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Face Milling: 45 or 90 Degree Face Mill? - CNCCookbook: Be A Better CNC'er
The angle is the angle of that cutting edge-45 degrees for the face mill on the left and 90 degrees for the one on the right. That angle is also called the lead angle of the cutter. And here are the optimal operating ranges for different geometries: By the way, round insert cutters are also called "Button Cutters".