Having discussed the wire line socket, drilling jars (which we may or may not use) and the drill stem, we come to the most important part of our cable tool string: the drill bit. Of course, all parts of the drill string have important roles, and we fail without any one of them, but the drill bit probably ranks first among equals. Whether in unconsolidated or consolidated (rock) formations, the drill bit makes the hole. Cable tool drillers can choose from several styles of bits. I’ll start with what we call a “regular” bit — by far the most popular — and discuss some different bit designs in future columns.
The end, or worm’s eye view, of a regular drill bit looks like a letter H with rounded sides. On a well-dressed bit, the sides make up about two-thirds of the circumference of the hole. The remaining open area is called the water course. This water course allows the drill bit to drop and creates mixing of the slurry (part of what the drill bit does). The bottom of the bit from a side view formed a flat V shape. The harder the drilling — especially in rock — the flatter the V wants to be. In unconsolidated drilling, the V can be a little sharper, but if it is too sharp the mixing effect of the bit suffers. In my experience mostly drilling in clay, sand and gravel, the V needs to be at least 90 degrees minimum and up to 135 degrees.