| 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 sub-terranean rock formation using a rotary drill string and bit incorporating a tool insert of the invention. |
