Robbins & Laramie
Drill bit for core boring
 A drill bit for core boring of the type provided with a multi-layered bit head, so constructed that when the blades of the upper-layered bit head have become worn out, it is possible to easily withdraw them without removing the bit from the bore. The core boring operation is then continued without delay by using the blades of the newly exposed lower-layered bit head. The drill bit is also so constructed that it is possible to eliminate any displacement of the upper layer blades from their fitted position on the blades of the lower layer blades during drilling by using the upper layer blades and at the same time, it is possible to easily disengage the engagement between the bit body and the upper layer blades when the upper layer blades have become worn out. 
Tone Boring Company Limited
E E
E21B E21B
inventional inventional
Makoto
Saito