Originally Posted by ChevroletSS
Check this out >>> GOODSON : Porting & Polishing : Rotary Files
<<< it shows the difference between rotary files for iron versus aluminum. Iron uses a finer flute than those for aluminum. This is because aluminum melts into the clearance between flutes plugging and gumming them up till they won't cut. Iron just makes solid chips that fly off the cutting face.
You can use a coarse aluminum bit on iron all that will happen is a rougher surface finish from some chatter the wider flutes create, but they'll cut just fine. Aluminum will gum up the smoother fine iron flute almost instantly. You can kind of get away with the fine flute on aluminum if you frequently run the bit into a piece of bar bath soap. This lubricates the flutes preventing the aluminum from sticking. Same thing if you're cutting aluminum with a rotary stone or drum sander, run the stone or drum sander into piece of bar soap every few cuts and it'll stay pretty clean of imbedded aluminum. This of course leaves your bits and stones smelling clean and refreshing.