Quote Originally Posted by xbow View Post
I checked with "The font of all (engineering) knowledge", (Tim!) at work and he offered the following: "you won't get far trying to lap aluminium with a steel bar, the grinding paste sticks to the softer material and laps the harder one"!
Slightly off-topic . . I'm familiar with the effect that Tim is describing. However, I've always presumed that the cause is the fact that all aluminium will have a very thin surface coat (once exposed to air) of aluminium oxide, which is of course extremely hard and often used as abrasive. Unless this surface layer is broken through, the aluminium oxide will resist the grinding paste better than the steel. Well that's my theory anyway.