In my last column, I promised to write about some uncommon drill bits this time, so here goes.
The first is called an underdigger or eccentric bit, also sometimes referred to as an underdog. (How it got this name is beyond me, and really makes no difference.) The cross section of an underdigger bit looks more like a T than the bent H section of a regular bit. You only dress it on the wide side of the T, and the bottom of the tall side is finished at an angle of around 30 to 45 degrees. This bit, by design, goes down through casing and, when it encounters drillable formations, the dressed-out portion is forced into the wall of the drill hole. If properly dressed, it makes a hole larger than the diameter of the couplings and drive shoe on the casing.