1/2″ Spoilboard Surfacing & Rabbeting Bit Insert Cutter. 1-1/2″ diameter. 1/2″ Shank. The ideal bit to keep your spoilboard perfectly flat for the utmost accuracy in your projects, this bit is also great for routing precise rabbets. The replaceable solid carbide inserts can be rotated so all 4 edges are used, providing extremely long life, even in abrasive materials. Ideal for CNC, mortising & dadoes.