TOOL INSERT

摘要

A tool insert comprises a substrate (40), a layer of ultra-hard abrasive material (52) bonded to the substrate and presenting a primary cutting edge (54) for the tool insert, and a protective layer (56) bonded to the ultra-hard abrasive material so as to protect the primary cutting edge thereof. The protective layer provides a secondary cutting edge (58) for the tool insert, the depth of the protective layer being selected so as to be sufficient to protect the primary cutting edge whilst cutting, milling or drilling a window through a first substance, such as a casing or lining of a borehole or shaft, but to expose the primary cutting edge upon encountering a second substance, such as a rockbed. The invention extends to a method of drilling a horizontal or angled hole in a subterranean rock formation using a rotary drill string and bit incorporating a tool insert of the invention.