Cubic Boron Nitride (CBN) also known as Polycrystalline Cubic Boron Nitride (PCBN) are ideal for cutting hardened tool steel (>HRC40). When not to use CBN - Aluminium and Aluminium Alloys - CBN contains trace amounts of Aluminium Nitride which chemically reacts with aluminium which causes it to bond with the cutter, which both leaves a terrible surface finish and rapidly wears the cutter.
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