On the outside edge of the rotor, if it is worn there will be a substantial lip. This lip could make contact with the metal base of the pad and grind even if you have good pad material left. If that is the case you probably are overdue for a new rotor.

Tom W

__________________
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.