don't use a rotating cutter - grind a cutter in a V with appropriate clearance and rake and set it up in the mill - but don't turn the mill on, you're going to use the X axis to create cutting action like a shaper. I usually use a bit of duct tape (handi mans secret weapon) to 'lock' the spindle brake on for ops like this.
The following shot should make it clear, these are 1 degree increments on about a 2.5" Dia piece. you get nice deep lines (i usually go 5-8 thou irrc, but just start off and raise the knee until it looks right) that are thin and well defined. this piece is steel, looks sort of like AL, but it was sandblasted, I had it normalized after the welding
