My cog tags are made out of 3mm aluminum and were both engraved and cut out using a CNC Mill
Step 1: Engraving And Cutting Out
I used a 60 degrees vbit for the engraving and a 3mm ball nose bit for the cut.
Found that cutting it out at a slower speed gave the best results.
Step 2: Sanding and Polishing
I then went and polished it on a buffering wheel