CBN is short for CarbonBoronNitrate. This is an extremely hard man-made material that is used to make long life milling tips. They are in the form of a small disc that can be rotated in the tool holder to present a new cutting edge whenever required. They are normally double sided so they can also be turned over when all edge points are dull on the first side. The cost of these tips is high upwards of $400, but they produce a good finish over a long time period and are more economical and versatile than conventional tool steel or carbide tips. Being extremely hard they can successfully mill hardened materials such as pre-chambers or exposed inserts.