work flow for us 'Forgers will be to mask the metal, use the laser to cut the mask, remove the pieces where you want to mark your metal, spray it with the moly, put it back in the laser and run the laser to burn the moly onto the metal (that way you don’t need to clean up the over spray). I also tried stainless steel (not sure specifically what grade it was). I did the same setup (sprayed the lube, dried, burned 3 times, rinsed) and achieved similar results as the aluminum. I’m not sure why I didn’t do it then, but I would think one might have better results if they did vector engraving at 100% (which I would say is vector cutting). Perhaps then to protect the mark, one could apply a wax or sealant over the metal (or clear coat?).