Monolithic roof cutting bit insert
A rotary drill bit for penetrating earth strata wherein the drill bit includes a drill bit body that has an axial forward end. The drill bit body has a hard insert, which is preferably monolithic, that is affixed to the drill bit body at the axial forward end thereof. The hard insert presents at least three discrete leading cutting edges for cutting the earth strata. Each cutting edge is stepped.
1. A rotary drill bit for penetrating earth strata, the drill bit comprising: an elongate bit body having an axial forward end; and a monolithic hard insert being affixed to the bit body at the axial forward end thereof, said hard insert having a trio of symmetric lobes, each said lobe having a discrete stepped leading cutting edge, each said stepped leading cutting edge having a straight upper step, a transition portion connected to said upper step and a straight lower step connected to said transition portion, said upper step of said leading cutting edge having an upper leading surface, said upper leading surface disposed at a rake angle with the vertical of between about five degrees to about negative fifteen degrees, said lower step and said transition portion of said leading cutting edge having a lower leading surface disposed at a rake angle with the vertical of between about zero degrees to about negative ten degrees.
2. The rotary drill bit of claim 1 wherein said upper step and said lower step on each leading cutting edge are parallel.
3. The rotary drill bit of claim 2 wherein said lower step and said upper step on each said leading cutting edge are oriented at an angle of about 5 to 35 degrees with respect to the horizontal.
4. The rotary drill bit of claim 1 wherein said cutting edge transition portion positioned between said lower step and said upper step rises a vertical height of generally between {fraction (1/16)}-⅛ inches.
5. The rotary drill bit of claim 1 wherein the rotary drill bit has a central longitudinal axis passing through the hard insert, the bit body having a peripheral surface, and each one of the leading cutting edges for cutting the earth strata begins at a point radially outward of the central axis of the hard insert and extends in a direction away from the central axis.
6. The rotary drill bit of claim 5 wherein each of said stepped leading cutting edges has a radially inward upper step and a radially outward lower step.
7. The rotary drill bit of claim 6 wherein each of said upper steps have a length of generally between ⅛-¼ inches.
8. The rotary drill bit of claim 1 wherein said upper step of each said leading cutting edge has a top surface relief angle of about 15 to 40 degrees, said transition portion of each said leading cutting edge has a top surface relief angle of 5 to 30 degrees and said lower step of each said leading cutting edge has a top surface relief angle of about 5 to 30 degrees.
9. The hard insert of claim 1 wherein each said lobe has a side clearance cutting edge corresponding to each one of said leading cutting edges.