I am machining a vertical surface 2" tall that follows a gradual spline profile using a Haas VF2SS vertical CNC mill and looking for the best possible finish. The material is 4340 annealed. I have tried a high performance 4 flute 20mm coated carbide cutter but I am not satisfied with the finish. I can't surface grind the piece because of the spline profile.

Does anyone have a recommendation for an end mill, an insert cutter or process that can give me a superior surface on 4340 with a finish grade N8 (125 micro-inch) or better that is truly perpendicular (no taper) to the horizontal surface.

Thanks for your ideas!