- You can't cut metal in the laser cutter: if you wish to mark the metal, please use Thermark / Cermark, or use anodised aluminium.
TODO: add somthing about laser ablation.
Akki did a "cut" line design (as opposed to etching which is filled in) on a black Dell laptop and it was done in less than 30 seconds.
MacBook Air 2013-08-07
Simon etched the back of his macbook air using the Engraving setting.
Speed was 200, Power was 80.
The effect looks good in bright light (when it looks black) The 'whiteness' of the etching is inconsistent, looking dirty/smokey in areas where it was denser. I'm guessing maybe some of the smoke? got annodized into the metal? Anyway it was on the back and just a test.
A couple of pictures are up at 
Planning on what to do to the front. 
MacBook Air 2014-03-08
Patrick engraved the front of a MacBook-pro
Speed was sloooow, power was 100, and the design was barely visible after over an hour of lasing time.
Maybe the anodised aluminium has changed in recent years?