I've done logo and text engraving in Aluminum that turns out great. The trick is to get the part perfectly level. I use a dial indicator mounted to a bracket on my spindle. I can jog the dial indicator back and forth across the surface to make sure I have it within .003 inch flatness.
I've also used a "spring loaded" V bit end mill with good results; especially works great on uneven surfaces. Leveling doesn't need to be perfect. Here's a pic where I engraved on a piece of quartz.