Cross drilling is a machining process
used to help automotive brake rotor
heat better. A series of small diameter holes are bored through the rotor to provide more surface area to radiate heat. The holes are either cut perpendicular to the rotor
surface or at a 45 degree angle.
This process actually weakens the structural integrity of the rotor so it is generally seen in high performance/racing applications only where expensive alloy rotors are required.